Available online at www.cpronline.in

Current Pharma Research 3 (4), 2013, 1031-1037.

Review Article

Janus Particles Recent and Novel Approach in Drug Delivery: An Overview. Y.R. Indalkara,*, S.S. Gaikwada, A. T. Ubalea aCollege of Pharmacy, Medha, Satara, Maharashtra, India.

Received 25 May 2013; received in revised form 16 Aug 2013; accepted 17 Aug 2013

Available online 15 September 2013

Abstract Janus particles are microscopic particles composed of two different sides. It is an analogue to the surfactant molecule, which has a hydrophilic head group and hydrophobic tail. Janus particle shows fundamental as well as commercial application in drug delivery system. Janus partials have collectively considerable attention in drug delivery system due to their self-organization into complex and well-defined assemblies and advanced surface-active properties with a segregated corona over particles with a uniform wettability. More recently its applications are in Pickering , electro spinning, and up scaled micro fluidic devices. Along with it has commercial application in areas such as painting, ceramics, and Photonic materials. In this article we highlight several aspects of Janus particles regarding their general properties, synthesis and its application in future opportunity.

Keywords: Janus particles, drug delivery, surface-active, Self-assembly.

One year later, Casagrande and Veyssie reported 1. Introduction [1-4] the synthesis of glass beads that were made A few years ago, Janus particals comes by hydrophobic on only one hemisphere using suggestion of de Gennes. These are special class of octadecyl trichlorosilane, while the other hemisphere colloidal particles with different chemical makeups was protected with a cellulose varnish. The glass on their two hemispheres that is hydrophilic and beads were studied for their potential to stabilize hydrophobic. Originally, the term Janus particle was emulsification processes. Recently, Binks and coined by C. Casagrande et al. in 1988 to describe Flechter investigated the wettability of Janus beads spherical glass particles with one of the at the interface between oil and water and conclude hemispheres hydrophilic and the other hydrophobic. that Janus particles are both surface-active and In that work, the amphiphilic beads were amphiphilic, whereas homogeneous particles are synthesized by protecting one hemisphere with only surface-active. Colloidal particles are varnish and chemically treating the other fundamental to nature and technology. In terms hemisphere with a silane reagent. This method dimension they present in small molecules form resulted in a particle with equal hydrophilic and (A°), which can only be approached by modern tools hydrophobic areas. In 1991, Pierre-Gilles de of electron microscopy, and big objects (100 mm). Gennes mentioned the term "Janus" particle in his They are pushed around by the random collisions Nobel lecture. Janus particles are named after the from thermal movement of liquid molecules two faced Roman god Janus because these surrounding them (Brownian motion).Its Chemical particles may be said to have "two faces" since they composition can incorporate both inorganic and possess two distinct types of properties. In Chinese organic species upon design. Janus particles can be philosophy, seemingly opposite forces are viewed divided into three categories as polymeric, as naturally interconnected, a concept known as Yin inorganic, and polymeric–inorganic, and each kind and Yang. Hence, Janus particles can also be of Janus particles can be present in various forms viewed as Yin− Yang particles. De Gennes pushed like spherical, dumbbell, half raspberry, cylindrical, for the advancement of Janus particles by pointing disk, or any of a variety of other shapes. Janus out these "Janus grains" have the unique property of particals are considered in two types as Janus densely self-assembling at liquid-liquid interfaces, ellipsoids and Janus dumbbells Out of Janus while allowing material transport to occur through ellipsoids can be kinetically trapped in a metastable the gaps between the solid amphiphilic particles. state due to the presence of secondary energy Although the term "Janus particles" was not yet minimum while jaus dumbbells possess only a used, Lee and coworkers reported the first particles primary energy minimum, showing that these matching this description in 1985. They introduced particles prefer to be in a single orientation. asymmetric / polymethylmethacrylate Their potential applications even at that time as lattices from seeded . surfactants and membrane applications, a “skin” that

can “breathe”. Such particles are expected to *Corresponding author. +91-09860694788 adsorb to interfaces and foam surfaces even more E-mail address: [email protected] (Y.R.Indalkar) strongly than particles whose chemical makeup is 2230-7842 / © 2013 CPR. All rights reserved. isotropic. Beyond this applications Janus particles

