Review Functionalization of by Inorganic Entities

Frédéric Dumur 1,* , Eddy Dumas 2 and Cédric R. Mayer 3,4,*

1 Aix Marseille Univ, CNRS, ICR, UMR 7273, F-13397 Marseille, France 2 Institut Lavoisier de Versailles, UMR CNRS 8180, Université de Versailles Saint-Quentin-en-Yvelines, F-78035 Versailles, France; [email protected] 3 Laboratoire LuMin, FRE CNRS 2036, CNRS, Université Paris-Sud, ENS Paris-Saclay, Université Paris-Saclay, F-91405 Orsay CEDEX, France 4 Département de Chimie, UFR des Sciences, Université de Versailles Saint-Quentin-en-Yvelines, F-78035 Versailles, France * Correspondence: [email protected] (F.D.); [email protected] (C.R.M.); Tel.: +33-0491289059 (F.D.)  Received: 25 February 2020; Accepted: 13 March 2020; Published: 18 March 2020 

Abstract: The great affinity of gold surface for numerous electron-donating groups has largely contributed to the rapid development of functionalized gold nanoparticles (Au-NPs). In the last years, a new subclass of has emerged, based on the association of inorganic molecular entities (IME) with Au-NPs. This highly extended and diversified subclass was promoted by the synergy between the intrinsic properties of the shell and the gold core. This review—divided into four main parts—focuses on an introductory section of the basic notions related to the stabilization of gold nanoparticles and defines in a second part the key role played by the functionalizing agent. Then, we present a wide range of inorganic molecular entities used to prepare these (NCs). In particular, we focus on four different types of inorganic systems, their topologies, and their current applications. Finally, the most recent applications are described before an overview of this new emerging field of research.

Keywords: Au◦-Nanoparticles; d and f metallic coordination and organometallic complexes; SiO2 coating; p- and s-block elements

1. Introduction In nanosciences, gold nanoparticles (Au-NPs) constitute a specific field of research, as chalcogenides or metallic oxides. Gold has always been a source of interest, which dates from antiquity [1]. Since the emergence of modern chemistry, Au-NPs chemistry has known two major steps in its development. In 1951, Turkevich presented the first synthesis of gold nanoparticles in aqueous solution, resulting in polydisperse nanoparticles [2]. In 1994, Brust described a unique and simple method to obtain well-monodispersed and functionalized NPs, by use of a two-phase liquid-liquid system—since, commonly named “Brust method”[3]. Starting from this date, functionalization of gold nanoparticles has known an ever-increasing research effort. This development has notably been supported by the intrinsic physicochemical properties of Au-NPs playing a key role in numerous fields of both fundamental and applied research. In particular, Au-NPs display fascinating electronic and optical properties [4–8]. Their abilities to ensure controllable interactions with various organic electron-donating groups have also largely contributed to their developments. Nowadays, use of Au-NPs has been extended to domains ranging from biomedical applications [9–12] to electronics [13,14], as a result of the great advances in their functionalizations and their stabilizations. Since the pioneering works concerning

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NanomaterialsNanomaterials 2020 2020, 10, 10, x, FORx FOR PEER PEER REVIEW REVIEW 2 of2 of 34 34 the functionalization processes of gold surfaces, the nature of the functionalizing agents is now nearly unlimited, but strongly depends on targeted applications. InIn this this review, review, we we focus focus our our attention attention on on inorganic inorganic molecular molecular entities entities (IME) (IME) as as functionalizing functionalizing In this review, we focus our attention on inorganic molecular entities (IME) as functionalizing agents.agents. NotionsNotions ofof stabilizationstabilization ofof goldgold nanoparticlesnanoparticles andand functionalizationfunctionalization ofof Au-NPsAu-NPs areare agents. Notions of stabilization of gold nanoparticles and functionalization of Au-NPs are preliminary preliminarypreliminary presented. presented. Then, Then, four four types types of of IME IME ar are edetailed detailed with with their their topologies topologies and and potential potential presented. Then, four types of IME are detailed with their topologies and potential applications: bloc applications:applications: bloc bloc p ,p ,metallic metallic oxoclusters, oxoclusters, organometallic organometallic co complexes,mplexes, and and coordination coordination complexes. complexes. p, metallic oxoclusters, organometallic complexes, and coordination complexes. Finally, we conclude Finally,Finally, we we conclude conclude with with significant significant applicat applicationsions concerning concerning these these nanocomposites nanocomposites and and their their with significant applications concerning these nanocomposites and their futures. futures.futures. 2. Notions on Stabilization of Gold and Silver Nanoparticles 2.2. Notions Notions on on Stabilization Stabilization of of Gold Gold and and Silver Silver Nanoparticles Nanoparticles 2.1. Role and Nature of the Stabilizing Agent 2.1.2.1. Role Role and and Nature Nature of of the the Stabilizing Stabilizing Agent Agent Nowadays, preparation of gold nanoparticles is relatively easy, due to the different reduction processesNowadays,Nowadays, at our preparation disposalpreparation for of of gold their gold nanoparticles syntheses.nanoparticles is To is relatively relatively introduce easy, easy, a specificdue due to to the function—orthe different different reduction reduction to get size-monodispersedprocessesprocesses at at our our disposal disposal NPs infor for a their desiredtheir syntheses. syntheses. To To at introduce aintroduce desired a a concentrationspecific specific function—or function—or with an to to expectedget get size- size- morphology—knowledgemonodispersedmonodispersed NPs NPs in in a adesired ofdesired the rolesolvent solvent and at the at a a naturedesi desiredred of concentration concentration the stabilizing with with agent an an expected is expected crucial. morphology— Stabilizationmorphology— ofknowledgeknowledge colloidal suspensionsof of the the role role and resultsand the the fromnature nature two of of specificthe the stabilizing stabilizing processes: agent agent the is is firstcrucial. crucial. is based Stabilization Stabilization on steric of repulsion,of colloidal colloidal inducedsuspensionssuspensions by long results results alkyl from chainsfrom two two grafted specific specific onto processes: processes: the gold the surface the fi rstfirst [is15 is based], based and on the on steric second steric repulsion, repulsion, results from induced induced electrostatic by by long long [ [ ] ] repulsionsalkylalkyl chains chains resulting grafted grafted from onto onto thethe the use gold gold of surface charged surface 15ligands15, and, and [16the the], second as second shown results results in the from Schemefrom electrostatic electrostatic1. repulsions repulsions resultingresulting from from the the use use of of charged charged ligands ligands [16 [16], ]as, as shown shown in in the the Scheme Scheme 1. 1.

Scheme 1. Both processes of nanoparticles (NP) repulsion by electrostatic or steric repulsion, induced bySchemeScheme charged 1. 1. Both molecules Both processes processes or long of of nanoparticles chains, nanoparticles respectively. (NP) (NP) repulsion repulsion by by electrostatic electrostatic or or steric steric repulsion, repulsion, induced induced byby charged charged molecules molecules or or long long chains, chains, respectively. respectively. Concerning the stabilization of by steric repulsions, the interaction of the stabilizing agent with theConcerningConcerning NPs surface the the is stabilization usuallystabilization ensured of of colloids bycolloids use of by electronby steric steric donatingrepulsions, repulsions, groups. the the interaction interaction Different anchoringof of the the stabilizing stabilizing groups haveagentagent been with with employed the the NPs NPs surface in surface the literature is is usually usually [17 ensured ensured], but the by by mostus use eof commonof electron electron ones donating donating are, without groups. groups. contest, Different Different alkylthiol anchoring anchoring or arylthiolgroupsgroups have groups have been been [17 ]employed (Schemeemployed2 ).in in Thethe the sulfurliterature literature atom [ 17[ is17],the ]but, but element the the most most that common common ensures ones theones strongest are, are, without without interaction contest, contest, withalkylthiolalkylthiol gold atoms. or or arylthiol arylthiol Other functionsgroups groups [ 17 have[17] ](Scheme also(Scheme been 2). 2). used The The to sulfur graftsulfur aatom stabilizingatom is is the the agent element element onto that thethat NPsensures ensures surface: the the phosphinestrongeststrongest interaction [ 18interaction–21], amine with with [22 gold gold], pyridine atoms. atoms. Other [23 Other], and function function carboxylates haves have [also24 also] (See been been Scheme used used to2 to). graft Tograft prevent a astabilizing stabilizing undesired agent agent desorptionontoonto the the NPs NPs of asurface: stabilizingsurface: phosphine phosphine agent, an[18-21 [18-21 efficient], ]amine, amine method [22 [22], ]pyridine consists, pyridineof [23 [ using23], ]and, and molecules carboxylate carboxylate possessing [24 [24] (See] (See atScheme Scheme least two2).2). To electron-donatingTo prevent prevent undesired undesired groups, desorption desorption such as of disulphideof a astabilizin stabilizin [25g g],agent, agent, dithiocarbamate an an efficient efficient functionsmethod method consists [consists26], or of trithiolof using using speciesmoleculesmolecules [27]. possessing Mixedpossessing donating at at least least groups two two electron-donating electron-donating such as mercaptopyridine, groups, groups, such such [28 ]as mercaptothiadiazoleas disulphide disulphide [25 [25], ]dithiocarbamate, dithiocarbamate [29], or lipoic acidfunctionsfunctions derivatives [ 26[26], ] [,30or or ]trithiolthat trithiol possess species species two [ anchoring27[27]. ].Mixed Mixed groups donating donating also egroupsffi groupsciently such ensuresuch as as amercaptopyridine, strongmercaptopyridine, interaction with [ 28][28] themercaptothiadiazolemercaptothiadiazole NP surface. These polydentate[ 29[29], ],or or lipoic lipoic ligands acid acid derivatives render derivatives functionalized [30] [30] th thatat possess NPspossess effectively two two anchoring anchoring more stable groups groups towards also also ligandefficientlyefficiently exchange. ensure ensure a astrong strong interaction interaction with with th the eNP NP surface. surface. These These polydentate polydentate ligands ligands render render functionalizedfunctionalized NPs NPs effectively effectively more more stable stable towards towards ligand ligand exchange. exchange.

Scheme 2. Nature of the anchoring groups. SchemeScheme 2. 2. Nature Nature of of the the anchoring anchoring groups. groups.

ToTo control control the the size size of of spherical spherical NPs, NPs, the the best best process process consists consists of of adjusting adjusting the the ratio ratio between between the the GoldGold (III) (III) salt salt and and the the stabilizing stabilizing agent agent [31–32 [31–32]. ]High. High concentrations concentrations of of well-monodispersed well-monodispersed NPs NPs are are [ ] [ ] generallygenerally obtained obtained with with long long alkyl alkyl chains chains (length (length > >C C8H8H17)17 )33,34 [33,34 or] or with with polymers polymers 35 [35. ]Small. Small NPs NPs

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To control the size of spherical NPs, the best process consists of adjusting the ratio between the Gold (III) salt and the stabilizing agent [31,32]. High concentrations of well-monodispersed NPs are generally obtained with long alkyl chains (length > C8H17)[33,34] or with polymers [35]. Small NPs with diameters around 1.5 to 3 nm are also obtained in these conditions. Preparation of large or small and monodispersed Au-NPs remains still a challenge at present. However, Wei et al. succeeded to synthesize a resorcinarene tetrathiol that provided very monodispersed NPs with a large size [36]. Another way to get a similar result consists of using hydrothermal conditions from Au-NPs seeds [37]. The monodispersity and size of NPs are currently controlled by the initial molar ratio of the gold (III) salt and the stabilizing agent, due to the fact that both parameters are highly dependent. When a strong coordination with gold atoms is ensured by the ligand—as in the case of sulfur atoms—all gold-clusters are covered by the ligands, immediately stopping the growth of the different crystalline faces. Small particles are thus obtained. On the other hand, when an electron-donating group ensuring a weak interaction with the gold surface is used, desorption of the ligand can occur, allowing the growth of a crystallization face, or the aggregation of particles. Polydispersed particles and aggregates are thus obtained. Other parameters can have a non-negligible influence on both the surface and morphology, such as the topology of the ligand, the solvent, the temperature, the strength of reducing agent, or the pH, in the case of an aqueous medium. Due to the influence of these different parameters, the use of a new stabilizing agent implies a preliminary control of these different points (ratio, nature of the solvent, temperature of the medium, nature, and ratio of the reducing agent).

2.2. Synthetic Methods Gold nanoparticles are used in a multitude of applications ranging from , to , optics, electronics, and 1D-3D-assemblies. With few exceptions, the most important prerequisite for all these applications remains the preparation of highly uniform-sized monodispersed NPs and well-stabilized NPs. A wide range of applications such as optical and electrochemical applications, requires monodispersed NPs of controlled size to analyze single NPs. Moreover, the control of self-assemblies of NPs also necessitates to have very monodispersed NPs [38]. Various processes may be used to prepare Au-NPs. The dominant synthetic approach consists of the reduction of a gold (III) salt (currently AuCl4−) in the presence of a stabilizing agent by use of a reducing agent (classically, NaBH4). Sometimes, the stabilizing agent and the reducing agent may be the same reactant. This is the case when citrate anions [2] or aniline is used: the reducing agent stabilizes the particles [22]. Two variants of these two synthetic procedures are commonly used to prepare particles. Thus, the reduction process can occur in a one-phase system (all reactants are solubilized in the same solvent) [39] or in a two-phase system (where the reduction is done at the interface between both , following a preliminary transfer of the AuCl4− anion in the same solvent as the stabilizing agent) [40]. Other modifications of these initial procedures allow, for instance, the control of the morphology of NPs: such is the case when CTAB (cetyltrimethylammonium bromide) is used as a stabilizing agent, resulting in the formation of [41]. Different sources of reduction may also be used instead of the usual chemical reducing agent, NaBH4. (c)-irradiation is thus able to produce NPs of different morphologies, determined by the experimental process [42]. Electrochemical [43] and photochemical (by UV irradiation) [44] procedures are other alternatives for the reduction process [45]. More recently, ultrasonication has also been used as a reduction method for gold ions [46,47].

