applied sciences

Review Perspectives in Liquid-Crystal-Aided Nanotechnology and Nanoscience

Yuan Shen and Ingo Dierking * School of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, UK; [email protected] * Correspondence: [email protected]



Received: 29 May 2019; Accepted: 14 June 2019; Published: 20 June 2019


Featured Application: Nanomaterial-doped liquid crystals.

Abstract: The research field of liquid crystals and their applications is recently changing from being largely focused on display applications and optical shutter elements in various fields, to quite novel and diverse applications in the area of nanotechnology and nanoscience. Functional nanoparticles have recently been used to a significant extent to modify the physical properties of liquid crystals by the addition of ferroelectric and magnetic particles of different shapes, such as arbitrary and spherical, rods, wires and discs. Also, particles influencing optical properties are increasingly popular, such as quantum dots, plasmonic, semiconductors and metamaterials. The self-organization of liquid crystals is exploited to order templates and orient nanoparticles. Similarly, nanoparticles such as rods, nanotubes and graphene oxide are shown to form lyotropic liquid crystal phases in the presence of isotropic host solvents. These effects lead to a wealth of novel applications, many of which will be reviewed in this publication.

Keywords: liquid crystal; ferroelectric nanoparticle; quantum dot; graphene oxide

1. Introduction For many materials, the transition between the liquid and the solid phase is not a single-step process, but a range of various mesophases, which are called liquid crystals (LCs). LCs are self-organized anisotropic fluids that are thermodynamically located between the isotropic liquid and the crystalline phase, exhibiting the fluidity of liquids as well as the long-range lattice order that can only be found in crystalline solids [1–3]. Generally, LCs are composed of anisotropic building blocks (usually of rod or disc shape), which are spontaneously oriented along a specific direction, called the director n [4]. Without an external alignment force, the director of a nematic LC, the simplest phase of LC, whose molecules are only orientationally ordered, is usually spatially changed continuously but randomly over large spatial extensions (except for defects, where the director may vary suddenly and drastically) [5,6]. Normally, one can split LCs into two typical categories, i.e., thermotropic LCs and lyotropic LCs [7–10]. Thermotropic LCs are usually further distinguished according to the shape of their constituent molecules, being called calamitic for rod-like, discotic for disk-like and sanidic for brick- or lath-like molecules (Figure1a) [ 11]. A typical calamitic mesogen is generally composed of a rigid core, often incorporating phenyl and biphenyl groups, and two flexible endgroups, often alkyl or alkoxy chains. A common structure of discotic mesogens is a rigid, disk-like core to which six flexible endgroups are attached. Apart from these conventional mesogens, research attention has been recently focused on the so-called non-conventional LCs [12], which are neither rod- nor disk-shaped. Among them, bent-core LCs [13], in particular, have received attention due to their unique effects of

Appl. Sci. 2019, 9, 2512; doi:10.3390/app9122512 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 2512 2 of 47 Appl. Sci. 2019, 9, x FOR PEER REVIEW 2 of 47 theunique observation effects of chiralitythe observation from achiral of chirality molecules, from resulting achiral from molecules, sterically resulting induced from packing sterically of the bent-coreinduced packing mesogens of [the10], bent-core such as ferroelectricity mesogens [10], or such the formationas ferroelectricity of helical or superstructures the formation of in helical the B7 phasesuperstructures [14]. Thermotropic in the B7 LCsphase are [14]. commonly Thermotropic constituted LCs byare single commonl organicy constituted entities or by mixtures single thereof,organic whichentities exhibit or mixtures various mesophasesthereof, which at di ffexhibiterent temperatures various mesophases or pressures at [15different], illustrated temperatures in Figure 1b.or Aspressures the temperature [15], illustrated rises, a in typical Figure thermotropic 1b. As the LCtemperature passes through rises, highera typical ordered thermotropic phases, alsoLC passes called softthrough crystals, higher the ordered hexatic phases, smectic also phases called with soft positional crystals, orderthe hexatic as well smectic as bond phases orientational with positional order, throughorder as thewell fluid as bond smectic orientational phases (SmC order, and through SmA), the which fluid exhibit smectic both phases positional (SmC andand orientationalSmA), which order,exhibit and both finally positional to the and nematic orient phaseational (N) order, with purelyand finally orientational to the nematic order, into phase the (N) isotropic with purely phase. Theorientational number of order, different into phases the isotropic observed phase. depends The onnumber the chemical of different composition, phases observed symmetry depends and order on ofthe the chemical LC molecules. composition, About 25symmetry different thermotropicand order of phases the LC are knownmolecules. to date, About and they25 different are still increasingthermotropic in number.phases are known to date, and they are still increasing in number.

FigureFigure 1.1. ((aa)) TheThe variousvarious shapes shapes of of LC LC molecules. molecules. (b ()b A) A typical typical illustration illustration of diofff differenterent thermotropic thermotropic LC phasesLC phases observed observed on heating on heating from crystalline from crystalline state. Reproduced state. Reproduced with permission with permission from [7]. Copyright from [7]. (2017)Copyright MDPI. (2017) MDPI.

InIn contrast,contrast, lyotropiclyotropic LCsLCs consistconsist ofof atat leastleast twotwo didifferentfferent kindskinds ofof components:components: aa collectioncollection ofof anisodiametricanisodiametric moleculesmolecules andand particlesparticles disperseddispersed inin aa suitablesuitable solventsolvent (Figure(Figure2 2).). These These systems systems will will thusthus alwaysalways be be mixtures, mixtures, with thewith main the control main variablecontrol beingvariable the concentration.being the concentration. Unlike thermotropics, Unlike thethermotropics, phase transitions the phase of lyotropic transitions LCs of are lyotropic not merely LCs are dependent not merely on thedependent temperature on the of temperature the system, butof the also, system, mainly, but on thealso, relative mainly, concentration on the relative of each concentration component of [15 each]. Being component mixtures, [15]. there Being will alwaysmixtures, be two-phasethere will regionsalways atbe the two-phase phase boundaries regions at between the phase two boundaries different lyotropic between phases. two different One can distinguishlyotropic phases. lyotropic One LCs can into distinguish three different lyotropic kinds: (i)LCs amphiphilic into three lyotropics,different (ii)kinds: colloidal (i) amphiphilic lyotropics, andlyotropics, (iii) chromonics, (ii) colloidal where lyotropics, the constituent and (iii) moleculeschromonics, are where dye molecules the constituent in a suitable molecules solvent. are dye As themolecules name indicates, in a suitable amphiphilic solvent. lyotropic As the LCsname are indicates, usually composed amphiphilic of amphiphilic lyotropic LCs molecules are usually upon additioncomposed of aof solvent amphiphilic [16], often molecules water. Asupon Figure addition2 shows, of witha solvent increasing [16], amphiphileoften water. concentration, As Figure 2 dueshows, to thewith segregation increasing of amphiphile hydrophobic concentration, and hydrophilic due regions, to the thesesegregation molecules of self-assemblehydrophobic intoand hydrophilic regions, these molecules self-assemble into spherical or rod-like micelles, leading to the formation of the hexagonal phase, cubic phase or lamellar phase. Nevertheless, the solute Appl. Sci. 2019, 9, 2512 3 of 47

sphericalAppl. Sci. 2019 or, rod-like 9, x FOR PEER micelles, REVIEW leading to the formation of the hexagonal phase, cubic phase or lamellar3 of 47 phase. Nevertheless, the solute component of a lyotropic LC need not always be molecular in nature, butcomponent may also of consist a lyotropic of much LC larger need (solid)not always particles be molecular with anisotropic in nature, shapes. but may It can also also consist be of colloidalof much sizelarger [7], (solid) as we particles will discuss with further anisotropic in Section shapes.3. SuchIt can materials also be of would colloidal then size be [7], known as we as will inorganic discuss liquidfurther crystals, in Section LC 3. clays, Such but materials also nanotubes, would then graphene be known oxide as orinorganic biological liquid structures crystals, such LC as clays, viruses but (Figurealso nanotubes,3). graphene oxide or biological structures such as viruses (Figure 3).

FigureFigure 2.2.A A typical typical lyotropic lyotropic LC LC phase phase diagram diagram of amphiphilic of amphiphilic molecules molecules dissolved dissolved in a solvent. in a solvent. Cubic phasesCubic mayphases be observedmay be atobserved different positionsat different in thepo phasesitions diagram. in the Atphase very diagram. high concentrations, At very high the inverseconcentrations, phases are the located. inverse Reproduced phases are withlocated. permission Reproduced from [7with]. Copyright permission (2017) from MDPI. [7]. Copyright (2017) MDPI. The fact that the director of LCs, which is often equivalent to the optic axis of the material, is easily influencedThe fact by that a variety the director of external of LCs, stimuli which such is asoften mechanical, equivalent magnetic, to the optic electric, axis orof opticthe material, fields, as is welleasily as influenced temperature, by makes a variety liquid of crystalsexternal attractive stimuli such to both as themechanical, industry magnetic, and academia electric, [6,17 or–26 optic], as exemplifiedfields, as well by theas temperature, $100 billion industrymakes liquid built aroundcrystals displays attractive and to large-screen,both the industry flat-panel and consumeracademia electronics.[6,17–26], as Due exemplified to the great by achievement the $100 inbillion displays industry since thebuilt 1970s, around LCs aredisplays one of and the mostlarge-screen, popular materialsflat-panel around consumer the worldelectronics. [27]. However, Due to the with great the achievement rapid development in displays and impressive since the 1970s, advantages LCs are of organicone of the light-emitting most popular displays materials (OLEDs), around there the is world some competition[27]. However, emerging. with the Over rapid the development last decade, moreand impressive and more scientists advantages have of moved organic their light-emitting attention away displays from display (OLEDs), materials there is to some a diversity competition of new fields—foremerging. instance,Over the new last optical decade, devices, more telecommunication,and more scientistsinformation have moved storage, their attention energy conservation, away from elastomerdisplay materials robots, sensors,to a diversity biotechnologies, of new fields—for nano-/micromanipulation, instance, new optical just devices, to name telecommunication, a few [15,28–45]. Theseinformation new fields storage, can be quiteenergy different conservation, from displays andelastomer more stimulating robots, duesensors, to their biotechnologies, novelty, leading tonano-/micromanipulation, a new era for LCs, materials just design to name and a technology. few [15,28–45]. These new fields can be quite different from displays and more stimulating due to their novelty, leading to a new era for LCs, materials design and technology. Appl. Sci. 2019, 9, 2512 4 of 47 Appl. Sci. 2019, 9, x FOR PEER REVIEW 4 of 47

FigureFigure 3.3. (a(a)) SEM SEM image image of aof collection a collection of tobacco of tobacco mosaic virusmosaic particles. virus particles. Scale bar: 0.2Scaleµm. bar: Reproduced 0.2 μm. Reproducedfrom Wikimedia from Commons, Wikimedia with Commons, no author with name no author supplied. name (b) supplied. The orientational (b) The orientational order parameter order of parameterthe viruses of increases the viruses as their increases concentration as their inconcen the solventtration is in increased. the solvent Reproduced is increased. with Reproduced permission withfrom permission [46]. Copyright from (1988)[46]. Copyrigh Americant (1988) Physical American Society. Physical Society.

AtAt the same time,time, revolutionaryrevolutionary developments developments have have been been made made in thein the fields fields of nanotechnology of nanotechnology and andnanoscience, nanoscience, leading leading to the birthto the of abirth series of of a novel series nanomaterials of novel nanomaterials [10,47–55]. These [10,47–55]. nanostructured These nanostructuredmaterials, whose materials, size in at whose least onesize dimensionin at least on ise in dimension the range is from in the 1 nm rang toe 100 from nm, 1 nm have to gained100 nm, a havewealth gained of academic a wealth and of industrial academic attention and industrial due to attention their size-dependent due to their electronic, size-dependent optical, electronic, magnetic, optical,and chemical magnetic, properties, and chemical which areproperties, significantly which di ffareerent significantly from those different of bulk materialsfrom those as of well bulk as materialsfrom individual as well atoms as from or moleculesindividual [ 55atoms–62]. or These molecules nanostructured [55–62]. These materials nanostructured have been extensively materials haveapplied been in nearlyextensively every applied field of in science nearly from every energy, field optics,of science computing from energy, to catalysis, optics, biosciences computing and to catalysis,medical sciencesbiosciences [48,63 –and65]. Undoubtedly,medical sciences when these[48,63–65]. novel nanometre-scaleUndoubtedly, when structures these encounter novel nanometre-scaleliquid crystals, a structures highly interesting encounter and liquid unique crystals, synergy a willhighly be observed,interesting leading and unique to an abundancesynergy will of beentirely observed, new leading and potential to an abundance applications of [ 2entirely,8,42,66 –new70]. and potential applications [2,8,42,66–70]. TheThe addition ofof nanomaterials nanomaterials to to a LCa LC material material produces produces a composite a composite or colloidal or colloidal dispersion dispersion [71,72]. [71,72].The new The materials new materials are expected are toexpected behave ditoff erentlybehave from differently their individual from their components individual (nanomaterials components (nanomaterialsand LCs) both and on the LCs) microscopic both on the as microscopic well as the as macroscopic well as the macroscopic scale [63–77]. scale There [63–77]. are three There basic are threeobjectives basic toobjectives making ato LC–nanomaterial making a LC‒nanomaterial composite system:composite (i) tosystem: modify (i) theto modify primary the physical primary or physicalchemical propertiesor chemical of theproperties pristine LCsof the [78 –pristine80]; (ii) manipulatingLCs [78–80]; and(ii) orderingmanipulating nanomaterials and ordering in LCs nanomaterialsto modify the propertiesin LCs to modify of nanomaterials the properties [81]; of and nanomaterials (iii) to obtain [81]; additional and (iii) functionalities to obtain additional that are functionalitiesavailable from that neither are theavailable LCs nor from the neither nanomaterials the LCs in nor their the intrinsic nanomaterials states [82 in]. their intrinsic states [82]. A fundamental investigation of the nanophysics of LCs was published by Brochard and de Gennes, whoA discussed fundamental a theory investigation of magnetic of nanoparticles the nanophysics suspended of LCs inwas a nematic published LCs by in 1970Brochard [83]. Laterand de in Gennes,the same who year, discussed Rault et al. a theory reported of themagnetic first suspension nanoparticles composed suspended of nematic in a nematic LCs and LCs small in 1970 magnetic [83]. Laterparticles in the [84 same]. The year, main Rault idea ofet theseal. reported pioneering the first studies suspension was to control composed the directorof nematic of theLCs LC and hosts small by magnetica coupling particles between [84]. LC moleculesThe main idea and elongatedof these pioneering magnetic particles,studies was producing to control what the hasdirector been of called the LCmagneto-nematics, hosts by a coupling i.e., nematicbetween liquidLC molecules crystals withand elongated ferromagnetic magnetic properties. particles, In fact,producing it should what be hasmentioned been called that this magneto-nematics, conclusion is wrong, i.e., asnematic the produced liquid systemcrystals is with not ferromagnetic. ferromagnetic As properties. the magnetic In fact,field isit removed,should be the mentioned magnetization that doesthis inconclusion fact decrease is towrong, zero, whileas the it shouldproduced retain system a finite is value not ferromagnetic.for ferromagnetics. As the Nevertheless, magnetic field one is could removed, call the th producede magnetization system does an anisotropic in fact decrease ferrofluid. to zero, Still, whilethis idea it should led the retain trend a of finite studies value on for LC–nanomaterial ferromagnetics. suspensions Nevertheless, for one the could next few call decades.the produced With systemthe rapid an development anisotropic offerrofluid. composite Still, LC materials,this idea itled was the recognized trend of that studies long-range on LC orientation‒nanomaterial and suspensionsinteractions infor mesophase the next few can decades. lead to aWith strong the impact rapid ofdevelopment nanomaterials of oncomposite the properties LC materials, of the LC it washosts recognized [85–87] and that vice long-range versa; the LCorientation matrix canand rearrange interactions the orientationalin mesophase and can positional lead to a order strong of impactnanomaterials of nanomaterials [88,89]. The on coupling the properties of long-range of the LC ordering hosts in[85–87] LCs and and the vice unique versa; properties the LC matrix of dopant can rearrangenanomaterials the allowsorientational us to change and orpositional even impose orde uniquer of physicalnanomaterials properties [88,89]. on the The LC–nanomaterial coupling of long-rangecomposites ordering by doping in diLCsfferent and kindsthe unique of nanomaterials properties of in dopant LCs [70 nanomaterials]. allows us to change or even impose unique physical properties on the LC‒nanomaterial composites by doping different kinds of nanomaterials in LCs [70]. Appl. Sci. 2019, 9, 2512 5 of 47

Up to now, there have been four primary groups of nanomaterials used as dopants in LCs, i.e., metal nanoparticles, ferroelectric nanoparticles, semi-conductor nanoparticles, and carbon nanoparticles. Among these dispersions, improved physical characteristics and novel functionalities can be obtained depending on the physical and chemical characteristics of the dopants as well as the interaction between LCs and nanomaterials [90–95]. For example, there are abundant reports declaring that doping metallic, semiconducting, oxide and ferroelectric nanoparticles into LCs can efficiently modify the electrical and mechanical (viscoelastic) properties of LC host, leading to large dielectric and optical anisotropy, low threshold voltage, and an improved electro-optical response [87,92,94,96,97]. In addition, ferroelectric nanoparticles can also increase the order parameter as well as the clearing point of the LC host due to the interaction between the elastic forces of LC molecules and the spontaneous polarization of ferroelectric nanoparticles [86,98–101]. Doping nematic LCs with ferromagnetic particles can effectively reorient the director of LCs by magnetic fields induced by the coupling of magnetic particles with LC molecules [83,102–106]. At the same time, there have been reports claiming the exact opposite behaviour: an increase in threshold voltage, a slower optical response, and a decrease in order and in transition temperatures. This indicates that the properties of the composite materials are very much dependent on the dopant size, materials employed, preparation conditions and time. It further indicates that a lot more research is needed to fully understand these systems. With largely varying results, even on a qualitative basis, the field of liquid crystal nanoparticle composites is only in its infancy. Metal and semiconducting nanoparticles have been used to stabilise the mesophase known as the blue phase, effectively broadening the temperature range of this frustrated phase from 1 K to up to 20 K [107,108]. Silica, ferroelectric and metal nanoparticles can effectively influence the electro-optical response of nematic LCs and induce a memory effect, i.e., a residual transmittance can be maintained without external electric fields being applied [109–112]. Moreover, a frequency-dependent electro-optical response can be obtained in suspensions of metal nanoparticles and nematic LCs due to the coupling of the dielectric properties of the nanoparticles and the LCs [78,113]. It was also reported that carbon nanotubes dispersed in nematic LCs would align along the direction of the LC director, therefore largely changing the conductivity depending on the orientation of the tubes [114]. In fact, it could be shown that the nanotubes can be reoriented elastically by reorienting the director through applied electric or magnetic fields. Fullerenes could effectively improve the switching speed of a nematic host LC [95], and improved electro-optic responses could be induced by doping graphene oxides in LCs [115]. In addition to changing the intrinsic physical properties of the pristine LC materials mentioned above, intriguing photonic functionalities can also be introduced into the composites by combining nanomaterials with LCs, which is considered a promising method for the building of novel metamaterials [116]. Metamaterials are artificial engineered bulks with regular nanostructures that show exotic optical properties [117]. Combining the emerging field of optical metamaterials with LCs provides an extremely attractive quality, i.e., tunability, which is of the utmost importance in applications such as optically addressed spatial light modulators, tunable photonic materials and dynamic holography [118–120]. When integrating LCs with metamaterials, one can modify the director alignment of LCs by applying external stimuli and therefore manipulating the overall optical characteristics of the composite [119]. For instance, upon periodically embedding particular metal nanomaterials, such as gold nanoparticles, into LC matrixes, a localized surface plasmon resonance (LSPR) will be obtained, which can be tuned by adjusting the birefringence of the surrounding LC matrix [77,81,121–125]. Apart from the abovementioned technologies, novel photonic properties and applications can be further exploited for nanodopants including semiconducting nanomaterials, dyes, oxides, quantum dots (QDs), etc. [126–129]. Instead of the SPR effect in the metallic nanomaterial–LC colloidal systems, the main property of interest in these cases is photoluminescence. The optical excitation and emission of these nanomaterials could be tuned effectively through the interaction between nanomaterials and the long-range ordered LC molecules, leading to a series of potential applications such as information storage, displays, LC lasers, etc. [130–134]. Appl. Sci. 2019, 9, 2512 6 of 47

At present, one of the central challenges faced by the development of novel LC–nanomaterial composites is reliable assembly of nanoscale building blocks into functional bulk materials by using distorted LCs. It has already been demonstrated that the localisation of a nanoparticle in a distorted LC region allows the decrease of the free energy, and the directional motion of nanomaterials in LCs can be driven by the collective long-range interactions mediated by the LC director field. As a result, the energetic cost of LC defects enables them to entrap and reorient nanomaterials in a reversible manner and with well-manipulated position and orientation [135–138]. In addition, nanomaterials that are suspended in LCs and have a given anchoring energy at the surface will induce a deformation of the orientation in the surrounding LCs, producing topological defects in the vicinity of the nanoparticle. The elastic distortion exerts a force on neighbouring nanomaterials at a range of up to a few micrometres, resulting in defect lines or points around nanomaterials [139–144]. This LC-mediated interaction can be either repulsive or attractive and, as a consequence, can be used as an effective method to manipulate the spatial distance between nanoparticles [88,145–148]. It is thus anticipated that, by coupling these two LC-mediated effects will result in the assembly of nanomaterials with one-, two- or even three-dimensional (3D) periodic superstructures [143,149–154]. Due to the great potential in both fundamental science and applications, this top-down assembly approach, where spontaneous or artificially generated LC textures are utilized as templates for organizing nanoparticles on controlled lattices or confining them in designed defects, has been increasingly coming into the spotlight. Another interesting research field of LC–nanomaterial technology that is worthwhile to discuss is the self-assembly of lyotropic LCs composed of 1- or 2D anisotropic nanomaterials [3,7]. This topic has been investigated from time to time as people have realized that suspensions of anisodiametric nanomaterials can form LC phases, even at a very low concentration, due to Bernal’s seminal work on suspensions of tobacco mosaic virus [155] and Onsager’s theory of the excluded volume mechanism [156]. However, with the boom in the development of nanotechnology and nanoscience in the last decade, various types of novel nanomaterials have emerged. The size dispersion, shape anisotropy, and surface morphology of the nanomaterials are now well controlled. All of these developments will definitely lead to a dramatic increase in the use of nanoparticle-based LCs. Nowadays, it is well known that suspensions of anisotropic nanorods or nanoplates, viruses, nano-cellulose, carbon nanotubes and graphene oxide can form lyotropic LC phases, which are usually accompanied by surprising physical properties [157–165]. In this review, we introduce recent studies on the interactions between LCs and nanomaterials leading to improved or novel physical properties for new applications in various research fields. We first discuss some results on improved electro-optical and other physical properties of LCs by doping with various nanoparticles. This is followed by an outline of the additional exotic photonic functionalities of nanoparticle-doped LCs. In the next section we emphasize the use of LC matrixes for nanoparticle assembly as well as the self-assembly of anisotropic nanoparticles into lyotropic LC phases. In the final part of this review our attention is focused on biological applications of LC–nanoparticle composites, i.e., biosensor and drug delivery systems. We anticipate reaching a wider readership, beyond the liquid crystal community, by presenting some recent examples that are illustrative and representative. The attention of this review is mainly focused on the functionalities of nanomaterial-doped LCs, but the rich LC world also provides further nanotechnology applications such as organic semiconductors, nanoporous materials, ion conducting materials, etc. Readers who are interested in these areas can find more information in a recently published review by Kato et al. [166] and the book by Li [167].

2. Modification of Physical Properties of LC Materials by Nanodopants LCs have been well identified with the applications in commercial displays due to their outstanding electro-optic performance, which is a consequence of their anisotropy of the dielectric constant and the optical refractive index in combination with their elasticity and ready response to external stimuli. However, with the rapid development of technology, liquid crystal displays (LCDs) can no longer meet Appl. Sci. 2019, 9, 2512 7 of 47 the increasingly demanding visual requirements with traditional LC mixtures. In the search for LCDs with lower driving voltage, higher colour contrast ratio and shorter response times, novel materials need to be employed to meet increasing demands. At the same time, for progress in other research and technology areas, such as novel optical devices, advanced materials, nanotechnologies and biosciences, LCs with improved physical properties are needed. Currently, one of the effective primary solutions is introducing various functional nanodopants into existing high-performance LC materials. As reported by many studies, nanodopants, such as ferromagnetic, ferroelectric, semiconducting particles of various shapes, as well as carbon nanoparticles, can effectively modify electro-optic responses as well as other physical characteristics of LCs. Since Reznikov et al. first reported that dispersing low concentrations of submicron ferroelectric particles (Sn2P2S6) in commercial nematic LCs (ZLI-4801) could enhance the dielectric response, lower the operating voltage and induce a linear response to the applied electric field [168], ferroelectric nanoparticle dispersions have been studied extensively as dopants for LC modifications. It was argued that the dispersion of ferroelectric nanoparticles can induce an enhancement of the orientational order of LCs because of the coupling between the spontaneous polarization of the ferroelectric nanoparticles and the liquid crystal via an elastic field, and hence effectively improve the electro-optical properties of LCs [86,169,170] through increased molecular order. Recently, Singh et al. systemically investigated the physical characteristics, including dielectric constants, threshold voltage and elastic constants, of the composite consisting of the nematic LC 6CHBT and ferroelectric nanoparticles of barium titanate (BaTiO3)[94]. The ferroelectric nanoparticle–LC composite in their system showed an improved electro-optical characteristic, accompanied by an increase in the dielectric anisotropy as well as a decrease in the threshold voltage. Mishra et al. found that the physical properties of suspensions of ferroelectric nanoparticles of barium titanate dispersed in the nematic liquid crystal 5CB are largely dependent on the concentration of nanoparticles [171]. For samples with a low concentration (<1 wt %), the threshold voltage, elastic constants, dielectric coefficients and ionic conductivity could be effectively changed. For those with high concentrations (>1 wt %), the changes were relatively small. Although there have been numerous papers proposing that doping ferroelectric nanoparticles will improve the physical properties of the dispersion, the experimental results still appear sketchy, sometimes even contradictory, and improvements are not straightforward to achieve. As pointed out above, the effect of ferroelectric particle doping strongly depends on the preparation conditions, time, size of the dopant particles, and various other uncontrollable parameters; for example, barium titanate exhibits a particle size-dependent transition between ferroelectric and paraelectric behaviour, whose transition, in addition, depends on the actual milling process employed to produce the nanoparticles. Glushchenko et al. found that doping ferroelectric nanoparticles (BaTiO3) did not change the threshold voltage of nematic LCs (5CB) [169], while Klein et al. even presented an increase in the Fréedericksz threshold voltage and no change in the dielectric anisotropy [172]. The reasons for the different behaviour observed may be the uncontrollable ionic contamination, broad size distribution and irregular shapes of particles, poor dispersibility, etc. For example, it has been reported that large ferroelectric particles (>100 nm) can form a polydomain structure but very small particles (<10 nm) may lose their ferroelectricity [98]. A recent study by Zangana et al. systemically investigated the dependence of the electric-optic and dielectric properties of the LC (nematic 5CB and ferroelectric SmC* Felix M4851/050), suspended [173] with barium titanate nanoparticles of different size and concentration (Figure4). They demonstrated that the ferroelectricity of nanoparticles was highly dependent on the particles’ sizes, and the particles did not influence the nematic LC properties unless the concentration was high enough (>0.5 vol %), where a reduction of electro-optic response time was observed, which was due to the enhancement of dielectric anisotropy. Dubey et al. did a low-frequency dielectric investigation of a Schiff-based LC compound dispersed with BaTiO3 nanoparticles [174]. They found that the doped LC–ferroelectric nanoparticle suspension exhibited a temperature- and frequency-dependent dielectric permittivity. In addition, due to the spontaneous dipole moment, a huge number of free ions in LCs were captured by Appl. Sci. 2019, 9, 2512 8 of 47 theAppl. ferroelectric Sci. 2019, 9, x nanodopants,FOR PEER REVIEW leading to a diminishing of the Maxwell–Wagner–Sillars effect. This8 of 47 will in fact have a pronounced effect on the threshold voltage, and ion capture would be expected to reduceand ion the capture threshold would voltage. be expected Alternatively, to reduce this impliesthe threshold that when voltage. using Alternatively, a pure, commercially this implies cleaned that liquidwhen crystal, using a the pure, ion commercially concentration cleaned is already liquid low, crystal, such that the onlyion concentration minute effects is onalready the threshold low, such voltagethat only would minute be expected, effects on explaining the threshold some ofvoltage the discrepancies would be reportedexpected, in explaining different publications. some of the Furthermore,discrepancies the reported authors in found different in general publications. that the Fu conductivity,rthermore, the dielectric authors anisotropy found in andgeneral relaxation that the conductivity, dielectric anisotropy and relaxation frequency of the compound decreased due to the frequency of the compound decreased due to the strong interaction between BaTiO3 particles and LCstrong molecules. interaction between BaTiO3 particles and LC molecules.