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can also assist chemical catalysis, boosting catalytic After the particles have fully adsorbed, the activity. They find applications in drug delivery as a temperature is lowered to solidify the wax variety of drugs can be loaded and subsequently and lock the particles in place. Subsequently, released either simultaneously to achieve synergy chemical modification is carried out at temperatures between them or in controlled sequence if demand where the oil phase is solid, thus ensuring the changes with time , in display technology, used by modification of only those faces of the particle that the controlled orientational switch between the dark are not buried in wax. The use of a solidified oil and bright sides, by external magnetic or electric phase offers several advantages. First, it freezes the fields. In the field of -filled composites, its particles into fixed positions during the chemical applications to assemble stress networks and modification step, avoiding the possibility that tailored pathways of electrical conductivity. In the adsorbed particles might wobble or rotate at the field of template-directed synthesis, the unique liquid–liquid interface. Indeed, particle rotation at the structures formed from their self-assembly can liquid–air interface has been reported by others. serve as templates by growing chemicals Second, the presence of a solid oil phase presents conformally around these exotic structures and advantages at the stage of separation and removing them afterward. Article includes review of purification of the particles. Third, when two stages progress in development of synthetic methods to of chemical modification are intended, this form Janus particles along with its application eliminates the need to find a liquid–liquid combination where the reactant is soluble in only Synthesis of Janus particles [3] one liquid. In experiment, scanning electron In the early days, methods to synthesize Janus microscopy (SEM) images reveal that most of the particles were not much advanced than today’s particles adsorb at the emulsion droplet surfaces, standards. Initially Janus particles were synthesized whereas only few particles are found inside the wax by fictionalization in which particle break into droplet (as viewed after the wax droplet is broken). fragments and then giving automatic Janus The excess of particles dispersed in water are character to the fragments. This generates filtered and rinsed away. The Janus geometry is heterogeneous particles without control over the confirmed by visualizing particles that contain shape, size, or purity; but remains an easy, low-cost fluorescent dye attached to one hemisphere. This approach for synthesis. While considering synthesis method has several advantages first; the reaction of Janus particles two important points should be can easily be scaled up to synthesize large addressed. First is the ability to control the geometry quantities of Janus particles, since large amounts of of the Janus particles, i.e., the relative areas of their interfaces can be created during emulsification. two sides and second is the ability to produce Janus Second, the Janus geometry can be particles in large quantities. So far, various ways to thermodynamically controlled by the synthesize Janus particles have been developed. between the particle surface and the liquids used in the emulsion. Compared to alternative methods Pickering emulsions [4] where the surface coverage of modified chemical This method can synthesize Janus particles with makeup is controlled kinetically, this is easier to controllable geometry in a wide range of sizes and achieve, and the monodispersity of surface can be scaled up to synthesize particles in gram- coverage is better from batch to batch. Also, a wide sized quantities. A schematic representation of the range of Janus geometries can be achieved by method is summarized in Fig. 1. At liquid–liquid simply tuning the amount of surfactants added interface particles whose surface energy is between during the synthesis. Third, the that can that of the two liquids will adsorb onto the interface, be applied to this method is versatile. Besides the thereby lowering the total free energy. In the case of amphiphilic Janus particles mentioned here, also our wax–water system, untreated hydrophilic fused bipolar Janus particles, with opposite charges on silica particles adsorb to the oil-in-water emulsion each side, can be produced via this method. interface at an elevated temperature where the wax is molten.

Fig. 1. Pickering emulsions: Method of synthesis of JANUS particle.