3. Prerequisite for Functionalizing Agents

3.1. Definition The term “functionalization” is not precisely defined in the literature. Different notions can be underly this term: definition of this term, role (interest of the functionalization), or grafting mode (way how to functionalize the particles). Hence, in the case of a metallic oxocluster, functionalization simply Nanomaterials 2020, 10, 548 4 of 36

Nanomaterialsmeans to2020 link, 10, in x FOR a covalent PEER REVIEW way an organic function to the oxocluster [48]. But in the case of4 Au-NPs, of 34 Nanomaterialstheir existence 2020, 10 and, x FOR stability PEER REVIEW are mostly due to the organic function, ensuring only a role of stabilizing4 of 34 or anagent entity (See able Scheme to exhibit3). Herein, specific we properties define the in words, addition “functionalizing to the simple agent” ability as to a stabilize molecule particles. or an entity or an entity able to exhibit specific properties in addition to the simple ability to stabilize particles. Theseable properties to exhibit specificcan be a properties specific organic in addition function to the simple(as an abilityalcohol to or stabilize an amine particles. function These or propertiesother These properties can be a specific organic function (as an alcohol or an amine function or other groups)can be[49,50 a specific], an organicadditional function physico-chemical (as an alcohol properties or an amine (such function as optical or other or electrochemical groups) [49,50], an groups) [49,50], an additional physico-chemical properties (such as optical or electrochemical properties)additional [51 physico-chemical], or lead to aproperties specific application (such as optical such or electrochemicalas in biomedicine properties) by grafting [51], or leadof a to a properties) [51], or lead to a specific application such as in biomedicine by grafting of a chemotherapeuticspecific application agent such [52]. as in biomedicine by grafting of a chemotherapeutic agent [52]. chemotherapeutic agent [52].

Scheme 3. Functionalization of the gold core. Scheme 3. Functionalization of the gold core. Scheme 3. Functionalization of the gold core. The possibility to combine intrinsic properties of the coating agent (functionalizing agent) with The possibility to combine intrinsic properties of the coating agent (functionalizing agent) with theThe intrinsic possibility properties to combine of the goldintrinsic core properties can generate of the innovative coating agent advanced (functionalizing materials with agent) promising with the intrinsic properties of the gold core can generate innovative advanced materials with promising theapplications intrinsic properties expected of in the many gold fields, core can including generate optics, innovative electronics, advanced catalysis, materials sensors, with biology,promising and applications expected in many fields, including optics, electronics, catalysis, sensors, biology, and applicationsothers. Another expected important in many point fields, concerns including the properties optics, electronics, of the functionalizing catalysis, sensors, agent. In biology, order to and keep others. Another important point concerns the properties of the functionalizing agent. In order to keep others.the stability Another of important NPs after functionalization,point concerns the the properties functionalizing of the functionalizing agent must induce agent. su ffiIncient order repulsions to keep the stability of NPs after functionalization, the functionalizing agent must induce sufficient thebetween stability NPs of as NPs a simple after stabilizing functionalization, agent could the do. functionalizing Another important agent point must also consistsinduce ofsufficient avoiding repulsions between NPs as a simple stabilizing agent could do. Another important point also consists repulsionsa modification between of theNPs size as a and simple the morphologystabilizing agent of the could final do. NPs Another after functionalization important point also of preformed consists of avoiding a modification of the size and the morphology of the final NPs after functionalization of ofnanoparticles avoiding a modification by ligand exchange. of the size and the morphology of the final NPs after functionalization of preformed nanoparticles by ligand exchange. preformedIn the nanoparticles case of the directby ligand synthesis exchange. of gold nanoparticles in the presence of the functionalizing In the case of the direct synthesis of gold nanoparticles in the presence of the functionalizing agent,In the the case goal of is tothe directly direct getsynthesis the most of monodispersedgold nanoparticles NPs in and the the presence most stable of the colloidal functionalizing solution by agent, the goal is to directly get the most monodispersed NPs and the most stable colloidal solution agent,optimizing the goal the is experimentalto directly get conditions. the most monodi In this review,spersed an NPs overview and the of most all inorganic stable colloidal molecular solution entities by optimizing the experimental conditions. In this review, an overview of all inorganic molecular by(IME) optimizing reported the as experimental functionalizing conditions. agents is presented.In this review, an overview of all inorganic molecular entities (IME) reported as functionalizing agents is presented. entities (IME) reported as functionalizing agents is presented. 3.2. Method to Functionalize Au◦-Nanoparticles 3.2. Method to Functionalize Au°-Nanoparticles 3.2. MethodIntroduction to Functionalize of the functionalizing Au°-Nanoparticles agent onto the gold surface can be realized according to two Introduction of the functionalizing agent onto the gold surface can be realized according to two diffIntroductionerent procedures. of the functionalizing The first process agent corresponds onto the gold to direct surface functionalization can be realized byaccording the only to usetwo of different procedures. The first process corresponds to direct functionalization by the only use of the differentthe functionalizing procedures. agent;The first the process second corresponds one by use ofto adirect mixture functionalization of the functionalizing by the only agent use with of the the functionalizing agent; the second one by use of a mixture of the functionalizing agent with the functionalizingstabilizing agent. agent; In thethe last second case—and one by to use avoid of thea mixture formation of ofthe two functionalizing types of NPs (theagent first with batch the of stabilizing agent. In the last case—and to avoid the formation of two types of NPs (the first batch of stabilizingNPs stabilized agent. only In the with last the case—and stabilizing to agent,avoid SAthe, formation and the second of two one types with of the NPs functionalizing (the first batch agent, of NPs stabilized only with the stabilizing agent, SA, and the second one with the functionalizing agent, NPsFA )—itstabilized is necessary only with to use the similarstabilizing anchorage agent, SA groups, and forthe both second agents. one with In this the process, functionalizing reduction agent, occurs FA)—it is necessary to use similar anchorage groups for both agents. In this process, reduction occurs FAin)—it the is presence necessary of FA to useor in similar the presence anchorage of mixed groupsFA /forSA bothwith agents. the desired In this ratio. process, Another reduction type of occurs process in the presence of FA or in the presence of mixed FA/SA with the desired ratio. Another type of inconsists the presence of the post-functionalizationof FA or in the presence of preformedof mixed FA NPs/SA (See with Scheme the desired4). ratio. Another type of process consists of the post-functionalization of preformed NPs (See Scheme 4). process consists of the post-functionalization of preformed NPs (See Scheme 4).

Scheme 4. The functionalizing agent (FA) is grafted to the extremity of the long alkyl chain that Schemeensuring 4. The steric functionalizing repulsion. agent (FA) is grafted to the extremity of the long alkyl chain that Scheme 4. The functionalizing agent (FA) is grafted to the extremity of the long alkyl chain that ensuring steric repulsion. ensuringThis process steric repulsion. can occur following two methods: by ligand exchange of SA by FA [18,53], and by reactivityThis process onto can Au-NPs occur following [54]. In the two last methods: case, prefunctionalized by ligand exchange NPs of SA are by synthesized FA [18], [53 and], and then by the This process can occur following two methods: by ligand exchange of SA by FA [18], [53], and by reactivityreaction onto is performedAu-NPs [54 onto]. In the the Au last◦-NP case, platform. prefunctionaliz As lasted point NPs requiring are synthesized different and approaches, then the the reactivity onto Au-NPs [54]. In the last case, prefunctionalized NPs are synthesized and then the reactionlength is ofperformed the spacer onto between the Au°-NP NPs and platform. the FA is As of cruciallast point importance requiring (See different Scheme approaches,5). the reaction is performed onto the Au°-NP platform. As last point requiring different approaches, the length of the spacer between NPs and the FA is of crucial importance (See Scheme 5). length of the spacer between NPs and the FA is of crucial importance (See Scheme 5).

Scheme 5. Chemistry directly realized onto prefunctionalized NPs. Scheme 5. Chemistry directly realized onto prefunctionalized NPs.

Nanomaterials 2020, 10, x FOR PEER REVIEW 4 of 34 or an entity able to exhibit specific properties in addition to the simple ability to stabilize particles. These properties can be a specific organic function (as an alcohol or an amine function or other groups) [49,50], an additional physico-chemical properties (such as optical or electrochemical properties) [51], or lead to a specific application such as in biomedicine by grafting of a chemotherapeutic agent [52].

Scheme 3. Functionalization of the gold core.

The possibility to combine intrinsic properties of the coating agent (functionalizing agent) with the intrinsic properties of the gold core can generate innovative advanced materials with promising applications expected in many fields, including optics, electronics, catalysis, sensors, biology, and others. Another important point concerns the properties of the functionalizing agent. In order to keep the stability of NPs after functionalization, the functionalizing agent must induce sufficient repulsions between NPs as a simple stabilizing agent could do. Another important point also consists of avoiding a modification of the size and the morphology of the final NPs after functionalization of preformed nanoparticles by ligand exchange. In the case of the direct synthesis of gold nanoparticles in the presence of the functionalizing agent, the goal is to directly get the most monodispersed NPs and the most stable colloidal solution by optimizing the experimental conditions. In this review, an overview of all inorganic molecular entities (IME) reported as functionalizing agents is presented.

3.2. Method to Functionalize Au°-Nanoparticles Introduction of the functionalizing agent onto the gold surface can be realized according to two different procedures. The first process corresponds to direct functionalization by the only use of the functionalizing agent; the second one by use of a mixture of the functionalizing agent with the stabilizing agent. In the last case—and to avoid the formation of two types of NPs (the first batch of NPs stabilized only with the stabilizing agent, SA, and the second one with the functionalizing agent, FA)—it is necessary to use similar anchorage groups for both agents. In this process, reduction occurs in the presence of FA or in the presence of mixed FA/SA with the desired ratio. Another type of process consists of the post-functionalization of preformed NPs (See Scheme 4).

Scheme 4. The functionalizing agent (FA) is grafted to the extremity of the long alkyl chain that ensuring steric repulsion.

This process can occur following two methods: by ligand exchange of SA by FA [18], [53], and by reactivity onto Au-NPs [54]. In the last case, prefunctionalized NPs are synthesized and then the Nanomaterials 2020, 10, 548 5 of 36 reaction is performed onto the Au°-NP platform. As last point requiring different approaches, the length of the spacer between NPs and the FA is of crucial importance (See Scheme 5).

SchemeScheme 5. Chemistry 5. Chemistry directly directly realized realized onto onto prefunctionalized prefunctionalized NPs. NPs. Nanomaterials 2020, 10, x FOR PEER REVIEW 5 of 34 The close proximity of both components can significantly alter the physical properties of FA (as The close proximity of both components can significantly alter the physical properties of FA (as the possible quenching of the luminescence of FA), or can influence the stability of the final composite. the possible quenching of the luminescence of FA), or can influence the stability of the final composite. For all these reasons, many studies of the literature used long thiolated alkyl chains, terminated by AF. For all these reasons, many studies of the literature used long thiolated alkyl chains, terminated by The group ensures a strong interaction with the gold surface and the long alkyl chain induces a AF. The thiol group ensures a strong interaction with the gold surface and the long alkyl chain good repulsion between NPs [55]. induces a good repulsion between NPs [55]. 3.3. Technical Characterization of Functionalized Au-NPs 3.3. Technical Characterization of Functionalized Au-NPs Characterization of NPs can be performed using different complementary techniques, each Characterization of NPs can be performed using different complementary techniques, each technique bringing its specific information (size, shape, composition of the sample, surface technique bringing its specific information (size, shape, composition of the sample, surface functionalization, charge, etc.). Methods to characterize NPs can also vary, depending on whether functionalization, charge, etc.). Methods to characterize NPs can also vary, depending on whether the ligand is charged or not. A complete characterization of the particles can only result from the the ligand is charged or not. A complete characterization of the particles can only result from the combination of several techniques. One of the most classical techniques to characterize Au-NPs is combination of several techniques. One of the most classical techniques to characterize Au-NPs is the the transmission electronic microscopy (TEM) for imagery with analysis by X-ray diffraction and transmission electronic microscopy (TEM) for imagery with analysis by X-ray diffraction and depending the sample coupled with energy dispersive X-ray to get a qualitative elementary analysis. depending the sample coupled with energy dispersive X-ray to get a qualitative elementary analysis. Another conventional technique is the UV-visible spectroscopy and for anisotropic forms near infra-red Another conventional technique is the UV-visible spectroscopy and for anisotropic forms near infra- spectroscopy [56]. Gold nanoparticles exhibiting a typical band named resonance plasmon band, red spectroscopy [56]. Gold nanoparticles exhibiting a typical band named resonance plasmon band, which is strongly dependent of the shape, the size, and the nature of the coating agent in the UV-visible, which is strongly dependent of the shape, the size, and the nature of the coating agent in the UV- and sometime the NIR region, position of this band can be detected by UV-visible spectroscopy [57]. visible, and sometime the NIR region, position of this band can be detected by UV-visible But in many cases, the resonance plasmon band can be hardly detected due to the fact that FA exhibit spectroscopy [57]. But in many cases, the resonance plasmon band can be hardly detected due to the visible or UV bands superimposing the plasmon band. Another method for an easy control of the size fact that FA exhibit visible or UV bands superimposing the plasmon band. Another method for an distribution is the dynamic light [58]. When FA is charged, zeta potential can constitute a easy control of the size distribution is the [58]. When FA is charged, zeta good technique to evaluate the hydrodynamic radius of NPs in solution [59]. 1H-NMR spectroscopy potential can constitute a good technique to evaluate the hydrodynamic radius of NPs in solution has also been used to characterize FA-capped Au-NPs (See Scheme6)[ 60]. The enhancement of [59]. 1H-NMR spectroscopy has also been used to characterize FA-capped Au-NPs (See Scheme 6) broader bands constituted a proof of coating. As an ultimate technique mainly used for small NPs [60]. The enhancement of broader bands constituted a proof of coating. As an ultimate technique (<2 nm), mass spectrometry, with the use of specific MALDI-TOF, is a powerful technique to analyze mainly used for small NPs (< 2 nm), mass spectrometry, with the use of specific MALDI-TOF, is a functionalized NPs [61,62]. powerful technique to analyze functionalized NPs [61,62]. i- ii-