FigureFigure 4. 4.( a()a) A A Schlieren Schlieren texture texture (left (left) of) of 0.5 0.5 vol vol % % BaTiO BaTiO3 +3 +5CB 5CB observed observed by by polarizing polarizing optical optical microscopy,microscopy, POM. POM. Observation Observation between between parallel parallel polarizers polarizers (right) provides(right) provides evidence thatevidence the dispersed that the particlesdispersed are particles collected are in the collected topological in the defects. topological (b,c) Permittivitydefects. (b,c components) Permittivityε components0 and ε 0, respectively. ε⊥′ and ε‖′, ⊥ k (drespectively.) The relationship (d) The between relationship the electro-optic between the response electro-opti time andc response the concentration time and the of BaTiOconcentration3 with (d )of theBaTiO field3 on-time with (d and) the (e) thefield off -timeon-time for largerand ( (ferroelectric)e) the off-time and smallerfor larger (non-ferroelectric) (ferroelectric) particles.and smaller (f) The(non-ferroelectric) relationship between particles. theconcentration (f) The relationship of BaTiO between3 and threshold the concentration voltage V ofth, BaTiO and (g3) and splay threshold elastic constantvoltage KV11th., Reproducedand (g) splay with elastic permission constant from K11. [ 173Reproduced]. Copyright with (2017) permissi AIP.on from [173]. Copyright (2017) AIP. It seems that ferroelectric nanoparticles do modify the physical properties of LC matrixes, sometimesIt seems leading that to anferroelectric improvement nanoparticles in the electro-optic do modify performance; the physical however, properties due to theof complicatedLC matrixes, size-dependentsometimes leading ferroelectricity to an improvement and aggregation in the behaviourelectro-optic of ferroelectricperformance; nanoparticles however, due in to LCs, the additionalcomplicated research size-dependent is necessary. ferroelectricity and aggregation behaviour of ferroelectric nanoparticles in LCs,Except additional for ferroelectric research nanoparticles,is necessary. inorganic nanoparticles, such as metallic nanoparticles, quantumExcept dots for (QDs) ferroelectric and oxides, nanoparticles, can also effectively inorganic modify nanoparticles, the physical such parameters as metallic of LCnanoparticles, materials, sometimesquantum leadingdots (QDs) to an and improvement oxides, can also in the effectively electrical modify and electro-optic the physical performance. parameters of LC materials, sometimesJust like leading ferroelectric to an nanoparticles, improvement metallic in the electrical nanoparticles, and electro-optic such as gold nanoparticles,performance. are expected to potentiallyJust like improve ferroelectric the physical nanoparticles, properties metallic of liquid nanoparticles, crystal materials such due as togold the nanoparticles, anticipation that are theexpected electrostatic to potentially interactions improve between the LC moleculesphysical prop anderties metal nanoparticlesof liquid crystal can significantlymaterials due enhance to the theanticipation orientational that order the electrostatic of LCs [175 –interactions181]. The argument between ofLC performance molecules and enhancement metal nanoparticles through ancan increasesignificantly in orientational enhance the order orientational has been used order in manyof LCs publications [175–181]. toThe explain argument the observed of performance results. Nevertheless,enhancement to through our knowledge an increase such in anorientational increase has order not beenhas been demonstrated used in many quantitatively publications by ato stronglyexplain reliablethe observed experimental results. method, Nevertheless, such as X-rayto our di ffknowledgeraction at equalsuch reducedan increase temperatures, has not been for example,demonstrated with values quantitatively for the neat by anda strongly the doped reliable system experimental being well outsidemethod, the such limits as X-ray of error. diffraction Recently, at Elkhalgiequal reduced et al. investigated temperatures, the thermodynamics,for example, with electro-optic values for the response neat and and the dielectric doped system permittivity being of well a nematicoutside LC the (6CHBT)-gold limits of error. nanoparticle Recently, Elkhalgi composite et al. [182 investigated] (Figure5). the It wasthermodynamics, found that the electro-optic threshold voltageresponse required and dielectric for switching permittivity LC molecules of a nematic from LC homogeneous (6CHBT)-gold to homeotropicnanoparticle alignmentcomposite was[182] decreased,(Figure 5). while It was the firstfound phase that transition the threshold temperature voltage (isotropic required to for nematic) switching as well LC asmolecules the dielectric from permittivityhomogeneous for theto homeotropic composite, was alignment raised due was to decrease the increasedd, while nematic the first ordering phase of transition the host LC temperature medium. (isotropic to nematic) as well as the dielectric permittivity for the composite, was raised due to the increased nematic ordering of the host LC medium. Apart from that, the presence of gold nanoparticles also promoted the reorientational motion of LC molecules by applying an electric field. Furthermore, an increase of dielectric permittivity and ionic conductivity was found in the Appl. Sci. 2019, 9, 2512 9 of 47

Apart from that, the presence of gold nanoparticles also promoted the reorientational motion of LC Appl. Sci. 2019, 9, x FOR PEER REVIEW 9 of 47 molecules by applying an electric field. Furthermore, an increase of dielectric permittivity and ionic conductivitygold nanoparticle-columnar was found in the discotic gold nanoparticle-columnarLC (HAT4) suspensions discotic [183]. LCSilver (HAT4) nanoparticles suspensions are [183also]. Silverquite popular nanoparticles among are the also metal quite nanoparticles popular among and the ha metalve been nanoparticles widely used and as have nanodopants been widely for used the asmodification nanodopants of LC for thephysical modification properties of LC[179] physical due to propertiestheir high [179electric] due and to theirthermal high conductivity, electric and thermalnano-scale conductivity, calorimetric nano-scale effect, excellent calorimetric chemic effect,al excellentstability, chemical and various stability, medical and various and catalysts medical andapplications catalysts [184–186]. applications The [184 thermodynamic,–186]. The thermodynamic, dielectric and dielectric electro-optic and electro-opticcharacteristics characteristics of a nematic ofLC a (6CHBT) nematic mixed LC (6CHBT) with silver mixed nanoparticles with silver with nanoparticles different sizes with have different been sizesstudied have by Tripathi been studied et al. byThey Tripathi found et that al. Theythe first-order found that phase the first-order transition phase temperature transition of temperature the mixture of thewas mixture marginally was marginallyincreased, while increased, the dielectric while the permittivity, dielectric permittivity, and elastic and constants elastic constantsdecreased decreased compared compared to the pure to theLC pure[187]. LC [187].

Figure 5. (a) VariationVariation ofof longitudinallongitudinal ((εε‖’)’) andand transversetransverse ((εε⊥0′) components of the relative dielectric k ⊥ permittivity with temperature for pure LC and gold-nanoparticle-dispersed 6CHBT. ( b) Frequency dependence of the longitudinal component of lossloss ((εε‖”)″) for purepure andand gold-nanoparticle-dispersedgold-nanoparticle-dispersed k 6CHBT. ReproducedReproduced withwith permissionpermission fromfrom [[182].182]. CopyrightCopyright (2018)(2018) TaylorTaylor && Francis.Francis.

QDs are semiconductor nanoparticles in the range from two to 10 nanometres in diameter. They are usuallyusually composedcomposed of of thousands thousands of of atoms atoms of groupof group II and II and VI elements VI elements (e.g., (e.g., CdSe, CdSe, CdTe, CdTe, ZnO), ZnO), group IIIgroup and III V elementsand V elements (e.g., InP, (e.g., InAs), InP, and InAs), group IV–VIand group elements IV‒ (e.g.,VI elements PbS) [188 (e.g.,]. As PbS) a semiconductor [188]. As a material,semiconductor the electrons material, of QDsthe willelectrons be promoted of QDs from will thebe valencepromoted band from to the the conduction valence band band to by the irradiationconduction of band photons by the with irradiation energies largerof photons than the with band energies gap. Since larger these than electrons the band are gap. not stableSince inthese the conductionelectrons are band, not theystable will in transitthe conduction back to the band, valence they band will and transit recombine back withto the the valence holes. Asband a result, and therecombine extra energy with willthe holes. be emitted As a asresult, photons the extra with aener wavelengthgy will be related emitted to as the photons bandgap. with According a wavelength to the developedrelated to the bulk bandgap. Wannier-Hamiltonian According to the model developed of Luis Brusbulk [189Wannier-Hamiltonian,190], this bandgap ismodel dependent of Luis on Brus the size[189,190], of the this employed bandgap QDs. is Theirdependent unique on size- the and size shape-dependent of the employed electro-optic QDs. Their characteristics unique size- made and QDsshape-dependent the focal point electro-optic of research charac in variousteristics fields made including QDs the photonics, focal point sensors, of research medicine, in various information fields storage,including etc. photonics, sensors, medicine, information storage, etc. The specialspecial physical physical properties properties have have made made QDs QDs a promising a promising nanodopant nanodopant for modifying for modifying physical propertiesphysical properties and improving and improving the electro-optic the electro-optic responses responses of LC materials of LC materials [191–194 ].[191–194]. Singh et al.Singh recently et al. foundrecently that found the inclusion that the ofinclusion QDs (CdSe-QDs) of QDs (CdSe-QD would effs)ectively would influence effectively phase influence transition phase temperatures transition andtemperatures enhancethe and dielectric enhance anisotropythe dielectric of anisotropy nematic LCs of (4PP4OB). nematic LCs Moreover, (4PP4OB). it increased Moreover, the it transitionincreased voltagethe transition for switching voltage LC for molecules switching from LC homogeneous molecules from alignment homogeneous to homeotropic alignment alignment, to homeotropic decreased thealignment, splay elastic decreased constant, the splay and increased elastic constant, the relaxation and increased frequency the of relaxation the LC matrix frequency [195]. of Roy the et LC al. reportedmatrix [195]. that Roy nematic et al. LCreported 1832A that dispersed nematic with LC 1832A QDs (InPdispersed/ZnS) exhibitedwith QDs a(InP/ZnS) promoted exhibited molecule a alignment,promoted molecule leading to alignment, an enhanced leading non-linear to an opticalenhanced anisotropy. non-linear They optical also foundanisotropy. that the They physical also characteristicsfound that the physical of the composite, characteristics including of the elasticcomposite, constants, including switching elastic constants, time, and switching viscosity, time, were distinctlyand viscosity, decreased, were while distinctly faster electro-optical decreased, response,while faster enhanced electro-optical birefringence response, and larger enhanced dielectric anisotropybirefringence were and observed larger dielectric [196]. anisotropy were observed [196]. Oxides such as zinc and titanium oxide nanoparticles have also regularly been used as dopants in LC materials due to their exotic electrical and optical properties. Numerous works focusing on the physical properties of LCs with oxide entities demonstrated new or improved electro-optic performance, including altered dielectric anisotropy, lower threshold voltage or shorter response Appl. Sci. 2019, 9, 2512 10 of 47

Oxides such as zinc and titanium oxide nanoparticles have also regularly been used as dopants in LC materials due to their exotic electrical and optical properties. Numerous works focusing on the physical properties of LCs with oxide entities demonstrated new or improved electro-optic performance, including altered dielectric anisotropy, lower threshold voltage or shorter response and relaxation time [197–199]. In a recent publication, Oh et al. demonstrated the effects on the physical characteristics of nematic LCs (ZSM-50087XX, JNC) by doping with titanium silicon oxide (TiSiO4) nanoparticles. The authors found that the electro-optic characteristics of the composite were apparently improved. By increasing the doping concentration, the electro-optic properties can be further improved until the doping concentration was too large (0.2 wt %), which would influence the LC behaviour and deteriorate the electro-optic behaviour. In addition, the authors also demonstrated that the dispersed TiSiO4 nanoparticles could effectively improve the thermal stability of the pristine LC system, capture impurity ions and thus lead to hysteresis-free voltage-transmittance characteristics [200]. Another kind of nanomaterial that has been widely used as a dopant for influencing the physical properties of LC materials is carbon nanomaterials. Carbon nanomaterials, including fullerene, carbon nanotube, graphene and graphene oxides, have attracted great attention due to their exotic physical and chemical properties. Fullerenes are hollow spherical molecules of nano-sized diameter, generally composed of 60–70 carbon atoms (C60,C70). Among the various fullerenes, C60 has been studied particularly extensively. The molecule of C60 is a strong acceptor capable of accepting from one to six electrons to form the corresponding anions [201]. In order to improve the electrical and electro-optic properties of LC materials, great efforts have been made to attach C60 covalently to mesogens as well as to dope C60 into LC matrices [95,202–204]. The recent, significant developments in fullerene-containing LCs have been reviewed by Zhang et al. [205]. In 1991, the discovery of quasi-1D carbon nanotubes (CNTs) was reported by Iijima [206], which ignited scientific interest in 1D nanostructures. CNTs are crystalline allotropes of carbon with atoms hexagonally arranged in curved lattices, forming hollow cylinders with diameters on the nanometre scale and aspect ratios up to 108. The CNTs composed of a single layer are called Single-Walled CNTs (SWCNTs), which can be either metallic or semi-conductive, depending on the diameter and rolling angle, while the ones formed by several concentric cylinders are called Multi-Walled CNTs (MWCNTs). Due to their exceptional anisotropic thermal, electrical, optical and mechanical properties, CNTs have been found to have great potential as additives to LCs, which will result in the improvement of electro-optic properties. A few papers have demonstrated the beneficial effects of CNTs doping on the physical properties of LC materials, such as reduced amplitude of residual DC, altered dielectric anisotropy, lower threshold voltage and shorter response time [207–213]. This improved electro-optical performance can be attributed to the higher dielectric anisotropy of the suspension, due to the extremely high permittivity and anisotropy of CNT inclusions as well as the ion-trapping effect of CNTs reducing the free ionic concentration of the solvent, which leads to an increase in the effective applied cell voltage (a reduction of electric field shielding). The inclusion of CNTs in LCs can also lead to other modifications of physical properties, such as viscosity, elastic constants and phase transition temperature, to name just a few. Very recently, Singh et al. reported the enhancement of orientational order, increase of dielectric anisotropy and decrement of threshold voltage in the composite of SWCNTs and nematic LCs (3017) [214,215] (Figure6). However, just like ferroelectric nanoparticles, the experimental results of the CNT-LC switching behaviour are not consistent. Some authors claimed neither a decrease in threshold voltage nor an increase in dielectric anisotropy was found in respective composites. On the contrary, a higher concentration of CNTs could even increase the switching voltage [216,217]. Appl. Sci. 2019, 9, 2512 11 of 47 Appl. Sci. 2019, 9, x FOR PEER REVIEW 11 of 47

FigureFigure 6. 6. (a(a) )Transmission Transmission voltage voltage function function of of the the sample sample device device with with single-walled single-walled carbon carbon nanotube nanotube doping. (b) Relative permittivities (transverse (ε 0) and longitudinal (ε 0) components) with doping. (b) Relative permittivities (transverse (ε⊥⊥′) and longitudinal (ε‖k′) components) with temperature for pure and dispersed samples. Reproduced with permission from [215]. Copyright temperature for pure and dispersed samples. Reproduced with permission from [215]. Copyright (2018) ELSEVIER. (2018) ELSEVIER. Another allotrope of carbon, graphene, which is a monolayer of sp2-hybridized carbon atoms Another allotrope of carbon, graphene, which is a monolayer of sp2-hybridized carbon atoms arranged into a 2D honeycomb lattice, has been one of the most attractive nanomaterials since arranged into a 2D honeycomb lattice, has been one of the most attractive nanomaterials since its its discovery in 2004 [218]. Due to the excellent mechanical, electrical, optical, thermal and discovery in 2004 [218]. Due to the excellent mechanical, electrical, optical, thermal and electronic electronic properties, such as high thermal conductivity, charge mobility, mechanical strength, optical properties, such as high thermal conductivity, charge mobility, mechanical strength, optical transmittance and chemical stability, graphene has been widely applied in a variety of applications transmittance and chemical stability, graphene has been widely applied in a variety of applications such as electronics, energy storage, biomedical engineering, new composite materials, or waste water such as electronics, energy storage, biomedical engineering, new composite materials, or waste management. However, graphene is known for its notoriously poor dispersibility and solubility in water management. However, graphene is known for its notoriously poor dispersibility and solvents, organic liquids, including LCs, which greatly inhibits their use in LC composites so far, solubility in solvents, organic liquids, including LCs, which greatly inhibits their use in LC despite large efforts having been made to increase dispersibility via surface treatment and surface composites so far, despite large efforts having been made to increase dispersibility via surface decoration with covalently bound mesogens. In contrast to graphene, graphene oxide (GO), obtained treatment and surface decoration with covalently bound mesogens. In contrast to graphene, by treating graphite with strong oxiders, is readily dispersible in solvents, including water, due to its graphene oxide (GO), obtained by treating graphite with strong oxiders, is readily dispersible in hydrophilic nature. In fact, in certain concentration ranges, GO actually forms a lyotropic liquid crystal solvents, including water, due to its hydrophilic nature. In fact, in certain concentration ranges, GO in many such solvents, as well as in water. Although GO is electrically insulating, it can be converted actually forms a lyotropic liquid crystal in many such solvents, as well as in water. Although GO is into conducting graphene (rGO) by chemical or thermal reduction methods, at least partially regaining electrically insulating, it can be converted into conducting graphene (rGO) by chemical or thermal the conductive properties of neat graphene. reduction methods, at least partially regaining the conductive properties of neat graphene. So far, a series of studies have shown some improvement in the electro-optic performance of So far, a series of studies have shown some improvement in the electro-optic performance of thermotropic LCs doped with graphene oxide (GO) [219–224]. The reason for this improvement may thermotropic LCs doped with graphene oxide (GO) [219–224]. The reason for this improvement may be the coupling of electrical dipoles on the surface of the GO with the ones of mesogens and the be the coupling of electrical dipoles on the surface of the GO with the ones of mesogens and the trapping of free ionic contamination by GO. Zangana et al. have dispersed GO of different sizes in a trapping of free ionic contamination by GO. Zangana et al. have dispersed GO of different sizes in a nematic LC (5CB) with a wide range of concentrations (Figure7). They found that the LC molecules nematic LC (5CB) with a wide range of concentrations (Figure 7). They found that the LC molecules would be strongly anchored on the surface of GO flakes. As a result, the threshold voltage and would be strongly anchored on the surface of GO flakes. As a result, the threshold voltage and elastic constants were strongly increased, while the elastically driven electro-optic off-response time elastic constants were strongly increased, while the elastically driven electro-optic off-response time decreased when increasing the concentration of GO flakes [225,226]. Similar results in GO-doped decreased when increasing the concentration of GO flakes [225,226]. Similar results in GO-doped nematic LC (E5CN7) were reported by Dalir et al. They found that the capacity of charge storage was nematic LC (E5CN7) were reported by Dalir et al. They found that the capacity of charge storage was expanded, while the mobility of free ions and the diffusion coefficient were significantly decreased expanded, while the mobility of free ions and the diffusion coefficient were significantly decreased due to the strong interaction between LC molecules and GO flakes [227]. In another publication, they due to the strong interaction between LC molecules and GO flakes [227]. In another publication, they also reported that GO doping would lead to an increase in the nematic-isotropic phase transition also reported that GO doping would lead to an increase in the nematic-isotropic phase transition temperature, which could be shifted by changing the GO concentration [224]. Moreover, Lapanik temperature, which could be shifted by changing the GO concentration [224]. Moreover, Lapanik et et al. reported a 30–50% reduction of the threshold voltage and switching time in partially reduced al. reported a 30–50% reduction of the threshold voltage and switching time in partially reduced graphene oxide (PRGO)-doped nematic LCs and a 20–25% increment of the spontaneous polarization graphene oxide (PRGO)-doped nematic LCs and a 20–25% increment of the spontaneous in PRGO-doped ferroelectric LCs [228]. Ozgan et al. found that the dielectric anisotropy of nematic LC polarization in PRGO-doped ferroelectric LCs [228]. Ozgan et al. found that the dielectric anisotropy (6CB) would be increased with an increasing concentration of GO [115]. In a very recent investigation, of nematic LC (6CB) would be increased with an increasing concentration of GO [115]. In a very Mrukiewicz et al. systematically investigated the physical properties of nematic LC (5CB) dispersed recent investigation, Mrukiewicz et al. systematically investigated the physical properties of nematic with GO at a wide concentration range from 0.05 to 0.3 wt %. They observed a decrease in the threshold LC (5CB) dispersed with GO at a wide concentration range from 0.05 to 0.3 wt %. They observed a decrease in the threshold voltage in electro-optic and dielectric spectroscopy measurements. This was attributed to the disrupted homogeneous alignment induced by the strong π‒π stacking effect Appl. Sci. 2019, 9, 2512 12 of 47

Appl. Sci. 2019, 9, x FOR PEER REVIEW 12 of 47 voltage in electro-optic and dielectric spectroscopy measurements. This was attributed to the disrupted betweenhomogeneous the graphene alignment oxide induced and the by theLC strongmolecules’π–π stackingbenzene erings.ffect betweenIn addition, the graphenethey presented oxide andthe dependencethe LC molecules’ of the benzenefirst-order rings. phase In addition,transition they temper presentedature, elastic the dependence constants, ofdielectric the first-order permittivity phase andtransition switching temperature, duration elasticon the constants,GO doping dielectric concentration permittivity [229]. Additionally, and switching except duration for fullerenes, on the GO CNTsdoping and concentration GOs, modified [229]. physical Additionally, properties except of for LC fullerenes, hosts can CNTs also and arise GOs, from modified other physicalcarbon nanoparticlesproperties of LCincluding hosts cancarbon also dots arise [230], from deto othernation carbon nanodiamonds nanoparticles [231 including] and graphene carbon dotsquantum [230], dotsdetonation [232]. nanodiamonds [231] and graphene quantum dots [232].

FigureFigure 7. 7. ((aa)) Threshold Threshold voltage voltage ( (toptop left left),), splay splay elastic elastic constant constant ( (toptop right right),), and and electro-optic electro-optic response response timetime ( (bottombottom)) as as functions of GO concentration in 5CB, for smallsmall andand largelarge sizes.sizes. ( (bb)) Schematic Schematic illustrationillustration of of the the Fréedericksz Fréedericksz transition transition of of a a device device cell cell filled filled with with a a GO GO flakes flakes dispersed nematic nematic liquidliquid crystal. crystal. Reproduced Reproduced with with permission permission from from [225]. [225]. Copyright Copyright (2016) (2016) John John Wiley Wiley and and Sons. Sons.

AA small small number number of of added added nanomaterials nanomaterials in in LCs LCs can can produce produce composite composite materials materials with with modified modified physicalphysical propertiesproperties and and improved improved electro-optical electro-optica performance—forl performance—for example, example, a shorter a shorter response response time, a time,higher a birefringence,higher birefringence, a lower a threshold lower threshold voltage, voltage, a larger dielectrica larger dielectric anisotropy, anisotropy, better contrast, better enhancedcontrast, enhancednonlinear-optical nonlinear-optical properties, etc.,properties, which hasetc., great which potential has great for the potential design of for next-generation the design LCof next-generationequipment including LC equipment LC displays, including tunable LC LC displays lasers and, tunable filters, electro-opticalLC lasers andswitchers filters, electro-optical and shutters, switchersnonlinear-optical and shutters, valves fornonlinear-optical photonic information valves processing for photonic systems, information telecommunication processing andsystems, many telecommunicationother aspects of modern and many technology. other aspects At present, of modern though, technology. experimental At present, results though, are still experimental inconclusive, resultslacking are reproducibility. still inconclusive, Thus, furtherlacking research reproducibilit is neededy. Thus, in the further near future, research both experimentalis needed in asthe well near as future,theoretical, both to experimental identify the mainas well underlying as theoretical, aspects to thatidentify lead tothe the main modification underlying of aspects liquid-crystal-based that lead to thecomposite modification materials’ of liquid-crystal-based properties, as well as composite the causes ma forterials’ the range properties, of behaviours as well observed. as the causes Developing for the rangea good of understanding behaviours observed. of the latter Developing will clear a the good way understanding for an optimized of the development latter will ofclear novel the materials way for anfor optimized improved development future technologies. of novel materials for improved future technologies.

3.3. Photonic Photonic Applications Applications SurfaceSurface plasmon plasmon waves, waves, which which are are the the collective collective oscillation oscillation in inelectron electron density density at the at the interface interface of aof dielectric a dielectric and and a metallic a metallic material, material, have have received received great great attention attention since since the the1960s 1960s [233]. [233 They]. They offer o theffer opportunitythe opportunity to merge to merge electronics electronics and and photonics photonics at atnanoscale nanoscale dimensions dimensions due due to to their their particular capabilitycapability to to guide guide and and localize electromagnetic waves in sub-wavelength metallic metallic structures, structures, and and havehave been been widely exploited in various research fieldsfields including transforming photo-conversion processesprocesses [234,235], [234,235], improving weak fluorescence fluorescence si signalsgnals for biological imaging [236,237], [236,237], enabling nano-lasersnano-lasers or or spontaneous spontaneous amplifie amplifiedd irradiation irradiation [238,239], [238,239], realizing realizing photo-thermal photo-thermal conversion conversion [240] [240] and so on. Moreover, a series of plasmonic devices, such as light sources, waveguides, lenses, filters, or dynamic colour displays, have already been demonstrated [241–244]. Appl. Sci. 2019, 9, 2512 13 of 47 and so on. Moreover, a series of plasmonic devices, such as light sources, waveguides, lenses, filters, or dynamic colour displays, have already been demonstrated [241–244]. In the past few decades, metal nanoparticles have already been widely utilized as functional building blocks for obtaining new generations of nanomaterials with tunable surface plasmonic resonance (SPR). Metal nanoparticles show SPR when stimulated by electromagnetic radiation of an appropriate wavelength due to the quantum confinement effect of free electrons. In particular, gold nanoparticles have received great attention and been found to be pivotal for SPR applications such as scanning near-field optical microscopy [245] and biosensors [246] for several reasons. First, the plasmonic resonance of most metals occurs in the ultraviolet (UV) region of the spectrum, while gold shows this resonance in the visible region. Secondly, gold is chemically inert and does not oxidize, which is a very important property in optical applications since oxide layers will drastically complicate the calculation of optical properties. Thirdly, gold nanoparticles are compatible with biological systems, which will be quite important for investigations and in vivo analysis. Last but not least, there are now many new physical and chemical methods for the synthesis of gold nanoparticles with a wide range of diameters and shapes [247]. The optical characteristics of gold nanoparticles in visible and near-infrared (NIR) wavelength ranges are mainly determined by the collective response of electrons. The strong interactions between the incident electro-magnetic field and free electron clouds at the gold nanoparticle’s surface will displace the electron clouds from their equilibrium position, thus inducing surface polarization charges that act as a restoring force on the electron clouds. This collective oscillation of free electrons will enhance the electric field that is produced at the interfaces of nanoparticles, and also confine it to a scale shorter than the wavelength of the incident electromagnetic field. At a macroscopic scale, such resonant excitation will lead to both strong scattering and absorption of light [247–249]. As a result, SPR is highly dependent on the diameter, shape, and dielectric constant of gold nanoparticles as well as the surrounding medium [248,250–253], implying that the SPR of gold nanoparticles can be tuned by changing these parameters. Although recent progress in the fabrication of nanomaterials has enabled us to produce metal nanoparticles with arbitrary size and shape, it is still time-consuming and costly to design plasmonic architectures with complicated structures for different optical applications. New methods for externally tuning SPR are still in demand. Among the various methods, one feasible approach is combining plasmonic units with host matrixes whose dielectric characteristics can be manipulated through external stimuli [254]. To this end, LCs have come into the spotlight. LCs hold great potential for enabling effective manipulation over the optic characteristics of plasmonic architectures due to their large refractive index anisotropy, real-time response to the external stimuli and the unique ability to act as dynamic templates for the self-assembly of nanomaterials. As a result, LCs have been widely used to manipulate SPR effects, not only in nanoparticles, but also in binary gratings, antenna arrays, patterned conductive surfaces and metamaterials. To date, a wealth of gold nanoparticle–LC composites with tunable SPR have been reported [81,82,124,125,181,254–263]. In recent years, among the different kinds of gold nanoparticles, due to the anisotropic mechanical and optical properties, gold nanorods have especially attracted attention [263,264]. Very recently, a LC modulated tunable filter based on SPR was designed and discussed by Li et al. [265]. The filter consisted of Au nanorod arrays and an Au film separated by a dielectric ITO glass layer. The numerical simulation demonstrated that the filter could achieve double absorption peaks at NIR wavelengths, which were generated by the strong plasmonic coupling between the plasmon-induced transparency mode of the Au nanorod dimer and the cavity mode of the Au cavity structure. A tunable band range of 160 nm was confirmed. On the other hand, apart from the size and shape of single nanoparticles, the SPR also depends on the interparticle spacing and spatial structure of nanoparticles. Rozic et al. demonstrated a self-organization of gold nanorods into end-to-end chains by using oriented linear arrays of smetic A defects (see Figure8). These nano-chains led to strongly anisotropic SPR absorption, which varied from 530 nm to 920 nm when end-to-end coupled gold nanorods were employed [266]. Appl. Sci. 2019, 9, 2512 14 of 47 Appl. Sci. 2019, 9, x FOR PEER REVIEW 14 of 47

FigureFigure 8. ExtinctionExtinction measurementmeasurement of of gold-nanorods gold-nanorods (GNRs) (GNRs)/8CB/8CB samples samples deposited deposited on a PVA on polymera PVA polymerwith (a, dwith) nonpolarized (a,d) nonpolarized incident incident light and light with and an wi incidentth an incident light polarized light polarized (b,e) parallel (b,e) andparallel (c,f ) andperpendicular (c,f) perpendicular to the oily to streaks.the oily Reproducedstreaks. Reproduced with permission with permission from [266 from]. Copyright [266]. Copyright (2017) American (2017) AmericanChemistry Chemistry Society. Society.

AnotherAnother possible possible SPR SPR effect, effect, which which is is attractive attractive but but as as yet yet largely largely unexploited, unexploited, arises arises from from the the designdesign and and realization realization of of tunable tunable emissive emissive materials, materials, in inwhich which the the SPR-related SPR-related losses losses of light of light can canbe compensatedbe compensated for forby bythe the corresponding corresponding enhancemen enhancementt effect. effect. For For instance, instance, in in [238], [238], nanoparticles nanoparticles composedcomposed of of gold cores andand dye-dopeddye-doped silicasilica shells shells could could e ffiefficientlyciently o ffoffsetset the the loss loss of SPRof SPR through through the thegain gain ofdyes, of dyes, thus thus enabling enabling surface surface plasmon plasmon amplification amplification by stimulated by stimulated emission emission of radiation. of radiation. Based Basedon this on idea, this the idea, authors the authors of [267] of successfully [267] successfully realized therealized control the of co bothntrol the of intensity both the and intensity decay ratesand decayof fluorescence rates of fluorescence by combining by the combining absorption the of absorp SPR andtion the of fluorescenceSPR and the anisotropy fluorescence of dyesanisotropy (Figure of9). dyesIn this (Figure system, 9). theIn this LC mediumsystem, the acts LC as medium an active ac template,ts as an active imposing template, orientational imposing ordering orientational on the orderingnanorods on and the mediating nanorods synchronous and mediating switching synchronous of the anisotropic switching particle of the orientation. anisotropic Although particle orientation.gold nanoparticles Although have gold been nanoparticles widely used have as plasmonic been widely building used blocks as plasmonic for various building plasmonic blocks optical for variousapplications, plasmonic they are optical rarely applications, applied in applications they are suchrarely as applied optical shutters,in applications smart windows such as or optical spatial shutters,light modulators. smart windows The main or spatial reason islight due modulators to the confinement. The main of thereason size is and due shape. to the The confinement SPR peaks of of thethese size gold and nanoparticles shape. The SPR are normallypeaks of limitedthese gold to within nanoparticles the near-infrared are normally (NIR) limited spectral to range.within Onethe near-infraredeffective solution (NIR) to thisspectral problem range. was One put forwardeffectiv bye solution Sheetah etto al.this [268 problem]. They developedwas put forward a guest–host by Sheetahmesostructured et al. [268]. composite They developed in which a dye guest molecules‒host mesostructured and plasmonic composite nanorods in spontaneously which dye molecules aligned andeither plasmonic parallel nanorods or orthogonally spontaneousl to the directory aligned of either the LC parallel (5CB and or orth AMLC-0010)ogonally to host. the director Such a systemof the LCallows (5CB the and feasible AMLC-0010) tunability host. of theSuch SPR a system peaks from allows the the NIR feasible to the visibletunability spectral of the range. SPR Bypeaks changing from thethe NIR geometry to the ofvisible nanoparticles spectral range. and di Byfferent changing kinds the of LCgeometry materials of nanopart (nematicicles and cholestericand different LCs), kinds the ofauthors LC materials also showed (nematic that and the cholesteric switching LCs), time the of the authors composite also showed could be that tuned the switching from milliseconds time of the to compositeseconds and could even be thetuned polarization from milliseconds of the transmitted to seconds lightand even could the be polarization manipulated of accordingthe transmitted to the lightapplication could be demands. manipulated Another according solution, to which the application was proposed demands. by the sameAnother group, solution, was co-dispersing which was proposeddifferent anisotropicby the same nanoparticles group, was co-dispersing in LCs [269]. different The authors anisotropic used gold nanoparticles nanorods with in LCs varying [269]. aspect The authorsratios in used a nematic gold nanorods LC host (5CB).with varying By applying aspect appropriate ratios in a surfacenematic treatments LC host (5CB). to the By particles, applying the appropriatenanorods oriented surface eithertreatments parallel to the or orthogonalparticles, the to nanorods the LC director. oriented With either the parallel application or orthogonal of an electric to thefield, LC the director. nanorods With would the application rotate their of short an electric or long field, axes the due nanorods to elastic would coupling rotate between their short particles or longand theaxes LC due director, to elastic thus coupling leading between to a tunable particle SPRs spectral and the range LC director, as well asthus polarization-dependent leading to a tunable SPRoptical spectral characteristics. range as well as polarization-dependent optical characteristics. Appl. Sci. 2019, 9, 2512 15 of 47 Appl. Sci. 2019, 9, x FOR PEER REVIEW 15 of 47

Figure 9.9. Electric switching of SPR-enhanced fluorescencefluorescence intensityintensity ofof gold–silica–dyegold‒silica‒dye nanoparticlesnanoparticles (GSDs)(GSDs) inin aa nematicnematic LCLC matrix.matrix. (a) FluorescenceFluorescence micrographmicrograph showingshowing the GSD–LCGSD‒LC composite in a planarplanar glassglass cell.cell. (b) FluorescenceFluorescence spectrumspectrum of GSDs inin thethe nematicnematic LC.LC. ((cc)) Polarization-dependentPolarization-dependent fluorescencefluorescence intensity. ( (dd)) Voltage-dependent Voltage-dependent fluorescence fluorescence of of GSDs GSDs in in a anematic nematic LC. LC. (e) ( eSwitching) Switching of ofGSDs’ GSDs’ fluorescence fluorescence intensity intensity by an by electric an electric field. field. (f) Voltage-dependent (f) Voltage-dependent ON and ON OFF and switching OFF switching times

timesobtained obtained from the from change the change of transmission of transmission I-I0 versus I-I0 versus time curves. time curves. Reproduced Reproduced with permission with permission from from[267]. [ 267Copyright]. Copyright (2016) (2016) Amer Americanican Chemical Chemical Society. Society.