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Block [5] on the particle surface occurred by comparing the A schematic representation of the method is contact angles of water on the surface of a summarized in Fig. 2. Block method was of the Janus particles with first described by Erhardt et al. in 2001.This method made with only the hydrophobic or hydrophobic produce block with well-defined ligands. Instead the results of this experiment geometries and compositions across a large variety showed that while there was some phase of substrates. They produced a triblock polymer separation, it was not complete. This result from polymethylacrylate, polystyrene and low- highlights that the ligand choice is extremely molecular-weight . The polystyrene important and any changes may result in incomplete and polymethylacrylate formed alternating layers in phase separation. between which polybutadiene sat in nanosized spheres. The blocks were then cross-linked and dissolved in THF, and after several washing steps, Phase separation [6] yielded spherical Janus particles with polystyrene on A schematic representation of the method is one face and polymethylacrylate on the other, with a summarized in Fig. 3.This method involves the polybutadiene core. The production of Janus mixing of two or more incompatible substances spheres, cylinders, sheets, and ribbons is possible which then separate into their own domains while using this method by adjustment of the starting still part of a single . These methods material's molecular weights and degree of cross- can involve the production of Janus nanoparticles of linking. two inorganic, as well as two organic, substances. In typical organic phase separation methods use cojetting of polymers to produce Janus nanoparticles. This technique is exemplified by the work of Yoshid et al. to produce Janus nanoparticles where one hemisphere has affinity for human cells, while the other hemisphere has no affinity for human cells. This was achieved by cojetting polyacrylamide/poly (acrylic acid) copolymers which have no affinity for human cells with biotinylated polyacrylamide/poly (acrylic acid) copolymers, which when exposed to streptavidin-modified ,

obtain an affinity for human cells. While in inorganic phase separation methods are diverse and vary greatly depending on the application. The most Fig. 2: Synthesis of Janus nanoparticles using the common method uses the growth of a crystal of one block copolymer self-assembly method. inorganic substance on or from other inorganic nanoparticles. A unique method has been developed by Gu et al., where iron- Competitive [7] nanoparticles were coated with reacted with The core aspect of competitive absorption involves acetylacetonate, trioctylphosphineoxide, two substrates that phase-separate due to one or and hexadecane-1,2-diol at 100°C to produce more opposite physical or chemical proprieties. nanoparticles with an iron-platinum core and an When these substrates are mixed with a amorphous cadmium-sulfur shell. The mixture was nanoparticle, typically gold, they maintain their then heated to 280°C, resulting in a separation and form two faces. A good example of and a partial eruption of the Fe-Pt from the core, this technique has been demonstrated by Vilain et creating a pure Fe-Pt sphere attached to the CdS- al., where phosphinine-coated gold nanoparticles coated nanoparticles. A new method of synthesizing were exposed to long-chain , resulting in inorganic Janus nanoparticles by phase separation substitution of the phosphinine ligands in a phase- has recently been developed by Zhao and Gao. In separated manner to produce Janus nanoparticles. this method, they explored the use of the common Phase separation was proven by showing the thiols homogeneous nanoparticles synthetic method of formed one locally pure domain on the nanoparticle flame synthesis. They found when a using FT-IR. Jakobs et al. demonstrated a major solution containing ferric triacetylacetonate and issue with the competitive adsorption method when tetraethylorthosilicate was burned, the iron and they attempted to synthesize amphiphilic gold Janus silicon components formed an intermixed solid, nanoparticles using the competitive adsorption of which undergoes phase separation when heated to hydrophobic and hydrophilic thiols. The synthesis approximately 1100°C to produce -silica demonstrated was quiet simple and only involved Janus nanoparticles. Additionally, they found it was two steps. First gold nanoparticles capped with possible to modify the silica after producing the tetra-n-octylammonium were produced. Janus nanoparticles, making it hydrophobic by Then the capping agent was removed followed by reacting it with . the addition of various ratios of hydrophilic disulfide functionalized and hydrophobic disulfide functionalized oligo(p-phenylenevinylene). They then attempted to prove that phase separation