Scheme 6. Example of technical methods to characterize smaller gold nanoparticles. (i)- by MALDI-TOF mass spectrometry, published in [61], ACS 2007 or (ii)- by 1H-NMR spectrometry, with figure showing inScheme (A), the 6. free Example ligand of and technical in (B), themethods same ligandto characterize surrounding smaller the gold Au-NPs. nanoparticles. From [60], ( RSCi)- by 2003. MALDI- TOF mass spectrometry, published in [61], ACS 2007 or (ii)- by 1H-NMR spectrometry, with figure Finally,showing other in A, etheffi cientfree ligand characterization and in B, the techniquessame ligand suchsurrounding as Fourier-transform the Au-NPs. From infrared [60], RSC spectroscopy 2003. (FT-IR) and surface-enhanced Raman spectroscopy (SERS) can be cited to characterize gold nanoparticlesFinally, [other62,63 ]. efficient characterization techniques such as Fourier-transform infrared spectroscopy (FT-IR) and surface-enhanced Raman spectroscopy (SERS) can be cited to characterize gold nanoparticles [62,63].

4. Functionalization by Inorganic Entities from P- Block Due to the covalent character of the bonds formed by the p-block elements of the periodic table, organic molecules are nearly exclusively prepared with these elements, but also with elements coming from columns V to VII. Few inorganic entities based on p-block elements have been used to functionalize Au-NPs and most of them are obtained under the form of clusters, like , carborane clusters or others. In this part, we describe in a first paragraph the functionalization of Au- NPs by these clusters and in a second one, we focus our attention on silica coatings to design

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4. Functionalization by Inorganic Entities from P- Block Due to the covalent character of the bonds formed by the p-block elements of the periodic table, organic molecules are nearly exclusively prepared with these elements, but also with elements coming from columns V to VII. Few inorganic entities based on p-block elements have been used to functionalize Au-NPs and most of them are obtained under the form of clusters, like fullerenes, carborane clusters or others. In this part, we describe in a first paragraph the functionalization of Au-NPs by these clusters and in a second one, we focus our attention on silica coatings to design functionalized Au◦core/SiO2shell nanocomposites. We end this subpart with a new area in full expansion that concerns the association of carbon nanotubes to Au-NPs.

4.1. Molecular Clusters

In the literature, three main molecular clusters have been extensively studied: fullerenes (C60), carborane clusters, and polyhedral oligomeric silsesquioxanes (POSS). We described herein their associations with Au-NPs, with a main part devoted to clusters.

4.1.1. Fullerene (C60) Clusters Fullerenes have been extensively studied since the discovery in 1985 of a new allotropic form of elemental carbon: C60 [64]. Due to their electronic, spectroscopic, and structural properties as well as their controllable functionalizations, C60 has been, among all existing fullerenes, the most widely used for the design of composite materials [65]. The first report concerning the association of C60 with Au-NPs was published to the best of our knowledge in 1998 by Mathias Brust [66]. In this work, C60 was used to mediate the aggregation of free gold nanoparticles in . Then, progress in the functionalization of C60 and the possibility to introduce only one covalent function onto C60 led to Au-NPs functionalized by fullerenes. In 2001, H. Fujihara et al. described the first thiolated fullerene-functionalized Au-NPs. In this nanocomposite, Au-NPs were co-stabilized by fullerene-thiol and octanethiol (See Scheme7)[ 67,68]. A similar approach was developed by K.G. Thomas et al. in 2002 by use of an alkyl chain between C60 and Au-NPs (Scheme7)[ 69], contrary to Y.-S. Shon et al. who used an aromatic aminomercaptophenol as a linker [70]. However, with Fujihara, fullerene- were grafted on the particles with other alkanethiols as co-stabilizing agents (C8H17SH or C12H25SH). Introduction of the fullerene-thiol moiety occurred via a ligand exchange process except for Shon, who tried the direct method by use of a mixture of C60-Ph-SH and C8H17SH. All these nanocomposites (Au-S-R-C60) were prepared for electrochemical or photoelectrochemical applications. Nanomaterials 2020, 10, x FOR PEER REVIEW 6 of 34 functionalized Au°core / SiO2shell nanocomposites. We end this subpart with a new area in full expansion that concerns the association of carbon nanotubes to Au-NPs.

4.1. Molecular Clusters

In the literature, three main molecular clusters have been extensively studied: fullerenes (C60), carborane clusters, and polyhedral oligomeric silsesquioxanes (POSS). We described herein their associations with Au-NPs, with a main part devoted to fullerene clusters.

4.1.1. Fullerene (C60) Clusters Fullerenes have been extensively studied since the discovery in 1985 of a new allotropic form of elemental carbon: C60 [64]. Due to their electronic, spectroscopic, and structural properties as well as their controllable functionalizations, C60 has been, among all existing fullerenes, the most widely used for the design of composite materials [65]. The first report concerning the association of C60 with Au- NPs was published to the best of our knowledge in 1998 by Mathias Brust [66]. In this work, C60 was used to mediate the aggregation of free gold nanoparticles in toluene. Then, progress in the functionalization of C60 and the possibility to introduce only one covalent function onto C60 led to Au- NPs functionalized by fullerenes. In 2001, H. Fujihara et al. described the first thiolated fullerene- functionalized Au-NPs. In this nanocomposite, Au-NPs were co-stabilized by fullerene-thiol and octanethiol (See Scheme 7) [67,68]. A similar approach was developed by K.G. Thomas et al. in 2002 by use of an alkyl chain between C60 and Au-NPs (Scheme 7) [69], contrary to Y.-S. Shon et al. who used an aromatic aminomercaptophenol as a linker [70]. However, with Fujihara, fullerene-thiols were grafted on the particles with other alkanethiols as co-stabilizing agents (C8H17SH or C12H25SH). Introduction of the fullerene-thiol moiety occurred via a ligand exchange process except for Shon, Nanomaterialswho tried the2020 direct, 10, 548 method by use of a mixture of C60-Ph-SH and C8H17SH. All these nanocomposites7 of 36 (Au-S-R-C60) were prepared for electrochemical or photoelectrochemical applications.

Scheme 7.7. SchemeScheme ofof oneone exampleexample ofof fullerenethiol-functionalizedfullerenethiol-functionalized goldgold .nanoparticle. ReproducedReproduced withwith permissionpermission fromfrom [[6969],], ACS,ACS, 2002.2002.

Other developments involving C60 and Au-NPs concern their combinations with Other developments involving C60 and Au-NPs concern their combinations with γ- γ-cyclodextrines [71] to form network aggregates or with porphyrins to design photovoltaic solar cells cyclodextrines [71] to form network aggregates or with porphyrins to design photovoltaic solar cells (See Scheme8)[ 72]. Recently, Au-NPs stabilized with fullerene—offering multiple binding modes [73] (See Scheme 8) [72]. Recently, Au-NPs stabilized with fullerene—offering multiple binding modes orNanomaterials functionalized 2020, 10 with, x FOR fulleropyrrolidine PEER REVIEW [74]—were reported. 7 of 34 [73] or functionalized with fulleropyrrolidine [74]—were reported.

Scheme 8. Scheme of nanocomposites based on the association of porphyrins and C60 onto Au-NPs Scheme 8. Scheme of nanocomposites based on the association of porphyrins and C60 onto Au-NPs surface. Reproduced with permission from [72], ACS 2003. surface. Reproduced with permission from [72], ACS 2003. 4.1.2. Carborane Clusters 4.1.2. Carborane Clusters Polyhedral carborane clusters have been extensively studied due to their potential applications in areasPolyhedral such as medicine, carborane catalysis, clusters and have materials been extensively science [75, 76studied]. Intense due etofforts their have potential also been applications devoted [ ] toin reachareas a such controlled as medicine, functionalization catalysis, [and77,78 materials]. In particular, science this 75,76 cluster. Intense can be directlyefforts have functionalized also been devoted to reach a controlled functionalization [77,78]. In particular, this cluster can be directly functionalized with one- or two-thiol functions through -B-SH bonding [79,80]. To date, only two articles describe the direct functionalization of Au-NPs by carboranethiol (See Figure 1). Both studies were done by Baše et al., in which they studied the nature of the interaction between these carboranethiol clusters and Au-NPs. Electrochemical properties of these nanocomposites were also investigated [81,82].

Figure 1. SE TEM images (A and B) of Au-NPs functionalized by the carboranethiol 1,12-(HS)2-1,12- C2B10H10 and the side view of space-filling models of same carboranethiol drawing in the capture on a gold (111) surface. Reproduced with permission from [82], ACS 2008.

The potential applications of carborane-based Au-NPs is extremely broad since examination of ion transport across biological membranes [83] or cancer applications [84] were notably developed.

4.1.3. POSS Clusters Polyhedral oligomeric silsesquioxanes (POSS) are clusters that have been intensively employed in materials sciences, particularly as mineral charge. They are considered nano-building blocks due to their diameters (1.5 nm), and are easily functionalized by one or several organic groups [45]. In

Nanomaterials 2020, 10, x FOR PEER REVIEW 7 of 34

Scheme 8. Scheme of nanocomposites based on the association of porphyrins and C60 onto Au-NPs surface. Reproduced with permission from [72], ACS 2003.

4.1.2. Carborane Clusters Polyhedral carborane clusters have been extensively studied due to their potential applications Nanomaterials 2020, 10, 548 8 of 36 in areas such as medicine, catalysis, and [75,76]. Intense efforts have also been devoted to reach a controlled functionalization [77,78]. In particular, this cluster can be directly withfunctionalized one- or two-thiol with one- functions or two-thiol through functions -B-SH bonding through [79 -B-SH,80]. To bonding date, only [79,80 two]. articles To date, describe only two the articlesdirect functionalization describe the direct of function Au-NPsalization by carboranethiol of Au-NPs (See by carboranethi Figure1). Bothol (See studies Figure were 1). done Both by studies Baše wereet al., indone which by theyBaše studiedet al., thein which nature they of the studied interaction the betweennature of these the carboranethiolinteraction between clusters these and Au-NPs.carboranethiol Electrochemical clusters and properties Au-NPs. of Electrochemica these nanocompositesl properties were of alsothese investigated nanocomposites [81,82 ].were also investigated [81,82].

Figure 1. SE TEM images (A and B) of Au-NPs functionalized by the carboranethiol 1,12-(HS)2-1,12- Figure 1. SE TEM images (A,B) of Au-NPs functionalized by the carboranethiol 1,12-(HS)2-1,12-C2B10H10C2B10H10 and the side view and of the space-filling side view of models space-filling of same models carboranethiol of same carboranethiol drawing in the drawing capture in [ ] onthe a capture gold (111) on asurface. gold (111) Repr surface.oduced Reproduced with permission with from permission 82 , ACS from 2008. [82 ], ACS 2008.

The potential applications of carborane-based Au Au-NPs-NPs is extremely broad since examination of ion transport across biological membranes [[83]83] oror cancercancer applicationsapplications [[84]84] werewere notablynotably developed.developed.