Recent progressprogress has has demonstrated demonstrated that that SPR SPR also existsalso exists in monolayers in monolayers of graphene of graphene [270], graphene [270], ribbonsgraphene [271 ribbons] and [271] other and 2D other materials 2D materials [272,273 [272,273].]. Compared Compared to the to SPR the inSPR gold in gold nanoparticles, nanoparticles, the SPRthe SPR in graphene in graphene possesses possesses lower lower propagation propagation loss loss and and stronger stronger mode mode confinement confinementin in thethe mid-IRmid-IR spectraspectra [[218,274,275].218,274,275]. Reshetnyak et al. recently di discussedscussed the theoretical possibility of manipulating thethe SPRSPR eeffectffect ofof graphenegraphene monolayersmonolayers andand ribbonsribbons byby modulatingmodulating thethe dielectricdielectric propertiesproperties of thethe neighbouring LC media [[276].276]. According to their modelling, the graphene monolayersmonolayers/ribbons/ribbons were coated withwith a a LC LC layer layer and, and, by by altering altering the appliedthe applie electricd electric field orfield the or surface the surface anchoring, anchoring, the LC directorthe LC woulddirector be would reoriented be reoriented and thus vary and the thus SPR vary spectra the dueSPR to spectra changes due in the to dielectricchanges characteristicsin the dielectric of thecharacteristics LC layer on of the the graphene LC layer monolayers on the graphene/ribbons andmonolayers/ribbons the polarization ofand the the incident polarization light. of the incidentOn thelight. other hand, due to the enhanced light absorption of SPR, gold nanoparticles and graphene can alsoOn actthe as other heat nano-sources hand, due to when the excitedenhanced by a specificlight absorption wavelength of ofSPR, light, gold i.e., actnanoparticles as photothermal and agents.graphene Over can the also past act decade,as heat thenano-sources photothermal when eff ectexcited has beenby a aspecific fertile groundwavelength for fundamentally of light, i.e., act new as scientificphotothermal research. agents. Among Over the the various past decade, inorganic the (e.g.,photothermal metal and effect carbon has nanomaterials) been a fertile andground organic for (e.g.,fundamentally indocyanine new green scientific and polyaniline) research. photothermal Among the agents,various gold inorganic nanoparticles (e.g., andmetal graphene and carbon have attractednanomaterials) particular and attentionorganic (e.g., due toindocyanine their high photothermalgreen and polyaniline) conversion photothermal efficiency as wellagents, as theirgold NIR-absorbingnanoparticles and spectra, graphene which makehave themattracted extraordinarily particular popularattention in due biological to their applications high photothermal [277–280]. Remoteconversion manipulation efficiency ofas LCs well through as their the NIR-absorbing photothermal effspectra,ects induced which by make particular them nanomaterialsextraordinarily is anpopular emerging in biological field of promisingapplications materials [277–280]. [281 Remo–285].te Themanipulation functional of nanoparticle–LC LCs through the composites photothermal are driveneffects byinduced light when by particular the photothermal nanomaterials nanomaterial is an emerging absorbs thefield light of energypromising and materials convertsit [281–285]. into heat, leadingThe functional to a local nanoparticle increase in‒LC temperature. composites Consequently,are driven by light the remotelywhen the triggeredphotothermal local nanomaterial temperature increaseabsorbs inducesthe light morphological energy and andconverts physical it changesinto heat, [23 leading]. Recently, to Palermoa local increase et al. presented in temperature. an optical manipulationConsequently, of the the remotely plasmonic triggered thermal efflocalect inducedtemperature by randomly increase distributed induces morphological gold nanoparticles and coupledphysical withchanges a nematic [23]. Recently, LCs director Palermo field et (E7)al. presented as an active an optical medium. manipulation The authors of reorientedthe plasmonic the LCthermal director effect by employinginduced by a randomly photoalignment distributed material, gold which nanoparticles produced coupled changes with in both a nematic the thermal LCs conductivitydirector field and (E7) the as refractive an active index medium. of the The LCs au surroundingthors reoriented the nanoparticles, the LC director thus by enabling employing optical a photoalignment material, which produced changes in both the thermal conductivity and the refractive index of the LCs surrounding the nanoparticles, thus enabling optical control of their SPR Appl. Sci. 2019, 9, x FOR PEER REVIEW 16 of 47 Appl. Sci. 2019, 9, 2512 16 of 47 and of the resulting heating [286]. Moreover, a tunable helical pitch and a reversible handedness inversioncontrol of of their cholesteric SPR and LCs of the(CLCs), resulting enabled heating by the [286 photothermal]. Moreover, effect, a tunable were helical demonstrated pitch and by a Wangreversible et al. handedness The authors inversion dispersed of cholesteric mesogen-func LCs (CLCs),tionalized enabled gold by nanorods the photothermal into CLCs. effect, Upon were irradiationdemonstrated with by NIR Wang light, et al. the The photothermal authors dispersed effect mesogen-functionalized of the dispersed gold gold nanorods nanorods commenced, into CLCs. triggeringUpon irradiation the change with of NIR the light, helical the pitch photothermal and the handedness effect of the inversion dispersed of gold the nanorodshelical superstructure commenced, fromtriggering left-handedness the change to of right-handedness the helical pitch and through the handedness an untwisted inversion transient of thestate helical (see Figure superstructure 10). The effectfrom left-handednessis based on the to temperature right-handedness dependence through of an the untwisted pitch of transientthe helical state superstructure (see Figure 10 of). Thethe cholestericeffect is based (chiral on the nematic) temperature liquid dependence crystal. To ofachi theeve pitch an inversion of the helical of the superstructure helical handedness, of the cholesteric special materials(chiral nematic) or mixtures liquid are crystal. needed, To achievewhich exhibit an inversion a temperature-induced of the helical handedness, twist inversion. special materialsTogether withor mixtures the optical are needed,properties which of the exhibit cholesteric a temperature-induced phase, known as twist selective inversion. reflection, Together the structure with the reflectsoptical left- properties or right-handed of the cholesteric circular polarized phase, known light, asdepending selective on reflection, the handedness the structure of thereflects CLC helix, left- whileor right-handed the wavelength circular of polarizedthe reflected light, circular depending polarized on the light handedness depends ofon the the CLC pitch helix, and while average the refractivewavelength index of the of reflected the helix. circular By polarizedremoving light the depends NIR irradiation, on the pitch the and reverse average process refractive would index spontaneouslyof the helix. By occur removing [287]. the In NIR other irradiation, recent work, the reverse the authors process designed would spontaneously an adaptive smart occur window [287]. In whoseother recent optical work, transmittance the authors could designed be switched an adaptive between smart transparent window whose and optical opaque transmittance states due to could the phasebe switched transition between between transparent the SmA* and opaque and the states N* duephase to theinduced phase transitionby the photothermal between the SmA*effect and of dispersedthe N* phase graphene induced (Figure by the 10c) photothermal [288]. effect of dispersed graphene (Figure 10c) [288].

FigureFigure 10. ((aa)) Schematic Schematic figure figure of of the photothermal proc processess of reversible handedness inversion. ( (bb)) DynamicDynamic photonic photonic reflection reflection colours colours of of self-organiz self-organizeded helical helical superstructure superstructure upon upon NIR NIR irradiation. irradiation. ReproducedReproduced withwith permission permission from from [287]. Copyright[287]. Copyright (2016) John (2016) Wiley andJohn Sons. Wiley (c) Polymer-stabilizedand Sons. (c) Polymer-stabilizedgraphene-containing graphene-containing CLC thin film as a photothermal CLC thin film material. as a photothermal Reproduced ma withterial. permission Reproduced from [with288]. permissionCopyright (2017)from [288]. ELSEVIER. Copyright (2017) ELSEVIER.

Light-drivenLight-driven phase phase transitions transitions in in LCs LCs are are a a fascinating fascinating topic topic from from both both the the scientific scientific and and the technologicaltechnological point point of of view. view. Apart Apart from from the the photothermal photothermal effect effect [289], [289], such such phase phase transition transition can can also also bebe obtained by by the photoisomerization ability ability of of azo azo benzene benzene dyes dyes [290–294], [290–294], which, which, at at the the same same time,time, is is also also usually usually used used for for fabricating fabricating LC LC lasers lasers [32,295–298]. [32,295–298]. However, However, because because these these dyes dyes are are not not nanoparticles,nanoparticles, theythey willwill notnot be be discussed discussed within within the the framework framework of thisof this review. review. However, However, since since we have we havementioned mentioned LC lasers LC lasers on several on several occasions, occasions, it is sensible it is sensible to go into to somewhat go into somewhat more detail more by discussing detail by discussingquantum dots quantum (QDs) dots in this (QDs) respect. in this respect. AsAs wewe mentioned mentioned above, above, QDs areQDs semiconductor are semiconductor nanoparticles nanoparticles with unique with physical unique characteristics physical characteristicssuch as high quantumsuch as yields,high largequantum absorption yields, cross-section, large absorption tunability cross-section, of fluorescence tunability emission, of fluorescence emission, excellent colour purity, broad excitation spectra, good stability and long Appl. Sci. 2019, 9, 2512 17 of 47 Appl. Sci. 2019, 9, x FOR PEER REVIEW 17 of 47 lifetimes.excellent colourAs a result, purity, QDs broad have excitation already spectra, been wide goodly stabilityused as anda nanodopant long lifetimes. in LC As materials a result, QDs for introducinghave already additional been widely photonic used as functionalities a nanodopant to in LCLCs materials including for LC introducing lasers. LC additional lasers are photonicusually madefunctionalities by doping to laser LCs includingdyes (e.g., LC DCM) lasers. into LC ch lasersolesteric are LCs usually (CLCs), made which, by doping due to laser the dyesselective (e.g., reflection,DCM) into are cholesteric photonic band LCs (CLCs),gap materials. which, A due CLC to has the a selective 1D modulation reflection, of the are refractive photonic index band due gap tomaterials. its unique A CLCself-organized has a 1D modulation helical superstructure of the refractives, in indexwhich due rod-like to its uniqueLC molecules self-organized self-assemble helical intosuperstructures, monomolecular in which layers, rod-like with LCtheir molecules long axes self-assemble continuously into monomolecularrotating along the layers, helical with axis, their perpendicularlong axes continuously to the director. rotating The along helical the superstructure helical axis, perpendicularis described by to its the pitch director. length TheP and helical the helicalsuperstructure handedness, is described where P byis the its pitchdistance length alongP and the thehelix helical axis over handedness, which the where directorP is rotates the distance 360°, whilealong the helixhelix axishandedness over which is the directordirection rotates of rotation. 360◦, while According the helix to handednessBragg’s law, is thesuch direction helical superstructuresof rotation. According can selectively to Bragg’s reflect law, circularly such helical polarized superstructures light with can the selectively same handedness reflect circularly as the helixpolarized and the light reflection with the samewavelength handedness is determined as the helix by and its pitch the reflection λ = P wavelength, where is is determined the average by refractiveits pitch λ index.= PThus,, where CLCs isare the usually average considered refractive index. a 1D Thus,photonic CLCs crystal are usually with considered an adjustable a 1D photonicphotonic bandgap crystal with (PBGs) an adjustable via the te photonicmperature bandgap dependence (PBGs) of via the the pitch temperature P. After dependence doping the ofgain the mediumpitch P. After(e.g., dopingdyes and the QDs), gain medium the CLC (e.g., helix dyes acts and as a QDs), resonator the CLC necessary helix acts for as laser a resonator emission, necessary which canfor laserbe tuned emission, by various which external can be tuned stimuli. by various Compared external to conventional stimuli. Compared LC lasers, to conventional which are fabricated LC lasers, bywhich dispersing are fabricated laser dyes by dispersing into CLCs, laser LC dyeslasers into based CLCs, on LC QDs lasers can based lase emissions on QDs can with lase improved emissions featureswith improved such as narrower featuressuch line width, as narrower lower lasing line width, threshold lower and lasing low intensity threshold to and noise low ratios, intensity as well to asnoise high ratios, stability as welldue asto highthe 3D stability quantum due confinemen to the 3D quantumt effect of confinement QDs [131,299–301] effect of (Figure QDs [131 11).,299 On– 301the] other(Figure hand, 11). amplified On the other spontaneous hand, amplified emission spontaneous (ASE) from emission colloids (ASE) of QDs from dispersed colloids of in QDs LCs dispersed has also beenin LCs investigated has also been widely investigated [301]. widelyCao [et301 al.]. Caoreported et al. reportedan ASE an effect ASE e ffofect QDs of QDs doped doped into into polymer-dispersedpolymer-dispersed LCs LCs (PDLCs). (PDLCs). They They found found that, that, due due to to the the multiple multiple scattering scattering in in the the LC LC–polymer‒polymer matrix,matrix, the the dwell dwell time time and and path path distance distance of of light light in in the the active active media media was was drastically drastically increased, increased, thus thus leadingleading to to enhanced enhanced ASE ASE and and s s decreased decreased threshold. Such Such a a QD-PDLC QD-PDLC system system is is promising promising for for applicationsapplications suchsuch asas random random fibre fibre lasers lasers and and laser laser amplifiers amplifiers [302]. Recently,[302]. Recently, tunable tunable ASE in grapheneASE in grapheneQD-doped QD-doped CLCs have CLCs also have been also realized been by realized the authors by the [303 authors]. [303].

FigureFigure 11. 11. ((aa)) POM POM ( (II)) and and fluorescence fluorescence microscope microscope ( (IIII)) images images of of the the QD-CLC QD-CLC cell. cell. ( (bb)) Fluorescence Fluorescence emissionemission andand absorptionabsorption spectra spectra of theof 8.7the wt8.7 % wt QDs % dissolved QDs dissolved in toluene in and toluene the measured and the fluorescence measured fluorescenceemission of the emission QD-CLC of cellthe QD-CLC in the isotropic cell in phase the isotropic (red curve). phase (c )(red Optical curve). tuning (c) Optical of the lasing tuning emission of the lasingof the emission QD-CLC of laser thewith QD-CLC the added laser with chiral-azobenzene the added chiral-azobenzene moiety under UV moiety irradiation under (UVI) and irradiation optically (inducedI) and optically red-shift induced of the reflection red-shift band of the of thereflecti chiral-azobenzene-addedon band of the chiral-azobenzene-added QD-CLC under UV irradiationQD-CLC under(II). Reproduced UV irradiation with (II permission). Reproduced from with [300 ].permission Copyright from (2014) [300]. Royal Copyright Society of(2014) Chemistry. Royal Society of Chemistry. Appl. Sci. 2019, 9, 2512 18 of 47

Nanomaterials with SPR effects, fluorescence emission and other particular optical properties can introduce unique photonic functionalities to LC materials. Vice versa, LCs with large refractive index and long-range elastic anisotropies can function as dynamic medium for externally tuning the optical properties of nanomaterials. Such multifunctional nanomaterial–LC composites pave the way for the realization of various novel optic devices.

4. LC-Aided-Assembly and Self-Assembly of Nanomaterials As we have seen, nanomaterials show unique physical and chemical properties that are significantly different from bulk materials as well as from individual atoms and molecules. However, single nanomaterial entities cannot be directly applied in functional devices. The generation of macroscopic assemblies of nanomaterials with controlled localization and orientation is necessary for their practical application. A common method is to use a “bottom-up” approach whereby functional nanomaterials are designed and directed through self-organization into a particular structure, which requires the targeted ability to manipulate interactions between nanomaterials. To this end, the properties of self-organization of LCs may be exploited. The unique ability of LC materials to self-assemble into long-range ordered superstructures has been widely used as a potential method for synthesizing various functional materials with complicated spatial structures. By virtue of the chemical reactions performed in aqueous lyotropic LCs, different mesoporous materials have been successfully obtained [304]. In addition, thermotropic LCs can also act as an appropriate medium to design “top-down” approaches. LCs composed of anisotropic molecules possess a long-range orientational or even positional order and a high susceptibility to various external stimuli. Thanks to these properties, anisotropic torques and forces will be imposed on the micro-additives when they are dispersed in a LC matrix, which will also direct them on a macroscopic scale or even arrange their position on specific micro-scale locations. In the past several decades, a large number of studies have demonstrated the interactions between microparticles in LCs [145,147,148,305,306] as well as between microparticles and LC defects [137,307,308]. Some of them even reported the successful fabrication of complex periodic superstructures consisting of microparticles [146,149,150,154,308,309]. However, when it comes to the nanoscale, the problem becomes much more complicated because of the uncontrolled aggregation as well as the limited number of effective characterization methods. Nanoparticles localized in distorted LC regions can effectively decrease the free energy of the system due to the release of the elastic energy generated by the volume occupied by the nanoparticles. In the presence of a distortion gradient, the nanoparticles tend to be automatically dragged into the centre of the distortion [310–313]. As a result, topological defects, which are the cores surrounded by strongly elastically distorted regions, are usually used as templates for the bottom-up assembly of nanoparticles [314]. Moreover, in the case of anisotropic nanoparticles, the surface anchoring energy plays a vital role in the orientation of nanoparticles. With tangential anchoring, the particles tend to align along the nematic director, while, with radial anchoring, the alignment of the particles tends to be perpendicular to the director. Furthermore, nanoparticles suspended in LCs with a given anchoring energy at the surface will induce a local deformation of LC director field. According to the studies of Koenig et al. [142], the elastic distortion exerts a weak force (<5 kBT) on neighbouring nanoparticles at a range of up to a few micrometres. Coupling the LC-mediated elastic interaction between nanoparticles with the long-range drag force exerted by the distortion gradient of director field, nanoparticle assemblies with ordered mesostructures in LCs are expected to be obtained, which will potentially be useful in applications connected with the fields of photonics and electronics [128,254,261,263,264,315–319]. It has been reported that gold nanoparticles can be aligned along linear disclinations [316,320] and trapped in the centre of toric focal conic defects of smectic phases [321]. Moreover, they can be captured by the topological defects induced by the inclusion of microparticles in nematic phases [311] (Figure 12). Such periodically organized gold nanoparticles are especially interesting for optical applications due to their intriguing SPR effects. Recently, Rozic et al. demonstrated that the orientation of gold nanorods Appl. Sci. 2019, 9, 2512 19 of 47

Appl. Sci. 2019, 9, x FOR PEER REVIEW 19 of 47 with a large range of diameters could be oriented along the direction of the linear arrays of smectic A thedefects, linear so-called arrays smecticof smectic oily streaks.A defects, The goldso-ca nanorodslled smectic self-organized oily streaks. into chains,The gold increasing nanorods the self-organizeddensity of nanorods. into chains, The increasing gap between the nanorods density of could nanorods. be controlled The gap throughbetween the nanorods balance could between be controlledthe steric repulsionthrough andthe balance the van derbetween Waals the attraction. steric repulsion Furthermore, and itthe was van found der that Waals these attraction. nanorods Furthermore,were more likely it was to befound trapped that by these the smecticnanorods dislocations were more with likely respect to be to ribbon-liketrapped by defects.the smectic Such dislocationsspecial microstructures with respect of to gold ribbon-like nanorods defects. could leadSuch to special an electromagnetic microstructures coupling of gold controlled nanorods by could light leadpolarization to an electromagnetic [266]. coupling controlled by light polarization [266].

FigureFigure 12. 12. (a(a) )POM POM images images of of (I ()I )a a0.1 0.1 wt wt % % Au Au dispersion dispersion of of AuNPs AuNPs in in the the smectic smectic A A phase phase showing showing anan array array of of very very faint faint lines lines in in the the directio directionn indicated indicated by by the the double-headed double-headed arrow, arrow, and and (II (II) )1.0 1.0 wt wt % % AuAu dispersion ofof AuNPsAuNPs in in the the smectic smectic phase phase between between homeotropic homeotropic glass slidesglass showingslides showing an example an exampleof a curved of domaina curved where domain the blackwhere arrows the black indicate arrows the direction indicate of the AuNP dire arrays.ction of Reproduced AuNP arrays. with Reproducedpermission fromwith [320permission]. Copyright from (2012) [320]. TheCopyri Royalght Society (2012) ofThe Chemistry. Royal Society (b) Schematic of Chemistry. illustration (b) Schematicof a linear illustration array of straight of a linear parallel array oily of straight streaks (parallelI); (II) POM oily streaks image of(I); a ( singleII) POM array image of 8CB of a filmsingle on

arrayMoS 2of.( III8CB) SEM film imageon MoS of2. the (III MoS) SEM2 substrate image of with the gold-nanoparticleMoS2 substrate with chains gold-nanoparticle and (IV) the enlarged chains view.and (IVReproduced) the enlarged with view. permission Reproduced from [316 with]. Copyright permission (2012) from John [316]. Wiley Copyright and Sons. (2012) (c) Optical John Wiley microscopy and Sons.image (c ()I Optical) of the densemicroscopy population image of (I toric) of the focal dense conic population domains (TFCDs) of toric focal on Si conic microchannels domains (Inset(TFCDs) is a onschematic Si microchannels diagram of(Inset a silica is a particle schematic trapped diagram in the of centrea silica of particle a TFCD); trapped (II) Confocal in the centre microscopy of a TFCD); image (IIof) fluorescentConfocal microscopy silica particles image trapped of fluorescent in TFCDs. silica Reproduced particles withtrapped permission in TFCDs. from Reproduced [321]. Copyright with permission(2007) Springer from Nature. [321]. Copyri (d) A sequenceght (2007) of Springer dark-field Nature. images showing(d) A sequence a gold-nanorod of dark-field moving images into a showingtopological a gold-nanorod singularity near moving the microsphere into a topo (I)logical and polarizing, singularity bright-field, near the dark-fieldmicrosphere microscopy (I) and polarizing,images and bright-field, a schematic diagramdark-field showing microscopy chains im ofages microsphere-gold and a schematic nanoparticle diagram dimersshowing aligned chains along of microsphere-goldthe director (II). Reproduced nanoparticle with dimers permission aligned from along [311 the]. Copyrightdirector (II (2012)). Reproduced American with Chemical permission Society. from [311]. Copyright (2012) American Chemical Society. Additionally, as mentioned above, CNTs are famous for their remarkable anisotropic thermal, electrical,Additionally, optical andas mentioned mechanical above, characteristics CNTs are asfamous well asfor their their exceptional remarkable thermal anisotropic and thermal, chemical electrical,stability andoptical have and attracted mechanical interest characteristics in relation toas variouswell as their applications exceptional such thermal as sensors, and electronicchemical stabilitydevices, and conductive have attracted and transparent interest films,in relation as well to as various strong, lightapplications and conductive such as fibres. sensors, The electronic realization devices,of macroscopic conductive ordered and assembliestransparent of films, CNTs as is significantwell as strong, for the light development and conductive of electronics, fibres. The new realizationmaterials and of energymacroscopic storage ordered devices. Inassemblies the past fewof years,CNTs someis significant studies have for reported the development on the ordered of electronics,alignment andnew assemblies materials of and CNTs energy in lyotropic storage LCs devices. [322–324 In]. the CNTs past dispersed few years, in films some of polymerizablestudies have reportedlyotropic on LC the surfactants ordered canalignment be aligned and in assembli the film thicknesses of CNTs direction in lyotropic by the alignmentLCs [322–324]. of a suitableCNTs dispersedhost mesophase in films under of polymerizable an external magnetic lyotropic field LC [325 surfactants], and filaments can be of aligned uniformly in alignedthe film CNTs thickness can be directionextracted by from the CNT-dispersedalignment of a lyotropicsuitable host LCs mesophase [326]. In a recentunder investigation,an external magnetic Kasprzak field et al. [325], reported and filamentsthat, by using of uniformly photo-polymerization, aligned CNTs can a large be extrac groupted of from SWCNT-based CNT-dispersed polymer lyotropic nanocomposites LCs [326]. In was a recentobtained investigation, from polymerizable Kasprzak quaternary et al. reported ammonium that, by surfactants.using photo-polymeri Efficient dispersionzation, a large of CNTs group within of SWCNT-based polymer nanocomposites was obtained from polymerizable quaternary ammonium surfactants. Efficient dispersion of CNTs within cylindrical micelles was observed due to the physical adsorption of the nonpolar tails of the surfactants. After photo-polymerization, the Appl. Sci. 2019, 9, 2512 20 of 47

Appl. Sci. 2019, 9, x FOR PEER REVIEW 20 of 47 cylindrical micelles was observed due to the physical adsorption of the nonpolar tails of the surfactants. compositesAfter photo-polymerization, with SWCNTs are the organized composites into with ordere SWCNTsd nanostructures are organized with into bo orderedth lamellar nanostructures and gyroid nanostructureswith both lamellar [327]. and Apart gyroid from nanostructures this, the al [327ignment]. Apart and from assembly this, the of alignment CNTs in and traditional assembly thermotropicof CNTs in traditional LCs [2,328–331] thermotropic and macromolecular LCs [2,328–331 LCs] and [332,333] macromolecular have also been LCs reported. [332,333] have also beenApart reported. from the relatively simple one- (organized by nematics, smectics and cholesterics) and two- (columnarApart from phases) the relatively dimensional simple nanoparticle one- (organized structures, by nematics, the structurally smectics rich and world cholesterics) of LCs also and providestwo- (columnar phases phases) with complicated dimensional nanoparticlespatial modulation structures, of thedensity structurally and molecular rich world orientation, of LCs also periodicprovides in phases three with(blue complicated and cubic phases) spatial dimensions modulation [334]. of density As a anddistinct molecular class of orientation, frustrated periodicphases, Bluein three Phases (blue (BPs) and are cubic composed phases) of dimensions double-twist [334 cylinders,]. As a distinct energetically class offavoured frustrated at high phases, enough Blue molecularPhases (BPs) chirality, are composed but always of double-twist leading to cylinders, geometric energetically frustration favouredsince it atis highimpossible enough to molecular tile the wholechirality, 3D but space always with leading double-twist to geometric cylinders frustration without since itintroducing is impossible disclination to tile the whole lines. 3D These space disclinationwith double-twist lines either cylinders self-assemble without introducinginto a 3D periodic disclination regular lines. lattice These (BPI disclination and BPII) linesor remain either amorphouslyself-assemble intodistorted a 3D periodic (BPIII), regularwhich latticemake (BPI BPs and potential BPII) or candidat remain amorphouslyes for the 3D distorted template (BPIII), of trappingwhich make sites BPs for potentialnanoparticles candidates [335]. forAccording the 3D templateto the computer of trapping simulations sites for nanoparticlesof Stratford et [335 al.]. [318],According nanoparticle to the computer‒BPLC composites simulations can of Stratfordgive rise etto al.regular [318], nanoparticle–BPLCcrystals, glasses, percolating composites gels, can isolatedgive rise clusters, to regular twisted crystals, rings glasses, and percolatingundulating gels,colloidal isolated ropes clusters, (Figure twisted 13a). ringsThe wide and undulatingvariety of structurescolloidal ropescan be (Figure tuned 13 viaa). Thethe wideconcentration variety of and structures anchoring can becondition tuned via ofthe particles concentration as well andas externalanchoring stimuli. condition Such of a particles new class as wellof composites as external will stimuli. be promising Such a new for class switchable, of composites multistable will be devicespromising for foroptical switchable, technologies multistable such as devices smart forglass optical and technologiese-paper. Furthermore, such as smart particle glass trapping and e-paper. can alsoFurthermore, substantially particle increase trapping the thermal can also stability substantially range increaseof the blue the phase, thermal by stability a factor range of two of or the more blue [107,108,336]phase, by a factor (Figure of two 13b). or moreApart [ 107from,108 the,336 disclinations] (Figure 13b). in Apart BPs, fromother thedefects disclinations such as infocal BPs, conic other domainsdefects such in smectic as focal conicLCs [321], domains fingerprint in smectic textures LCs [321 in], CLCs fingerprint [337–339], textures or ineven CLCs the [337 LC–339‒water], or interfaceeven the [340], LC–water as well interface as the topological [340], as well defects as the indu topologicalced by microparticles defects induced [311], by microparticlescan also be used [311 to], trapcan alsonanoparticles be used to and trap produce nanoparticles periodic and arrays. produce periodic arrays.

FigureFigure 13. 13. ((aa)) Snapshots Snapshots of of the the steady steady states states obtained obtained wh whenen a a dispersion dispersion of of colloids colloids with with different different concentrationconcentration in thethe isotropicisotropic phase phas ise is placed placed in ain sandwich a sandwich geometry geometry and thenand quenchedthen quenched into a regimeinto a regimewhere BPwhere I is BP stable I is instable the bulk.in theReproduced bulk. Reprod withuced kind with permission kind permissi fromon [ 318from]. [318]. Copyright Copyright (2014) (2014)Springer Springer Nature. Nature. (b) The (b) phase The phase diagram diagram for the for BPLC the /BPLC/nannanoparticleoparticle composite. composite. Reproduced Reproduced with withpermission permission from from [336]. [336]. Copyright Copyrigh (2018)t (2018) Optical Optical Society Society of America. of America.