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behave like classical dipoles, since their size is much larger than the distance at which electrostatic attractions are strongly felt. The study of zwitterionic Janus particles once again demonstrates their ability to form defined clusters. However, this particular type of Janus particle prefers to aggregate into larger clusters since this is more energetically favorable because each cluster carries a macroscopic dipole which allows the aggregation of already-formed clusters into larger assemblies. Compared to aggregates formed through van der Waals interactions for homogenous particles, the Fig. 3. Phase separation method of producing Janus shapes of the Zwitterionic Janus nanoclusters are nanoparticles. different and the Janus clusters are less dense and more asymmetric. The structures formed by Janus amphiphilic particles are analogous to the micellar Applications [7,8,9] shapes adopted by standard molecular . Self-assembly This similarity can provide an approximate idea of Janus particles' two or more distinct faces give them the fundamental rules followed by even more special properties in solution. In particular, they complex molecular systems such as the phase have been observed to self-assemble in a specific behavior of diblock copolymers and functionality of way in aqueous or organic solutions. In the case of biological membranes assembled by phospholipids. spherical Janus having hemispheres of A more detailed understanding of these systems polystyrene (PS) and poly (methyl methacrylate) would be built upon the answers to several more (PMMA), aggregation into clusters has been delicate questions. A comprehensive study of the observed in various organic , such as phase behavior of the amphiphilic Janus particles, . Similarly, Janus discs composed of how they evolve from dilute solution to concentrated sides of PS and poly (tert-butyl methacrylate) suspensions, might go beyond the conventional (PtBMA) can undergo back-to-back stacking into phase diagram for homogeneous colloids by superstructures when in an organic solution. revealing new behavior such as microphase Interestingly, these particular Janus particles form separation. Inspired by the assembly behavior of aggregates in organic solvents considering that both surfactants, we also expect that the self-assembled sides of these particles are soluble in the organic structures of amphiphilic Janus particles depend on . It appears that the slight selectivity of the and can even be controlled by the Janus balance, solvent is able to induce self-assembly of the the relative surface areas of the hydrophobic and particles into discrete clusters of Janus particles. charged patches. Eventually the question that needs This type of aggregation does not occur for either to be answered is what new assembled structures standard block copolymers nor for homogeneous can form when Janus amphiphilic particles of particles and thus is a feature specific to Janus different size, surface chemistry and possibly Janus particles. Additionally, the behavior of Janus particle balance are mixed together, since real systems in aqueous solutions is also very interesting. In an typically contain more than one component. On the , two kinds of biphasic particles can application side, amphiphilic Janus particles can be distinguished. The first type is particles which are serve as solid surfactants that adhere to the oil– truly amphiphilic and possess one hydrophobic and water interface, thereby stabilizing emulsions and one hydrophilic side. The second type has two foams. Moreover, the wormlike strings, or even the water-soluble, yet chemically distinct, sides. To clusters, may also modify suspension rheology to illustrate the first case, extensive studies have been produce different glass or gel dynamics and yielding carried out with spherical Janus particles composed behaviors, and they may be used as environment- of one hemisphere of water-soluble PMAA and responsive vehicles within which to encapsulate another side of water-insoluble polystyrene. In these cargo for subsequent controlled release. The idea of studies, the Janus particles were found to aggregate having solid surfactants generalizes the concept of on two hierarchical levels. The first type of self- molecular amphiphilicity that is so pervasive and assembled aggregates look like small clusters, useful in nature and technology. similar to what is found for the case of Janus particles in an organic solution. The second type is Stabilizers in emulsions [9] noticeably larger than the first and has been termed Application of Janus particle in stabilization of 'super micelles'. Unfortunately, the structure of the emulsion due its amphiphilic properties.In 2006, super micelles is unknown so far; however, they Janus nanoparticles, made from gold and iron may be similar to multilamellar vesicles. For the oxides, were compared with their homogeneous second case of Janus particles which contain two counterparts by measuring the ability of the particles distinct, but still water-soluble sides, the work of to reduce the interfacial tension between water and Granick’s group provides some insight. Their n-. Experimental results indicated Janus research deals with the clustering of dipolar nanoparticles are considerably more surface-active (zwitterionic), micronsized Janus particles, whose than homogeneous particles of comparable size and two sides are both fully water soluble. Zwitterionic chemical nature. Furthermore, increasing the Janus particles are interesting because they do not amphiphilic character of the particles can increase 1034