4.1.3. POSS POSS Clusters Polyhedral oligomeric silsesquioxanes (POSS) are clusters that have been intensively employed in materials sciences, particularly as mineral charge.charge. They are considered nano-building blocks due to their diameters (1.5 nm), and are easilyeasily functionalizedfunctionalized by one or severalseveral organicorganic groupsgroups [[4545].]. In fact, POSS were used for the first time as functionalizing agents by G. Schmid et al., who successfully introduced alkylthiol groups on the clusters. Another feature of that study was related to the nature of the nanoparticles which were Au55Cl6 clusters [85]. Another strategy was used by Rotello et al. who used electrostatic or hydrogen bonding interactions to link POSS to Au-NPs in order to obtain self-assemblies. For hydrogen bonding interaction, the approach consisted of the functionalization of Au-NPs by thymine functions (Thy-Au) and in the monofunctionalization of POSS by a diaminopyridine group (POSS-DAP) (See Scheme9). Both functions being complementary, the recognition process of POSS towards Au-Thy resulted in the generation of nanocomposites exhibiting spherical aggregation resulting from the crystalline packing of non-polar POSS-DAP [86]. The second study consisted to devise POSS with eight ammonium groups (octa-ammonium POSS®, OA-POSS), and to generate self-assemblies with carboxylate capped Au-NPs. Syntheses of these aggregates were performed, in both studies, with two sizes of Au-NPs [87]. Over the years, potential applications of POSS-based Au-NPs broadened, extending from colorimetric detection [88] to reduction reactions [89]. Nanomaterials 2020, 10, x FOR PEER REVIEW 8 of 34

fact, POSS were used for the first time as functionalizing agents by G. Schmid et al., who successfully introduced alkylthiol groups on the clusters. Another feature of that study was related to the nature of the nanoparticles which were Au55Cl6 clusters [85]. Another strategy was used by Rotello et al. who used electrostatic or hydrogen bonding interactions to link POSS to Au-NPs in order to obtain self- assemblies. For hydrogen bonding interaction, the approach consisted of the functionalization of Au- NPs by thymine functions (Thy-Au) and in the monofunctionalization of POSS by a diaminopyridine group (POSS-DAP) (See Scheme 9). Both functions being complementary, the recognition process of POSS towards Au-Thy resulted in the generation of nanocomposites exhibiting spherical aggregation resulting from the crystalline packing of non-polar POSS-DAP [86]. The second study consisted to devise POSS with eight ammonium groups (octa-ammonium POSS®, OA-POSS), and to generate self- Nanomaterialsassemblies 2020with, 10 carboxylate, 548 capped Au-NPs. Syntheses of these aggregates were performed, in9 both of 36 studies, with two sizes of Au-NPs [87].

Schema 9. Scheme illustrating POSS clusters (b) in interaction with Au°-nanoparticles, functionalized Scheme 9. Scheme illustrating POSS clusters (b) in interaction with Au◦-nanoparticles, functionalized byby thyminethymine throughthrough three-pointthree-point hydrogenhydrogen bondingbonding recognitionrecognition (a(a).). ReproducedReproduced withwith permissionpermission [ ] fromfrom [8686],, RSC,RSC, 2002.

4.2. SilicaOver Coating the years, potential applications of POSS-based Au-NPs broadened, extending from colorimetricDuring thedetection design [ of88] a to new reduction nanocomposite, reactions whatever [89]. its nature, two parameters are fundamental: colloidal stability of the resulting solution and ease of functionalization. Another important point 4.2. Silica Coating concerns the possible desorption of the ligands from Au-NPs. In this field, silica appears an excellent candidateDuring to preventthe design that of problem a new and nanocomposite, particles coalescence. whatever Moreover, its nature, silica two is chemicallyparameters inert, are opticallyfundamental: transparent, colloidal and stability easilyfunctionalizable. of the resulting Inso thislution context, and silica-Auease of functionalization. NPs have been developed Another forimportant numerous point applications concerns the [90 ].possible Due to thesedesorption different of the properties, ligands numerousfrom Au-NPs. efforts In were this devotedfield, silica to coatappears Au-NPs an excellent with silica candidate and to controlto prevent the that thickness problem of the and coating particles and coalescence. on different Moreover, form of NPs silica [91 is]. Mulvaneychemically and inert, Liz-Marz opticallyán transpar thus developedent, and an easily efficient functionalizable. way to prepare In Authis◦core context,/SiO2shell silica-Aunanocomposites NPs have ofbeen controllable developed thickness. for numerous The firstapplications step of the [90] synthesis. Due to these consisted different in functionalizing properties, numerous citrate-capped efforts Au-NPswere devoted with aminopropyl-trimethoxysilane.to coat Au-NPs with silica and to Aminecontrol functionsthe thickness ensuring of the acoating good interactionand on different with Auform◦ surface, of NPs.[91 a complete] Mulvaney coverage and Liz-Marzán of Au◦-surface thus developed by alkoxysilane an efficient groups way wasto prepare obtained, Au°core followed/SiO2shell bynanocomposites the condensation of controllable of siloxane thickness. groups. Finally,The first the step thickness of the synthesis of the silica consisted coating in wasfunctionalizing controlled bycitrate-capped the complementary Au-NPs addition with aminopropyl-trimeth of sodium silicate [92oxysilane.]. Since thisAmine initial functions report, many ensuring studies a weregood devotedinteraction to findwith alternatives Au° surface, to this a complete synthetic procedurecoverage of [93 Au°-surface,94], to control by the alkoxysilane optical properties groups of was the nanocompositesobtained, followed [95 by,96 the] or condensation to functionalize of siloxane silica shell groups. by polymers Finally, the [97 thickness] or by chromophores of the silica coating to get Nanomaterialsfluorescencewas controlled 2020, enhancement10 ,by x FOR the PEER complementar REVIEW (See Scheme y addition 10)[98]. of sodium silicate [92]. Since this initial report,9 of many 34 studies were devoted to find alternatives to this synthetic procedure [93,94], to control the optical properties of the nanocomposites [95,96] or to functionalize silica shell by polymers [97] or by chromophores to get fluorescence enhancement (See Scheme 10) [98].

Scheme 10. Scheme showing the method to coat Au -NP by SiO through a mercaptosilane group, Scheme 10. Scheme showing the method to coat Au°-NP◦ by SiO2 through2 a mercaptosilane group, with TEM image and UV-Vis figure of these Au /SiO , reproduced with permission from [96], with TEM image and UV-Vis figure of these Au°core◦core/SiO2shell2shell, reproduced with permission from [96], ACSACS 2007. 2007. An original device was reported by Xia et al. in 2003 [99]. Their device consisted to first perform An original device was reported by Xia et al. in 2003 [99]. Their device consisted to first perform an Au◦core/SiO2shell and then to functionalize its surface with a uniform polymer as a second shell (poly an Au°core/SiO2shell and then to functionalize its surface with a uniform polymer as a second shell (poly (benzyl methacrylate)). By treatment with an aqueous solution of HF, the SiO2 shell dissolved, leading (benzyl methacrylate)). By treatment with an aqueous solution of HF, the SiO2 shell dissolved, leading to a hollow bead with movable gold cores (See Scheme 11). to a hollow bead with movable gold cores (See Scheme 11).

Scheme 11. Schematic procedure used by Xia et al. to device PBzMA hollow beads containing movable gold cores (C), from Au@SiO2@PBzMA NP that realized from routes (A and B). The

representative SEM before HF etching (D) and TEM images of Au@SiO2@PBzMA particles (E) after HF etching. Reproduced with permission from [99], ACS 2003.

4.3. Combination Au-NPs and Carbon Nanotubes. As for fullerenes, carbon nanotubes, which were discovered in 1991 [100], have largely been studied for their unique structural, electrical, and mechanical properties, paving the way towards numerous practical applications [101]. In the specific case of these nanocomposites—and due to the difference of size—it is more obvious that it is the nanotube (NT) that is functionalized by Au-NPs. However, these composites are very promising materials for various applications, ranging from optic to electronic, or catalysis. [101,102]. Different strategies were investigated to decorate nanotubes by Au-NPs. One of the first methods consisted to reduce a AuIII salt onto the surface of carbon nanotubes by thermal decomposition [103]. However, the main developments concerned the electrostatic and covalent anchorage of gold nanoparticles on nanotubes. In all these approaches, a pre-functionalization of the NT surface by oxidative treatment with HNO3 or H2SO4-HNO3 was required, leading to the formation of carboxylic acid groups [104]. Then, conventional organic reactions could be performed onto NT’s surface due to the presence of these carboxylic groups or other functional groups. Another similar approach consisted to introduce amino groups on NT surface with boron nitride nanotubes (See Scheme 12) [105]. Another aspect concerning the presence of carboxylic acid groups is the modification of the electrostatic charge of the nanotube. Thus, the

Nanomaterials 2020, 10, x FOR PEER REVIEW 9 of 34

Scheme 10. Scheme showing the method to coat Au°-NP by SiO2 through a mercaptosilane group,

with TEM image and UV-Vis figure of these Au°core/SiO2shell, reproduced with permission from [96], ACS 2007.

An original device was reported by Xia et al. in 2003 [99]. Their device consisted to first perform an Au°core/SiO2shell and then to functionalize its surface with a uniform polymer as a second shell (poly Nanomaterials 2020, 10, 548 10 of 36 (benzyl methacrylate)). By treatment with an aqueous solution of HF, the SiO2 shell dissolved, leading to a hollow bead with movable gold cores (See Scheme 11).

SchemeScheme 11.11.Schematic Schematic procedure procedure used used by Xiaby etXia al. et to al. device to device PBzMA PBzMA hollow beadshollow containing beads containing movable

goldmovable cores gold (C), fromcores Au@SiO (C), from2@PBzMA Au@SiO NP2@PBzMA that realized NP that from realized routes (Afrom,B). Theroutes representative (A and B). SEM The beforerepresentative HF etching SEM (D )before and TEM HF imagesetching of (D Au@SiO) and TEM2@PBzMA images particles of Au@SiO (E) after2@PBzMA HF etching. particles Reproduced (E) after withHF etching. permission Reproduced from [99 ],with ACS permission 2003. from [99], ACS 2003.

4.3.4.3. Combination Au-NPs and Carbon Nanotubes.Nanotubes. AsAs forfor fullerenes,fullerenes, carboncarbon nanotubes,nanotubes, whichwhich werewere discovereddiscovered inin 19911991 [[100100],], havehave largelylargely beenbeen studiedstudied forfor theirtheir uniqueunique structural,structural, electrical,electrical, andand mechanicalmechanical properties,properties, pavingpaving thethe wayway towardstowards numerousnumerous practicalpractical applicationsapplications [[101101].]. In thethe specificspecific casecase ofof thesethese nanocomposites—andnanocomposites—and duedue toto thethe didifferencefference ofof size—itsize—it isis moremore obviousobvious thatthat itit isis thethe nanotubenanotube (NT)(NT) thatthat isis functionalizedfunctionalized by Au-NPs. However,However, thesethese compositescomposites areare veryvery promisingpromising materialsmaterials forfor variousvarious applications,applications, rangingranging fromfrom opticoptic toto electronic,electronic, biosensorsbiosensors oror catalysis.catalysis. [[101101,102,102].]. DiDifferentfferent strategiesstrategies werewere investigatedinvestigated toto decoratedecorate III nanotubesnanotubes byby Au-NPs.Au-NPs. One of the firstfirst methodsmethods consistedconsisted toto reducereduce aa AuAuIII salt ontoonto thethe surfacesurface ofof carboncarbon nanotubesnanotubes byby thermalthermal decompositiondecomposition [[103103].]. However,However, thethe mainmain developmentsdevelopments concernedconcerned thethe electrostaticelectrostatic and covalent anchorage of of gold gold nanopa nanoparticlesrticles on on nanotubes. nanotubes. In In all all these these approaches, approaches, a apre-functionalization pre-functionalization of of the the NT NT surf surfaceace by oxidative treatmenttreatment withwith HNOHNO33 oror HH2SO2SO44-HNO-HNO33 waswas required,required, leadingleading to to the the formation formation of carboxylic of carboxylic acid groupsacid groups [104]. Then,[104] conventional. Then, conventional organic reactions organic couldreactions be performed could be ontoperformed NT’s surface onto NT’s due tosurface the presence due to of the these presence carboxylic of these groups carboxylic or other functionalgroups or groups.other functional Another groups. similar approachAnother consistedsimilar approach to introduce consisted amino to groups introduce on NT amino surface groups with on boron NT nitridesurface nanotubeswith boron (See nitride Scheme nanotubes 12)[105 (See]. Another Scheme aspect12) [105 concerning]. Another theaspect presence concerning of carboxylic the presence acid groupsof carboxylic is the modificationacid groups is of the the modification electrostatic chargeof the ofelectrostatic the nanotube. charge Thus, of the the nanotube. anionic character Thus, the of NTs allowed the adsorption of cationic polyelectrolyte chains and subsequently the interaction with negatively charged Au-NPs [106,107]. Nanomaterials 2020, 10, x FOR PEER REVIEW 10 of 34 Nanomaterials 2020, 10, x FOR PEER REVIEW 10 of 34 Nanomaterials 2020, 10, 548 11 of 36 anionic character of NTs allowed the adsorption of cationic polyelectrolyte chains and subsequently anionic character of NTs allowed the adsorption of cationic polyelectrolyte chains and subsequently the interaction with negatively charged Au-NPs [106,107]. the interaction with negatively charged Au-NPs [106,107].

Scheme 12. (i) Schematic representation of decorative boron nitride nanotubes, functionalized by SchemeScheme 12. 12.(i) Schematic(i) Schematic representation representation of decorative of decorative boron bo nitrideron nitride nanotubes, nanotubes, functionalized functionalized by thiol by thiol pendant groups, by Au-NPs, i- route of synthesis and (ii) corresponding TEM image, reproduced pendantthiol pendant groups, groups, by Au-NPs, by Au-NPs, i- route ofi- route synthesis of synthesis and (ii) and corresponding (ii) corresponding TEM image, TEM reproducedimage, reproduced with with permission from [105], ACS 2004. permissionwith permission from [105 from], ACS [105 2004.], ACS 2004. AnotherAnother approach approach consisted consisted in in introducing introducing a a silica silica coating coating onto onto NTs NTs surface surface via via thiol thiol or or amino amino Another approach consisted in introducing a silica coating onto NTs surface via thiol or amino functionsfunctions [108 [108].]. This This silica silica coating coating can can be be functionalized functionalized in in a asecond second step, step, as as reported reported by by Bottini Bottini et et al. al. functions [108]. This silica coating can be functionalized in a second step, as reported by Bottini et al. (See(See Scheme Scheme 13) 13)[ [109,110109,110]].. (See Scheme 13) [109,110].