OnOn the the other other hand, hand, it it is is well well kn knownown that that 1D 1D or or 2D 2D anisotropic anisotropic nanoparticles, nanoparticles, such such as as CNTs CNTs and and GOs,GOs, can can act act as as LC LC building building blocks blocks in in isotropic isotropic so solventslvents and and will will entropically entropically form form lyotropic lyotropic liquid liquid crystallinecrystalline phasesphases aboveabove a criticala critical volume volume fraction fraction due todue the excludedto the excluded volume interactions,volume interactions, according accordingto Onsager’s to Onsager’s theory. According theory. toAccording the simple to steric the simple model, steric the molecular model, the configurations molecular configurations are dominated areby stericdominated effects, by which steric are effects, dependent which on theare moleculedependent concentration. on the molecule In a dilute concentration. solution, the In molecular a dilute solution,configuration the molecular behaves isotropically configuration due behaves to the large isotropically free volume. due Into athe condensed large free solution, volume. the In free a condensedvolume is inhibitedsolution, bythe the free concentrated volume is inclusions inhibited and, by asthe a consequence,concentrated the inclusions suspension and, becomes as a consequence, the suspension becomes liquid crystalline. In such suspensions, the nanoparticles exhibit the long-range orientational order with the thermodynamic stability [341]. Such nanoparticle LCs are particularly interesting and hold great potential in industrial applications such as thin films, fibres and various multifunctional composites. A variety of different materials have Appl. Sci. 2019, 9, 2512 21 of 47 liquid crystalline. In such suspensions, the nanoparticles exhibit the long-range orientational order with the thermodynamic stability [341]. Such nanoparticle LCs are particularly interesting and hold great potential in industrial applications such as thin films, fibres and various multifunctional composites. A variety of different materials have recently been studied to an increasing extent, such as carbon nanotubes, graphene oxide, nanorods and nanowires, viruses, clays and nanocrystalline cellulose. Because of their anisotropic shape, CNTs may form lyotropic nematic LC phases when they are suspended in an isotropic liquid medium at a sufficiently high concentration [342–344]. At the same time, such carbon nanotube liquid crystals (CNTLCs) can also be formed with the assistance of external forces such as magnetic field [345], electric field [331], and especially shear [346]. Nevertheless, suspending CNTs at a high concentration in a solvent is not always straightforward. CNTs are notoriously difficult to disperse in any medium and the long-term stability of the dispersion is poor because of the strong attraction from van der Waals forces between CNTs. To obtain a homogeneous suspension of CNTs, various chemical approaches have been employed [347–351]. In addition, CNTLCs usually show numerous topological defects that do not coarsen with time due to their length and waviness [352,353], as well as the high viscosity (Figure 14a–d). This is unfavourable for the optimum performance of CNTs and thus seriously hinders their applications. Methods such as mechanical shearing [354], as well as changing the morphology of CNTs [355], have been applied to obtain homogeneously aligned CNTLCs. CNTs are excellent conductors and have exceptional conductivity anisotropy. The electrical conductivity along the main axis of the tube can typically be several orders of magnitude larger than that in the tube’s radial direction. As a consequence, with the assistance of self-assembly CNTLCs, CNT films with specific structures, vertically aligned CNT arrays or horizontally aligned CNT arrays, can be obtained, which will have considerable potential to be used as transparent electrodes [356–359]. Compared to the traditional ITO electrodes, which are suffering from difficulties such as the brittleness of ITO, the rising cost of indium, and the high vacuum and temperature processing during the production, CNT electrodes have great advantages in terms of their flexibility, low cost, safe fabrication process and special conductivity anisotropy, which make them a good substitute for ITO in the LCD industries [360,361]. CNT films can also act as alignment layers to orient LC molecules via the CNT substrates due to the orientational structure and the π–π interaction between them [212,362–364] (Figure 14e,f). Apart from that, CNTs also show anisotropic optical responses due to the anisotropy and the graphite π electrons. When the polarization of the incident light is perpendicular to the tube axis, there will be no attenuation; in contrast, if they are parallel to each other, there will be strong absorption. Such anisotropic absorption not only exists in the visible region, but also in ultraviolet and infrared regions. This optical property produces a highly aligned CNT film to be used in various optical applications [365–367] (Figure 14i,j). Last but not least, CNTLCs can also be used to fabricate continuous and multifilament fibres. These lightweight fibres show a uniform morphology with few defects and a high degree of CNT alignment (Figure 14g,h). They have excellent mechanical, electrical and thermal properties and can be applied in the field of smart and strong materials including smart textiles, conductive composites and electric wires [368–370]. Appl. Sci. 2019, 9, x FOR PEER REVIEW 21 of 47 recently been studied to an increasing extent, such as carbon nanotubes, graphene oxide, nanorods and nanowires, viruses, clays and nanocrystalline cellulose. Because of their anisotropic shape, CNTs may form lyotropic nematic LC phases when they are suspended in an isotropic liquid medium at a sufficiently high concentration [342–344]. At the same time, such carbon nanotube liquid crystals (CNTLCs) can also be formed with the assistance of external forces such as magnetic field [345], electric field [331], and especially shear [346]. Nevertheless, suspending CNTs at a high concentration in a solvent is not always straightforward. CNTs are notoriously difficult to disperse in any medium and the long-term stability of the dispersion is poor because of the strong attraction from van der Waals forces between CNTs. To obtain a homogeneous suspension of CNTs, various chemical approaches have been employed [347–351]. In addition, CNTLCs usually show numerous topological defects that do not coarsen with time due to their length and waviness [352,353], as well as the high viscosity (Figure 14a–d). This is unfavourable for the optimum performance of CNTs and thus seriously hinders their applications. Methods such as mechanical shearing [354], as well as changing the morphology of CNTs [355], have been applied to obtain homogeneously aligned CNTLCs. CNTs are excellent conductors and have exceptional conductivity anisotropy. The electrical conductivity along the main axis of the tube can typically be several orders of magnitude larger than that in the tube’s radial direction. As a consequence, with the assistance of self-assembly CNTLCs, CNT films with specific structures, vertically aligned CNT arrays or horizontally aligned CNT arrays, can be obtained, which will have considerable potential to be used as transparent electrodes [356–359]. Compared to the traditional ITO electrodes, which are suffering from difficulties such as the brittleness of ITO, the rising cost of indium, and the high vacuum and temperature processing during the production, CNT electrodes have great advantages in terms of their flexibility, low cost, safe fabrication process and special conductivity anisotropy, which make them a good substitute for ITO in the LCD industries [360,361]. CNT films can also act as alignment layers to orient LC molecules via the CNT substrates due to the orientational structure and the π‒π interaction between them [212,362–364] (Figure 14e,f). Apart from that, CNTs also show anisotropic optical responses due to the anisotropy and the graphite π electrons. When the polarization of the incident light is perpendicular to the tube axis, there will be no attenuation; in contrast, if they are parallel to each other, there will be strong absorption. Such anisotropic absorption not only exists in the visible region, but also in ultraviolet and infrared regions. This optical property produces a highly aligned CNT film to be used in various optical applications [365–367] (Figure 14i,j). Last but not least, CNTLCs can also be used to fabricate continuous and multifilament fibres. These lightweight fibres show a uniform morphology with few defects and a high degree of CNT alignment (Figure 14g,h). They have excellent mechanical, electrical and thermal properties and can be applied in the field of smartAppl. Sci. and2019 strong, 9, 2512 materials including smart textiles, conductive composites and electric wires 22[368– of 47 370].

FigureFigure 14. ((aa)) Optical Optical images images of of a a CNT CNT nematic nematic LC LC schlieren schlieren texture. texture. ( (bb)) Optical Optical images images of of a a dried dried CNTLCCNTLC phase. phase. SEM SEM images images of of topologi topologicalcal disclinations disclinations of of strength +1/2+1/2 ( (cc)) and and −1/21/2 are are observable observable − ((dd).). Reproduced Reproduced with with permission permission from from [162]. [162]. Co Copyrightpyright (2003) (2003) The The American American Association Association for for the the AdvancementAdvancement of of Science. Science. (e ()e A) A schematic schematic illustration illustration of ofthe the aligning aligning LC LC molecules molecules by a by CNT a CNT film. film. (f) (f) The SEM images of the CNT alignment film on a glass substrate. Reproduced with permission from [356]. Copyright (2010) ELSEVIER. (g) Winding drums with collected fibres. (h) SEM images of corresponding CNT fibres. Reproduced with permission from [368]. Copyright (2013) The American Association for the Advancement of Science. (i) Sketch of an optical CNT polarizer. (j) Schematic cell structures of a TN-LCD by the aligned CNT polarizer. Reproduced with permission from [341]. Copyright (2018) Royal Society of Chemistry.

Furthermore, since GO has an extremely large aspect ratio, it is expected that aqueous GO dispersions can also self-assemble into lyotropic LCs according to Onsager’s theory (Figure 15a,b). Although there have been a fair number of studies focused on the liquid crystallinity of aqueous GO dispersions, it should be noticed that the first relevant study for graphene LCs was that of Behabtu et al., who dispersed graphene in chlorosulfonic acid [163]. One should recall that graphene is hardly soluble in water and organic solvents, which severely limits its application in LCs. Instead, GO has long been known to disperse well in water due to its tunable amphiphilic properties. In 2011, Kim et al. first reported the liquid crystallinity of GO [164]. The liquid crystalline properties of GO dispersions have since been studied intensively. Early investigations found that the liquid crystalline properties of GO dispersions are highly dependent on the concentration [165,371] and aspect ratio of GO flakes [372]. A high concentration of GO can lead to a “pseudo” lamellar phase [165] and a tunable structural colour can be obtained by varying the concentration [371], while a large aspect ratio of GO flakes can efficiently decrease the critical concentration for the spontaneous formation of the nematic phase [373]. Furthermore, Jalili et al. found that the ability of GO to form lyotropic LCs is largely due to the polarity and the capacity of the solvents to form hydrogen bonds with GO flakes [374], while the groups of Xu et al. [375] and Tkacz et al. [376] discovered that the LC behaviour of GO dispersions was also related to the salt concentration and pH value, respectively. Kim et al. [164] and Shen et al. [377] showed that the macroscopic alignment of GO LC could be induced by magnetic and/or electric fields. Very recently, a systematic analysis of the electro-optical response of GO LCs under alternating fields was presented by Guerrero et al. [378] and a shear-induced stripe assembly was reported by Hong et al. [379]. Due to these intriguing physical properties, various potential applications based on GO LCs (Figure 15c–e), such as GO LC gels with “shape memory” effect [380], photoluminescence imaging [381], nanocomposites [382–384], sensitive strain sensors [385], transparent electrodes [386], conductive fibres [387–390], multifunctional yarns [391] and membranes [392], flame-retardant nanocoatings [393], optical devices [394] and biomedical applications [395], have been anticipated and some of them have been realized. More relevant information can be found in a recent review published by Sasikala et al. [396]. Appl. Sci. 2019, 9, 2512 23 of 47 Appl. Sci. 2019, 9, x FOR PEER REVIEW 23 of 47

FigureFigure 15. ((aa)) POM POM images images and and ( (bb)) macroscopic macroscopic photographs photographs between between crossed crossed polarizers polarizers of of GO GO −4 4 −3 3 −3 3−3 −33 aqueousaqueous dispersions dispersions in in planar planar cells cells with with mass mass fraction fraction fm fofm of5 ×5 10 10, 1 × ,10 1 , 310 × 10, 3, 5 10× 10 ,, 5 8 × 1010 , , × − × − × − × − 18 × 1010−2 3(from, 1 101 to2 6).(from Reproduced 1 to 6). Reproduced with permission with permissionfrom [165]. fromCopyright [165]. (2011) Copyright American (2011) Chemistry American × − × − Society.Chemistry (c) Society. Schematic (c) Schematic illustration illustration of GO LC of GO membrane LC membrane in protic in protic ionic ionic liquid liquid for for nanofiltration. nanofiltration. ReproducedReproduced with with permission from [392]. [392]. Copy Copyrightright (2018) (2018) American American Chemistry Chemistry Society. Society. ( (dd)) SEM SEM imagesimages of of an as-spunas-spun GOGO fibre.fibre. Reproduced with permission fromfrom [[387].387]. Copyright Copyright (2013) (2013) John John Wiley and Sons. Sons. (e (e) )Illustration Illustration of of fire fire igniti ignitionon retardation retardation behaviour behaviour for for ( (II) )neat neat PU PU foam foam and and ( (IIII)) PDA/GO-coatedPDA/GO-coated PU PU foam. Reproduced Reproduced with with perm permissionission from from [393]. [393]. Copyright Copyright (2018) (2018) John John Wiley Wiley andand Sons. Sons.

BeforeBefore the the conclusion conclusion of of this this section, section, it is it necessary is necessary to introduce to introduce another another nanomaterial nanomaterial that thatcan self-assemblecan self-assemble into intocholesteric cholesteric lyotropic lyotropic LCs, LCs, i.e., i.e., cellulose cellulose nanocrystals nanocrystals (Figure (Figure 16).16). Cellulose nanocrystalsnanocrystals areare rod-likerod-like nanoparticles nanoparticles with with a relativelya relatively high high aspect aspect ratio ratio (from (from 10 to 10 over to 100)over that 100) can that be canextracted be extracted from an from extensive an extensive range of range materials of mate includingrials including wood, bacteria, wood, algaebacteria, and algae tunicate and [ 397tunicate–399]. [397–399].Cellulose LCsCellulose have attractedLCs have much attracted interest much due tointerest their mechanicaldue to their and mechanical optical properties. and optical The properties.individual celluloseThe individual nanocrystal cellulose (CNC) nanocrystal rods are transparent(CNC) rods across are transparent the visible spectrumacross the and visible have spectrumlow density and and have excellent low mechanicaldensity and strength. excellent It ismechanical well known strength. that suspensions It is well of celluloseknown withthat suspensionsan appropriate of concentrationcellulose with canan spontaneouslyappropriate concentration form a CLC can phase spontaneously [159,400,401]. form However, a CLC it phase is still [159,400,401].not completely However, understood it is how still the not transfer completely of chirality understood from asymmetric how the transfer carbons of in chirality the molecular from asymmetricstructure of carbons CNCs to in the the macroscopic molecular structure scale of theof CNCs LC phases to the occurs. macroscopic A recent scale study of the by LC Usov phases et al. occurs.confirmed A recent that the study mechanism by Usov ofet chiralityal. confirmed transfer that involves the mechanism multiple of steps, chirality including transfer a geometric involves multipleeffect whereby steps, theincluding CNCs acquirea geometric a uniformly effect wh right-handedereby the CNCs twisted acquire morphology a uniformly and transfer right-handed this to twistedthe helical morphology modulation and of transfer the macroscopic this to the phase helical [402 modulation]. The induced of the helicalmacroscopic pitch ofphase the cholesteric[402]. The inducedCNC suspension helical pitch is also of the intricate. cholesteric Although CNC manysuspen studiession is havealso intricate. demonstrated Although a decrease many ofstudies pitch havewith increasingdemonstrated CNC a concentrationdecrease of pitch [403 –with405], incr theeasing CNCs inCNC all theseconcentration cases are electrostatically[403–405], the CNCs charged. in allIt isthese thus cases unlikely are electrostatically that the decrease charged. of the pitchIt is thus is solely unlikely due that to the the concentration decrease of the increase. pitch is Itsolely may duealso to be the related concentration to electrostatic increase. screening It may ealsoffects be [ 406related–408 ].to Soelectrostatic far, the manipulation screening effects of the [406–408]. pitch has Sobeen far, successfully the manipulation realized by of controlling the pitch various has been parameters successfully including realized the concentration by controlling of ions various [409], parametersdifferent kinds including of solvents the [concentration410], ultrasound of treatmentions [409], [ 411different], etc. However,kinds of thesolvents methods [410], for ultrasound controlling treatment [411], etc. However, the methods for controlling the orientation of the helix axis are still limited. Recently, a cholesteric CNC film with controllable helix was achieved by Petesic et al. The Appl. Sci. 2019, 9, 2512 24 of 47

Appl. Sci. 2019, 9, x FOR PEER REVIEW 24 of 47 the orientation of the helix axis are still limited. Recently, a cholesteric CNC film with controllable helix authorswas achieved successfully by Petesic controlled et al. The the authors cholesteric successfully domains controlled by using thesmall cholesteric commercial domains magnets. by using By manipulatingsmall commercial the direction magnets. of By the manipulating magnetic field, the directionthe orientation of the magneticof the helix field, can thebe orientationadjusted from of the0° tohelix 17°, can leading be adjusted to interesting from 0 ◦opticalto 17◦ ,properties leading to [412 interesting]. The geometrical optical properties characteristics [412]. The of CNCs geometrical [413], ascharacteristics well as the ofnature CNCs of [ 413the], solvents as well as[414], the natureappear of to the play solvents an important [414], appear role toin playthe behaviour an important of pitch.role in A the most behaviour desirable of feature pitch. of A the most cholesteric desirable CNC feature suspensions of the cholesteric is the drying CNC to suspensions solid films. is The the helicaldrying superstructure to solid films. Thewill helicalnot vani superstructuresh due to kinetic will arrest; not vanish in contrast, due to kineticthe pitch arrest; of the in helix contrast, will thebe decreasedpitch of the to helix the range will be of decreased a visible selective to the range reflection, of a visible making selective the films reflection, like photonic making crystal the films paper like [415]photonic that crystalcan be paperused as [415 a colorimetric] that can be humidity used as a colorimetricsensor [416] humidityor temperat sensorure and [416 PH] or sensor temperature [409], forand optical PH sensor encryption [409], for [417,418], optical encryptionin chiral plasmonics [417,418], in[419], chiral as plasmonics light scattering [419], asshutters light scattering[420], as nano-templatesshutters [420], as[421], nano-templates and even in [421 mirrorless], and even lasing in mirrorless [160]. Rece lasingntly, [160 Rofouie]. Recently, et al. Rofouie reported et al.a cholestericreported a cholestericCNC photonic CNC semi-spherical photonic semi-spherical film with a filmpolydomain with a polydomain morphology morphology where the thickness, where the structurethickness, and structure optical and properties optical properties of the film of the ch filmanged changed along along the thesurface, surface, leading leading to toa abroadband broadband reflectionreflection covering covering the the enti entirere spectral range [422]. [422].

FigureFigure 16. 16. ((aa)) TEM TEM images images of of dried dried CNCs CNCs and and the the formatio formationn of of a a CLC CLC phase phase and and its its coexistence coexistence with with thethe isotropic phasephase inin a a CNC CNC suspension suspension (upper (upper image). image). The The typical typical CLC CLC fingerprint fingerprint appearance appearance and andthe helicalthe helical arrangement arrangement of nanorods of nanorods are illustratedare illustrated below. below. Reproduced Reproduced with with permission permission from from [160 , [160,400,421,423].400,421,423]. Copyright Copyright (2010), (2010), (2012), (2012), (2014), (2014), (2016) Springer(2016) Springer Nature. Nature. (b) Colour (b) changeColour observed change observedwhen a dried when CNC a dried film is CNC immersed film inis waterimmersed and thein correspondingwater and the spectrum. corresponding Reproduced spectrum. with Reproducedpermission fromwith [permission416]. Copyright from (2013)[416]. Copyright Elsevier. ( c(2013)) Nanoparticle Elsevier. templating(c) Nanoparticle of the templating CNC-derived of thehelical CNC-derived structure. Reproducedhelical structure. with permissionReproduced from with [424 permission,425]. Copyright from [424,425]. (2012) John Copyright Wiley and (2012) Sons Johnand (2017)Wiley Nationaland SonsAcademy and (2017) of National Sciences. Academy of Sciences.

5.5. Biological Biological Applications: Applications: Biosensors Biosensors and and Drug Drug Delivery Delivery

5.1. Biosensors 5.1. Biosensors With increasingly improved medical techniques, it is expected that people will live longer. With increasingly improved medical techniques, it is expected that people will live longer. However, the current circumstances are not that promising. Due to the emerging problem of a “graying However, the current circumstances are not that promising. Due to the emerging problem of a society” as well as the rapid spread of “illnesses of affluence,” chronic conditions such as overweight, “graying society” as well as the rapid spread of “illnesses of affluence,” chronic conditions such as hypertension, heart disease, diabetes, etc. are still severely threatening people’s health. Therefore, overweight, hypertension, heart disease, diabetes, etc. are still severely threatening people’s health. the advancement of more compelling techniques for immune detection of different kinds of disease Therefore, the advancement of more compelling techniques for immune detection of different kinds biomarkers is necessary. LCs, which are susceptible to weak external stimuli, such as chemistry, of disease biomarkers is necessary. LCs, which are susceptible to weak external stimuli, such as solvent vapour, humidity or surface forces, have received increased attention for use in biological chemistry, solvent vapour, humidity or surface forces, have received increased attention for use in sensing since Abbott and his co-workers initiated the research field of using LC materials as sensing biological sensing since Abbott and his co-workers initiated the research field of using LC materials elements for detecting biomolecules [426]. The key idea of this study is using a nematic LC film with a as sensing elements for detecting biomolecules [426]. The key idea of this study is using a nematic specific alignment, either homogeneous or homeotropic, at an interface with an aqueous phase. Under LC film with a specific alignment, either homogeneous or homeotropic, at an interface with an aqueous phase. Under polarizing optical microscopy, the LCs will show a particular texture. By introducing some “impurity” molecules into the aqueous phase, the alignment of the LC molecules Appl. Sci. 2019, 9, x FOR PEER REVIEW 25 of 47 at the interface will be altered due to the binding between the biomolecules and the doped impurity, thus leading to a texture transition (Figure 17). This transition is easily seen by the naked eye. Such LC-based biosensors can localize biomolecules to the micrometre scale and the procedure can be executed under ambient light even without an external electric power source. All these advantages lend LC-based biosensors great potential for the next generation of high-sensitivity, low-costAppl. Sci. 2019 and, 9 ,label-free 2512 bioassays. Up to now, various investigations of LC-based biosensors25 have of 47 been reported [427–432]. Very recently, Lee et al. reported a label-free protein (bovine serum albumin, BSA) quantitative detection by a dual-frequency LC (DFLC)-based biosensor [433]. The spectrapolarizing of opticalthe dielectric microscopy, properties the LCs of willDFLC show in th a particulare presence texture. of BSA By at introducingvaried concentrations some “impurity” were measuredmolecules intoand theanalysed. aqueous They phase, demonstrated the alignment that of the LCdifference molecules in dielectric at the interface properties will be between altered duethe low-to the and binding high-frequency between the regimes biomolecules was andrelated the dopedto the impurity,concentration thus leadingof BSA, to which a texture permitted transition a quantitative(Figure 17). Thismeasurement transition of is the easily BSA seen concentration. by the naked eye.

Figure 17. ((aa)) Schematic Schematic illustration illustration of of the LC-based biosensor. ( (bb––dd)) Optical Optical image image and and cartoon representationrepresentation ofof the the anchoring anchoring transition transition of 5CB of and5CB the and state the of thestate aqueous-5CB of the aqueous-5CB interface immediately interface immediatelyafter injection after of a dispersioninjection of of a vesicles dispersion formed of fromvesicles 0.1 mMformed L-α-dilauroyl from 0.1 phosphatidylcholinemM L-α-dilauroyl phosphatidylcholine(L-DLPC) in tris-buff ered(L-DLPC) saline, in (c tris-buffered) after ~10–20 saline, min of (c contact) after ~10 with‒20 the min vesicle of contact dispersion with ofthe L-DLPC vesicle dispersionand (d) after of 2L-DLPC h of contact. and (d Reproduced) after 2 h of withcontact. permission Reproduced from wi [427th ].permission Copyright from (2003) [427]. The Copyright American (2003)Association The American for the Advancement Association fo ofr Science.the Advancement of Science.

TheSuch simplicity LC-based and biosensors versatility can of localize LC-based biomolecules biosensors to and the micrometretheir sensitivity scale is and restricted the procedure by the sizecan and be executed amount of under biomolecules, ambient lightwhich even may without restrict their an external applications electric in the power bioassay source. range All of these low concentrationsadvantages lend and LC-based trace molecules. biosensors It is great thus potentialsignificant for to theseek next signal-enhancement generation of high-sensitivity, strategies for LC-basedlow-cost andsensors label-free to circumvent bioassays. the Upproblem. to now, Severa variousl studies investigations have demonstrated of LC-based that biosensors the disruption have ofbeen the reported LC orientation [427–432 can]. Very be enhanced recently, Lee by etintroducing al. reported nanoparticles, a label-free protein thus leading (bovine to serum an amplified albumin, opticalBSA) quantitative signal [434–440] detection (Figure by a dual-frequency18a,b). A recent LC study (DFLC)-based by Wei biosensoret al. demonstrated [433]. The spectraa nickel of nanoparticle-assistedthe dielectric properties LC ofcell DFLC system in that the can presence be used of BSAto visualized at varied the concentrations enzymatic activities were measured between cholylglycineand analysed. hydrolase They demonstrated (CGH) and that cholylglycine the difference (CG). in dielectricThe trace properties doping of between nickel nanoparticles the low- and inducedhigh-frequency a uniform regimes homeotropic was related LC alignment to the concentration that could ofbe BSA,disturbed which by permitted introducing a quantitative CG to the binding-immobilizedmeasurement of the BSA CGH concentration. surface. Such a system had a detection limit at the “10 pM” level, and couldThe quantitatively simplicity anddefine versatility a working of LC-basedrange from biosensors 0.1 nM to and1 μM their [439]. sensitivity The investigation is restricted by Kim by the et al.size reported and amount a different of biomolecules, biosensor based which on may self-rep restrictorting their and applications self-regulating in the LCs bioassay (Figure range 18c,d). of low In thisconcentrations interesting work, and trace the authors molecules. showed It is thusthat a significant range of stimuli, to seek including signal-enhancement temperature, strategies mechanical for shearLC-based as well sensors as cationic to circumvent amphiphiles, the problem. can induce Several a studiescontinuous have demonstratedor transient release that the of disruption microcargo of (micro-particlesthe LC orientation or can-droplets) be enhanced that initially by introducing is trapped nanoparticles, within the thus LC leadingmatrix. toBy an pre-programming amplified optical signal [434–440] (Figure 18a,b). A recent study by Wei et al. demonstrated a nickel nanoparticle-assisted LC cell system that can be used to visualized the enzymatic activities between cholylglycine hydrolase (CGH) and cholylglycine (CG). The trace doping of nickel nanoparticles induced a uniform homeotropic LC alignment that could be disturbed by introducing CG to the binding-immobilized CGH surface. Such a system had a detection limit at the “10 pM” level, and could quantitatively define a working range from 0.1 nM to 1 µM[439]. The investigation by Kim et al. reported a different biosensor based Appl. Sci. 2019, 9, 2512 26 of 47

on self-reporting and self-regulating LCs (Figure 18c,d). In this interesting work, the authors showed that a range of stimuli, including temperature, mechanical shear as well as cationic amphiphiles, can Appl. Sci. 2019, 9, x FOR PEER REVIEW 26 of 47 induce a continuous or transient release of microcargo (micro-particles or -droplets) that initially throughis trapped an withininterplay the of LC elasticity, matrix. By electrical pre-programming double layers, through buoyancy an interplay and shear of elasticity, forces in electrical diverse geometries,double layers, the buoyancy LC matrixes and could shear forcesself-report in diverse and se geometries,lf-regulate thetheir LC chemical matrixes response could self-report to targeted and chemical,self-regulate physical their chemicaland biological response events to targeted in particular chemical, ways. physical These LC and materials biological can events detect in particularthe weak mechanicalways. These shear LC materials stress generated can detect by themotile weak bacter mechanicalia and then shear respond stress generated in a self-regulated by motile manner bacteria viaand a thenfeedback respond loop in in a self-regulatedorder to release manner the mini viamum a feedback amount loop of biocidal in order agent to release required the minimum to cause bacterialamount ofcell biocidal death [441]. agent required to cause bacterial cell death [441].

FigureFigure 18. 18. (a)( aThe) The orientations orientations of 5CB of 5CB in the in LC the cells LC fabricated cells fabricated with DMOAP-coated with DMOAP-coated glass slides glass (upperslides (upperslides) slides)and andmodified modified slides slides (I– (IIII–III; ;lowerlower): ):(I ()I )(3-aminopropyl)trimethoxysilane (APS)/DMOAP-coated(APS)/DMOAP-coated glassglass slide,slide, ( II(II) streptavidin) streptavidin alkaline alkaline phosphatase phosphatase (Sv-ALP) (Sv-ALP) modified modified glass glass slide, slide,and ( IIIand) silver-deposited (III) silver-deposited glass slide. glass (slide.b) Optical (b) Optical images images under crossed under crossed polarizers polarizers of LC cells of withLC cells 5CB withsandwiched 5CB sandwiched between a DMOAP-coatedbetween a DMOAP-coated glass slide and glas as silver-deposited slide and a silver-deposited glass slide with glass complementary slide with complementarytarget DNA at concentrations target DNA at of concentrations (I) 0, (II) 0.1, (III of) 1,(I ()IV 0, )( 10,II) (0.1,V) 100(III pM,) 1, ( andIV) (10,VI )(V two-base) 100 pM, mismatched and (VI) two-baseDNA at concentrationmismatched DNA of 100 at pM. concentration Reproduced of with100 pM. permission Reproduced from [with434]. permission Copyright from (2010) [434]. John CopyrightWiley and Sons.(2010) (c )John Sequential Wiley micrographsand Sons. ((Ic–)III Sequential) and corresponding micrographs illustrations (I–III) and (IV –VIcorresponding) of a LC film illustrationscontaining microdroplets (IV‒VI) of a LC before film (containiI,IV) andng after microdroplets addition of before cationic (I, amphiphileIV) and after DTAB addition to the of overlying cationic amphiphileaqueous phase, DTAB at 0to ( IIthe,V )overlying and 60 min aqueous (III,VI phase,). (d) Release at 0 (II of,V microcargo) and 60 min from (III LCs,VI). by (d the) Release heat of of a microcargohuman finger from or byLCs motile by the bacteria heat of that a human generate finger interfacial or by shear motile stresses bacteria and that interact generate with microcargointerfacial shearin a self-regulated stresses and interact manner. with Reproduced microcargo with in permission a self-regulated from [ 441manner.]. Copyright Reproduced (2018) with Springer permission Nature. from [441]. Copyright (2018) Springer Nature. 5.2. Drug Delivery 5.2. DrugAs oneDelivery of the key features providing biological systems with unique physical and chemical properties,As one theof self-assemblythe key features of biologicalproviding materials,biological suchsystems as amphiphilic with unique lipids, physical has beenand chemical the focus properties,of an increasing the self-assembly number of studies.of biological Lyotropic materials, LC dropletssuch as amphiphilic with diameters lipids, in has the been range the of focus 100 nm, of anusually increasing referred number to as cubosomesof studies. Lyotropic and hexosomes, LC droplets are potential with diameters candidates in forthe constructingrange of 100 novel nm, usuallymatrices referred mimicking to as biological cubosomes systems and hexosomes, in the formation are potential of new nanoparticlecandidates for carriers constructing for delivering novel matricesdrugs and mimicking nutrition. biological These self-assembled systems in nanostructuresthe formation of are new spontaneously nanoparticle formed carriers by for biocompatible delivering drugsamphiphilic and lipids,nutrition. such asThese phospholipids, self-assembled monoglycerides, nanostructures glycolipids are andspontaneously urea-based lipids,formed mainly by biocompatiblein water, and containedamphiphilic in a thinlipids, shell such of amphiphilic as phospholipids, block co-polymer monoglycerides, (Figure 19 ). glycolipids and urea-based lipids, mainly in water, and contained in a thin shell of amphiphilic block co-polymer (Figure 19). Appl. Sci. 2019, 9, 2512 27 of 47 Appl. Sci. 2019, 9, x FOR PEER REVIEW 27 of 47

Figure 19.19. TEMTEM images images of of different different nonlamella nonlamellarr lipid nanoparticles containing ( a–d) reversed bicontinuous, ( e,f) sponge, and ((g,,h)) reversedreversed hexagonalhexagonal lyotropiclyotropic LCLC phases.phases. Reproduced with permission from [[442].442]. Copyright (2005)(2005) American Chemical Society.