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the interfacial activity. The ability of Janus Janus Particles as Optical Probes for nanoparticles to lower interfacial tension between Microrheology [4] water and n-hexane confirmed previous theoretical Going beyond questions of self-assembly, transport predictions on their ability to stabilize Pickering problems are also worth emphasizing. Janus emulsions. In 2007, the amphiphilic nature of the particles can display anisotropic transport under Janus nanoparticles was examined by measuring certain conditions. This is usually due to the different the force between the atomic force local flow profiles or osmotic potential induced by microscopy (AFM) tip and the particle surface. The the external field or chemical reactions. It was found stronger interactions between the hydrophilic AFM that Au-coated Janus particles will diffuse tip and the hydrophilic side of the Janus perpendicular to the external electric field, due to the nanoparticles were reflected by a greater adhesion induced-charge electrophoresis flow close to the force. The Janus nanoparticles were drop cast onto particle surface of different hemispheres. The both hydrophobic ally and hydrophilic ally modified effective net force pushes particles to move in the substrates. The hydrophobic hemisphere of the direction perpendicular to the external electric field. Janus particles was exposed when a hydrophilic Another way to move Janus particles in a certain substrate surface was used, resulting in disparities direction is by self-propulsion induced by a in adhesion force measurements. Thus, the Janus catalytically active surface. This may find its nanoparticles adopted a confirmation that application in drug delivery systems, since self- maximized the interactions with the substrate motile particles can move more efficiently than surface. The nature of amphiphilic Janus conventional Brownian particles. Another potential nanoparticles to orient themselves spontaneously at utilization of Janus particles is to use them as the interface between oil and water has been well tracers to probe the local rheological properties of known. This behavior allows considering amphiphilic their environment. The contribution here is to use Janus nanoparticles as analogues of molecular modulated optical nanoprobes (MOONs) whose surfactants for the stabilization of emulsions. In optical properties are modulated by their rotational 2005, spherical silica particles with amphiphilic diffusion, thus augmenting the information about properties were prepared by partial modification of position that is obtained from conventional the external surface with an alkylsilane agent. These translational diffusion. From the rotational diffusion, particles form spherical assemblies encapsulating a more comprehensive understanding can be water-immiscible organic compounds in aqueous obtained in many complex environments where media by facing their hydrophobic alkylsilylated side translational and rotational diffusion are not simply to the inner organic phase and their hydrophilic side proportional to one another, such as glassy to the outer aqueous phase, thus stabilizing oil materials. Indeed, when Janus particles with high droplets in water. In 2009, hydrophilic surface of enough optical contrast are viewed and magnified in silica particles was made partially hydrophobic by an appropriate microscope, their images mimic the adsorbing cetyltrimethylammonium bromide. These moon in all its phases (Fig. 4). This enables the amphiphilic nanoparticles spontaneously assembled simultaneous tracking, at the single-particle level, of at the water- interface. In 2010, two rotational degrees of freedom. Special tracking Janus particles composed from silica and techniques have been developed to measure the polystyrene, with the polystyrene portion loaded with orientation of two axes of a MOON particle from a nanosized magnetite particles, were used to form single image, so that rotational dynamics can be kinetically stable oil-in-water emulsions that can be acquired from a series of video frames. Also, the spontaneously broken on application of an external location of the center of MOON particles can be magnetic field. Such Janus materials will find determined with modification of conventional applications in magnetically controlled optical tracking programs to give information about center- switches and other related areas. The first real of-mass diffusion. In some cases, the translational applications of Janus nanoparticles were in polymer and rotational dynamics are largely coupled as they synthesis. In 2008, spherical amphiphilic Janus basically convey the same rheological properties of nanoparticles, having one polystyrene and one the environment. They can, however, be decoupled poly(methyl methacrylate) side, were shown to be in more complex systems, for example in the case effective as compatibilizing agents of milligram scale of hopping of colloids on a patterned surface or for compatibilization of two immiscible polymer blends, asymmetric colloidal clusters. This illustrates the polystyrene and poly(methyl methacrylate). The richness of information that rotational diffusion of Janus nanoparticles oriented themselves at the Janus probes can provide. interface of the two polymer phases, even under high temperature and shear conditions, allowing the Applications in biological sciences [10] formation of much smaller domains of polymethyl The ground breaking progress in the biological methacrylate) in a polystyrene phase. The sciences has led to a drive towards custom made performance of the Janus nanoparticles as materials with precisely designed physical - compatibilizing agents was significantly superior to chemical properties at the nanoscale level. other state-of-the-art compatibilizers, such as linear Inherently Janus nanoparticles play a crucial role in block copolymers. such applications. In 2009, a new type of bio-hybrid material composed of Janus nanoparticles with spatially controlled affinity towards human endothelial cells was reported. 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with one hemisphere exhibiting high binding affinity bear properties that are applicable in bioimaging, for human endothelial cells and the other targeted drug delivery, in vivo diagnosis, and hemisphere being resistant towards binding. therapy. The purpose of the nanothin SiO2 shell was The Janus nanoparticles were fabricated via to reduce the release of toxic Ag+ ions from the electrohydrodynamic jetting of two polymer liquid nanoparticles surface to live cells. As a result, these solutions. When incubated with human endothelial hybrid nanoparticles showed no cyctotoxicity during cells, these Janus nanoparticles exhibited expected bioimaging and remained stable in suspension with behavior, where one face binds toward human no signs of agglomeration or settling, thus enabling endothelial cells, while the other face was not these nanoparticles as biocompatible multifunctional bonding. These Janus nanoparticles not only bound probes for bioimaging. Next, by labeling their to the top of the human endothelial cells, but also surfaces and selectively binding them on the associated all around the perimeter of cells forming membrane of live-tagged Raji and HeLa cells, this a single particle lining. The biocompatibility between demonstrated the nanoparticles as and the Janus nanoparticles and cells was excellent. their detection under dark-field illumination was The concept is to eventually design probes based achieved. These new hybrid Janus nanoparticles on Janus nanoparticles to attain directional overcame the individual limitations of Fe2O3 (poor information about cell-particle interactions. particle stability in suspension) and of Ag (toxicity) nanoparticles, while retaining the desired magnetic