SchemeScheme 13. 13. SchematicSchematic illustration illustration of a of non-covalent a non-covalent functionalizat functionalizationion of CNTs of CNTs comprising comprising (1) Scheme 13. Schematic illustration of a non-covalent functionalization of CNTs comprising (1) polymer(1) polymer wrapping wrapping using using poly(sod poly(sodiumium 4-styrenesulfonate) 4-styrenesulfonate) (PSS), (PSS), (2) ( 2)self-assembly self-assembly of of poly- poly- polymer wrapping using poly(sodium 4-styrenesulfonate) (PSS), (2) self-assembly of poly- (diallyldimethylammonium(diallyldimethylammonium chloride) chloride) (PDDA), ((3)) nanoparticlenanoparticle depositiondeposition and and ( 4(4)) SEM SEM image image (top) (top) of (diallyldimethylammonium chloride) (PDDA), (3) nanoparticle deposition and (4) SEM image (top) ofone one monolayer monolayer of Au@SiOof Au@SiO2 nanoparticles2 nanoparticles assembled assembled onto onto a carbon a , nanotube, reproduced reproduced from from [108], of one monolayer of Au@SiO2 nanoparticles assembled onto a carbon nanotube, reproduced from [108RSC], 2006.RSC 2006. [108], RSC 2006. An alternative and elegant approach consisted to used π–π stacking interactions of aromatic An alternative and elegant approach consisted to used π−π stacking interactions of aromatic moleculesAn suchalternative as pyrene and toelegant functionalize approach NTs. consisted This strategyto used π−π was stacking used by interactions Huang et al. of witharomatic a molecules such as pyrene to functionalize NTs. This strategy was used by Huang et al. with a 1- 1-pyrene-methylaminemolecules such as pyrene as linker to betweenfunctionalize NTs andNTs. Au-NPs This strategy [111]. was used by Huang et al. with a 1- pyrene-methylamine as linker between NTs and Au-NPs [111]. pyrene-methylamine as linker between NTs and Au-NPs [111]. 5. Functionalization by Polyoxometalate Compounds (POM) 5. Functionalization by Polyoxometalate Compounds (POM) 5. Functionalization by Polyoxometalate Compounds (POM) Smaller than gold nanoparticles but lying between the colloidal and the molecular range, the polyoxometalates (POM) species based on Mo or W constitute a full class of nanobuilding blocks. These

Nanomaterials 2020, 10, x FOR PEER REVIEW 11 of 34

NanomaterialsSmaller2020 than, 10, 548gold nanoparticles but lying between the colloidal and the molecular range,12 ofthe 36 polyoxometalates (POM) species based on Mo or W constitute a full class of nanobuilding blocks. Thesemetallic metallic oxo-clusters oxo-clusters play a greatplay rolea great in many role areas.in many Their areas. applications Their applications are due to theare combination due to the combinationof several value-adding of several properties, value-adding and to properties, their ability and to behave to their as fully ability oxidized to /(photo)-reduciblebehave as fully [ ] oxidized/(photo)-reduciblecompounds [112]. compounds 112 . Due to their anionic charge, main links between POMs and metallic nanoparticles are conducted by electrostatic interactions. In In this this case, POMs play protecting-ligand shell role surround metallic [ ] nanoparticles [113–116113–116].. The The development of of organic-inorganic hybrids derivatives POMs allowed another class of covalent POMs surrounding metall metallicic nanoparticles. These These organic-inorganic organic-inorganic hybrids derivatives POMsPOMs areare designed designed from from a lacunara lacunar POM, POM, containing containing a surface a surface of more of nucleophilicmore nucleophilic oxides, [ ] oxides,on which on organosilyl which organosilyl groups (RSi groups (OR) (RSitype), (OR) can3 type), ensure can a covalent ensure a link covalent W-O-Si link [117 W-O-Si]. Mayer 117 et al.. Mayer [118] [ ] 3 etused al. this118 hybrid used this POM, hybrid to introduce POM, to thiolintroduce groups. thiol Using grou thisps. Using strategy, this gold strategy, nanoparticles gold nanoparticles are covalently are covalentlysurrounded surrounded by hybrid POM. by hybrid And thePOM. mercaptoorganosilyl And the mercaptoorganosilyl group ensures group the link ensures between the nanoparticleslink between [ ] nanoparticlesand POMs [117 and]. OtherPOMs similar 117 . Other developments similar developments have been later have reported been later by Shweta reported et al.by [Shweta119]. et al. [ ] 119 .Di fferent strategies have also been used to modify POMs such as the functionalization of the POMDifferent core by organo-aminostrategies have groups, also been reported used into 2019modify by thePOMs Leroy’s such Group as the [120functionalization]. In this case, POMsof the [ ] POMcould core be used by organo-amino as a reducing andgroups, coating reported agent in to 2019 design by the these Leroy’s gold nanocomposites.Group 120 . In this Another case, POMs route couldconsists be inused using as aa reducedreducing polyoxovanadate and coating agent functionalized to design these with gold bisphosphonate nanocomposites. molecules Another in orderroute consiststo prepare in using in a single a reduced step hybridpolyoxovanadate organic–inorganic functionalized polyoxometalate with bisphosphonate decorated goldmolecules nanoparticles. in order Theseto prepare new in nanocomposites a single step hybrid were shownorganic–inorganic to strongly inhibitpolyoxometalate P. aeruginosa decorated and S. epidermidisgold nanoparticles. biofilm Thesegrowth new (See nanocomposites Scheme 14)[121 were]. shown to strongly inhibit P. aeruginosa and S. epidermidis biofilm growth (See Scheme 14) [121].

Scheme 14.14.Schematic Schematic representation representation of CitNPs, of CitNPs, CitNPs@POVred CitNPs@POVred and NPs@POV and NPs@POV synthesis, reproducedsynthesis, withreproduced permission with frompermission [121], RSC from 2019. [121], RSC 2019.

6. Functionalization Functionalization by Organometallic Complexes Functionalization of Au-NPs by organometallic (OM) complexes is predominantly trusted by ferrocene complexes due to their electronic properties valuated mainly in redoxredox basedbased sensors.sensors. But other OM based on ruthenium or rhodium as metals have also been grafted on ontoto Au-NPs for catalysis applications asas recentlyrecently summarized summarized by by Wilton-Ely Wilton-Ely [122 [122]. In]. In this this part, part, we we will will review review the ditheff erentdifferent OM OMcomplexes complexes and extendand extend our overviewour overview to metallodendritic to metallodendritic complexes. complexes. 6.1. Single Ferroncenyl Complexes 6.1. Single Ferroncenyl Complexes Brust’s method has considerably contributed to gold chemistry and particularly to the Brust’s method has considerably contributed to gold chemistry and particularly to the functionalization of Au-NPs by various groups. Notably, this method enabled the development functionalization of Au-NPs by various groups. Notably, this method enabled the development of of ferrocenated NPs, by use of ferrocene substituted thiols acting as functionalizing agents. For the ferrocenated NPs, by use of ferrocene substituted thiols acting as functionalizing agents. For the direct synthesis of Au-NPs, various linkers have been tested, such as ferrocenylhexanethiol (See direct synthesis of Au-NPs, various linkers have been tested, such as ferrocenylhexanethiol (See Scheme 15)[123]. This molecule exhibited a linker of sufficient length to induce a good control of the Scheme 15) [123]. This molecule exhibited a linker of sufficient length to induce a good control of the size monodispersity of NPs [124]. Linkers containing aromatic groups have also been used such as the ferrocenethiophenol group [125–127].

Nanomaterials 2020, 10, x FOR PEER REVIEW 12 of 34 Nanomaterials 2020, 10, x FOR PEER REVIEW 12 of 34 size monodispersity of NPs [124]. Linkers containing aromatic groups have also been used such as [ ] thesizeNanomaterials ferrocenethiophenol monodispersity2020, 10, 548 of group NPs [124125–127]. Linkers. containing aromatic groups have also been used such13 of as 36 the ferrocenethiophenol group [125–127].

Scheme 15. Scheme of a typical ferrocenethiophenol, figure reproduced with permission from [124] ACSScheme 2007. 15. Scheme of a typical ferrocenethiophenol, figure figure reproduced withwith permission from [124] ACS 2007. ACS 2007. The other strategy to functionalize Au-NPs consists in the ligand place-exchange reaction. The other strategy to functionalize Au-NPs consists in the ligand place-exchange reaction. Murray MurrayThe and other co-workers strategy wereto functionalize one of the Au-NPsfirst research consists groups in the to ligand use thisplace-exchange procedure forreaction. the and co-workers were one of the first research groups to use this procedure for the heterofunctionalization heterofunctionalizationMurray and co-workers of wereAu-NPs one ofwith the varifirstous research functional groups groups.to use thisIn procedurea first study,for the of Au-NPs with various functional groups. In a first study, ferrocenyloctanethiol was thus exchanged ferrocenyloctanethiolheterofunctionalization was thusof exchangedAu-NPs with with differentvarious alkanethiolsfunctional [groups.128]. In a Insecond, a first the samestudy, with different alkanethiols [128]. In a second, the same group devised mono-functionalized ferrocenated groupferrocenyloctanethiol devised mono-functionalized was thus exchanged ferrocenated with Au-NPs different by alkanethiols use of ferrocenyl [128] . octanethiolIn a second, [129 the]. Ansame Au-NPs by use of ferrocenyl octanethiol [129]. An alternative to this procedure was recently developed alternativegroup devised to this mono-functionalized procedure was recentlyferrocenated developed Au-NPs by useAstruc of ferrocenyl et al., octanethiolconsisting [in129 the]. An by Astruc et al., consisting in the functionalization of Au-NPs by cross olefin metathesis. In that recent functionalizationalternative to thisof Au-NPs procedure by crosswas olefinrecently metathesis. developed In bythat Astrucrecent procedure,et al., consisting Au-NPs in are the procedure, Au-NPs are prefunctionalized by alkene-terminated groups (methyl acrylate). Then, the prefunctionalizedfunctionalization by of alkene-terminated Au-NPs by cross groups olefin (metmetathesis.hyl acrylate). In that Then, recent the procedure,cross-metathesis Au-NPs of aare cross-metathesis of a ferrocenylmethyl acrylate and alkene-substituted Au-NPs using Grubbs’ catalyst, ferrocenylmethylprefunctionalized acrylate by alkene-terminated and alkene-substituted groups (met Au-NPshyl acrylate). using Then,Grubbs’ the catalyst,cross-metathesis gave the of a gave the nanocomposite [130]. nanocompositeferrocenylmethyl [130 ].acrylate and alkene-substituted Au-NPs using Grubbs’ catalyst, gave the Most of the ferrocenated Au-NPs were designed for electrochemical applications. However, nanocompositeMost of the ferrocenated[130]. Au-NPs were designed for electrochemical applications. However, parallel to this first research topic, ferrocenated Au-NPs were also prepared for sensors parallelMost to this of thefirst ferrocenated research topic, Au-NPs ferrocenated were designe Au-NPsd for were electrochemical also prepared applications. for redox However,sensors applications towards H2PO4− and HSO4− anions. One of the first researchers to develop this applicationsparallel to towards this first H 2researchPO4- and HSOtopic,4- anions.ferrocenated One of Au-NPs the first researcherswere also toprepared develop for this redox application sensors application was Astruc and co-workers, in 2000, who used amidoferrocenyldodecanethiol[ groups] [131]. wasapplications Astruc and towards co-workers, H2PO 4in- and 2000, HSO who4- anions. used Oneamidoferrocenyldodecanethiol of the first researchers to develop groups this 131 application. They They notably designed amidoferrocenated groups to check the recognition properties of these different notablywas Astruc designed and amidoferrocenated co-workers, in 2000, groups who usedto chec amidoferrocenyldodecanethiolk the recognition properties ofgroups these [different131]. They ferrocenated Au-NPs [132]. The recognition process was based on a double hydrogen-bonding ferrocenatednotably designed Au-NPs amidoferrocenated [132]. The recognition groups process to chec wask the based recognition on a doubleproperties hydrogen-bonding of these different interaction between the amido group of the amidoferrocenium and the anion. In Scheme 16, the interactionferrocenated between Au-NPs the amido[132]. Thegroup recognition of the amidof processerrocenium was based and on the a anion.double In hydrogen-bonding Scheme 16, the ferrocenated NPs used in this study is presented [133]. ferrocenatedinteraction NPsbetween used thein this amido study group is presented of the amidof[133]. errocenium and the anion. In Scheme 16, the ferrocenated NPs used in this study is presented [133].