Unlike traditional liposomes, which can only load a hydrophilic drug cargo and release it to the target immediately once the lipid membrane is broken, these LC nano nanoparticlesparticles have cores with equal amounts ofof polarpolar and and non-polar non-polar regimes, regimes, and and thus thus can can carry carry either eith hydrophilicer hydrophilic or hydrophobic or hydrophobic cargo, cargo,or even or both even simultaneously. both simultaneously. At the At same the same time, ti theme, interior the interior drugs drugs are protected are protected from from chemical chemical and andenzymatic enzymatic degradation degradation during during the passage the passage through throug the digestiveh the digestive system system to the siteto the of release,site of release, leading leadingto a sustained to a sustained release over release time [over15]. Duetime to[15]. such Due a slow to such release, a slow the drugs release, are the inclined drugs to are accumulate inclined asto accumulatemuch as possible as much in sickness-related as possible in tissuessickness-related or cells to generatetissues or a cells maximal to generate effect and a maximal reduce the effect negative and reduceeffects onthe human negative bodies effects such on as human the toxicity bodies of such the drugs,as the makingtoxicity theseof the LC drugs, colloidal making nanoparticles these LC particularlycolloidal nanoparticles interesting inparticularly medical treatments interesting [443 ,444in ].medical Solid lipidtreatments nanoparticles [443,444]. (SLNs) Solid have lipid also nanoparticlesattracted great attention(SLNs) have as cosmetic also attracted and pharmaceutical great attention formulations as cosmetic [445]. Dueand topharmaceutical their relatively formulationssmall particle [445]. size but Due large to their surface relatively area, SLNs small possess particle very size strong but adhesivelarge surface characteristics. area, SLNs Thispossess can veryinduce strong film adhesive formation characteristics. on the human This skin, can which induce may film be formation helpful to on keep the thehuman skin skin, moist which and evenmay berestore helpful the to previously keep the damagedskin moist protective and even lipidrestore film the on previously the body throughdamaged occlusion. protective Incorporation lipid film on theof active body ingredientsthrough occlusion. into a solid Incorporation lipid matrix of canactive provide ingredients protection into a against solid lipid hydrolytic matrix degradation,can provide protectionallowing drug against release hydrolytic at a controlled degradation, rate. This allowing implies drug either release a transient at a controlled or a continuous rate. This drug implies release, eitherdepending a transient on the or particle a continuous size and drug polymorphic release, depending transitions on of thethe lipidparticle matrix size [446and]. polymorphic Continuous transitionsrelease is critical of thefor lipid active matrix ingredients [446]. Continuous that may irritate release the is bodycritical if for the active concentration ingredients is too that high may or whenirritate a the prolonged body if the period concentration of drug transport is too high time or is when desired, a prolonged whereas transientperiod of releasedrug transport will be helpfultime is desired,to improve whereas the penetration transient release of drugs. will be In helpful addition to to improve LC phases the penetration and solid lipids, of drugs. a large In addition variety ofto LCmaterials phases can and be fabricatedsolid lipids, into a drug-carrierlarge variety nanoparticles. of materials Forcan instance, be fabricated both biodegradable into drug-carrier and nanoparticles.non-biodegradable For instance, polymers both have biodegradable been used as and drug non-bi carrierodegradable materials [447 polymers]. have been used as drug carrier materials [447]. 6. Summary 6. SummaryIn this paper, we summarized the influence of nanoparticle doping on the electro-optical and other physical properties, of liquid crystals, including molecule alignment, viscosity, clearing point, In this paper, we summarized the influence of nanoparticle doping on the electro-optical and or elastic constants, just to name a few of the properties introduced. Various kinds of nanodopants, other physical properties, of liquid crystals, including molecule alignment, viscosity, clearing point, such as ferroelectric nanoparticles, noble metallic nanoparticles, semiconductor nanoparticles and or elastic constants, just to name a few of the properties introduced. Various kinds of nanodopants, carbon nanoparticles, have been presented to induce modifications of the physical properties of LCs. such as ferroelectric nanoparticles, noble metallic nanoparticles, semiconductor nanoparticles and However, these studies create an inconsistent picture of the beneficial effects of nanoparticles on the carbon nanoparticles, have been presented to induce modifications of the physical properties of LCs. physical properties of LC hosts. This may be attributed to the effect of different nanoparticles’ size, However, these studies create an inconsistent picture of the beneficial effects of nanoparticles on the shape, dispersibility and functionalization. To thoroughly understand the influence of nanoparticle physical properties of LC hosts. This may be attributed to the effect of different nanoparticles’ size, shape, dispersibility and functionalization. To thoroughly understand the influence of nanoparticle Appl. Sci. 2019, 9, 2512 28 of 47 doping on the physical properties of LCs, further systematic investigations are required. We further demonstrated the exciting photonic functionalities, including SPR effect, photothermal effect and lasing, of LC–nanoparticle composites. The intrinsic optical properties of nanoparticles can be effectively modified by a LC medium, leading to a variety of multifunctional photonic applications. Various nanoparticle mesostructures, obtained either within distorted areas of LCs or topological defects, were discussed. The self-organization-assisted nanoparticle assembly within the disclination lines will effectively eliminate high-energy volume areas and reduce the elastic-free energy density of the LC system. Such an effect can even lead to a stabilization of the 3D blue phase defect lattice. Additionally, we demonstrated the self-assembly of nanoparticles into lyotropic LC phases. Colloidal suspensions of anisotropic particles, including rods, tubes, disks, flexible chains and wires, can self-organize from the isotropic disordered to the nematic ordered phase as the concentration exceeds a critical value according to Onsager’s model. These lyotropic LC materials have intriguing physical properties and can be applied to various functional devices. Finally, we briefly introduce the practical applications of LC–nanoparticle composites in biological systems. The aspects of biological sensors and drug delivery systems are of particular interest, and it is anticipated that this area of novel liquid crystal research will attract much interest in the liquid crystal community and soft matter community in particular.

Author Contributions: Y.S. conceived and wrote the manuscript, and prepared the figures and reproduction permissions. I.D. helped with discussing and writing the manuscript. Funding: This research received no external funding. Acknowledgments: Y.S. would like to thank the China Scholarship Council (CSC). Conflicts of Interest: The authors declare no conflict of interest.

References

1. Tschierske, C. Liquid Crystals: Materials Design and Self-Assembly; Springer Science & Business Media: Berlin, Germany, 2012; Volume 318. 2. Bisoyi, H.K.; Kumar, S. Liquid-crystal nanoscience: An emerging avenue of soft self-assembly. Chem. Soc. Rev. 2011, 40, 306–319. [CrossRef][PubMed] 3. Kato, T.; Mizoshita, N.; Kishimoto, K. Functional liquid-crystalline assemblies: Self-organized soft materials. Angew. Chem. Int. Ed. 2005, 45, 38–68. [CrossRef][PubMed] 4. Ohzono, T.; Fukuda, J.I. Zigzag line defects and manipulation of colloids in a nematic liquid crystal in microwrinkle grooves. Nat. Commun. 2012, 3, 701. [CrossRef][PubMed] 5. Chuang, I.; Durrer, R.; Turok, N.; Yurke, B. Cosmology in the laboratory: Defect dynamics in liquid crystals. Science 1991, 251, 1336–1342. [CrossRef][PubMed] 6. Pieranski, P.; Yang, B.; Burtz, L.-J.; Camu, A.; Simonetti, F. Generation of umbilics by magnets and flows. Liq. Cryst. 2013, 40, 1593–1608. [CrossRef] 7. Dierking, I.; Al-Zangana, S. Lyotropic liquid crystal phases from anisotropic nanomaterials. Nanomaterials 2017, 7, 305. [CrossRef][PubMed] 8. Saliba, S.; Mingotaud, C.; Kahn, M.L.; Marty, J.-D. Liquid crystalline thermotropic and lyotropic nanohybrids. Nanoscale 2013, 5, 6641–6661. [CrossRef] 9. Laschat, S.; Baro, A.; Steinke, N.; Giesselmann, F.; Hägele, C.; Scalia, G.; Judele, R.; Kapatsina, E.; Sauer, S.; Schreivogel, A.; et al. Discotic liquid crystals: From tailor-made synthesis to plastic electronics. Angew. Chem. Int. Ed. 2007, 46, 4832–4887. [CrossRef] 10. Hegmann, T.; Qi, H.; Marx, V.M. Nanoparticles in liquid crystals: Synthesis, self-assembly, defect formation and potential applications. J. Inorg. Organomet. Polym. Mater. 2007, 17, 483–508. [CrossRef] 11. Dierking, I. Textures of Liquid Crystals; John Wiley & Sons: Hoboken, NJ, USA, 2003. 12. Tschierske, C. Non-conventional liquid crystals—the importance of micro-segregation for self-organisation. J. Mater. Chem. 1998, 8, 1485–1508. [CrossRef] 13. Takezoe, H.; Eremin, A. Bent-Shaped Liquid Crystals: Structures and Physical Properties; CRC Press: Boca Raton, FL, USA, 2017. Appl. Sci. 2019, 9, 2512 29 of 47

14. Coleman, D.; Fernsler, J.; Chattham, N.; Nakata, M.; Takanishi, Y.; Körblova, E.; Link, D.; Shao, R.-F.; Jang, W.; Maclennan, J. Polarization-modulated smectic liquid crystal phases. Science 2003, 301, 1204–1211. [CrossRef] [PubMed] 15. Lagerwall, J.P.F.; Scalia, G. A new era for liquid crystal research: Applications of liquid crystals in soft matter nano-, bio- and microtechnology. Curr. Appl. Phys. 2012, 12, 1387–1412. [CrossRef] 16. Park, H.-S.; Lavrentovich, O.D. Lyotropic chromonic liquid crystals: Emerging applications. Liq. Cryst. Beyond Disp. 2012.[CrossRef] 17. Zheng, Z.G.; Li, Y.; Bisoyi, H.K.; Wang, L.; Bunning, T.J.; Li, Q. Three-dimensional control of the helical axis of a chiral nematic liquid crystal by light. Nature 2016, 531, 352–356. [CrossRef][PubMed] 18. Li, S.-S.; Shen, Y.; Chang, Z.-N.; Li, W.-S.; Xu, Y.-C.; Fan, X.-Y.; Chen, L.-J. Dynamic cholesteric liquid crystal superstructures photoaligned by one-step polarization holography. Appl. Phys. Lett. 2017, 111, 231109. [CrossRef] 19. Zheng, Z.G.; Zola, R.S.; Bisoyi, H.K.; Wang, L.; Li, Y.; Bunning, T.J.; Li, Q. Controllable dynamic zigzag pattern formation in a soft helical superstructure. Adv. Mater. 2017, 29, 1701903. [CrossRef][PubMed] 20. Bisoyi, H.K.; Bunning, T.J.; Li, Q. Stimuli-driven control of the helical axis of self-organized soft helical superstructures. Adv. Mater. 2018, 30, e1706512. [CrossRef][PubMed] 21. Shen, Y.; Xu, Y.C.; Ge, Y.H.; Jiang, R.G.; Wang, X.Z.; Li, S.S.; Chen, L.J. Photoalignment of dye-doped cholesteric liquid crystals for electrically tunable patterns with fingerprint textures. Opt. Express 2018, 26, 1422–1432. [CrossRef] 22. Subacius, D.; Bos, P.J.; Lavrentovich, O.D. Switchable diffractive cholesteric gratings. Appl. Phys. Lett. 1997, 71, 1350–1352. [CrossRef] 23. Bisoyi, H.K.; Urbas, A.M.; Li, Q. Soft materials driven by photothermal effect and their applications. Adv. Opt. Mater. 2018, 6, 1800458. [CrossRef] 24. Ma, L.-L.; Li, S.-S.; Li, W.-S.; Ji, W.; Luo, B.; Zheng, Z.-G.; Cai, Z.-P.; Chigrinov, V.; Lu, Y.-Q.; Hu, W.; et al. Rationally designed dynamic superstructures enabled by photoaligning cholesteric liquid crystals. Adv. Opt. Mater. 2015, 3, 1691–1696. [CrossRef] 25. Lin, Y.; Yang, Y.; Shan, Y.; Gong, L.; Chen, J.; Li, S.; Chen, L. Magnetic nanoparticle-assisted tunable optical patterns from spherical cholesteric liquid crystal bragg reflectors. Nanomaterials 2017, 7, 376. [CrossRef] [PubMed] 26. O’Neill, M.; Kelly, S.M. Ordered materials for organic electronics and photonics. Adv. Mater. 2010, 23, 566–584. [CrossRef][PubMed] 27. Sano, S.; Miyama, T.; Takatoh, K.; Kobayashi, S. Enhancement of the characteristics of lcds by doping nanoparticles: Reduction of the operating voltage, viscosity, and response times. In Proceedings of the Integrated Optoelectronic Devices 2006, San Jose, CA, USA, 21–26 January 2006; SPIE: Bellingham, WA, USA, 2006; p. 613501. 28. Stannarius, R. Liquid crystals: More than display fillings. Nat. Mater. 2009, 8, 617–618. [CrossRef][PubMed] 29. Ryabchun, A.; Bobrovsky, A. Cholesteric liquid crystal materials for tunable diffractive optics. Adv. Opt. Mater. 2018, 6, 1800335. [CrossRef] 30. Li, W.-S.; Ma, L.-L.; Gong, L.-L.; Li, S.-S.; Yang, C.; Luo, B.; Hu, W.; Chen, L.-J. Interlaced cholesteric liquid crystal fingerprint textures via sequential uv-induced polymer-stabilization. Opt. Mater. Express 2016, 6, 19–28. [CrossRef] 31. Li, W.-S.; Shen, Y.; Chen, Z.-J.; Cui, Q.; Li, S.-S.; Chen, L.-J. Demonstration of patterned polymer-stabilized cholesteric liquid crystal textures for anti-counterfeiting two-dimensional barcodes. Appl. Opt. 2017, 56, 601–606. [CrossRef] 32. Huang, W.; Yuan, C.L.; Shen, D.; Zheng, Z.G. Dynamically manipulated lasing enabled by a reconfigured fingerprint texture of a cholesteric self-organized superstructure. J. Mater. Chem. C 2017, 5, 6923–6928. [CrossRef] 33. Price, A.D.; Schwartz, D.K. DNA hybridization-induced reorientation of liquid crystal anchoring at the nematic liquid crystal/aqueous interface. J. Am. Chem. Soc. 2008, 130, 8188–8194. [CrossRef] 34. Sivakumar, S.; Wark, K.L.; Gupta, J.K.; Abbott, N.L.; Caruso, F. Liquid crystal emulsions as the basis of biological sensors for the optical detection of bacteria and viruses. Adv. Funct. Mater. 2009, 19, 2260–2265. [CrossRef] Appl. Sci. 2019, 9, 2512 30 of 47

35. Chen, C.-H.; Yang, K.-L. Detection and quantification of DNA adsorbed on solid surfaces by using liquid crystals. Langmuir 2010, 26, 1427–1430. [CrossRef] 36. Spicer, P.T. Progress in liquid crystalline dispersions: Cubosomes. Curr. Opin. Colloid Interface Sci. 2005, 10, 274–279. [CrossRef] 37. Sagalowicz, L.; Leser, M.E.; Watzke, H.J.; Michel, M. Monoglyceride self-assembly structures as delivery vehicles. Trends Food Sci. Technol. 2006, 17, 204–214. [CrossRef] 38. Yaghmur, A.; Glatter, O. Characterization and potential applications of nanostructured aqueous dispersions. Adv. Colloid Interface Sci. 2009, 147–148, 333–342. [CrossRef] 39. Mushenheim, P.C.; Trivedi, R.R.; Weibel, D.B.; Abbott, N.L. Using liquid crystals to reveal how mechanical anisotropy changes interfacial behaviors of motile bacteria. Biophys. J. 2014, 107, 255–265. [CrossRef] [PubMed] 40. Zhou, S.; Sokolov, A.; Lavrentovich, O.D.; Aranson, I.S. Living liquid crystals. Proc. Natl. Acad. Sci. USA 2014, 111, 1265. [CrossRef][PubMed] 41. Kim, Y.H.; Yoon, D.K.; Jeong, H.S.; Lavrentovich, O.D.; Jung, H.-T. Smectic liquid crystal defects for self-assembling of building blocks and their lithographic applications. Adv. Funct. Mater. 2011, 21, 610–627. [CrossRef] 42. Tschierske, C. Liquid crystal engineering—new complex mesophase structures and their relations to polymer morphologies, nanoscale patterning and crystal engineering. Chem. Soc. Rev. 2007, 36, 1930–1970. [CrossRef] 43. Zhou, S. Living liquid crystals. In Lyotropic Chromonic Liquid Crystals: From Viscoelastic Properties to Living Liquid Crystals; Zhou, S., Ed.; Springer International Publishing: Cham, Switzerland, 2017; pp. 77–92. 44. Wang, L.; Ge, S.; Hu, W.; Nakajima, M.; Lu, Y. Tunable reflective liquid crystal terahertz waveplates. Opt. Mater. Express 2017, 7, 2023. [CrossRef] 45. Kularatne, R.S.; Kim, H.; Boothby, J.M.; Ware, T.H. Liquid crystal elastomer actuators: Synthesis, alignment, and applications. J. Polym. Sci. B Polym. Phys. 2017, 55, 395–411. [CrossRef] 46. Oldenbourg, R.; Wen, X.; Meyer, R.B.; Caspar, D.L.D. Orientational distribution function in nematic tobacco-mosaic-virus liquid crystals measured by X-ray diffraction. Phys. Rev. Lett. 1988, 61, 1851–1854. [CrossRef][PubMed] 47. Daniel, M.-C.; Astruc, D. Gold nanoparticles: Assembly, supramolecular chemistry, quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology. Chem. Rev. 2004, 104, 293–346. [CrossRef][PubMed] 48. Kim, F.S.; Ren, G.; Jenekhe, S.A. One-dimensional nanostructures of π-conjugated molecular systems: Assembly, properties, and applications from photovoltaics, sensors, and nanophotonics to nanoelectronics. Chem. Mater. 2010, 23, 682–732. [CrossRef] 49. Hamley, I.W. Nanotechnologie mit weichen Materialien. Angew. Chem. 2003, 115, 1730–1752. [CrossRef] 50. Anisa, M.; Abdallah, S.D.; Peter, A.S. ‘Mind the gap’: Science and ethics in nanotechnology. Nanotechnology 2003, 14, R9.

51. Gupta, S.M.; Tripathi, M. A review of TO2 nanoparticles. Chin. Sci. Bull. 2011, 56, 1639. [CrossRef] 52. Koch, U.; Fojtik, A.; Weller, H.; Henglein, A. Photochemistry of semiconductor colloids. Preparation of extremely small ZnO particles, fluorescence phenomena and size quantization effects. Chem. Phys. Lett. 1985, 122, 507–510. [CrossRef] 53. Özgür, Ü.; Alivov, Y.I.; Liu, C.; Teke, A.; Reshchikov, M.A.; Do˘gan,S.; Avrutin, V.; Cho, S.J.; Morkoç, H. A comprehensive review of ZnO materials and devices. J. Appl. Phys. 2005, 98, 041301. [CrossRef]

54. Zhao, Y.; Li, C.; Liu, X.; Gu, F.; Jiang, H.; Shao, W.; Zhang, L.; He, Y. Synthesis and optical properties of TiO2 nanoparticles. Mater. Lett. 2007, 61, 79–83. [CrossRef] 55. Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M.A. Chemistry and properties of nanocrystals of different shapes. Chem. Rev. 2005, 105, 1025–1102. [CrossRef] 56. Liu, J.; He, H.; Xiao, D.; Yin, S.; Ji, W.; Jiang, S.; Luo, D.; Wang, B.; Liu, Y. Recent advances of plasmonic nanoparticles and their applications. Materials 2018, 11, 1833. [CrossRef][PubMed] 57. Zhang, S.; Pelligra, C.I.; Feng, X.; Osuji, C.O. Directed assembly of hybrid nanomaterials and nanocomposites. Adv. Mater. 2018, 30, 1705794. [CrossRef][PubMed] 58. Meng, Q.B.; Fu, C.H.; Einaga, Y.; Gu, Z.Z.; Fujishima, A.; Sato, O. Assembly of highly ordered

three-dimensional porous structure with nanocrystalline TiO2 semiconductors. Chem. Mater. 2002, 14, 83–88. [CrossRef] Appl. Sci. 2019, 9, 2512 31 of 47

59. Furube, A.; Du, L.; Hara, K.; Katoh, R.; Tachiya, M. Ultrafast plasmon-induced electron transfer from gold

nanodots into TiO2 nanoparticles. J. Am. Chem. Soc. 2007, 129, 14852–14853. [CrossRef][PubMed] 60. Robel, I.; Kuno, M.; Kamat, P.V. Size-dependent electron injection from excited CdSe quantum dots into TiO2 nanoparticles. J. Am. Chem. Soc. 2007, 129, 4136–4137. [CrossRef][PubMed] 61. Hyun, B.-R.; Zhong, Y.-W.; Bartnik, A.C.; Sun, L.; Abruña, H.D.; Wise, F.W.; Goodreau, J.D.; Matthews, J.R.; Leslie, T.M.; Borrelli, N.F. Electron injection from colloidal PbS quantum dots into titanium dioxide nanoparticles. ACS Nano 2008, 2, 2206–2212. [CrossRef][PubMed] 62. Tripathi, A.K.; Singh, M.K.; Mathpal, M.C.; Mishra, S.K.; Agarwal, A. Study of structural transformation in

TiO2 nanoparticles and its optical properties. J. Alloy Compd. 2013, 549, 114–120. [CrossRef] 63. Chik, M.W.; Hussain, Z.; Zulkefeli, M.; Tripathy, M.; Kumar, S.; Majeed, A.B.A.; Byrappa, K. Polymer-wrapped single-walled carbon nanotubes: A transformation toward better applications in healthcare. Drug Deliv. Transl. Res. 2018, 9, 578–594. [CrossRef] 64. Farokhzad, O.C.; Langer, R. Impact of nanotechnology on drug delivery. ACS Nano 2009, 3, 16–20. [CrossRef] 65. Grzelczak, M.; Vermant, J.; Furst, E.M.; Liz-Marzán, L.M. Directed self-assembly of nanoparticles. ACS Nano 2010, 4, 3591–3605. [CrossRef] 66. Goodby, J.W.; Saez, I.M.; Cowling, S.J.; Görtz, V.; Draper, M.; Hall, A.W.; Sia, S.; Cosquer, G.; Lee, S.-E.; Raynes, E.P. Transmission and amplification of information and properties in nanostructured liquid crystals. Angew. Chem. Int. Ed. 2008, 47, 2754–2787. [CrossRef][PubMed] 67. Qi, H.; Hegmann, T. Impact of nanoscale particles and carbon nanotubes on current and future generations of liquid crystal displays. J. Mater. Chem. 2008, 18, 3288–3294. [CrossRef] 68. Blanc, C.; Coursault, D.; Lacaze, E. Ordering nano- and microparticles assemblies with liquid crystals. Liq. Cryst. Rev. 2013, 1, 83–109. [CrossRef] 69. Shivakumar, U.; Mirzaei, J.; Feng, X.; Sharma, A.; Moreira, P.; Hegmann, T. Nanoparticles: Complex and multifaceted additives for liquid crystals. Liq. Cryst. 2011, 38, 1495–1514. [CrossRef] 70. Dierking, I. Nanomaterials in liquid crystals. Nanomaterials 2018, 8, 453. [CrossRef] 71. Garbovskiy, Y.A.; Glushchenko, A.V. Liquid crystalline colloids of nanoparticles. In Solid State Physics; Academic Press: Cambridge, MA, USA, 2010; Volume 62, pp. 1–74. 72. Poulin, P.; Stark, H.; Lubensky, T.C.; Weitz, D.A. Novel colloidal interactions in anisotropic fluids. Science 1997, 275, 1770. [CrossRef] 73. Khoo, I.; Chen, K.; Williams, Y.Z. Orientational photorefractive effect in undoped and cdse nanorods-doped nematic liquid crystal—Bulk and interface contributions. IEEE J. Sel. Top. Quantum Electron. 2006, 12, 443–450. [CrossRef] 74. Khoo, I.C.; Williams, Y.Z.; Lewis, B.; Mallouk, T. + Photorefractive CdSe and gold nanowire-doped liquid − crystals and polymer-dispersed-liquid-crystal photonic crystals. Mol. Cryst. Liq. Cryst. 2006, 446, 233–244. [CrossRef] 75. Kinkead, B.; Hegmann, T. Effects of size, capping agent, and concentration of CdSe and CdTe quantum dots doped into a nematic liquid crystal on the optical and electro-optic properties of the final colloidal liquid crystal mixture. J. Mater. Chem. 2010, 20, 448–458. [CrossRef] 76. Bartkiewicz, S.; Matczyszyn, K.; Miniewicz, A.; Kajzar, F. High gain of light in photoconducting polymer–nematic liquid crystal hybrid structures. Opt. Commun. 2001, 187, 257–261. [CrossRef] 77. Müller, J.; Sönnichsen, C.; von Poschinger, H.; von Plessen, G.; Klar, T.A.; Feldmann, J. Electrically controlled light scattering with single metal nanoparticles. Appl. Phys. Lett. 2002, 81, 171–173. [CrossRef] 78. Tomohiro, M.; Jirakorn, T.; Hiroyuki, S.; Yoshio, S.; Yukihide, S.; Naoki, T.; Shunsuke, K. Fast switching of frequency modulation twisted nematic liquid crystal display fabricated by doping nanoparticles and its mechanism. Jpn. J. Appl. Phys. 2004, 43, 2580. 79. Rajiv, M.; Sat Prakash, Y.; Abhishek Kumar, S.; Abhishek Kumar, M.; Kamal Kumar, P.; Prashant, K.S.; Avinash Chand, P. Zinc oxide (1% Cu) nanoparticle in nematic liquid crystal: Dielectric and electro-optical study. Jpn. J. Appl. Phys. 2009, 48, 101501. 80. Li, X.; Yang, C.; Wang, Q.; Jia, D.; Hu, L.; Peng, Z.; Xuan, L. Enhanced birefringence for metallic nanoparticle doped liquid crystals. Opt. Commun. 2013, 286, 224–227. [CrossRef] 81. Park, S.Y.; Stroud, D. Surface-enhanced plasmon splitting in a liquid-crystal-coated gold nanoparticle. Phys. Rev. Lett. 2005, 94, 217401. [CrossRef][PubMed] Appl. Sci. 2019, 9, 2512 32 of 47

82. Kossyrev, P.A.; Yin, A.; Cloutier, S.G.; Cardimona, D.A.; Huang, D.; Alsing, P.M.; Xu, J.M. Electric field tuning of plasmonic response of nanodot array in liquid crystal matrix. Nano Lett. 2005, 5, 1978–1981. [CrossRef] [PubMed] 83. Brochard, F.; De Gennes, P. Theory of magnetic suspensions in liquid crystals. J. Phys. 1970, 31, 691–708. [CrossRef] 84. Rault, J.; Cladis, P.E.; Burger, J.P. Ferronematics. Phys. Lett. A 1970, 32, 199–200. [CrossRef] 85. Kaczmarek, M.; Buchnev, O.; Nandhakumar, I. Ferroelectric nanoparticles in low refractive index liquid crystals for strong electro-optic response. Appl. Phys. Lett. 2008, 92, 103307. [CrossRef] 86. Li, F.; Buchnev, O.; Cheon, C.I.; Glushchenko, A.; Reshetnyak, V.; Reznikov, Y.; Sluckin, T.J.; West, J.L. Orientational coupling amplification in ferroelectric nematic colloids. Phys. Rev. Lett. 2006, 97, 147801. [CrossRef] 87. Kaur, S.; Singh, S.P.; Biradar, A.M.; Choudhary, A.; Sreenivas, K. Enhanced electro-optical properties in gold nanoparticles doped ferroelectric liquid crystals. Appl. Phys. Lett. 2007, 91, 023120. [CrossRef] 88. Yada, M.; Yamamoto, J.; Yokoyama, H. Direct observation of anisotropic interparticle forces in nematic colloids with optical tweezers. Phys. Rev. Lett. 2004, 92, 185501. [CrossRef] 89. Smalyukh, I.I.; Kuzmin, A.N.; Kachynski, A.V.; Prasad, P.N.; Lavrentovich, O.D. Optical trapping of colloidal particles and measurement of the defect line tension and colloidal forces in a thermotropic nematic liquid crystal. Appl. Phys. Lett. 2005, 86, 021913. [CrossRef] 90. Lapanik, A.; Rudzki, A.; Kinkead, B.; Qi, H.; Hegmann, T.; Haase, W. Electrooptical and dielectric properties of alkylthiol-capped gold nanoparticle–ferroelectric liquid crystal nanocomposites: Influence of chain length and tethered liquid crystal functional groups. Soft Matter 2012, 8, 8722–8728. [CrossRef] 91. Shukla, R.K.; Feng, X.; Umadevi, S.; Hegmann, T.; Haase, W. Influence of different amount of functionalized bulky gold nanorods dopant on the electrooptical, dielectric and optical properties of the flc host. Chem. Phys. Lett. 2014, 599, 80–85. [CrossRef] 92. Joshi, T.; Kumar, A.; Prakash, J.; Biradar, A.M. Low power operation of ferroelectric liquid crystal system dispersed with zinc oxide nanoparticles. Appl. Phys. Lett. 2010, 96, 253109. [CrossRef] 93. Lisetski, L.N.; Minenko, S.S.; Zhukov, A.V.; Shtifanyuk, P.P.; Lebovka, N.I. Dispersions of carbon nanotubes in cholesteric liquid crystals. Mol. Cryst. Liq. Cryst. 2009, 510, 43–50. [CrossRef] 94. Singh, U.B.; Dhar, R.; Dabrowski, R.; Pandey, M.B. Enhanced electro-optical properties of a nematic liquid

crystals in presence of BaTiO3 nanoparticles. Liq. Cryst. 2014, 41, 953–959. [CrossRef] 95. San, S.E.; Okutan, M.; Köysal, O.; Yerli, Y. Carbon nanoparticles in nematic liquid crystals. Chin. Phys. Lett. 2008, 25, 212. [CrossRef] 96. Zhao, D.; Xu, L.; Shang, Y.; Li, X.; Guo, L. Facet-dependent electro-optical properties of cholesteric liquid

crystals doped with Cu2O nanocrystals. Nano Res. 2018, 11, 4836–4845. [CrossRef] 97. Williams, Y.; Chan, K.; Park, J.H.; Khoo, I.C.; Lewis, B.; Mallouk, T.E. Electro-Optical and Nonlinear Optical Properties of Semiconductor Nanorod Doped Liquid Crystals. In Proceedings of the Optics and Photonics 2005, San Diego, CA, USA, 22 July 2005; SPIE: Bellingham, WA, USA, 2005; p. 593613. 98. Garbovskiy, Y.; Glushchenko, A. Ferroelectric nanoparticles in liquid crystals: Recent progress and current challenges. Nanomaterials 2017, 7, 361. [CrossRef] 99. Shelestiuk, S.M.; Reshetnyak, V.Y.; Sluckin, T.J. Frederiks transition in ferroelectric liquid-crystal nanosuspensions. Phys. Rev. E 2011, 83, 041705. [CrossRef] 100. Kurochkin, O.; Buchnev, O.; Iljin, A.; Park, S.K.; Kwon, S.B.; Grabar, O.; Yu, R. A colloid of ferroelectric nanoparticles in a cholesteric liquid crystal. J. Opt. A Pure Appl. Opt. 2009, 11, 024003. [CrossRef] 101. Lopatina, L.M.; Selinger, J.V. Maier-saupe-type theory of ferroelectric nanoparticles in nematic liquid crystals. Phys. Rev. E 2011, 84, 041703. [CrossRef] 102. Chen, S.-H.; Amer, N.M. Observation of macroscopic collective behavior and new texture in magnetically doped liquid crystals. Phys. Rev. Lett. 1983, 51, 2298–2301. [CrossRef] 103. Martínez-Miranda, L.J.; McCarthy, K.; Kurihara, L.K.; Harry, J.J.; Noel, A. Effect of the surface coating on the magnetic nanoparticle smectic-a liquid crystal interaction. Appl. Phys. Lett. 2006, 89, 161917. [CrossRef] 104. Kopˇcanský, P.; Tomašoviˇcová, N.; Koneracká, M.; Závišová, V.; Timko, M.; Džarová, A.; Šprincová, A.; Éber, N.; Fodor-Csorba, K.; Tóth-Katona, T.; et al. Structural changes in the 6chbt liquid crystal doped with spherical, rodlike, and chainlike magnetic particles. Phys. Rev. E 2008, 78, 011702. [CrossRef] Appl. Sci. 2019, 9, 2512 33 of 47