Nanocorals [11] properties of Fe2O3 and the plasmonic optical In 2010, a new type of cellular probe synthesized properties of Ag. from Janus nanoparticles called a nanocoral, combining cellular specific targeting and biomolecular sensing, was presented. Nanocoral is Catalyst in decomposition [14] composed of polystyrene and gold hemispheres. In 2010, spherical silica Janus nanoparticles with The polystyrene hemisphere of the nanocoral was one side coated with platinum were used for the first selectively functionalized with antibodies to target time to catalyze the decomposition of hydrogen receptors of specific cells. This was demonstrated peroxide (H2O2). The platinum particle catalyzes the by functionalizing the polystyrene region with surface chemical reaction: 2H2O2 → O2 + H2O. The antibodies that specifically attached to breast cancer decomposition of hydrogen peroxide created Janus cells. The gold region of the nanocoral surface was catalytic nanometers, the motion of which was used for detecting and imaging. Thus, the targeting analyzed experimentally and theoretically using and sensing mechanisms were decoupled and could computer simulations. The motion of the spherical be separately engineered for a particular Janus nanoparticles was found to agree with the experiment. Additionally, the polystyrene region may predictions of computed simulations. Ultimately, also be used as a carrier for drugs and other catalytic nanomotors have practical applications in chemicals by surface hydrophobic adsorption or delivering chemical payloads in micro fluidic chips, encapsuation, making the nanocoral a possible eliminating pollution in aquatic media, removing multifunctional . toxic chemicals within biological systems, and performing medical procedures. Imaging and magnetolytic therapy [12,13] Also in 2010, Janus nanoparticles synthesized from Water-repellent fibers [15] hydrophobic magnetic nanoparticles on one side In 2011, Janus nanoparticles were shown to be and polystyrene-block-allyl alcohol) on the other applicable in textiles. Water-repellent fibers can be side were used for imaging and magnetolytic prepared by coating polyethylene terephthalate therapy. The magnetic side of the Janus fabric with amphiphilic spherical Janus nanoparticles nanoparticles responded well to external magnetic [10]. The Janus particles bind with the hydrophilic stimuli. The nanoparticles were quickly attached to reactive side of the textile surface, while the the cell surfaces using a magnetic field. hydrophobic side is exposed to the environment, Magnetolytic therapy was achieved through thus providing the water-repellent behavior. Janus magnetic field-modulated cell membrane damage. particle sizes of 200 nm were found to deposit on First, the nanoparticles were brought close in the surface of fibers and were very efficient for the contact with the tumor cells, and then a spinning design of water-repellent textiles. magnetic field was applied. After 15 minutes, the majority of the tumor cells were killed. Magnetic Applications in electronics [16,17] Janus nanoparticles could serve as the basis for The potential application of Janus particles was first potential applications in medicine and electronics. demonstrated by Nisisako et al., who made use of Quick responses to external magnetic fields could the electrical of Janus particles filled with become an effective approach for targeted imaging, white and black pigments in both hemispheres. therapy in vitro and in vivo, and cancer treatment. These particles were used to make switchable Similarly, a quick response to magnetic fields is also screens by placing a thin layer of these spheres desirable to fabricate smart displays, opening new between two electrodes. Upon changing the applied opportunities in electronics and spintronics. In 2011, electric field, the particles orient their black sides to silica-coated Janus nanoparticles, composed of the and their white sides to the . Thus oxide and (Fe2O3), were prepared in the orientation and the color of the display can be one step with scalable flame aerosol technology. changed by simply reversing the electric field. With These hybrid plasmonic-magnetic nanoparticles 1036