- SchemeScheme 16. 16. SchemeScheme of ofamidoferrocenated-Au amidoferrocenated-Au NPs NPs and and the the recognition recognition of of H H2PO2PO4 4anions,− anions, reproduced reproduced [ ] withwithScheme permission permission 16. Scheme from from of133 [ 133amidoferrocenated-Au ACS] ACS 2002. 2002. NPs and the recognition of H2PO4- anions, reproduced with permission from [133] ACS 2002. ToTo improve improve the the redox redox properties properties or orthe the sensitivit sensitivityy in inanions anions sensing, sensing, different different poly-ferrocene poly-ferrocene complexescomplexesTo improve have have been the been proposed.redox proposed. properties The simplest or The the simplestsensitivitone was y onebiferrocene. in anions was biferrocene.sensing, Nishirada different et al. Nishirada poly-ferrocenesynthesized et al. biferrocene-terminatedcomplexessynthesized have biferrocene-terminated been alkanethiol proposed. toThe functionalize alkanethiolsimplest one Au-NPs to was functionalize biferrocene. and analyzed Au-NPs Nishirada their and electrodeposition et analyzedal. synthesized their [134–136biferrocene-terminatedelectrodeposition]. Another study [134–136 combinedalkanethiol]. Another ferrocenyl to functionalize study and combined biferrocenyl Au-NPs ferrocenyl andgroups an andalyzed with biferrocenyl terpyridine their electrodeposition groupsligands withto generate[terpyridine134–136 a ]biferrocenyl-functionalized. Another ligands tostudy generate combined a biferrocenyl-functionalized rutheniumferrocenyl and(II) complex-Au-NPsbiferrocenyl ruthenium groups for (II) withredox complex-Au-NPs terpyridine applications ligands for [137 redox]. to Then,generateapplications to perform a biferrocenyl-functionalized [137 the]. recognition Then, to perform of H2PO theruthenium4- anion, recognition Astruc (II) complex-Au-NPs ofet Hal2. POsynthesized4− anion, for a Astruc dendronredox etapplications al.bearing synthesized three [137 ]. amidoferrocenyl,Then,a dendron to perform bearing or the three three recognition silylferrocenyl amidoferrocenyl, of H2 POgroups.4- anion, or threeThese Astruc silylferrocenyl three et ferrocenylal. synthesized groups. dendrons a Thesedendron were three bearingintroduced ferrocenyl three ontoamidoferrocenyl,dendrons Au-NPs were surface introduced or bythree a ligand ontosilylferrocenyl Au-NPs place-exchange surface groups. by processThese a ligand three [ place-exchange138 ]ferrocenyl. In particular, dendrons process they [138 wereextended]. In introduced particular, this approachontothey extendedAu-NPs to a larger surface this approachmetallodendr by a ligand to aon larger place-exchange containing metallodendron up process to nine containing [ 138ferroc]. Inenyl upparticular, tounits. nine In ferrocenylthey this extended way, units. they this In designedapproachthis way, ferrocenyl-dendron-Au-NPs theyto a designedlarger metallodendr ferrocenyl-dendron-Au-NPs onexhibiting containing 360 ferrocenylup exhibiting to nine units ferroc 360 (for ferrocenylenyl the largestunits. units In dendron) this (for theway, at largest thethey peripherydesigneddendron) of ferrocenyl-dendron-Au-NPs at the the nanocomposites. periphery of the With nanocomposites. these exhibiting nano composites,360 With ferrocenyl these various nanocomposites, units (foranions the couldlargest various be dendron) recognized, anions couldat the peripherybe recognized, of the such nanocomposites. as the well-known With these Adenosine-5 nanocomposites,0-triphosphate various [139 anions,140]. could Recycling be recognized, of organic compounds is an active research field and ferrocenyl-based Au-NPs have been notably used as catalysts

Nanomaterials 2020, 10, x FOR PEER REVIEW 13 of 34

Nanomaterials 2020, 10, 548 14 of 36 such as the well-known Adenosine-5’-triphosphate [139,140]. Recycling of organic compounds is an active research field and ferrocenyl-based Au-NPs have been notably used as catalysts for recycling for4-nitrophenol. recycling 4-nitrophenol. [141]. But recent [141]. advances But recent on advances surface onfunctionalization surface functionalization has recently has been recently obtained been obtainedwith ferrocenyl-Au with ferrocenyl-Au NPs for which NPs for a deformation which a deformation of the organic of the coating organic was coating observed. was observed. As a result, As a result,stable layer a stable or ferrocenyl-Au layer or ferrocenyl-Au NPs adsorbed NPs adsorbed at the metal at the surface metal could surface be could obtained be obtained [142]. [142].

6.2. Variety of Organometallics-Au-NPs. Examples ofof Au-NPs Au-NPs functionalized functionalized by organometallicby organometallic complexes complexes otherthan other ferrocene than ferrocene are relatively are scarce.relatively In thisscarce. field, In the this first field, example the first of this example category of is this a tetranuclear category is iron a complex.tetranuclear Relatively iron complex. close to 5 5 ferroceneRelatively in close structure, to ferrocene this organometallic in structure, complex this organometallic [Fe(η -C5H5)3( µcomplex3-CO)4(η [Fe(-C5Hη54-CCONH(CH5H5)3(µ3-CO)2)114SH](η5- wasC5H4 usedCONH(CH as redox2)11 sensorsSH] was for used two as typical redox sensors phosphate for two anions typical (ATP phosphate and H2PO anions4−). Palladium (ATP and (II) H2PO OM4- complexes). Palladium have (II) OM large complexes catalytic properties. have large Fratoddicatalytic properties. et al. recently Fratoddi synthesized et al. recently an OM Pdsynthesized (II) thiol complexan OM Pd in (II) which thiol the complex thiol function in which was the directly thiol function linked towas the directly Pd (II) center.linked to This the nanocomposite Pd (II) center. This was preparednanocomposite via the was direct prepared functionalization via the direct process functionalization (Brust’s method) process [143]. For(Brust’s applications method) in [143 catalysis,]. For someapplications OMs based in catalysis, on Ru (IIIsome or OMs II) or based Rh (I) on as Ru metals (III or were II) or synthesized Rh (I) as metals [144 ].were Dinuclear synthesized ruthenium [144]. complexesDinuclear ruthenium were prepared complexes with two were or prepared four alkylthiol with tw pendanto or four groups alkylthiol to ensure pendant a strong groups grafting to ensure of OMa strong onto grafting Au-NPs of surface OM onto [145 Au-NPs,146]. The surface rhodium [145,146 OM]. complex The rhodium was mononuclear OM complex andwas wasmononuclear linked to Au-NPsand was bylinked an amidododecanethiol to Au-NPs by an amidododecanethiol group (See Scheme group 17)[145 (See]. Scheme 17) [145].

Scheme 17.17. Route of functionalization of Au-NPs by trinucleartrinuclear rutheniumruthenium complexes,complexes, reproduced with permission from [[145145]] ACS 2006.2006. 7. Functionalization by Coordination Complexes from D-Block Elements 7. Functionalization by Coordination Complexes from D-Block Elements 7.1. Prussian Blue Derivatives 7.1. Prussian Blue Derivatives Several nanocomposites resulting from the association of gold nanoparticles with Prussian blue derivativesSeveral have nanocomposites been reported resulting in the literature. from the Various association strategies of gold have nanoparticles been developed with to Prussian generate blue the nanocomposite.derivatives have Thebeen first reported is based in onthe a literature. one-step procedure Various strategies combining have an electrochemicallybeen developed to controlled generate generationthe nanocomposite. of gold nanoparticles The first is based concomitant on a one- withstep the procedure electrochemical combining formation an electrochemically of Prussian blue (PB)controlled [147]. generation Such a modified of gold electrode nanoparticles has been concomitant successfully with used the for electrochemical catalyzing the formation reduction of Prussian blue (PB) [147]. Such a modified electrode has been successfully used for catalyzing the hydrogen peroxide and for the amperometric detection of H2O2 with a nanomolar sensitivity. PB@Au nanoparticlesreduction of withhydrogen a diameter peroxide ranging and from for 20the to amperometric 50 nm were thus detection obtained of (See H2 SchemeO2 with 18 a). nanomolar Previously, thesensitivity. same type PB@Au of functionalization nanoparticles with had beena diameter performed ranging with from gold 20 particles to 50 nm stabilized were thus with obtained dendrimers (See (PAMAM:Scheme 18). polyamidoamine) Previously, the same by potential type of cyclingfunctionalization electrodeposition had been [148 performed]. In that case,with particlesgold particles were [ ] smallerstabilized (3 nm)with and dendrimers exhibited (PAMAM: a narrower polyamidoamine) size distribution. PB@Auby potential nanocomposites cycling electrodeposition can also be prepared 148 . byIn athat chemical case, particles procedure were [149 ].smaller Hence, (3 reduction nm) an ofd ferricexhibited ion ina waternarrower in the size presence distribution. of Fe(CN) PB@Auand [ ] 6 preformednanocomposites gold particlescan also resultedbe prepared in PB-functionalized by a chemical procedure particles with149 . an Hence, average reduction size of50 of nm.ferric Then ion formationin water in of the PB@Au-multilayer presence of Fe(CN) thin films6 and by preformed the Langmuir–Blodgett gold particles technique resulted resulted in PB-functionalized in a hydrogen peroxideparticles sensor.with an In average both cases size (chemical of 50 nm. or electrochemicalThen formation method), of PB@Au-multilayer functionalization thin of films the particles by the resultedLangmuir–Blodgett from an electrostatic technique interaction resulted in between a hydrogen PB and peroxide gold particles. sensor. In both cases (chemical or electrochemical method), functionalization of the particles resulted from an electrostatic interaction between PB and gold particles.

Nanomaterials 2020, 10, 548 15 of 36 NanomaterialsNanomaterials 2020 2020, 10,, x10 FOR, x FOR PEER PEER REVIEW REVIEW 14 of 14 34 of 34

CN CN CN CN NC NC CN CN NC NC CN CN Fe Fe Fe Fe NC NC CN CN NC NC CN CN N N N N

HS HS N N HS HS

3- 3- 3- 3- [Fe(CN)[Fe(CN)5PZT]5PZT] [Fe(CN)[Fe(CN)5L] 5L] with withL : 2 L or : 24-mercaptopyridine or 4-mercaptopyridine

SchemeScheme 18. 18.Prussian Prussian blue-modified blue-modified blue-modified complexes. complexes.

GraftingGrafting of of metalof metal metal complexes complexescomplexes on onAu on Au particles Au particles particles can can also can also be also realizedbe realized be realized using using linkers usinglinkers such linkers such as 2-pyrazin-as such2-pyrazin- as 2-pyrazin-2-ylethanethiol2-ylethanethiol2-ylethanethiol [150 [150], or], 2-or [ and2-150 and ],4-mercaptopyridine or 4-mercaptopyridine 2- and 4-mercaptopyridine [151 [151]. In]. theIn the[ last151 last ].case, In case, the aggregation aggregation last case, aggregationof theof the particles particles of thewaswas particles observed observed was by observed byuse use of ofthe by the 2-mercaptomyridine, use 2-mercaptomyridine, of the 2-mercaptomyridine, whereas whereas stable whereas stable particles stableparticles particleswere were obtained obtained were obtained with with 4- 4- withmercaptopyridine.mercaptopyridine. 4-mercaptopyridine. 7.2. Terpyridinyl Metallic Complexes 7.2.7.2. Terpyridinyl Terpyridinyl Metallic Metallic Complexes Complexes Complexed and none-complexed terpyridines have also been used to prepare stable ComplexedComplexed and and none-complexed none-complexed terpyridines terpyridines have have also also been been used used to toprepare prepare stable stable nanocomposites. As a first example, a thiol pendant group was used to anchor the terpyridine [152]. nanocomposites.nanocomposites. As Asa first a first example, example, a thiol a thiol pendant pendant group group was was used used to anchorto anchor the the terpyridine terpyridine [152 [152]. ]. In that work, stable particles were obtained at low concentration of terpyridine by ligand exchange on In thatIn that work, work, stable stable particles particles were were obtained obtained at loatw lo concentrationw concentration of terpyridineof terpyridine by byligand ligand exchange exchange weakly stabilized particles. Increase of the concentration resulted of a none-negligible interaction of on onweakly weakly stabilized stabilized particles. particles. Increase Increase of theof the concentration concentration resulted resulted of aof none-negligible a none-negligible interaction interaction the terpyridine moieties with the surface, aggregating the particles (See Scheme 19). of theof the terpyridine terpyridine moieties moieties with with the the surface, surface, aggregating aggregating the the particles particles (See (See Scheme Scheme 19). 19).

SchemeScheme 19. 19.SomeSome Some examples examples examples of of bis bisof-terpyridine -terpyridinebis-terpyridine Ru Ru Ru(II) (II) (II)complexes, complexes, complexes, with with with direct direct direct link link between between Au-NPs Au-NPs and and metallicmetallic complex complex or throughor through an alkylan alkyl chain. chain.