105. Mertelj, A.; Lisjak, D.; Drofenik, M.; Copiˇc,M.ˇ Ferromagnetism in suspensions of magnetic platelets in liquid crystal. Nature 2013, 504, 237. [CrossRef] 106. Mertelj, A.; Osterman, N.; Lisjak, D.; Copiˇc,M.ˇ Magneto-optic and converse magnetoelectric effects in a ferromagnetic liquid crystal. Soft Matter 2014, 10, 9065–9072. [CrossRef] 107. Hiroyuki, Y.; Yuma, T.; Kosuke, K.; Hitoshi, K.; Tetsuya, T.; Akihiko, F.; Susumu, K.; Hirotsugu, K.; Masanori, O. Nanoparticle-stabilized cholesteric blue phases. Appl. Phys. Express 2009, 2, 121501. 108. Karatairi, E.; Rožiˇc,B.; Kutnjak, Z.; Tzitzios, V.; Nounesis, G.; Cordoyiannis, G.; Thoen, J.; Glorieux, C.; Kralj, S. Nanoparticle-induced widening of the temperature range of liquid-crystalline blue phases. Phys. Rev. E 2010, 81, 041703. [CrossRef] 109. Prakash, J.; Choudhary, A.; Kumar, A.; Mehta, D.S.; Biradar, A.M. Nonvolatile memory effect based on gold nanoparticles doped ferroelectric liquid crystal. Appl. Phys. Lett. 2008, 93, 112904. [CrossRef] 110. Dolgov, L.O.; Yaroshchuk, O.V. Electrooptic properties of liquid crystals filled with silica nanoparticles of different sorts. Colloid Polym. Sci. 2004, 282, 1403–1408. [CrossRef] 111. Basu, R. Soft memory in a ferroelectric nanoparticle-doped liquid crystal. Phys. Rev. E 2014, 89, 022508. [CrossRef] 112. Kempaiah, R.; Liu, Y.; Nie, Z.; Basu, R. Giant soft-memory in liquid crystal nanocomposites. Appl. Phys. Lett. 2016, 108, 083105. [CrossRef] 113. Shiraishi, Y.; Toshima, N.; Maeda, K.; Yoshikawa, H.; Xu, J.; Kobayashi, S. Frequency modulation response of a liquid-crystal electro-optic device doped with nanoparticles. Appl. Phys. Lett. 2002, 81, 2845–2847. [CrossRef] 114. Dierking, I.; Scalia, G.; Morales, P.; LeClere, D. Aligning and reorienting carbon nanotubes with nematic liquid crystals. Adv. Mater. 2004, 16, 865–869. [CrossRef] 115. Özgan, ¸S.;Eskalen, H.; Tapkıranlı, Y. Thermal and electro-optic properties of graphene oxide-doped hexylcyanobiphenyl liquid crystal. J. Theor. Appl. Phys. 2018, 12, 169–176. [CrossRef] 116. Gardner, D.F.; Evans, J.S.; Smalyukh, I.I. Towards reconfigurable optical metamaterials: Colloidal nanoparticle self-assembly and self-alignment in liquid crystals. Mol. Cryst. Liq. Cryst. 2011, 545, 1227–1245. [CrossRef] 117. Valentine, J.; Zhang, S.; Zentgraf, T.; Ulin-Avila, E.; Genov, D.A.; Bartal, G.; Zhang, X. Three-dimensional optical metamaterial with a negative refractive index. Nature 2008, 455, 376. [CrossRef][PubMed] 118. Zheludev, N.I.; Kivshar, Y.S. From metamaterials to metadevices. Nat. Mater. 2012, 11, 917–924. [CrossRef] [PubMed] 119. Si, G.; Leong, E.S.P.; Jiang, X.; Lv, J.; Lin, J.; Dai, H.; Liu, Y.J. All-optical, polarization-insensitive light tuning properties in silver nanorod arrays covered with photoresponsive liquid crystals. Phys. Chem. Chem. Phys. 2015, 17, 13223–13227. [CrossRef][PubMed] 120. Nemati, A.; Wang, Q.; Hong, M.; Teng, J. Tunable and reconfigurable metasurfaces and metadevices. Opto Electron. Adv. 2018, 1, 180009. [CrossRef] 121. Liu, Y.J.; Hao, Q.; Smalley, J.S.T.; Liou, J.; Khoo, I.C.; Huang, T.J. A frequency-addressed plasmonic switch based on dual-frequency liquid crystals. Appl. Phys. Lett. 2010, 97, 091101. [CrossRef] 122. Link, S.; El-Sayed, M.A. Optical properties and ultrafast dynamics of metallic nanocrystals. Annu. Rev. Phys. Chem. 2003, 54, 331–366. [CrossRef][PubMed] 123. Hutter, E.; Fendler, J.H. Exploitation of localized surface plasmon resonance. Adv. Mater. 2004, 16, 1685–1706. [CrossRef] 124. Chu, K.C.; Chao, C.Y.; Chen, Y.F.; Wu, Y.C.; Chen, C.C. Electrically controlled surface plasmon resonance frequency of gold nanorods. Appl. Phys. Lett. 2006, 89, 103107. [CrossRef] 125. Evans, P.R.; Wurtz, G.A.; Hendren, W.R.; Atkinson, R.; Dickson, W.; Zayats, A.V.; Pollard, R.J. Electrically switchable nonreciprocal transmission of plasmonic nanorods with liquid crystal. Appl. Phys. Lett. 2007, 91, 043101. [CrossRef] 126. Singh, D.P.; Gupta, S.K.; Srivastava, A.; Manohar, R. The phenomenon of induced photoluminescence in ferroelectric mesophase. J. Lumin. 2013, 139, 60–63. [CrossRef] 127. Tong, X.; Zhao, Y. Liquid-crystal gel-dispersed quantum dots: Reversible modulation of photoluminescence intensity using an electric field. J. Am. Chem. Soc. 2007, 129, 6372–6373. [CrossRef][PubMed] 128. Rodarte, L.A.; Cisneros, F.; Hein, E.J.; Ghosh, S.; Hirst, S.L. Quantum dot/liquid crystal nanocomposites in photonic devices. Photonics 2015, 2, 855–864. [CrossRef] Appl. Sci. 2019, 9, 2512 34 of 47

129. Danilov, V.V.; Artem’ev, M.V.; Baranov, A.V.; Ermolaeva, G.M.; Utkina, N.A.; Khrebtov, A.I. Fluorescence of semiconductor nanorods in liquid-crystal composites. Opt. Spectrosc. 2008, 105, 306–309. [CrossRef] 130. Ozaki, M.; Kasano, M.; Ganzke, D.; Haase, W.; Yoshino, K. Mirrorless lasing in a dye-doped ferroelectric liquid crystal. Adv. Mater. 2002, 14, 306–309. [CrossRef] 131. Chen, L.-J.; Lee, C.-R.; Chu, C.-L. Surface passivation assisted lasing emission in the quantum dots doped cholesteric liquid crystal resonating cavity with polymer template. RSC Adv. 2014, 4, 52804–52807. [CrossRef] 132. Lee, C.-R.; Lin, S.-H.; Guo, J.-W.; Lin, J.-D.; Lin, H.-L.; Zheng, Y.-C.; Ma, C.-L.; Horng, C.-T.; Sun, H.-Y.; Huang, S.-Y. Electrically and thermally controllable nanoparticle random laser in a well-aligned dye-doped liquid crystal cell. Opt. Mater. Express 2015, 5, 1469–1481. [CrossRef] 133. Rodarte, A.L.; Gray, C.; Hirst, L.S.; Ghosh, S. Spectral and polarization modulation of quantum dot emission in a one-dimensional liquid crystal photonic cavity. Phys. Rev. B 2012, 85, 035430. [CrossRef] 134. Wu, K.-J.; Chu, K.-C.; Chao, C.-Y.; Chen, Y.-F.; Lai, C.-W.; Kang, C.-C.; Chen, C.-Y.; Chou, P.-T. CdS nanorods imbedded in liquid crystal cells for smart optoelectronic devices. Nano Lett. 2007, 7, 1908–1913. [CrossRef] 135. Ruhwandl, R.W.; Terentjev, E.M. Long-range forces and aggregation of colloid particles in a nematic liquid crystal. Phys. Rev. E 1997, 55, 2958–2961. [CrossRef] 136. Stark, H. Physics of colloidal dispersions in nematic liquid crystals. Phys. Rep. 2001, 351, 387–474. [CrossRef] 137. Pires, D.; Fleury, J.-B.; Galerne, Y. Colloid particles in the interaction field of a disclination line in a nematic phase. Phys. Rev. Lett. 2007, 98, 247801. [CrossRef][PubMed] 138. Samitsu, S.; Takanishi, Y.; Yamamoto, J. Molecular manipulator driven by spatial variation of liquid-crystalline order. Nat. Mater. 2010, 9, 816. [CrossRef][PubMed] 139. Kuksenok, O.V.; Ruhwandl, R.W.; Shiyanovskii, S.V.; Terentjev, E.M. Director structure around a colloid particle suspended in a nematic liquid crystal. Phys. Rev. E 1996, 54, 5198–5203. [CrossRef] 140. Mondain-Monval, O.; Dedieu, J.C.; Gulik-Krzywicki, T.; Poulin, P.Weak surface energy in nematic dispersions: Saturn ring defects and quadrupolar interactions. Eur. Phys. J. B Condens. Matter Complex. Syst. 1999, 12, 167–170. [CrossRef] 141. Stark, H. Director field configurations around a spherical particle in a nematic liquid crystal. Eur. Phys. J. B Condens. Matter Complex. Syst. 1999, 10, 311–321. [CrossRef] 142. Koenig, G.M.; de Pablo, J.J.; Abbott, N.L. Characterization of the reversible interaction of pairs of nanoparticles dispersed in nematic liquid crystals. Langmuir 2009, 25, 13318–13321. [CrossRef][PubMed] 143. Škarabot, M.; Muševiˇc,I. Direct observation of interaction of nanoparticles in a nematic liquid crystal. Soft Matter 2010, 6, 5476–5481. [CrossRef] 144. Tomar, V.; Roberts, T.F.; Abbott, N.L.; Hernández-Ortiz, J.P.; de Pablo, J.J. Liquid crystal mediated interactions between nanoparticles in a nematic phase. Langmuir 2012, 28, 6124–6131. [CrossRef] 145. Kotar, J.; Vilfan, M.; Osterman, N.; Babiˇc,D.; Copiˇc,M.;ˇ Poberaj, I. Interparticle potential and drag coefficient in nematic colloids. Phys. Rev. Lett. 2006, 96, 207801. [CrossRef] 146. Škarabot, M.; Ravnik, M.; Žumer, S.; Tkalec, U.; Poberaj, I.; Babiˇc,D.; Osterman, N.; Muševiˇc,I. Interactions of quadrupolar nematic colloids. Phys. Rev. E 2008, 77, 031705. [CrossRef] 147. Vilfan, M.; Osterman, N.; Copiˇc,M.;ˇ Ravnik, M.; Žumer, S.; Kotar, J.; Babiˇc,D.; Poberaj, I. Confinement effect on interparticle potential in nematic colloids. Phys. Rev. Lett. 2008, 101, 237801. [CrossRef][PubMed] 148. Chernyshuk, S.B.; Lev, B.I. Elastic interaction between colloidal particles in confined nematic liquid crystals. Phys. Rev. E 2010, 81, 041701. [CrossRef][PubMed] 149. Muševiˇc,I.; Škarabot, M.; Tkalec, U.; Ravnik, M.; Žumer, S. Two-dimensional nematic colloidal crystals self-assembled by topological defects. Science 2006, 313, 954. [CrossRef][PubMed] 150. Škarabot, M.; Ravnik, M.; Žumer, S.; Tkalec, U.; Poberaj, I.; Babiˇc, D.; Osterman, N.; Muševiˇc, I. Two-dimensional dipolar nematic colloidal crystals. Phys. Rev. E 2007, 76, 051406. [CrossRef][PubMed] 151. Muševiˇc,I.; Škarabot, M. Self-assembly of nematic colloids. Soft Matter 2008, 4, 195–199. [CrossRef] 152. Ognysta, U.; Nych, A.; Nazarenko, V.; Škarabot, M.; Muševiˇc,I. Design of 2d binary colloidal crystals in a nematic liquid crystal. Langmuir 2009, 25, 12092–12100. [CrossRef] 153. Prathap Chandran, S.; Mondiot, F.; Mondain-Monval, O.; Loudet, J.C. Photonic control of surface anchoring on solid colloids dispersed in liquid crystals. Langmuir 2011, 27, 15185–15198. [CrossRef] 154. Nych, A.; Ognysta, U.; Škarabot, M.; Ravnik, M.; Žumer, S.; Muševiˇc,I. Assembly and control of 3d nematic dipolar colloidal crystals. Nat. Commun. 2013, 4, 1489. [CrossRef] Appl. Sci. 2019, 9, 2512 35 of 47

155. Bawden, F.C.; Pirie, N.W.; Bernal, J.D.; Fankuchen, I. Liquid crystalline substances from virus-infected plants. Nature 1936, 138, 1051. [CrossRef] 156. Onsager, L. The effects of shape on the interaction of colloidal particles. Ann. N. Y. Acad. Sci. 1949, 51, 627–659. [CrossRef] 157. Li, L.S.; Walda, J.; Manna, L.; Alivisatos, A.P. Semiconductor nanorod liquid crystals. Nano Lett. 2002, 2, 557–560. [CrossRef] 158. Li, L.S.; Alivisatos, A.P. Semiconductor nanorod liquid crystals and their assembly on a substrate. Adv. Mater. 2003, 15, 408–411. [CrossRef] 159. Revol, J.F.; Bradford, H.; Giasson, J.; Marchessault, R.H.; Gray, D.G. Helicoidal self-ordering of cellulose microfibrils in aqueous suspension. Int. J. Biol. Macromol. 1992, 14, 170–172. [CrossRef] 160. Lagerwall, J.P.F.; Schütz, C.; Salajkova, M.; Noh, J.; Hyun Park, J.; Scalia, G.; Bergström, L. Cellulose nanocrystal-based materials: From liquid crystal self-assembly and glass formation to multifunctional thin films. NPG Asia Mater. 2014, 6, e80. [CrossRef] 161. Shimoda, H.; Oh, S.J.; Geng, H.Z.; Walker, R.J.; Zhang, X.B.; Mcneil, L.E.; Zhou, O. Self-assembly of carbon nanotubes. Adv. Mater. 2002, 14, 899–901. [CrossRef] 162. Song, W.; Kinloch, I.A.; Windle, A.H. Nematic liquid crystallinity of multiwall carbon nanotubes. Science 2003, 302, 1363. [CrossRef] 163. Behabtu, N.; Lomeda, J.R.; Green, M.J.; Higginbotham, A.L.; Sinitskii, A.; Kosynkin, D.V.; Tsentalovich, D.E.; Parravasquez, A.N.G.; Schmidt, J.; Kesselman, E. Spontaneous high-concentration dispersions and liquid crystals of graphene. Nat. Nanotechnol. 2010, 5, 406–411. [CrossRef][PubMed] 164. Kim, J.E.; Han, T.H.; Lee, S.H.; Kim, J.; Ahn, C.W.; Yun, J.M.; Kim, S.O. Graphene oxide liquid crystals. Angew. Chem. 2011, 50, 3043–3047. [CrossRef][PubMed] 165. Xu, Z.; Gao, C. Aqueous liquid crystals of graphene oxide. ACS Nano 2011, 5, 2908–2915. [CrossRef] [PubMed] 166. Kato, T.; Uchida, J.; Ichikawa, T.; Sakamoto, T. Functional liquid crystals towards the next generation of materials. Angew. Chem. Int. Ed. 2018, 57, 4355–4371. [CrossRef][PubMed] 167. Li, Q. Nanoscience with Liquid Crystals; Springer: Berlin, Germany, 2016. 168. Reznikov, Y.; Buchnev, O.; Tereshchenko, O.; Reshetnyak, V.; Glushchenko, A.; West, J. Ferroelectric nematic suspension. Appl. Phys. Lett. 2003, 82, 1917–1919. [CrossRef] 169. Glushchenko, A.; Cheon, C.I.; West, J.; Li, F.; Büyüktanir, E.; Reznikov, Y.; Buchnev, A. Ferroelectric particles in liquid crystals: Recent frontiers. Mol. Cryst. Liq. Cryst. 2006, 453, 227–237. [CrossRef]

170. Blach, J.F.; Saitzek, S.; Legrand, C.; Dupont, L.; Henninot, J.F.; Warenghem, M. BaTiO3 ferroelectric nanoparticles dispersed in 5CB nematic liquid crystal: Synthesis and electro-optical characterization. J. Appl. Phys. 2010, 107, 074102. [CrossRef] 171. Mishra, M.; Dabrowski, R.S.; Dhar, R. Thermodynamical, optical, electrical and electro-optical studies

of a room temperature nematic liquid crystal 4-pentyl-40-cyanobiphenyl dispersed with barium titanate nanoparticles. J. Mol. Liq. 2016, 213, 247–254. [CrossRef] 172. Klein, S.; Richardson Robert, M.; Greasty, R.; Jenkins, R.; Stone, J.; Thomas Michael, R.; Sarua, A. The influence of suspended nanoparticles on the frederiks threshold of the nematic host. Philos. Trans. R. Soc. A Math. Phys. Eng. Sci. 2013, 371, 20120253. [CrossRef][PubMed] 173. Al-Zangana, S.; Turner, M.; Dierking, I. A comparison between size dependent paraelectric and ferroelectric

BaTiO3 nanoparticle doped nematic and ferroelectric liquid crystals. J. Appl. Phys. 2017, 121, 085105. [CrossRef] 174. Dubey, R.; Mishra, A.; Singh, K.N.; Alapati, P.R.; Dhar, R. Electric behaviour of a schiff’s base liquid crystal

compound doped with a low concentration of BaTiO3 nanoparticles. J. Mol. Liq. 2017, 225, 496–501. [CrossRef] 175. Prasad, S.K.; Kumar, M.V.; Shilpa, T.; Yelamaggad, C.V. Enhancement of electrical conductivity, dielectric anisotropy and director relaxation frequency in composites of gold nanoparticle and a weakly polar nematic liquid crystal. RSC Adv. 2014, 4, 4453–4462. [CrossRef] 176. Sridevi, S.; Prasad, S.K.; Nair, G.G.; D’Britto, V.; Prasad, B.L.V. Enhancement of anisotropic conductivity, elastic, and dielectric constants in a liquid crystal-gold nanorod system. Appl. Phys. Lett. 2010, 97, 151913. [CrossRef] Appl. Sci. 2019, 9, 2512 36 of 47

177. Pandey, A.S.; Dhar, R.; Kumar, S.; Dabrowski, R. Enhancement of the display parameters of

40-pentyl-4-cyanobiphenyl due to the dispersion of functionalised gold nano particles. Liq. Cryst. 2011, 38, 115–120. [CrossRef] 178. Holt, L.A.; Bushby, R.J.; Evans, S.D.; Burgess, A.; Seeley, G. A 106-fold enhancement in the conductivity of a discotic liquid crystal doped with only 1% (w/w) gold nanoparticles. J. Appl. Phys. 2008, 103, 063712. [CrossRef] 179. Singh, U.B.; Dhar, R.; Dabrowski, R.; Pandey, M.B. Influence of low concentration silver nanoparticles on the electrical and electro-optical parameters of nematic liquid crystals. Liq. Cryst. 2013, 40, 774–782. [CrossRef] 180. Liu, B.; Ma, Y.; Zhao, D.; Xu, L.; Liu, F.; Zhou, W.; Guo, L. Effects of morphology and concentration of cus nanoparticles on alignment and electro-optic properties of nematic liquid crystal. Nano Res. 2017, 10, 618–625. [CrossRef] 181. Khatua, S.; Manna, P.; Chang, W.-S.; Tcherniak, A.; Friedlander, E.; Zubarev, E.R.; Link, S. Plasmonic nanoparticles liquid crystal composites. J. Phys. Chem. C 2010, 114, 7251–7257. [CrossRef] − 182. Elkhalgi, H.H.M.; Khandka, S.; Singh, U.B.; Pandey, K.L.; Dabrowski, R.; Dhar, R. Dielectric and electro-optical properties of a nematic liquid crystalline material with gold nanoparticles. Liq. Cryst. 2018, 45, 1795–1801. [CrossRef] 183. Mishra, M.; Kumar, S.; Dhar, R. Effect of high concentration of colloidal gold nanoparticles on the thermodynamic, optical, and electrical properties of 2, 3, 6, 7, 10, 11-hexabutyloxytryphenylene discotic liquid crystalline material. Soft Matter 2017, 15, 34–44. [CrossRef] 184. Gittins, D.I.; Bethell, D.; Schiffrin, D.J.; Nichols, R.J. A nanometre-scale electronic switch consisting of a metal cluster and redox-addressable groups. Nature 2000, 408, 67. [CrossRef] 185. Haynes, C.L.; McFarland, A.D.; Zhao, L.; Van Duyne, R.P.; Schatz, G.C.; Gunnarsson, L.; Prikulis, J.; Kasemo, B.; Käll, M. Nanoparticle optics: The importance of radiative dipole coupling in two-dimensional nanoparticle arrays. J. Phys. Chem. B 2003, 107, 7337–7342. [CrossRef] 186. Crooks, R.M.; Zhao, M.; Sun, L.; Chechik, V.; Yeung, L.K. Dendrimer-encapsulated metal nanoparticles: Synthesis, characterization, and applications to catalysis. Acc. Chem. Res. 2001, 34, 181–190. [CrossRef] 187. Tripathi, P.; Mishra, M.; Kumar, S.; Dabrowski, R.; Dhar, R. Dependence of physical parameters on the size of silver nano particles forming composites with a nematic liquid crystalline material. J. Mol. Liq. 2018, 268, 403–409. [CrossRef] 188. Mirzaei, J.; Reznikov, M.; Hegmann, T. Quantum dots as liquid crystal dopants. J. Mater. Chem. 2012, 22, 22350–22365. [CrossRef] 189. Brus, L.E. Electron–electron and electron-hole interactions in small semiconductor crystallites: The size dependence of the lowest excited electronic state. J. Chem. Phys. 1984, 80, 4403–4409. [CrossRef] 190. Brus, L. Electronic wave functions in semiconductor clusters: Experiment and theory. J. Phys. Chem. 1986, 90, 2555–2560. [CrossRef] 191. Singh, U.B.; Dhar, R.; Pandey, A.S.; Kumar, S.; Dabrowski, R.; Pandey, M.B. Electro-optical and dielectric properties of cdse quantum dots and 6chbt liquid crystals composites. AIP Adv. 2014, 4, 117112. [CrossRef] 192. Konshina, E.A.; Galin, I.F.; Shcherbinin, D.P.; Gavrish, E.O. Study of dynamics and relaxation optical response of nematic liquid crystals doped with CdSe/ZnS quantum dots. Liq. Cryst. 2014, 41, 1229–1234. [CrossRef] 193. Gupta, S.K.; Singh, D.P.; Tripathi, P.K.; Manohar, R.; Varia, M.; Sagar, L.K.; Kumar, S. CdSe quantum dot-dispersed DOBAMBC: An electro-optical study. Liq. Cryst. 2013, 40, 528–533. [CrossRef] 194. Rastogi, A.; Pathak, G.; Srivastava, A.; Herman, J.; Manohar, R. Cd1 x ZnxS/ZnS core/shell quantum dots in − nematic liquid crystals to improve material parameter for better performance of liquid crystal based devices. J. Mol. Liq. 2018, 255, 93–101. [CrossRef] 195. Singh, U.B.; Pandey, M.B.; Dhar, R.; Verma, R.; Kumar, S. Effect of dispersion of cdse quantum dots on phase transition, electrical and electro-optical properties of 4pp4ob. Liq. Cryst. 2016, 43, 1075–1082. [CrossRef] 196. Roy, A.; Pathak, G.; Herman, J.; Inamdar, S.R.; Srivastava, A.; Manohar, R. InP/ZnS quantum-dot-dispersed nematic liquid crystal illustrating characteristic birefringence and enhanced electro-optical parameters. Appl. Phys. A 2018, 124, 273. [CrossRef] 197. Tripathi, P.K.; Misra, A.K.; Manohar, S.; Gupta, S.K.; Manohar, R. Improved dielectric and electro-optical parameters of ZnO nano-particle (8% Cu2+) doped nematic liquid crystal. J. Mol. Struct. 2013, 1035, 371–377. [CrossRef] Appl. Sci. 2019, 9, 2512 37 of 47

198. Ye, W.; Yuan, R.; Dai, Y.; Gao, L.; Pang, Z.; Zhu, J.; Meng, X.; He, Z.; Li, J.; Cai, M.; et al. Improvement of image

sticking in liquid crystal display doped with γ-Fe2O3 nanoparticles. Nanomaterials 2017, 8, 5. [CrossRef] [PubMed] 199. Elkhalgi, H.H.M.; Khandka, S.; Yadav,N.; Dhar, R.; Dabrowski, R. Effects of manganese (ii) titanium oxide nano

particles on the physical properties of a room temperature nematic liquid crystal 4-(trans-40-n-hexylcyclohexyl) isothiocyanatobenzene. J. Mol. Liq. 2018, 268, 223–228. [CrossRef] 200. Oh, C.-W.; Park, E.-G.; Park, H.-G. Enhanced electro-optical properties in titanium silicon oxide nanoparticle doped nematic liquid crystal system. Surf. Coat. Technol. 2019, 360, 50–55. [CrossRef] 201. Li, C.-Z.; Yip, H.-L.; Jen, A.K.Y. Functional fullerenes for organic photovoltaics. J. Mater. Chem. 2012, 22, 4161–4177. [CrossRef] 202. San, S.E.; Köysal, O.; Okutan, M. Laser-induced dielectric anisotropy of a hybrid liquid crystal composite made up of methyl red and fullerene C60. J. Non-Cryst. Solids 2005, 351, 2798–2801. [CrossRef] 203. Okutan, M.; Eren San, S.; Basaran, E.; Yakuphanoglu, F. Determination of phase transition from nematic to isotropic state in carbon nano-balls’ doped nematic liquid crystals by electrical conductivity-dielectric measurements. Phys. Lett. A 2005, 339, 461–465. [CrossRef] 204. Shukla, R.K.; Raina, K.K.; Haase, W. Fast switching response and dielectric behaviour of fullerene/ferroelectric liquid crystal nanocolloids. Liq. Cryst. 2014, 41, 1726–1732. [CrossRef] 205. Zhang, X.; Yu, B.; Cong, H. Recent developments in fullerene-containing thermotropic liquid crystals. Curr. Org. Chem. 2017, 21, 1600–1611. [CrossRef] 206. Iijima, S. Helical microtubules of graphitic carbon. Nature 1991, 354, 56. [CrossRef] 207. Baik, I.-S.; Jeon, S.Y.; Lee, S.H.; Park, K.A.; Jeong, S.H.; An, K.H.; Lee, Y.H. Electrical-field effect on carbon nanotubes in a twisted nematic liquid crystal cell. Appl. Phys. Lett. 2005, 87, 263110. [CrossRef] 208. Chi-Yen, H.; Chao-Yuan, H.; Hung-Chih, P.; Kuang-Yao, L. Electrooptical responses of carbon nanotube-doped liquid crystal devices. Jpn. J. Appl. Phys. 2005, 44, 8077. 209. Chen, H.-Y.; Lee, W. Suppression of field screening in nematic liquid crystals by carbon nanotubes. Appl. Phys. Lett. 2006, 88, 222105. [CrossRef] 210. Chen, H.-Y.; Lee, W.; Clark, N.A. Faster electro-optical response characteristics of a carbon-nanotube-nematic suspension. Appl. Phys. Lett. 2007, 90, 033510. [CrossRef] 211. Lee, W.; Wang, C.-Y.; Shih, Y.-C. Effects of carbon nanosolids on the electro-optical properties of a twisted nematic liquid-crystal host. Appl. Phys. Lett. 2004, 85, 513–515. [CrossRef] 212. Kumar, J.; Manjuladevi, V.; Gupta, R.K.; Kumar, S. Fast response in TN liquid-crystal cells: Effect of functionalised carbon nanotubes. Liq. Cryst. 2016, 43, 488–496. [CrossRef] 213. Shukla, R.K.; Chaudhary, A.; Bubnov, A.; Raina, K.K. Multi-walled carbon nanotubes-ferroelectric liquid crystal nanocomposites: Effect of cell thickness and dopant concentration on electro-optic and dielectric behaviour. Liq. Cryst. 2018, 45, 1672–1681. [CrossRef] 214. Singh, D.; Bahadur Singh, U.; Bhushan Pandey, M.; Dabrowski, R.; Dhar, R. Improvement of orientational order and display parameters of liquid crystalline material dispersed with single-wall carbon nanotubes. Mater. Lett. 2018, 216, 5–7. [CrossRef] 215. Singh, D.; Singh, U.B.; Pandey, M.B.; Dabrowski, R.; Dhar, R. Enhancement in electro-optical parameters of nematic liquid crystalline material with swcnts. Opt. Mater. 2018, 84, 16–21. [CrossRef] 216. Schymura, S.; Scalia, G. On the effect of carbon nanotubes on properties of liquid crystals. Phil. Trans. R. Soc. A 2013, 371, 20120261. [CrossRef] 217. Scalia, G.; Lagerwall, J.P.F.; Schymura, S.; Haluska, M.; Giesselmann, F.; Roth, S. Carbon nanotubes in liquid crystals as versatile functional materials. Phys. Status Solidi 2007, 244, 4212–4217. [CrossRef] 218. Novoselov, K.S.; Geim, A.K.; Morozov, S.V.; Jiang, D.; Zhang, Y.; Dubonos, S.V.; Grigorieva, I.V.; Firsov, A.A. Electric field effect in atomically thin carbon films. Science 2004, 306, 666. [CrossRef][PubMed] 219. Gökçen, M.; Yıldırım, M.; Köysal, O. Dielectric and ac electrical conductivity characteristics of liquid crystal doped with graphene. Eur. Phys. J. Appl. Phys. 2012, 60, 30104. [CrossRef] 220. Alam, T.M.; Pearce, C.J. Impact of graphene incorporation on the orientational order of graphene/liquid crystal composites. Chem. Phys. Lett. 2014, 592, 7–13. [CrossRef] 221. Basu, R. Effects of graphene on electro-optic switching and spontaneous polarization of a ferroelectric liquid crystal. Appl. Phys. Lett. 2014, 105, 112905. [CrossRef] Appl. Sci. 2019, 9, 2512 38 of 47