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this method, it may be possible to make very thin [13] Takasi Nisisako, T. Torii. Adv. Mater. 18 (2006) and environmentally friendly displays. 1152–1156. doi:10.1002/adma.200502431. [14] Wu Liz Y, Benjamin M Ross. Smll, 6 (2010) Janus particles handling by dielectrophoresis 503-507.doi:10.1002/smll.200901604. [17,18,19] [15] Synytska Alla, Rina Khanum. Appl. Mater. Janus particles Au/fluorescent polystyrene are Interfaces, 3 (2011) 1216–1220. fabricated and their flip/flop rotational effect is doi:10.1021/am2003u. studied in a micro fluidic channel thanks to [16] Thibault Honegger, O. Lecarme, K. Berton, D. dielectrophoresis, providing a new type of local light Peyrade. Microelectron. Eng, 87 (2010) 756–759. switch. A method for producing large amounts more [17] Thibault Honegger, O. Lecarme, K. Berton, D. than 106 particles/ml of Janus particles is first Peyrade. J. Vac. Sci. Technol, 5 (2010) 1 – 3. presented. Those particles were then injected in an [18] Erhardt Rainer. Macromolecules, 34 (2001) electromicrofluidic chip and stabilized in the fluid by 1069–1075. a dielectrophoretic trap. The spanning frequency of [19] Gu Hongwei, Zheng Rongkun, Zhang XiXiang, this trap allowed performing a “flip-flop” effect of the Xu Bing. Journal of the American Chemical Society, Janus particles by recording their fluorescent 126 (2004) 5664–5665. intensities. Flip Au top side and flop PS top side frequencies are identified. Experiments were performed on the time-triggered commutations between flip and flop frequencies to define the capability of each Janus particle to sustain speed control of their flip-flop.

Conclusion

Janus particles give desirable control over not only surface chemical composition, but also the capacity to produce large quantities of monodisperse .Janus particles. Looking to the future, Fundamental research of this kind will continue to elucidate our understanding of the principles of the self-assembly process. This in turn implies that the observed structures are likely to be equilibrated. To this end, it will be necessary to find synthetic pathways in the laboratory to implement the multivalent structures that have been proposed in large quantities and in monodisperse form.

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