TheThe same same reaction, reaction, done done in the in the presence presence of anof an excessexce excess of ss zincof zinc triflate,triflate, triflate, afforded aff ordedafforded in allin all cases cases stable stable particles,particles, with with stabilization stabilization of the of the particles particles being being ensured ensured by by electrostatic electrostatic repulsions repulsions generated generated by by complexes.complexes. The The dramatic dramatic dramatic influence influence influence of of the theof the counterion counterion counterion on on onthe the the stabilization stabilization stabilization process process process was was was also also also evidenced. evidenced. evidenced. A thiolA thiol group group can can also also be introducedbe introduced on on the the terpyridineterpyrid terpyridineine core core by by use use of aliphaticof aliphatic spacers spacers of different diofff differenterent lengthlength [153 [153].]. After After]. After anchorage anchorage anchorage of of theof the the terpyridine terpyridine terpyridine on on onthe the the pa particles, rticles,particles, and and and addition addition addition of of differentof di ffdifferenterent cations, cations, suchsuch as Fe(II),as Fe(II), Zn(II), Zn(II), Cu(I), Cu(I), or Ag(I),or Ag(I), self-assemblyself-assemb self-assembly of ly theof the particles particles was was observed, observed, resulting resulting from from the the coordinationcoordination processprocess process betweenbetween between terpyridinesterpyridines terpyridines linked linked linked to to separated separatedto separated particles. particles. particles. During During During that that work,that work, work, Rotello Rotello Rotello et al.et al.et controlled al.controlled controlled interparticle interparticle interparticle spacing spacing spacing by by modification bymodification modification of theof theof length the length length of theof theof spacer the spacer spacer and and the and the stability the stability stability of the of of aggregatesthethe aggregates aggregates by changingby bychanging changing the the cation. the cation. cation. The The sameThe same same strategy strategy strategy was was was developed developed developed by by another byanother another groupgroup group toto prepareto prepare didifferentffdifferenterent superstructuressuperstructures superstructures by by use byus eus of ofe copolymers copolymersof copolymers as as template templateas template [154 [154]. [154] The. The]. sizeThe size ofsize of the theof aggregates the aggregates aggregates can can also can also be also controlledbe becontrolled controlled by theby bythe nature the nature nature of the of cationtheof the cation [cation155 ].[155 By [155] use. By]. of Byuse alkali use of alkaliof cations, alkali cations, acations, weak a weak coordination a weak coordination coordination of the cationof theof the wascationcation observed, was was observed, observed, resulting resulting inresulting large in 3D largein assembly.large 3D 3D assembly assembly After. dissociation After. After dissociation dissociation of these of aggregates theseof these aggregates aggregates with DMF, with with whichDMF, DMF, gavewhichwhich individual gave gave individual individual gold particles—and gold gold particles—and particles—and addition addition addition of Co of (II) Coof cation, Co (II) (II) cation, inducing cation, inducing inducing a strong a strong a coordination strong coordination coordination with thewithwith terpyridine—a the the terpyridine—a terpyridine—a highly highly dispersed highly dispersed dispersed 3D spherical 3D 3D sphe assembly sphericalrical assembly was assembly obtained. was was obtained. It wasobtained. the It first wasIt examplewas the the first offirst transformationexampleexample of oftransformation oftransformation a 3D nanonetwork. of ofa 3Da 3D Morenanonetwork. nanonetwork. recently, a More 2D More network recently, recently, was a prepared 2Da 2D network network according was was toprepared aprepared novel accordingaccording to toa novela novel procedure procedure [156 [156]. Thiol-substituted]. Thiol-substituted terpyridin terpyridine weree were first first deposited deposited as asa a monolayermonolayer onto onto the the oxide oxide surface surface of aof Si/SiO a Si/SiO2 substrate2 substrate by bythe the Langmuir-Blodgett Langmuir-Blodgett method. method. Then, Then,

Nanomaterials 2020, 10, 548 16 of 36 procedureNanomaterials [2020156,]. 10 Thiol-substituted, x FOR PEER REVIEW terpyridine were first deposited as a monolayer onto the 15 oxide of 34 surface of a Si/SiO substrate by the Langmuir-Blodgett method. Then, addition of Fe (II) led to the addition of Fe (II) 2led to the formation of the complex with terpyridine grafted on adjacent particles. formation of the complex with terpyridine grafted on adjacent particles. The influence of the conjugated The influence of the conjugated linkers on the conductivity of the network was investigated. Metal linkers on the conductivity of the network was investigated. Metal complexes can also be generated to complexes can also be generated to stabilize particles without creating inter-particle interactions. stabilize particles without creating inter-particle interactions. Hence, reaction of the grafted terpyridine Hence, reaction of the grafted terpyridine with an excess of Ru(terpyridine)Cl3 gave the with an excess of Ru(terpyridine)Cl gave the nanocomposite [157]. Then, a partial ligand-exchange nanocomposite [157]. Then, a partial3 ligand-exchange reaction on the former nanocomposite with reaction on the former nanocomposite with thio-substituted pyrrole led, after electropolymerization, to thio-substituted pyrrole led, after electropolymerization, to the first example of a polypyrrole the first example of a polypyrrole nanocomposite containing metal complexes and metal nanoparticles. nanocomposite containing metal complexes and metal nanoparticles. A similar functionalization of A similar functionalization of particles was performed with a rigid fully conjugated linker between particles was performed with a rigid fully conjugated linker between the metal complex and the the metal complex and the particles (See Scheme 20)[158]. Communication between particles and particles (See Scheme 20) [158]. Communication between particles and complexes was complexes was electrochemically evidenced by modification of the oxidation potential of Ru (II) in electrochemically evidenced by modification of the oxidation potential of Ru (II) in the metal the metal complex. As final nanocomposite, a preformed Ru (II) terpyridine complex was attached to complex. As final nanocomposite, a preformed Ru (II) terpyridine complex was attached to gold gold nanoparticles by ligand exchange to prepare a voltage-driven molecular switch [159]. This Ru (II) nanoparticles by ligand exchange to prepare a voltage-driven molecular switch [159]. This Ru (II) complex was also associated to a second metal complex (ferrocene) and the mutual influence of both complex was also associated to a second metal complex (ferrocene) and the mutual influence of both metal complexes grafted on particles was investigated by electrochemistry [159]. metal complexes grafted on particles was investigated by electrochemistry [159].

SchemeScheme 20.20. Au-NPsAu-NPs stabilizedstabilized byby terpyridiniumterpyridinium ligandsligands forfor aa post-complexation,post-complexation, reproducedreproduced withwith permissionpermission fromfrom [[158158]] RSCRSC 2006.2006.

7.3.7.3. Polypyridyls Metallic Complexes PolypyridylPolypyridyl metal complexes complexes based based on on bidentate bidentate ligands ligands have have been been extensively extensively used used as IME as IME for forAu Aunanoparticles. nanoparticles. They Theycan be can immobilized be immobilized onto Au-N ontoPs Au-NPs either by either electrostati by electrostaticc interaction interaction or by use orof an by anchoring use of an group anchoring [160,161 group]. Grafting [160,161 of]. electrochemical Grafting of electrochemical or optical probes or optical onto the probes surface onto of gold the surfacenanoparticles of gold is nanoparticles another active is anotherfield ofactive research. field Among of research. all existing Among metal all existing complexes metal complexescombining combiningelectrochemical electrochemical and luminescence and luminescence properties, properties, the well-known the well-known tris(bipyridine)rutheniumtris(bipyridine)ruthenium (II) is a (II)complex is a complex of choice. of choice.Functionalization Functionalization of gold nanoparticles of gold nanoparticles by bipyridine by bipyridine complexes complexes can be realized can be realizedaccording according to two different to two di strategies.fferent strategies. In the first, In themetal first, complexes metal complexes directly interact directly with interact the withparticles the particlesvia electrostatic via electrostatic interactions. interactions. Several Several reports reports concern concern the thedirect direct grafting grafting of of the the unmodifiedunmodified tristris(bipyridine)ruthenium(bipyridine)ruthenium (II) (II) complex, complex, its intrinsicits intrinsi propertiesc properties making making it a convenient it a convenient probe to probe evaluate to theevaluate possible the energypossible transfer, energy electrontransfer, transfer electron or transf enhanceder or enhanced intersystem intersyste crossingm ratecrossing constant rate betweenconstant 2+ thebetween gold nanoparticlesthe gold nanoparticles and the complex. and Particularly,the complex. a luminescence-quenchingParticularly, a luminescence-quenching of Ru(bipy)3 onto of gold nanoparticles2+ was reported by Murray et al. [162], metal surfaces being known to e ciently Ru(bipy)3 onto gold nanoparticles was reported by Murray et al. [162], metal surfaces beingffi known quenchto efficiently molecular quench excited molecular states ex (Seecited Scheme states 21 (See). Scheme 21).

Nanomaterials 2020, 10, 548 17 of 36 Nanomaterials 2020, 10, x FOR PEER REVIEW 16 of 34

Scheme 21. Au-NPs stabilized by tris(bipyridine) Ru (II) complexes for a light-induced processes inside

theseScheme Au◦ -(S-C7-Ru),21. Au-NPs reproduced stabilized by with tris permission(bipyridine) from Ru [(II)162 ],complexes ACS 2006. for a light-induced processes inside these Au°-(S-C7-Ru), reproduced with permission from [162], ACS 2006. Quenching of fluorescence has been studied with tiopronin-protected gold nanoparticles of differentQuenching diameters. of Afluorescence clear increase has in been quenching studied e ffiwiciencyth tiopronin-protected with the diameter gold core nanoparticles was evidenced, of adifferent successful diameters. quantification A clear of increase this phenomenon in quenching being efficiency possible, with thanks the diameter to a reversible core was electrostatic evidenced, bindinga successful between quantification the particles of and this the phenomenon fluorophore. bein By additiong possible, or not thanks of KCl to electrolyte a reversible in theelectrostatic solution, electrostaticbinding between interactions the particles between and NPs the and fluorophore. the complex wereBy addition modified, or withnot theof KCl electrolyte electrolyte hiding in thethe carboxylatesolution, electrostatic binding sites interactions of the tiopronin. between Another NPs and study the complex was devoted were tomodified, quantifying with the the number electrolyte of adsorbedhiding the complexes carboxylate onto binding the gold sites surface of the of tiopro two dinin.fferently Another stabilized study was particles devoted using to time-correlatedquantifying the single-photonnumber of adsorbed counting complexes spectroscopy onto [163 the]. Thegold kinetics surface of of adsorption two differently of the complex, stabilized as wellparticles as the using size andtime-correlated temperature single-photon dependence of counting Au-NPs, spectroscopy were other parameters [163]. The investigated kinetics of adsorp for thetion comprehension of the complex, of theas luminescence-quenchingwell as the size and temperature [164]. Other dependence fundamental of Au-NPs, questions were were other studied parameters such as investigated chromophores for 2+ density,the comprehension size, or temperature of the luminescence-quenching dependence of optical and [164 surface]. Other properties fundamental for immobilized questions were Ru(bipy) studied3 complexsuch as chromophores on Au-NPs or nanorods density, size, via Au-S or temperatur bonding [165e dependence,166]. All former of optical studies and were surface realized properties with the for aimimmobilized to find an Ru(bipy) efficient control32+ complex of the on energetic Au-NPs or or the nanorods electronic via communication Au-S bonding between [165,166 the]. All particles former andstudies the complexwere realized for future with biological, the aim catalytic,to find an optical, efficient or electroniccontrol of applications. the energetic or the electronic 2+ communicationRu(bpy)3 complexbetween isthe also particles widely and studied the forcomplex its electrochemiluminescence for future biological, catalytic, (ECL) properties.optical, or Usingelectronic this property,applications. various modified electrodes were prepared as for example for solid-state ECL detectionRu(bpy) in capillary32+ complex electrophoresis is also widely [167 studied]. In order for its to electrochemiluminescence optimize the ECL detection (ECL) of the properties. indium 2+ tinUsing oxide this (ITO) property, electrode, various Ru(bpy) modified3 -AuNPs electrodes aggregates were prepared were prepared as for example and immobilized for solid-state on ECL the conductivedetection in support capillary via electrophoresis Au-S binding. [167]. In order to optimize the ECL detection of the indium 4 oxideThese (ITO) systems electrode, exhibited Ru(bpy) a significant32+-AuNPs improvement aggregates ofwere ECL prepared intensity withand aimmobilized detection 10 ontimes the moreconductive sensitive support than withoutvia Au-S grafted binding. gold nanoparticles. Modified electrodes were also used for the selectiveThese detection systems of exhibited biochemical a significant molecules improv such asement pentoxyverine of ECL intensity [168]. with Others a detection hybrid materials 104 times 2+ weremore prepared sensitive using than polypyridinylwithout grafted complexes gold nanopartic derived fromles. Modified Ru(bpy)3 electrodescomplex. were As aalso first used example, for the a complexselective bearing detection three of thiolbiochemical pendant groupsmolecules was such used as to pentoxyverine fabricate an ITO [168 electrode]. Others with hybrid self-assembled materials layerswere prepared of ruthenium using complexes polypyridinyl [169 complexes]. Formation derived of stable from and Ru(bpy) well-ordered32+ complex. 3D As structures a first example, at the electrodea complex surface bearing was three thus observedthiol pendant and agroups noticeable was increaseused to offabricate the photocurrent an ITO electrode response with with self- the numberassembled of layers layers was of ruthenium evidenced. complexes Improvement [169]. of suchFormation 3D-assemblies of stable couldand well-ordered be valuable in3D ruthenium structures (II)at dye-sensitizedthe electrode surface solar cells. was thus observed and a noticeable increase of the photocurrent response withModified the number electrodes of layers can was also evidenced. be prepared Improvem using anent electroactive of such 3D-assemblies spacer such as could a viologen be valuable group] in (Seeruthenium Scheme (II) 22 )[dye-sensitized170]. By deposition solar cells. of functionalized Au-NPs onto a gold electrode, a sensitivity moreModified than 15 times electrodes larger can for also the be electrode prepared was using observed an electroactive for the anodicspacer such photocurrent as a viologen detection. group] A(See participation Scheme 22) of [170]. the viologenBy deposition moiety of byfunctionalized its one-electron Au-NPs reduction onto a processgold electrode, was demonstrated. a sensitivity more than 15 times larger for the electrode was observed for the anodic photocurrent detection. A participation of the viologen moiety by its one-electron reduction process was demonstrated.