222. Basu, R.; Garvey, A.; Kinnamon, D. Effects of graphene on electro-optic response and ion-transport in a nematic liquid crystal. J. Appl. Phys. 2015, 117, 074301. [CrossRef] 223. Basu, R.; Kinnamon, D.; Garvey, A. Nano-electromechanical rotation of graphene and giant enhancement in dielectric anisotropy in a liquid crystal. Appl. Phys. Lett. 2015, 106, 201909. [CrossRef] 224. Javadian, S.; Dalir, N.; Kakemam, J. Non-covalent intermolecular interactions of colloidal nematic liquid crystals doped with graphene oxide. Liq. Cryst. 2017, 44, 1341–1355. [CrossRef] 225. Al-Zangana, S.; Iliut, M.; Turner, M.; Vijayaraghavan, A.; Dierking, I. Properties of a thermotropic nematic liquid crystal doped with graphene oxide. Adv. Opt. Mater. 2016, 4, 1541–1548. [CrossRef] 226. Al-Zangana, S.; Iliut, M.; Boran, G.; Turner, M.; Vijayaraghavan, A.; Dierking, I. Dielectric spectroscopy of isotropic liquids and liquid crystal phases with dispersed graphene oxide. Sci. Rep. 2016, 6, 31885. [CrossRef] 227. Dalir, N.; Javadian, S.; Kakemam, J.; Yousefi, A. Evolution of electro-chemical and electro-optical properties of nematic liquid crystal doped with graphene oxide. J. Mol. Liq. 2018, 265, 398–407. [CrossRef] 228. Lapanik, V.; Timofeev, S.; Haase, W. Electro-optic properties of nematic and ferroelectric liquid crystalline nanocolloids doped with partially reduced graphene oxide. Phase Transit. 2016, 89, 133–143. [CrossRef] 229. Mrukiewicz, M.; Kowiorski, K.; Perkowski, P.; Mazur, R.; Djas, M. Threshold voltage decrease in a thermotropic nematic liquid crystal doped with graphene oxide flakes. Beilstein J. Nanotechnol. 2019, 10, 71–78. [CrossRef][PubMed] 230. Shukla, R.K.; Mirzaei, J.; Sharma, A.; Hofmann, D.; Hegmann, T.; Haase, W. Electro-optic and dielectric properties of a ferroelectric liquid crystal doped with chemically and thermally stable emissive carbon dots. RSC Adv. 2015, 5, 34491–34496. [CrossRef] 231. Lapanik, V.; Lugouskiy, A.; Timofeev, S.; Haase, W. Influence of the size and the attached organic tail of modified detonation nanodiamond on the physical properties of liquid crystals. Liq. Cryst. 2014, 41, 1332–1338. [CrossRef] 232. Cho, M.-J.; Park, H.-G.; Jeong, H.-C.; Lee, J.-W.; Jung, Y.H.; Kim, D.-H.; Kim, J.-H.; Lee, J.-W.; Seo, D.-S. Superior fast switching of liquid crystal devices using graphene quantum dots. Liq. Cryst. 2014, 41, 761–767. [CrossRef] 233. Raether, H. Surface excitations. In Excitation of Plasmons and Interband Transitions by Electrons; Springer: Berlin, Germany, 1980; pp. 116–171. 234. Song, J.-H.; Atay, T.; Shi, S.; Urabe, H.; Nurmikko, A.V. Large enhancement of fluorescence efficiency from cdse/zns quantum dots induced by resonant coupling to spatially controlled surface plasmons. Nano Lett. 2005, 5, 1557–1561. [CrossRef][PubMed] 235. Zhang, H.; Li, Y.; Ivanov, I.A.; Qu, Y.; Huang, Y.; Duan, X. Plasmonic modulation of the upconversion fluorescence in nayf4:Yb/tm hexaplate nanocrystals using gold nanoparticles or nanoshells. Angew. Chem. Int. Ed. 2010, 49, 2865–2868. [CrossRef] 236. Fu, Y.; Zhang, J.; Lakowicz, J.R. Plasmon-enhanced fluorescence from single fluorophores end-linked to gold nanorods. J. Am. Chem. Soc. 2010, 132, 5540–5541. [CrossRef] 237. Kneipp, K.; Wang, Y.; Kneipp, H.; Perelman, L.T.; Itzkan, I.; Dasari, R.R.; Feld, M.S. Single molecule detection using surface-enhanced raman scattering (sers). Phys. Rev. Lett. 1997, 78, 1667–1670. [CrossRef] 238. Noginov, M.A.; Zhu, G.; Belgrave, A.M.; Bakker, R.; Shalaev, V.M.; Narimanov, E.E.; Stout, S.; Herz, E.; Suteewong, T.; Wiesner, U. Demonstration of a spaser-based nanolaser. Nature 2009, 460, 1110. [CrossRef] 239. Stockman, M.I. Spasers explained. Nat. Photonics 2008, 2, 327. [CrossRef] 240. Huang, X.; Jain, P.K.; El-Sayed, I.H.; El-Sayed, M.A. Plasmonic photothermal therapy (pptt) using gold nanoparticles. Lasers Med. Sci. 2007, 23, 217. [CrossRef] 241. Ebbesen, T.W.; Lezec, H.J.; Ghaemi, H.F.; Thio, T.; Wolff, P.A. Extraordinary optical transmission through sub-wavelength hole arrays. Nature 1998, 391, 667. [CrossRef] 242. Fang, N.; Lee, H.; Sun, C.; Zhang, X. Sub–diffraction-limited optical imaging with a silver superlens. Science 2005, 308, 534–537. [CrossRef] 243. Liu, Z.; Lee, H.; Xiong, Y.; Sun, C.; Zhang, X. Far-field optical hyperlens magnifying sub-diffraction-limited objects. Science 2007, 315, 1686. [CrossRef][PubMed] 244. Shao, L.; Zhuo, X.; Wang, J. Advanced plasmonic materials for dynamic color display. Adv. Mater. 2018, 30, 1704338. [CrossRef][PubMed] 245. Kalkbrenner, T.; Ramstein, M.; Mlynek, J.; Sandoghdar, V. A single gold particle as a probe for apertureless scanning near-field optical microscopy. J. Microsc. 2001, 202, 72–76. [CrossRef][PubMed] Appl. Sci. 2019, 9, 2512 39 of 47

246. Lazarides, A.A.; Lance Kelly, K.; Jensen, T.R.; Schatz, G.C. Optical properties of metal nanoparticles and nanoparticle aggregates important in biosensors. J. Mol. Struct. 2000, 529, 59–63. [CrossRef] 247. Myroshnychenko, V.; Rodríguez-Fernández, J.; Pastoriza-Santos, I.; Funston, A.M.; Novo, C.; Mulvaney, P.; Liz-Marzán, L.M.; García de Abajo, F.J. Modelling the optical response of gold nanoparticles. Chem. Soc. Rev. 2008, 37, 1792–1805. [CrossRef] 248. Choudhary, A.; Singh, G.; Biradar, A.M. Advances in gold nanoparticle–liquid crystal composites. Nanoscale 2014, 6, 7743–7756. [CrossRef] 249. Prasad, P.N. Nanophotonics; John Wiley & Sons: Hoboken, NJ, USA, 2004. 250. Jensen, T.; Kelly, L.; Lazarides, A.; Schatz, G.C. Electrodynamics of noble metal nanoparticles and nanoparticle clusters. J. Clust. Sci. 1999, 10, 295–317. [CrossRef] 251. Kelly, K.L.; Coronado, E.; Zhao, L.L.; Schatz, G.C. The optical properties of metal nanoparticles: The influence of size, shape, and dielectric environment. J. Phys. Chem. B 2003, 107, 668–677. [CrossRef] 252. Chen, B.; Zeng, X.; Baumeister, U.; Ungar, G.; Tschierske, C. Liquid crystalline networks composed of pentagonal, square, and triangular cylinders. Science 2005, 307, 96. [CrossRef][PubMed] 253. Mock, J.J.; Barbic, M.; Smith, D.R.; Schultz, D.A.; Schultz, S. Shape effects in plasmon resonance of individual colloidal silver nanoparticles. J. Chem. Phys. 2002, 116, 6755–6759. [CrossRef] 254. De Sio, L.; Caputo, R.; Cataldi, U.; Umeton, C. Broad band tuning of the plasmonic resonance of gold nanoparticles hosted in self-organized soft materials. J. Mater. Chem. 2011, 21, 18967–18970. [CrossRef] 255. Caldwell, M.E.; Yeatman, E.M. Surface-plasmon spatial light modulators based on liquid crystal. Appl. Opt. 1992, 31, 3880–3891. [CrossRef][PubMed] 256. Wang, Y. Voltage-induced color-selective absorption with surface plasmons. Appl. Phys. Lett. 1995, 67, 2759–2761. 257. Wang, Y.; Russell, S.D.; Shimabukuro, R.L. Voltage-induced broad-spectrum reflectivity change with surface-plasmon waves. J. Appl. Phys. 2004, 97, 023708. 258. Koenig, G.M.; Meli, M.-V.; Park, J.-S.; de Pablo, J.J.; Abbott, N.L. Coupling of the plasmon resonances of chemically functionalized gold nanoparticles to local order in thermotropic liquid crystals. Chem. Mater. 2007, 19, 1053–1061. [CrossRef] 259. Hsu, L.-H.; Lo, K.-Y.; Huang, S.-A.; Huang, C.-Y.; Yang, C.-S. Irreversible redshift of transmission spectrum of gold nanoparticles doped in liquid crystals. Appl. Phys. Lett. 2008, 92, 181112. 260. Pratibha, R.; Park, K.; Smalyukh, I.I.; Park, W. Tunable optical metamaterial based on liquid crystal-gold nanosphere composite. Opt. Express 2009, 17, 19459–19469. [CrossRef][PubMed] 261. Liu, Q.; Cui, Y.; Gardner, D.; Li, X.; He, S.; Smalyukh, I.I. Self-alignment of plasmonic gold nanorods in reconfigurable anisotropic fluids for tunable bulk metamaterial applications. Nano Lett. 2010, 10, 1347–1353. [CrossRef][PubMed] 262. Liu, Q.; Senyuk, B.; Tang, J.; Lee, T.; Qian, J.; He, S.; Smalyukh, I.I. Plasmonic complex fluids of nematiclike and helicoidal self-assemblies of gold nanorods with a negative order parameter. Phys. Rev. Lett. 2012, 109, 088301. [CrossRef][PubMed] 263. Liu, Q.; Yuan, Y.; Smalyukh, I.I. Electrically and optically tunable plasmonic guest–host liquid crystals with long-range ordered nanoparticles. Nano Lett. 2014, 14, 4071–4077. [CrossRef][PubMed] 264. Zhang, Y.; Liu, Q.; Mundoor, H.; Yuan, Y.; Smalyukh, I.I. Metal nanoparticle dispersion, alignment, and assembly in nematic liquid crystals for applications in switchable plasmonic color filters and e-polarizers. ACS Nano 2015, 9, 3097–3108. [CrossRef] 265. Li, D.; Liu, F.; Ren, G.J.; Fu, P.; Yao, J.Q. Liquid crystal-modulated tunable filter based on coupling between plasmon-induced transparency and cavity mode. Opt. Eng. 2018, 57, 097101. [CrossRef] 266. Rožiˇc,B.; Fresnais, J.; Molinaro, C.; Calixte, J.; Umadevi, S.; Lau-Truong, S.; Felidj, N.; Kraus, T.; Charra, F.; Dupuis, V.; et al. Oriented gold nanorods and gold nanorod chains within smectic liquid crystal topological defects. ACS Nano 2017, 11, 6728–6738. [CrossRef] 267. Jiang, L.; Mundoor, H.; Liu, Q.; Smalyukh, I.I. Electric switching of fluorescence decay in gold–silica–dye nematic nanocolloids mediated by surface plasmons. ACS Nano 2016, 10, 7064–7072. [CrossRef] 268. Sheetah, G.H.; Liu, Q.; Senyuk, B.; Fleury, B.; Smalyukh, I.I. Electric switching of visible and infrared transmission using liquid crystals co-doped with plasmonic gold nanorods and dichroic dyes. Opt. Express 2018, 26, 22264–22272. [CrossRef] Appl. Sci. 2019, 9, 2512 40 of 47

269. Sheetah, G.H.; Liu, Q.; Smalyukh, I.I. Self-assembly of predesigned optical materials in nematic codispersions of plasmonic nanorods. Opt. Lett. 2016, 41, 4899–4902. [CrossRef] 270. Jablan, M.; Soljaˇci´c,M.; Buljan, H. Plasmons in graphene: Fundamental properties and potential applications. Proc. IEEE 2013, 101, 1689–1704. [CrossRef] 271. Ju, L.; Geng, B.; Horng, J.; Girit, C.; Martin, M.; Hao, Z.; Bechtel, H.A.; Liang, X.; Zettl, A.; Shen, Y.R.; et al. Graphene plasmonics for tunable terahertz metamaterials. Nat. Nanotechnol. 2011, 6, 630. [CrossRef] [PubMed] 272. Garciía de Abajo, F.J. Special Issue “2d Materials for Nanophotonics”; ACS Publications: Washington, DC, USA, 2017. 273. Li, Y.; Li, Z.; Chi, C.; Shan, H.; Zheng, L.; Fang, Z. Plasmonics of 2d nanomaterials: Properties and applications. Adv. Sci. 2017, 4, 1600430. [CrossRef][PubMed] 274. Jablan, M.; Buljan, H.; Soljaˇci´c,M. Plasmonics in graphene at infrared frequencies. Phys. Rev. B 2009, 80, 245435. [CrossRef] 275. García de Abajo, F.J. Graphene plasmonics: Challenges and opportunities. ACS Photonics 2014, 1, 135–152. [CrossRef] 276. Reshetnyak, V.Y.; Bunning, T.J.; Evans, D.R. Using liquid crystals to control surface plasmons. Liq. Cryst. 2018, 45, 2010–2021. [CrossRef] 277. Zhu, M.; Baffou, G.; Meyerbröker, N.; Polleux, J. Micropatterning thermoplasmonic gold nanoarrays to manipulate cell adhesion. ACS Nano 2012, 6, 7227–7233. [CrossRef][PubMed] 278. Stehr, J.; Hrelescu, C.; Sperling, R.A.; Raschke, G.; Wunderlich, M.; Nichtl, A.; Heindl, D.; Kürzinger, K.; Parak, W.J.; Klar, T.A.; et al. Gold nanostoves for microsecond DNA melting analysis. Nano Lett. 2008, 8, 619–623. [CrossRef][PubMed] 279. Hirsch, L.R.; Stafford, R.J.; Bankson, J.A.; Sershen, S.R.; Rivera, B.; Price, R.E.; Hazle, J.D.; Halas, N.J.; West, J.L. Nanoshell-mediated near-infrared thermal therapy of tumors under magnetic resonance guidance. Proc. Natl. Acad. Sci. USA 2003, 100, 13549. [CrossRef][PubMed] 280. Huschka, R.; Neumann, O.; Barhoumi, A.; Halas, N.J. Visualizing light-triggered release of molecules inside living cells. Nano Lett. 2010, 10, 4117–4122. [CrossRef][PubMed] 281. Sun, Y.; Evans, J.S.; Lee, T.; Senyuk, B.; Keller, P.; He, S.; Smalyukh, I.I. Optical manipulation of shape-morphing elastomeric liquid crystal microparticles doped with gold nanocrystals. Appl. Phys. Lett. 2012, 100, 241901. [CrossRef] 282. Fong, W.-K.; Hanley, T.L.; Thierry, B.; Kirby, N.; Waddington, L.J.; Boyd, B.J. Controlling the nanostructure of gold nanorod–lyotropic liquid-crystalline hybrid materials using near-infrared laser irradiation. Langmuir 2012, 28, 14450–14460. [CrossRef][PubMed] 283. Fong, W.-K.; Hanley, T.L.; Thierry, B.; Tilley, A.; Kirby, N.; Waddington, L.J.; Boyd, B.J. Understanding the photothermal heating effect in non-lamellar liquid crystalline systems, and the design of new mixed lipid systems for photothermal on-demand drug delivery. Phys. Chem. Chem. Phys. 2014, 16, 24936–24953. [CrossRef] 284. Pezzi, L.; De Sio, L.; Veltri, A.; Placido, T.; Palermo, G.; Comparelli, R.; Curri, M.L.; Agostiano, A.; Tabiryan, N.; Umeton, C. Photo-thermal effects in gold nanoparticles dispersed in thermotropic nematic liquid crystals. Phys. Chem. Chem. Phys. 2015, 17, 20281–20287. [CrossRef][PubMed] 285. Luciano De, S.; Tiziana, P.; Roberto, C.; Maria Lucia, C.; Nelson, T.; Timothy, J.B. Plasmonic photoheating of gold nanorods in thermo-responsive chiral liquid crystals. J. Opt. 2016, 18, 125005. 286. Palermo, G.; Cataldi, U.; De Sio, L.; Bürgi, T.; Tabiryan, N.; Umeton, C. Optical control of plasmonic heating effects using reversible photo-alignment of nematic liquid crystals. Appl. Phys. Lett. 2016, 109, 191906. [CrossRef] 287. Wang, L.; Gutierrez-Cuevas, K.G.; Urbas, A.; Li, Q. Near-infrared light-directed handedness inversion in plasmonic nanorod-embedded helical superstructure. Adv. Opt. Mater. 2016, 4, 247–251. [CrossRef] 288. Wang, L.; Bisoyi, H.K.; Zheng, Z.; Gutierrez-Cuevas, K.G.; Singh, G.; Kumar, S.; Bunning, T.J.; Li, Q. Stimuli-directed self-organized chiral superstructures for adaptive windows enabled by mesogen-functionalized graphene. Mater. Today 2017, 20, 230–237. [CrossRef] 289. Gutierrez-Cuevas, K.G.; Wang, L.; Xue, C.; Singh, G.; Kumar, S.; Urbas, A.; Li, Q. Near infrared light-driven liquid crystal phase transition enabled by hydrophobic mesogen grafted plasmonic gold nanorods. Chem. Commun. 2015, 51, 9845–9848. [CrossRef] Appl. Sci. 2019, 9, 2512 41 of 47

290. Ikeda, T.; Tsutsumi, O. Optical switching and image storage by means of azobenzene liquid-crystal films. Science 1995, 268, 1873. [CrossRef] 291. Natansohn, A.; Rochon, P. Photoinduced motions in azo-containing polymers. Chem. Rev. 2002, 102, 4139–4176. [CrossRef][PubMed] 292. Tanaka, D.; Ishiguro, H.; Shimizu, Y.; Uchida, K. Thermal and photoinduced liquid crystalline phase transitions with a rod–disc alternative change in the molecular shape. J. Mater. Chem. 2012, 22, 25065–25071. [CrossRef] 293. Lin, T.-H.; Li, Y.; Wang, C.-T.; Jau, H.-C.; Chen, C.-W.; Li, C.-C.; Bisoyi, H.K.; Bunning, T.J.; Li, Q. Red, green and blue reflections enabled in an optically tunable self-organized 3d cubic nanostructured thin film. Adv. Mater. 2013, 25, 5050–5054. [CrossRef] 294. Fernandez-Palacio, F.; Poutanen, M.; Saccone, M.; Siiskonen, A.; Terraneo, G.; Resnati, G.; Ikkala, O.; Metrangolo, P.; Priimagi, A. Efficient light-induced phase transitions in halogen-bonded liquid crystals. Chem. Mater. 2016, 28, 8314–8321. [CrossRef][PubMed] 295. Coles, H.; Morris, S. Liquid-crystal lasers. Nat. Photonics 2010, 4, 676. [CrossRef] 296. Zheng, Z.-G.; Liu, B.-W.; Zhou, L.; Wang, W.; Hu, W.; Shen, D. Wide tunable lasing in photoresponsive chiral liquid crystal emulsion. J. Mater. Chem. C 2015, 3, 2462–2470. [CrossRef] 297. Furumi, S.; Yokoyama, S.; Otomo, A.; Mashiko, S. Phototunable photonic bandgap in a chiral liquid crystal laser device. Appl. Phys. Lett. 2004, 84, 2491–2493. [CrossRef] 298. Lucchetta, D.E.; Criante, L.; Francescangeli, O.; Simoni, F. Light amplification by dye-doped holographic polymer dispersed liquid crystals. Appl. Phys. Lett. 2004, 84, 4893–4895. [CrossRef] 299. Chen, L.-J.; Lin, J.-D.; Huang, S.-Y.; Mo, T.-S.; Lee, C.-R. Thermally and electrically tunable lasing emission and amplified spontaneous emission in a composite of inorganic quantum dot nanocrystals and organic cholesteric liquid crystals. Adv. Opt. Mater. 2013, 1, 637–643. [CrossRef] 300. Chen, L.-J.; Lin, J.-D.; Lee, C.-R. An optically stable and tunable quantum dot nanocrystal-embedded cholesteric liquid crystal composite laser. J. Mater. Chem. C 2014, 2, 4388–4394. [CrossRef] 301. Gao, S.; Zhang, C.; Liu, Y.; Su, H.; Wei, L.; Huang, T.; Dellas, N.; Shang, S.; Mohney, S.E.; Wang, J.; et al. Lasing from colloidal inp/zns quantum dots. Opt. Express 2011, 19, 5528–5535. [CrossRef][PubMed] 302. Mingxuan, C.; Yating, Z.; Xiaoxian, S.; Yongli, C.; Haiting, Z.; Chao, Y.; Haitao, D.; Guang, L.; Guizhong, Z.; Jianquan, Y. Enhanced amplified spontaneous emission in a quantum dot-doped polymer-dispersed liquid crystal. Nanotechnology 2016, 27, 26LT01. 303. Mingxuan, C.; Siwei, Y.; Yating, Z.; Xiaoxian, S.; Yongli, C.; Haiting, Z.; Yu, Y.; Guqiao, D.; Guizhong, Z.; Jianquan, Y. Tunable amplified spontaneous emission in graphene quantum dots doped cholesteric liquid crystals. Nanotechnology 2017, 28, 245202. 304. Cuiqing, W.; Dairong, C.; Xiuling, J. Lyotropic liquid crystal directed synthesis of nanostructured materials. Sci. Technol. Adv. Mater. 2009, 10, 023001. 305. Chernyshuk, S.B.; Lev, B.I. Theory of elastic interaction of colloidal particles in nematic liquid crystals near one wall and in the nematic cell. Phys. Rev. E 2011, 84, 011707. [CrossRef][PubMed] 306. Eskandari, Z.; Silvestre, N.M.; Tasinkevych, M.; Telo da Gama, M.M. Interactions of distinct quadrupolar nematic colloids. Soft Matter 2012, 8, 10100–10106. [CrossRef] 307. West, J.L.; Glushchenko, A.; Liao, G.; Reznikov, Y.; Andrienko, D.; Allen, M.P. Drag on particles in a nematic suspension by a moving nematic-isotropic interface. Phys. Rev. E 2002, 66, 012702. [CrossRef][PubMed] 308. Yoshida, H.; Asakura, K.; Fukuda, J.; Ozaki, M. Three-dimensional positioning and control of colloidal objects utilizing engineered liquid crystalline defect networks. Nat. Commun. 2015, 6, 7180. [CrossRef][PubMed] 309. Loudet, J.-C.; Barois, P.; Poulin, P. Colloidal ordering from phase separation in a liquid-crystalline continuous phase. Nature 2000, 407, 611. [CrossRef] 310. Voloschenko, D.; Pishnyak, O.P.; Shiyanovskii, S.V.; Lavrentovich, O.D. Effect of director distortions on morphologies of phase separation in liquid crystals. Phys. Rev. E 2002, 65, 060701. [CrossRef] 311. Senyuk, B.; Evans, J.S.; Ackerman, P.J.; Lee, T.; Manna, P.; Vigderman, L.; Zubarev, E.R.; van de Lagemaat, J.; Smalyukh, I.I. Shape-dependent oriented trapping and scaffolding of plasmonic nanoparticles by topological defects for self-assembly of colloidal dimers in liquid crystals. Nano Lett. 2012, 12, 955–963. [CrossRef] [PubMed] 312. Senyuk, B.; Smalyukh, I.I. Elastic interactions between colloidal microspheres and elongated convex and concave nanoprisms in nematic liquid crystals. Soft Matter 2012, 8, 8729–8734. [CrossRef] Appl. Sci. 2019, 9, 2512 42 of 47

313. Jose, R.; Skaˇcej,G.; Sastry, V.S.S.; Žumer, S. Colloidal nanoparticles trapped by liquid-crystal defect lines: A lattice monte carlo simulation. Phys. Rev. E 2014, 90, 032503. [CrossRef][PubMed] 314. Lagerwall, J.P.; Scalia, G. Liquid Crystals with Nano and Microparticles; World Scientific: Singapore, 2017. 315. Milette, J.; Cowling, S.J.; Toader, V.; Lavigne, C.; Saez, I.M.; Bruce Lennox, R.; Goodby, J.W.; Reven, L. Reversible long range network formation in gold nanoparticle-nematic liquid crystal composites. Soft Matter 2012, 8, 173–179. [CrossRef] 316. Coursault, D.; Grand, J.; Zappone, B.; Ayeb, H.; Lévi, G.; Félidj, N.; Lacaze, E. Linear self-assembly of nanoparticles within liquid crystal defect arrays. Adv. Mater. 2012, 24, 1461–1465. [CrossRef][PubMed] 317. Bitar, R.; Agez, G.; Mitov, M. Cholesteric liquid crystal self-organization of gold nanoparticles. Soft Matter 2011, 7, 8198–8206. [CrossRef] 318. Stratford, K.; Henrich, O.; Lintuvuori, J.S.; Cates, M.E.; Marenduzzo, D. Self-assembly of colloid-cholesteric composites provides a possible route to switchable optical materials. Nat. Commun. 2014, 5, 3954. [CrossRef] 319. Coursault, D.; Blach, J.-F.; Grand, J.; Coati, A.; Vlad, A.; Zappone, B.; Babonneau, D.; Lévi, G.; Félidj, N.; Donnio, B.; et al. Tailoring anisotropic interactions between soft nanospheres using dense arrays of smectic liquid crystal edge dislocations. ACS Nano 2015, 9, 11678–11689. [CrossRef] 320. Milette, J.; Relaix, S.; Lavigne, C.; Toader, V.; Cowling, S.J.; Saez, I.M.; Lennox, R.B.; Goodby, J.W.; Reven, L. Reversible long-range patterning of gold nanoparticles by smectic liquid crystals. Soft Matter 2012, 8, 6593–6598. [CrossRef] 321. Yoon, D.K.; Choi, M.C.; Kim, Y.H.; Kim, M.W.; Lavrentovich, O.D.; Jung, H.-T. Internal structure visualization and lithographic use of periodic toroidal holes in liquid crystals. Nat. Mater. 2007, 6, 866. [CrossRef] 322. Scalia, G.; von Bühler, C.; Hägele, C.; Roth, S.; Giesselmann, F.; Lagerwall, J.P.F. Spontaneous macroscopic carbon nanotube alignment via colloidal suspension in hexagonal columnar lyotropic liquid crystals. Soft Matter 2008, 4, 570–576. [CrossRef] 323. Schymura, S.; Enz, E.; Roth, S.; Scalia, G.; Lagerwall, J.P.F. Macroscopic-scale carbon nanotube alignment via self-assembly in lyotropic liquid crystals. Synth. Met. 2009, 159, 2177–2179. [CrossRef] 324. Sousa, M.; Cloutier, S.; Jian, K.; Weissman, B.; Hurt, R.; Crawford, G. Patterning lyotropic liquid crystals as precursors for carbon nanotube arrays. Appl. Phys. Lett. 2005, 87, 173115. [CrossRef] 325. Mauter, M.S.; Elimelech, M.; Osuji, C.O. Nanocomposites of vertically aligned single-walled carbon nanotubes by magnetic alignment and polymerization of a lyotropic precursor. ACS Nano 2010, 4, 6651–6658. [CrossRef] 326. Schymura, S.; Dölle, S.; Yamamoto, J.; Lagerwall, J. Filament formation in carbon nanotube-doped lyotropic liquid crystals. Soft Matter 2011, 7, 2663–2667. [CrossRef] 327. Kasprzak, C.R.; Scherzinger, E.T.; Sarkar, A.; Miao, M.; Porcincula, D.H.; Madriz, A.M.; Pennewell, Z.M.; Chau, S.S.; Fernando, R.; Stefik, M.; et al. Ordered nanostructures of carbon nanotube–polymer composites from lyotropic liquid crystal templating. Macromol. Chem. Phys. 2018, 219, 1800197. [CrossRef] 328. Scalia, G. Alignment of carbon nanotubes in thermotropic and lyotropic liquid crystals. ChemPhysChem 2010, 11, 333–340. [CrossRef] 329. Ji, Y.; Huang, Y.Y.; Terentjev, E.M. Dissolving and aligning carbon nanotubes in thermotropic liquid crystals. Langmuir 2011, 27, 13254–13260. [CrossRef][PubMed] 330. Lynch, M.D.; Patrick, D.L. Organizing carbon nanotubes with liquid crystals. Nano Lett. 2002, 2, 1197–1201. [CrossRef] 331. Tie, W.; Yang, G.H.; Bhattacharyya, S.S.; Lee, Y.H.; Lee, S.H. Electric-field-induced dispersion of multiwalled carbon nanotubes in nematic liquid crystal. J. Phys. Chem. C 2011, 115, 21652–21658. [CrossRef] 332. Bisoyi, H.K.; Kumar, S. Carbon nanotubes in triphenylene and rufigallol-based room temperature monomeric and polymeric discotic liquid crystals. J. Mater. Chem. 2008, 18, 3032–3039. [CrossRef] 333. Ahir, S.V.; Huang, Y.Y.; Terentjev, E.M. Polymers with aligned carbon nanotubes: Active composite materials. Polymer 2008, 49, 3841–3854. [CrossRef] 334. Lavrentovich, O.D. Liquid crystals, photonic crystals, metamaterials, and transformation optics. Proc. Natl. Acad. Sci. USA 2011, 108, 5143. [CrossRef] 335. Ravnik, M.; Alexander, G.P.; Yeomans, J.M.; Žumer, S. Three-dimensional colloidal crystals in liquid crystalline blue phases. Proc. Natl. Acad. Sci. USA 2011, 108, 5188. [CrossRef][PubMed] 336. Lin, P.; Yan, Q.; Wei, Z.; Chen, Y.; Chen, F.; Huang, Z.; Li, X.; Wang, H.; Wang, X.; Cheng, Z. All-inorganic perovskite quantum dots stabilized blue phase liquid crystals. Opt. Express 2018, 26, 18310–18319. [CrossRef] [PubMed] Appl. Sci. 2019, 9, 2512 43 of 47

337. Mitov, M.; Portet, C.; Bourgerette, C.; Snoeck, E.; Verelst, M. Long-range structuring of nanoparticles by mimicry of a cholesteric liquid crystal. Nat. Mater. 2002, 1, 229. [CrossRef][PubMed] 338. Michel, M.; Christian, B.; François de, G. Fingerprint patterning of solid nanoparticles embedded in a cholesteric liquid crystal. J. Phys. Condens. Matter 2004, 16, S1981. 339. Pendery, J.S.; Merchiers, O.; Coursault, D.; Grand, J.; Ayeb, H.; Greget, R.; Donnio, B.; Gallani, J.-L.; Rosenblatt, C.; Félidj, N.; et al. Gold nanoparticle self-assembly moderated by a cholesteric liquid crystal. Soft Matter 2013, 9, 9366–9375. [CrossRef] 340. Rahimi, M.; Roberts, T.F.; Armas-Pérez, J.C.; Wang, X.; Bukusoglu, E.; Abbott, N.L.; de Pablo, J.J. Nanoparticle self-assembly at the interface of liquid crystal droplets. Proc. Natl. Acad. Sci. USA 2015, 112, 5297. [CrossRef] 341. Chang, C.; Zhao, Y.; Liu, Y.; An, L. Liquid crystallinity of carbon nanotubes. RSC Adv. 2018, 8, 15780–15795. [CrossRef] 342. Bravo-Sanchez, M.; Simmons, T.J.; Vidal, M.A. Liquid crystal behavior of single wall carbon nanotubes. Carbon 2010, 48, 3531–3542. [CrossRef] 343. Lu, L.; Chen, W. Large-scale aligned carbon nanotubes from their purified, highly concentrated suspension. ACS Nano 2010, 4, 1042–1048. [CrossRef][PubMed] 344. Song, W.; Windle, A.H. Isotropic nematic phase transition of dispersions of multiwall carbon nanotubes. − Macromolecules 2005, 38, 6181–6188. [CrossRef] 345. Steinert, B.W.; Dean, D.R. Magnetic field alignment and electrical properties of solution cast pet–carbon nanotube composite films. Polymer 2009, 50, 898–904. [CrossRef] 346. Hobbie, E.K.; Fry, D.J. Nonequilibrium phase diagram of sticky nanotube suspensions. Phys. Rev. Lett. 2006, 97, 036101. [CrossRef][PubMed] 347. Davis, V.A.; Ericson, L.M.; Parra-Vasquez, A.N.G.; Fan, H.; Wang, Y.; Prieto, V.; Longoria, J.A.; Ramesh, S.; Saini, R.K.; Kittrell, C.; et al. Phase behavior and rheology of swnts in superacids. Macromolecules 2004, 37, 154–160. [CrossRef] 348. Rai, P.K.; Pinnick, R.A.; Parra-Vasquez, A.N.G.; Davis, V.A.; Schmidt, H.K.; Hauge, R.H.; Smalley, R.E.; Pasquali, M. Isotropic nematic phase transition of single-walled carbon nanotubes in strong acids. J. Am. − Chem. Soc. 2006, 128, 591–595. [CrossRef] 349. Pénicaud, A.; Poulin, P.; Derré, A.; Anglaret, E.; Petit, P. Spontaneous dissolution of a single-wall carbon nanotube salt. J. Am. Chem. Soc. 2005, 127, 8–9. [CrossRef] 350. Baskaran, D.; Mays, J.W.; Bratcher, M.S. Noncovalent and nonspecific molecular interactions of polymers with multiwalled carbon nanotubes. Chem. Mater. 2005, 17, 3389–3397. [CrossRef] 351. Haggenmueller, R.; Rahatekar, S.S.; Fagan, J.A.; Chun, J.; Becker, M.L.; Naik, R.R.; Krauss, T.; Carlson, L.; Kadla, J.F.; Trulove, P.C.; et al. Comparison of the quality of aqueous dispersions of single wall carbon nanotubes using surfactants and biomolecules. Langmuir 2008, 24, 5070–5078. [CrossRef] 352. Zhang, S.; Kinloch, I.A.; Windle, A.H. Mesogenicity drives fractionation in lyotropic aqueous suspensions of multiwall carbon nanotubes. Nano Lett. 2006, 6, 568–572. [CrossRef][PubMed] 353. Zhang, S.; Li, Q.; Kinloch, I.A.; Windle, A.H. Ordering in a droplet of an aqueous suspension of single-wall carbon nanotubes on a solid substrate. Langmuir 2010, 26, 2107–2112. [CrossRef][PubMed] 354. Zamora-Ledezma, C.; Blanc, C.; Maugey, M.; Zakri, C.; Poulin, P.; Anglaret, E. Anisotropic thin films of single-wall carbon nanotubes from aligned lyotropic nematic suspensions. Nano Lett. 2008, 8, 4103–4107. [CrossRef][PubMed] 355. Song, W.; Windle, A.H. Size-dependence and elasticity of liquid-crystalline multiwalled carbon nanotubes. Adv. Mater. 2008, 20, 3149–3154. [CrossRef] 356. Fu, W.; Liu, L.; Jiang, K.; Li, Q.; Fan, S. Super-aligned carbon nanotube films as aligning layers and transparent electrodes for liquid crystal displays. Carbon 2010, 48, 1876–1879. [CrossRef] 357. Roussel, F.; Brun, J.-F.; Allart, A.; Huang, L.; O’Brien, S. Horizontally-aligned carbon nanotubes arrays and their interactions with liquid crystal molecules: Physical characteristics and display applications. AIP Adv. 2012, 2, 012110. [CrossRef] 358. Park, H.-G.; Lee, M.-J.; Kim, K.; Seo, D.-S. Transparent conductive single wall carbon nanotube network films for liquid crystal displays. ECS Solid State Lett. 2012, 1, R31–R33. [CrossRef] 359. Rajasekharan, R.; Dai, Q.; Wilkinson, T.D. Electro-optic characteristics of a transparent nanophotonic device based on carbon nanotubes and liquid crystals. Appl. Opt. 2010, 49, 2099–2104. [CrossRef] Appl. Sci. 2019, 9, 2512 44 of 47

360. Yu, L.; Shearer, C.; Shapter, J. Recent development of carbon nanotube transparent conductive films. Chem. Rev. 2016, 116, 13413–13453. [CrossRef] 361. Song, Y.I.; Lee, J.W.; Kim, T.Y.; Jung, H.J.; Jung, Y.C.; Suh, S.J.; Yang, C.-M. Performance-determining factors in flexible transparent conducting single-wall carbon nanotube film. Carbon Lett. 2013, 14, 255–258. [CrossRef] 362. Lee, H.; Lee, J.-H.; Huynh, C.P.; Hawkins, S.C.; Musameh, M.; Kim, D.H.; Lee, S.H.; Choi, J. Orientational and electro-optical properties of liquid crystal aligned with a directly spinnable carbon nanotube web. Liq. Cryst. 2015, 42, 322–327. [CrossRef] 363. Russell, J.M.; Oh, S.; LaRue, I.; Zhou, O.; Samulski, E.T. Alignment of nematic liquid crystals using carbon nanotube films. Thin Solid Film 2006, 509, 53–57. [CrossRef] 364. Liu, Y.; Lim, Y.J.; Kundu, S.; Lee, S.H.; Lee, G.-D. Super-fast switching of twisted nematic liquid crystals with a single-wall-carbon-nanotube-doped alignment layer. J. Korean Phys. Soc. 2015, 66, 952–958. [CrossRef] 365. Ren, L.; Pint, C.L.; Booshehri, L.G.; Rice, W.D.; Wang, X.; Hilton, D.J.; Takeya, K.; Kawayama, I.; Tonouchi, M.; Hauge, R.H.; et al. Carbon nanotube terahertz polarizer. Nano Lett. 2009, 9, 2610–2613. [CrossRef][PubMed] 366. Shoji, S.; Suzuki, H.; Zaccaria, R.P.; Sekkat, Z.; Kawata, S. Optical polarizer made of uniaxially aligned short single-wall carbon nanotubes embedded in a polymer film. Phys. Rev. B 2008, 77, 153407. [CrossRef] 367. Byeong Gyun, K.; Young Jin, L.; Kwang-Un, J.; Kyu, L.; Young Hee, L.; Seung Hee, L. A tunable carbon nanotube polarizer. Nanotechnology 2010, 21, 405202. 368. Behabtu, N.; Young, C.C.; Tsentalovich, D.E.; Kleinerman, O.; Wang, X.; Ma, A.W.K.; Bengio, E.A.; ter Waarbeek, R.F.; de Jong, J.J.; Hoogerwerf, R.E.; et al. Strong, light, multifunctional fibers of carbon nanotubes with ultrahigh conductivity. Science 2013, 339, 182. [CrossRef] 369. Zhang, S.; Koziol, K.K.K.; Kinloch, I.A.; Windle, A.H. Macroscopic fibers of well-aligned carbon nanotubes by wet spinning. Small 2008, 4, 1217–1222. [CrossRef][PubMed] 370. Ericson, L.M.; Fan, H.; Peng, H.; Davis, V.A.; Zhou, W.; Sulpizio, J.; Wang, Y.; Booker, R.; Vavro, J.; Guthy, C.; et al. Macroscopic, neat, single-walled carbon nanotube fibers. Science 2004, 305, 1447. [CrossRef] 371. Li, P.; Wong, M.; Zhang, X.; Yao, H.; Ishige, R.; Takahara, A.; Miyamoto, M.; Nishimura, R.; Sue, H.-J. Tunable lyotropic photonic liquid crystal based on graphene oxide. ACS Photonics 2014, 1, 79–86. [CrossRef] 372. Dan, B.; Behabtu, N.; Martinez, A.; Evans, J.S.; Kosynkin, D.V.; Tour, J.M.; Pasquali, M.; Smalyukh, I.I. Liquid crystals of aqueous, giant graphene oxide flakes. Soft Matter 2011, 7, 11154–11159. [CrossRef] 373. Aboutalebi, S.H.; Gudarzi, M.M.; Zheng, Q.B.; Kim, J.-K. Spontaneous formation of liquid crystals in ultralarge graphene oxide dispersions. Adv. Funct. Mater. 2011, 21, 2978–2988. [CrossRef] 374. Jalili, R.; Aboutalebi, S.H.; Esrafilzadeh, D.; Konstantinov, K.; Moulton, S.E.; Razal, J.M.; Wallace, G.G. Organic solvent-based graphene oxide liquid crystals: A facile route toward the next generation of self-assembled layer-by-layer multifunctional 3d architectures. ACS Nano 2013, 7, 3981–3990. [CrossRef][PubMed] 375. Xu, Z.; Gao, C. Graphene chiral liquid crystals and macroscopic assembled fibres. Nat. Commun. 2011, 2, 571. [CrossRef] 376. Tkacz, R.; Oldenbourg, R.; Mehta, S.B.; Miansari, M.; Verma, A.; Majumder, M. Ph dependent isotropic to nematic phase transitions in graphene oxide dispersions reveal droplet liquid crystalline phases. Chem. Commun. 2014, 50, 6668–6671. [CrossRef][PubMed] 377. Shen, T.-Z.; Hong, S.-H.; Song, J.-K. Electro-optical switching of graphene oxide liquid crystals with an extremely large kerr coefficient. Nat. Mater. 2014, 13, 394. [CrossRef] 378. Arenas-Guerrero, P.; Delgado, Á.V.; Jiménez, M.L. Analysis of the electro-optical response of graphene oxide dispersions under alternating fields. Carbon 2018, 144, 395–401. [CrossRef] 379. Hong, S.-H.; Shen, T.-Z.; Song, J.-K. Shear-induced assembly of graphene oxide particles into stripes near surface. Liq. Cryst. 2018, 45, 1303–1311. [CrossRef] 380. Guo, F.; Kim, F.; Han, T.H.; Shenoy, V.B.; Huang, J.; Hurt, R.H. Hydration-responsive folding and unfolding in graphene oxide liquid crystal phases. ACS Nano 2011, 5, 8019–8025. [CrossRef][PubMed] 381. Senyuk, B.; Behabtu, N.; Pacheco, B.G.; Lee, T.; Ceriotti, G.; Tour, J.M.; Pasquali, M.; Smalyukh, I.I. Nonlinear photoluminescence imaging of isotropic and liquid crystalline dispersions of graphene oxide. ACS Nano 2012, 6, 8060–8066. [CrossRef] 382. Qi, B.; Yuan, Z.; Lu, S.; Liu, K.; Li, S.; Yang, L.; Yu, J. Mechanical and thermal properties of epoxy composites containing graphene oxide and liquid crystalline epoxy. Fibers Polym. 2014, 15, 326–333. [CrossRef] 383. Lu, S.; Li, S.; Yu, J.; Yuan, Z.; Qi, B. Epoxy nanocomposites filled with thermotropic liquid crystalline epoxy grafted graphene oxide. RSC Adv. 2013, 3, 8915–8923. [CrossRef] Appl. Sci. 2019, 9, 2512 45 of 47

384. Naficy, S.; Jalili, R.; Aboutalebi, S.H.; Gorkin Iii, R.A.; Konstantinov, K.; Innis, P.C.; Spinks, G.M.; Poulin, P.; Wallace, G.G. Graphene oxide dispersions: Tuning rheology to enable fabrication. Mater. Horiz. 2014, 1, 326–331. [CrossRef] 385. Wu, X.; Hou, K.; Huang, J.; Wang, J.; Yang, S. Graphene-based cellular materials with extremely low density and high pressure sensitivity based on self-assembled graphene oxide liquid crystals. J. Mater. Chem. C 2018, 6, 8717–8725. [CrossRef] 386. Zamora-Ledezma, C.; Puech, N.; Zakri, C.; Grelet, E.; Moulton, S.E.; Wallace, G.G.; Gambhir, S.; Blanc, C.; Anglaret, E.; Poulin, P.Liquid crystallinity and dimensions of surfactant-stabilized sheets of reduced graphene oxide. J. Phys. Chem. Lett. 2012, 3, 2425–2430. [CrossRef] 387. Park, H.; Lee, K.H.; Kim, Y.B.; Ambade, S.B.; Noh, S.H.; Eom, W.; Hwang, J.Y.; Lee, W.J.; Huang, J.; Han, T.H. Dynamic assembly of liquid crystalline graphene oxide gel fibers for ion transport. Sci. Adv. 2018, 4, eaau2104. [CrossRef][PubMed] 388. Jalili, R.; Aboutalebi, S.H.; Esrafilzadeh, D.; Shepherd, R.L.; Chen, J.; Aminorroaya-Yamini, S.; Konstantinov,K.; Minett, A.I.; Razal, J.M.; Wallace, G.G. Scalable one-step wet-spinning of graphene fibers and yarns from liquid crystalline dispersions of graphene oxide: Towards multifunctional textiles. Adv. Funct. Mater. 2013, 23, 5345–5354. [CrossRef] 389. Xu, Z.; Gao, C. Graphene in macroscopic order: Liquid crystals and wet-spun fibers. Acc. Chem. Res. 2014, 47, 1267–1276. [CrossRef][PubMed] 390. Kim, I.H.; Yun, T.; Kim, J.-E.; Yu, H.; Sasikala, S.P.; Lee, K.E.; Koo, S.H.; Hwang, H.; Jung, H.J.; Park, J.Y.; et al. Mussel-inspired defect engineering of graphene liquid crystalline fibers for synergistic enhancement of mechanical strength and electrical conductivity. Adv. Mater. 2018, 30, 1803267. [CrossRef] 391. Aboutalebi, S.H.; Jalili, R.; Esrafilzadeh, D.; Salari, M.; Gholamvand, Z.; Aminorroaya Yamini, S.; Konstantinov, K.; Shepherd, R.L.; Chen, J.; Moulton, S.E.; et al. High-performance multifunctional graphene yarns: Toward wearable all-carbon energy storage textiles. ACS Nano 2014, 8, 2456–2466. [CrossRef] 392. Mahalingam, D.K.; Wang, S.; Nunes, S.P. Graphene oxide liquid crystal membranes in protic ionic liquid for nanofiltration. ACS Appl. Nano Mater. 2018, 1, 4661–4670. [CrossRef] 393. Kim, H.; Kim, D.W.; Vasagar, V.; Ha, H.; Nazarenko, S.; Ellison, C.J. Polydopamine-graphene oxide flame retardant nanocoatings applied via an aqueous liquid crystalline scaffold. Adv. Funct. Mater. 2018, 28, 1803172. [CrossRef] 394. Kim, M.; Park, J.H.; Shahini, S.; Yamamoto, J.; Campidelli, S.; Kim, Y.S.; Scalia, G. Graphene: A new liquid crystal for high performance electro-optic applications. In Liquid Crystals XXII; International Society for Optics and Photonics: Bellingham, WA, USA, 2018; p. 107350N. 395. Huang, X.; He, J.; Sun, K.; Chen, Y.; Zha, Z.; Zhou, C. Liquid crystal behavior and cytocompatibility of graphene oxide dispersed in sodium alginate solutions. Carbon 2018, 129, 258–269. [CrossRef] 396. Sasikala, S.P.; Lim, J.; Kim, I.H.; Jung, H.J.; Yun, T.; Han, T.H.; Kim, S.O. Graphene oxide liquid crystals: A frontier 2d soft material for graphene-based functional materials. Chem. Soc. Rev. 2018, 47, 6013–6045. [CrossRef] 397. Klemm, D.; Kramer, F.; Moritz, S.; Lindström, T.; Ankerfors, M.; Gray, D.; Dorris, A. Nanocelluloses: A new family of nature-based materials. Angew. Chem. Int. Ed. 2011, 50, 5438–5466. [CrossRef][PubMed] 398. Shoda, M.; Sugano, Y. Recent advances in bacterial cellulose production. Biotechnol. Bioprocess. Eng. 2005, 10, 1. [CrossRef] 399. Eichhorn, S.J. Cellulose nanowhiskers: Promising materials for advanced applications. Soft Matter 2011, 7, 303–315. [CrossRef] 400. Li, Y.; Jun-Yan Suen, J.; Prince, E.; Larin, E.M.; Klinkova, A.; Thérien-Aubin, H.; Zhu, S.; Yang, B.; Helmy, A.S.; Lavrentovich, O.D.; et al. Colloidal cholesteric liquid crystal in spherical confinement. Nat. Commun. 2016, 7, 12520. [CrossRef] 401. Marchessault, R.; Morehead, F.; Walter, N. Liquid crystal systems from fibrillar polysaccharides. Nature 1959, 184, 632–633. [CrossRef] 402. Usov, I.; Nyström, G.; Adamcik, J.; Handschin, S.; Schütz, C.; Fall, A.; Bergström, L.; Mezzenga, R. Understanding nanocellulose chirality and structure–properties relationship at the single fibril level. Nat. Commun. 2015, 6, 7564. [CrossRef][PubMed] 403. Hirai, A.; Inui, O.; Horii, F.; Tsuji, M. Phase separation behavior in aqueous suspensions of bacterial cellulose nanocrystals prepared by sulfuric acid treatment. Langmuir 2009, 25, 497–502. [CrossRef][PubMed] Appl. Sci. 2019, 9, 2512 46 of 47

404. Schütz, C.; Agthe, M.; Fall, A.B.; Gordeyeva, K.; Guccini, V.; Salajková, M.; Plivelic, T.S.; Lagerwall, J.P.F.; Salazar-Alvarez, G.; Bergström, L. Rod packing in chiral nematic cellulose nanocrystal dispersions studied by small-angle x-ray scattering and laser diffraction. Langmuir 2015, 31, 6507–6513. [CrossRef][PubMed] 405. Shafeiei-Sabet, S.; Hamad, W.Y.; Hatzikiriakos, S.G. Influence of degree of sulfation on the rheology of cellulose nanocrystal suspensions. Rheol. Acta 2013, 52, 741–751. [CrossRef] 406. Dong, X.M.; Kimura, T.; Revol, J.-F.; Gray, D.G. Effects of ionic strength on the isotropic chiral nematic phase − transition of suspensions of cellulose crystallites. Langmuir 1996, 12, 2076–2082. [CrossRef] 407. Araki, J.; Kuga, S. Effect of trace electrolyte on liquid crystal type of cellulose microcrystals. Langmuir 2001, 17, 4493–4496. [CrossRef] 408. Song, W.; Lee, J.-K.; Gong, M.S.; Heo, K.; Chung, W.-J.; Lee, B.Y. Cellulose nanocrystal-based colored thin films for colorimetric detection of aldehyde gases. ACS Appl. Mater. Interfaces 2018, 10, 10353–10361. [CrossRef] [PubMed] 409. Kelly, J.A.; Shukaliak, A.M.; Cheung, C.C.Y.; Shopsowitz, K.E.; Hamad, W.Y.; MacLachlan, M.J. Responsive photonic hydrogels based on nanocrystalline cellulose. Angew. Chem. Int. Ed. 2013, 52, 8912–8916. [CrossRef] 410. Bruckner, J.R.; Kuhnhold, A.; Honorato-Rios, C.; Schilling, T.; Lagerwall, J.P.F. Enhancing self-assembly in cellulose nanocrystal suspensions using high-permittivity solvents. Langmuir 2016, 32, 9854–9862. [CrossRef] 411. Beck, S.; Bouchard, J.; Berry,R. Controlling the reflection wavelength of iridescent solid films of nanocrystalline cellulose. Biomacromolecules 2011, 12, 167–172. [CrossRef] 412. Frka-Petesic, B.; Guidetti, G.; Kamita, G.; Vignolini, S. Controlling the photonic properties of cholesteric cellulose nanocrystal films with magnets. Adv. Mater. 2017, 29, 1701469. [CrossRef] 413. Beck-Candanedo, S.; Roman, M.; Gray, D.G. Effect of reaction conditions on the properties and behavior of wood cellulose nanocrystal suspensions. Biomacromolecules 2005, 6, 1048–1054. [CrossRef][PubMed] 414. Dong, Y.-m.; Mao, W.; Wang, H.-w.; Zhao, Y.-q.; Li, X.-j.; Bi, D.-x.; Yang, L.-l.; Ge, Q.; Fang, X. Measurement of critical concentration for mesophase formation of chitosan derivatives in both aqueous and organic solutions. Polym. Int. 2006, 55, 1444–1449. [CrossRef] 415. Parker, R.M.; Guidetti, G.; Williams, C.A.; Zhao, T.; Narkevicius, A.; Vignolini, S.; Frka-Petesic, B. The self-assembly of cellulose nanocrystals: Hierarchical design of visual appearance. Adv. Mater. 2018, 30, 1704477. [CrossRef][PubMed] 416. Zhang, Y.P.; Chodavarapu, V.P.; Kirk, A.G.; Andrews, M.P.Structured color humidity indicator from reversible pitch tuning in self-assembled nanocrystalline cellulose films. Sens. Actuators B Chem. 2013, 176, 692–697. [CrossRef] 417. Zhang, Y.P.; Chodavarapu, V.P.; Kirk, A.G.; Andrews, M.P. Nanocrystalline cellulose for covert optical encryption. J. Nanophotonics 2012, 6, 063516. [CrossRef] 418. Bardet, R.; Roussel, F.; Coindeau, S.; Belgacem, N.; Bras, J. Engineered pigments based on iridescent cellulose nanocrystal films. Carbohydr. Polym. 2015, 122, 367–375. [CrossRef][PubMed] 419. Majoinen, J.; Hassinen, J.; Haataja, J.S.; Rekola, H.T.; Kontturi, E.; Kostiainen, M.A.; Ras, R.H.A.; Törmä, P.; Ikkala, O. Chiral plasmonics using twisting along cellulose nanocrystals as a template for gold nanoparticles. Adv. Mater. 2016, 28, 5262–5267. [CrossRef] 420. Almeida, P.L.; Kundu, S.; Borges, J.P.; Godinho, M.H.; Figueirinhas, J.L. Electro-optical light scattering shutter using electrospun cellulose-based nano- and microfibers. Appl. Phys. Lett. 2009, 95, 043501. [CrossRef] 421. Shopsowitz, K.E.; Qi, H.; Hamad, W.Y.; MacLachlan, M.J. Free-standing mesoporous silica films with tunable chiral nematic structures. Nature 2010, 468, 422. [CrossRef] 422. Rofouie, P.; Alizadehgiashi, M.; Mundoor, H.; Smalyukh, I.I.; Kumacheva, E. Self-assembly of cellulose nanocrystals into semi-spherical photonic cholesteric films. Adv. Funct. Mater. 2018, 28, 1803852. [CrossRef] 423. Majoinen, J.; Kontturi, E.; Ikkala, O.; Gray, D.G. Sem imaging of chiral nematic films cast from cellulose nanocrystal suspensions. Cellulose 2012, 19, 1599–1605. [CrossRef] 424. Shopsowitz, K.E.; Stahl, A.; Hamad, W.Y.; MacLachlan, M.J. Hard templating of nanocrystalline titanium dioxide with chiral nematic ordering. Angew. Chem. 2012, 124, 6992–6996. [CrossRef] 425. Li, Y.; Prince, E.; Cho, S.; Salari, A.; Mosaddeghian Golestani, Y.; Lavrentovich, O.D.; Kumacheva, E. Periodic assembly of nanoparticle arrays in disclinations of cholesteric liquid crystals. Proc. Natl. Acad. Sci. USA 2017, 114, 2137. [CrossRef][PubMed] 426. Gupta, V.K.; Skaife, J.J.; Dubrovsky, T.B.; Abbott, N.L. Optical amplification of ligand-receptor binding using liquid crystals. Science 1998, 279, 2077. [CrossRef][PubMed] Appl. Sci. 2019, 9, 2512 47 of 47

427. Brake, J.M.; Daschner, M.K.; Luk, Y.-Y.; Abbott, N.L. Biomolecular interactions at phospholipid-decorated surfaces of liquid crystals. Science 2003, 302, 2094. [CrossRef] 428. Hsiao, Y.-C.; Sung, Y.-C.; Lee, M.-J.; Lee, W. Highly sensitive color-indicating and quantitative biosensor based on cholesteric liquid crystal. Biomed. Opt. Express 2015, 6, 5033–5038. [CrossRef][PubMed] 429. Lin, C.-H.; Lee, M.-J.; Lee, W. Bovine serum albumin detection and quantitation based on capacitance measurements of liquid crystals. Appl. Phys. Lett. 2016, 109, 093703. [CrossRef] 430. Lee, M.-J.; Chang, C.-H.; Lee, W. Label-free protein sensing by employing blue phase liquid crystal. Biomed. Opt. Express 2017, 8, 1712–1720. [CrossRef] 431. Qi, L.; Hu, Q.; Kang, Q.; Yu, L. Fabrication of liquid-crystal-based optical sensing platform for detection of hydrogen peroxide and blood glucose. Anal. Chem. 2018, 90, 11607–11613. [CrossRef] 432. Zhang, J.; Su, X.; Yang, D.; Luan, C. Label-free liquid crystal biosensor for cecropin b detection. Talanta 2018, 186, 60–64. [CrossRef] 433. Lin, C.-M.; Wu, P.-C.; Lee, M.-J.; Lee, W. Label-free protein quantitation by dielectric spectroscopy of dual-frequency liquid crystal. Sens. Actuators B Chem. 2019, 282, 158–163. [CrossRef] 434. Tan, H.; Yang, S.; Shen, G.; Yu, R.; Wu, Z. Signal-enhanced liquid-crystal DNA biosensors based on enzymatic metal deposition. Angew. Chem. Int. Ed. 2010, 49, 8608–8611. [CrossRef][PubMed] 435. Hartono, D.; Qin, W.J.; Yang, K.-L.; Yung, L.-Y.L. Imaging the disruption of phospholipid monolayer by protein-coated nanoparticles using ordering transitions of liquid crystals. Biomaterials 2009, 30, 843–849. [CrossRef][PubMed] 436. Liao, S.; Qiao, Y.; Han, W.; Xie, Z.; Wu, Z.; Shen, G.; Yu, R. Acetylcholinesterase liquid crystal biosensor based on modulated growth of gold nanoparticles for amplified detection of acetylcholine and inhibitor. Anal. Chem. 2012, 84, 45–49. [CrossRef][PubMed] 437. Yang, S.; Liu, Y.; Tan, H.; Wu, C.; Wu, Z.; Shen, G.; Yu, R. Gold nanoparticle based signal enhancement liquid crystal biosensors for DNA hybridization assays. Chem. Commun. 2012, 48, 2861–2863. [CrossRef][PubMed] 438. Zhao, D.; Peng, Y.; Xu, L.; Zhou, W.; Wang, Q.; Guo, L. Liquid-crystal biosensor based on nickel-nanosphere-induced homeotropic alignment for the amplified detection of thrombin. ACS Appl. Mater. Interfaces 2015, 7, 23418–23422. [CrossRef][PubMed] 439. Wei, Y.; Jang, C.-H. Visualization of cholylglycine hydrolase activities through nickel nanoparticle-assisted liquid crystal cells. Sens. Actuators B Chem. 2017, 239, 1268–1274. [CrossRef] 440. Choudhary, A.; George, F.T.; Li, G. Conjugation of nanomaterials and nematic liquid crystals for futuristic applications and biosensors. Biosensors 2018, 8, 69. [CrossRef][PubMed] 441. Kim, Y.-K.; Wang, X.; Mondkar, P.; Bukusoglu, E.; Abbott, N.L. Self-reporting and self-regulating liquid crystals. Nature 2018, 557, 539–544. [CrossRef][PubMed] 442. Barauskas, J.; Johnsson, M.; Tiberg, F. Self-assembled lipid superstructures: Beyond vesicles and liposomes. Nano Lett. 2005, 5, 1615–1619. [CrossRef][PubMed] 443. Vallamkondu, J.; Corgiat, E.; Buchaiah, G.; Kandimalla, R.; Reddy, P. Liquid crystals: A novel approach for cancer detection and treatment. Cancers 2018, 10, 462. [CrossRef][PubMed] 444. Malmsten, M. Soft drug delivery systems. Soft Matter 2006, 2, 760–769. [CrossRef] 445. Nikam, S.; Chavan, M.; Sharma, P.H. Solid lipid nanoparticles: A lipid based drug delivery. Nanotechnology 2014, 1, 5. 446. Volkhard Jenning, S.H.G. Encapsulation of retinoids in solid lipid nanoparticles (sln). J. Microencapsul. 2001, 18, 149–158. [CrossRef][PubMed] 447. Müller-Goymann, C.C. Physicochemical characterization of colloidal drug delivery systems such as reverse micelles, vesicles, liquid crystals and nanoparticles for topical administration. Eur. J. Pharm. Biopharm. 2004, 58, 343–356. [CrossRef][PubMed]

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