Nanomaterials 2020, 10, 548 18 of 36

Nanomaterials 2020, 10, x FOR PEER REVIEW 17 of 34 Continuation of that initial work consisted to study the influence of the nanoparticles size on the Continuationphotocurrent response.of that initial work consisted to study the influence of the nanoparticles size on the photocurrent response.

N (CH2)7 N N (CH2)6S N N Ru2+ N N N

2 Scheme 22. ExampleExample of of Ru complex - viologen linked thiol to immobilize onto Au-NPs [ 170].170].

NanostructuredNanostructured assembliesassemblies ofof particlesparticles withwith diametersdiameters ranging from 50 to 100 nm presented the optimal optimal photocurrent photocurrent efficiencies efficiencies [171]. [171]. Based Based on on the the strategy strategy that that an an increase increase of ofthe the ionic ionic force force is knownis known to tocause cause flocculation flocculation of of Au-NPs, Au-NPs, a anovel novel method method to to elab elaborateorate these these was developed, still using these viologen-substituted ruthenium complex [[172].172]. In that work, influence influence of the nature of the electrolyte, in particular of the an anionicionic moiety was studied due to the fact that it it can can modify thethe morphology morphology of theseof these systems systems and subsequentlyand subsequently alter the alter photocurrent the photocurrent responses. responses. Preceding Precedingthese viologen-based these viologen-based systems anchored systems to anchored Au-NPs viato aAu-NPs thiol group, via a several thiol group, viologen several systems viologen based 2+ systemson electrostatic based interactionson electrostatic between interactions the Ru(bipy) between3 complex the Ru(bipy) and citrate-capped32+ complex and nanoparticles citrate-capped were nanoparticlesinvestigated [ 173were,174 investigated]. Photoelectrochemical [173,174]. Photoelectrochemical cells of low efficiencies cells were of obtained, low efficiencies resulting inwere an obtained,energy-transfer-quenching resulting in an ofenergy-transfer-quenchi the viologen sensitizerng by of the the gold viologen nanoparticles. sensitizer Gold by nanoparticle the gold nanoparticles.assemblies can Gold also be nanoparticle easily obtained assemblies by exploiting can thealso complexation be easily obtained of a metal by cation exploiting by pyridine the complexationmoieties [175]. of Using a metal this strategy,cation by thin pyridine films were moieti obtainedes [175]. and Using exhibited this diode-likestrategy, thin responses. films were obtainedAlways and withexhibited a polypyridinic diode-like environmentresponses. around the redox center, other anchoring groups have 2+ beenAlways used to functionalizewith a polypyridinic and stabilize environment gold nanoparticles. around the In redox a recent center, work, other a Ru(bipy) anchoring3 complex groups havewith abeen terminal used amine to functionaliz group wase usedand tostabilize rationalize gold the nanoparticles. effect of the nanoparticles In a recent work, on the a radiative Ru(bipy) and32+ complexthe none-radiative with a terminal rates of amine a phosphorescent group was used compound to rationalize [176]. Phosphorescent the effect of the molecules nanoparticles were graftedon the radiativeon the particles and the via anone-radiative selective recognition rates technique:of a phosphorescent nanoparticles compound were first prepared[176]. Phosphorescent in the presence moleculesof streptavidin were withgrafted an on excess the particles of bovine via serum a selective albumin. recognition Streptavidin technique: being nanoparticles known to be awere selective first preparedsensor for in biotin, the presence phosphorescent of streptavidin molecules with werean excess thus of functionalized bovine serum with albu biotin.min. Streptavidin Then, grafting being of knownthese molecules to be a selective onto NPs sensor was realized, for biotin, based phosphorescent on the biotin-streptavidin molecules were recognition thus functionalized process. Another with biotin.example, Then, also grafting based on of the these biotin-streptavidin molecules onto recognition, NPs was realized, was developed based foron thethe assembly biotin-streptavidin of recognitionand gold nanoparticles process. Another on DNA example, templates also [177 ].based Other on properties, the biotin-streptavidin such as the supramolecular recognition, control was developedof valence-tautomeric for the assembly equilibrium of proteins of a bistable and cobaltgold nanoparticles complex was alsoon DNA studied templates [178]. Influence [177]. Other of the properties,anchorage onsuch thermodynamic as the supramolecular parameters control was demonstrated,of valence-tautomeric caused by equilibrium surface confinement of a bistable resulting cobalt complexfrom the attachmentwas also studied of the valence[178]. Influence tautomer. of the anchorage on thermodynamic parameters was demonstrated,Until now, caused only nanocomposites by surface confinement obtained resultin by electrostaticg from the interactions attachment or of by the use valence of long tautomer. aliphatic chainsUntil with now, thiol only end nanocomposites groups have been obtained presented. by electrostatic In these systems, interactions no electronic or by use communication of long aliphatic was chainspossible with through thiol theend non-conjugated groups have been spacer. presented. Recently, In Mayerthese systems, and co-workers no electronic reported communication the synthesis wasof several possible polypyridinic through the ruthenium non-conjugated complexes spacer based. Recently, on phenanthroline Mayer and bidentate co-workers ligands reported with fully the synthesisconjugated of spacers,several polypyridinic enabling a direct ruthenium communication complexes between based on the phenanthroline particles and bidentate the complex ligands (See withScheme fully 23 )[conjugated158,179]. Modification spacers, enabling of the redoxa dire potentialct communication of the ruthenium between complexes the particles was observed,and the complexevidencing (See an Scheme electronic 23) communication. [158,179]. Modification of the redox potential of the ruthenium complexes was observed, evidencing an electronic communication. Bipyridyl complexes with conjugated linkers have also been investigated for surface-enhanced Raman scattering measurements [180]. In particular, the influence of the solvent was studied, and results demonstrated different ways of adsorption for the complex, with these phenomena depending on the solvents. The ruthenium complex used in this study was composed of a ruthenium center chelated by three bipyridine ligands modified either by the addition of a conjugated spacer of triphenylamine, acting as the pendant group, or by the addition carboxylate groups. In all former examples, only functionalizing agents with only one anchoring point have been presented. But they

Nanomaterials 2020, 10, x FOR PEER REVIEW 18 of 34

Nanomaterialscan also2020 have, 10 ,two 548 anchoring groups. This is the case for a complex prepared with a phenanthroline19 of 36 exhibiting two thiol groups [181]. 2+ COOH COOH N Bu4NOOC HOOC N N N NCS N N Ru Ru N N N NCS N N HOOC HOOC N COOH Nanomaterials 2020, 10, x FOR PEER REVIEW COONBu4 18 of 34 Scheme 23. Ruthenium (II) bipyridyl complexes immobilized onto Au-NPs, through amino or can alsoScheme have 23. twoRuthenium anchoring (II)groups. bipyridyl This complexes is the case immobilized for a complex onto prepared Au-NPs, with through a phenanthroline amino or isothiocyanate groups developed by Pérez Leon et al. [180]. exhibitingisothiocyanate two thiol groups groups developed [181]. by Pérez Leon et al. [180]. 2+ 7.4.Bipyridyl Coordination complexes Complexes with Grafted conjugated on Au-NPs linkers for have Multiple also beenApplications investigatedCOOH for surface-enhanced COOH Raman scattering measurements [180].N In particular, the influence of the solvent was studied, and Corresponding ruthenium complex was Buused4NOOC to create nanocomposite junctions between HOOC N results demonstrated differentN ways of adsorption for the complex,N withNCS these phenomena depending on electrodes. NanocompositesN N were prepared via a sulfur-exchange reaction and grafted between two the solvents. The rutheniumRu complex used in this study was composedRu of a ruthenium center chelated facing gold electrodesN withN a micrometric gap. ConductivityN measurementsNCS were investigated and by three bipyridine ligands modifiedN either by the addition of a conjugatedN spacer of triphenylamine, demonstrated theHOOC efficiency of such electric self-assemblies,HOOC with the presence of the ruthenium cation acting as the pendant group, or by theN addition carboxylate groups. In all former examples, only allowing an increased COOHconductivity and a lower energy barrier. Concerning other practical functionalizing agents with only one anchoring point have been presented.COONBu But they can also have two applications, bidentate ligands such as bisoxazolin e were used to prepare4 complexes valuable in anchoring groups. This is the case for a complex prepared with a phenanthroline exhibiting two thiol catalysis.Scheme 23.Hence, Ruthenium a chiral (II) copper-bisoxazine bipyridyl complexes complex immo covalentlybilized onto linked Au-NPs, on gold through colloids amino has or been used groups [181]. forisothiocyanate an enantioselective groups developed ene-reaction by Pérez between Leon et2-ph al. [180].enylpropene and ethylglyoxylate [182]. Use of this 7.4.original Coordination homogeneous Complexes catalyst Grafted onresulted Au-NPs in for reactions Multiple with Applications high yields and high enantiomeric excess. 7.4. AnotherCoordination advantage Complexes of Graftedthis catalyst on Au-NPs is that for it Multiple can be easilyApplications separated from the reaction mixture by Corresponding ruthenium complex was used to create nanocomposite junctions between filtration,Corresponding and is rutheniumthus reusable. complex Zinc was(II) phthalocyanineused to create wasnanocomposite anchored to junctions Au-NPs betweenfor another electrodes. Nanocomposites were prepared via a sulfur-exchange reaction and grafted between electrodes.application Nanocomposites (See Scheme 24)were [183]. prepared via a sulfur-exchange reaction and grafted between two two facing gold electrodes with a micrometric gap. Conductivity measurements were investigated facing gold electrodes with a micrometric gap. ConductivityC H C H measurements were investigated and and demonstrated the efficiency of such electric self-assemblies,6 13 6 13 with the presence of the ruthenium demonstrated the efficiency of such electric self-assemblies, with the presence of the ruthenium cation cation allowing an increased conductivity and a lower energy barrier. Concerning other practical allowing an increased conductivity and a lower energy barrier. Concerning other practical applications, bidentate ligands such asHS-(H bisoxazolineC) were usedC to6H13 prepare complexes valuable in applications, bidentate ligands such as bisoxazolin2 11 Ne wereN used to prepare complexes valuable in catalysis. Hence, a chiral copper-bisoxazine complexZ covalently linked on gold colloids has been catalysis. Hence, a chiral copper-bisoxazine complex covalentlyn linked on gold colloids has been used used for an enantioselective ene-reaction between 2-phenylpropeneN N C H and ethylglyoxylate [182]. Use for an enantioselective ene-reaction betweenC6H13 2-phenylpropene and6 ethylglyoxylate13 [182]. Use of this of this original homogeneous catalyst resulted in reactions with high yields and high enantiomeric original homogeneous catalyst resulted in reactions with high yields and high enantiomeric excess. excess. Another advantage of this catalyst is that it can be easily separated from the reaction mixture Another advantage of this catalyst is that it can beC HeasilyC H separated from the reaction mixture by by filtration, and is thus reusable. Zinc (II) phthalocyanine6 13 6 13 was anchored to Au-NPs for another filtration, and is thus reusable. Zinc (II) phthalocyanine was anchored to Au-NPs for another applicationScheme (See 24. Scheme Zinc (II) 24 )[phthalocyanine183]. complex used to coat Au-NPs for photodynamic therapy [183]. application (See Scheme 24) [183].

By grafting the photosensitizer onto Au-NPs,C6H13C6H 13the generation of singlet oxygen was obtained with an enhanced quantum yield, as compared to the free photosensitizer. Such systems could be potentially used for the delivery of photodynamic photosensitizerC H agents in photodynamic therapy. HS-(H2C)11 N N 6 13 An interesting example of anion sensors based on porphyrins was recently reported in the literature Zn [184]. The anchorage onto the NPs was realized inN this case by use of four pendant groups derived C H N C H from thiotic acid. The nanocomposite,6 13 when tested with six6 13different anions, exhibited a significant increase in anion-binding affinities, when compared to the free metalloporphyrin. This improved affinity for anions resulted from the pre-organizationC6H13 C6H13 of the porphyrins at the surface of the particles. Modified electrodes were also prepared with porphyrins. Based on a layer-by-layer deposition SchemeScheme 24. 24. ZincZinc (II) (II) phthalocyanine phthalocyanine co complexmplex used used to to coat coat Au-NPs Au-NPs for for photodynamic photodynamic therapy therapy [183]. [183]. process of an electron acceptor, Au-NPs covalently linked to an ITO electrode could be prepared. ExaminationBy grafting grafting the theof photosensitizer photosensitizerthese electrodes onto onto Au-NPs, Au-NPs,in photochemical the the generation generation experimentsof singlet of singlet oxygen oxygenrevealed was obtained was the obtained resultingwith anwith enhancedphotoelectrochemical an enhanced quantum quantum yield, cells yield, toas be compared as of comparedlow efficiency to tothe the withfree free thisphotosensitizer. photosensitizer. photosensitizer Such Such [173]. systems systems In 2007, could could Ozawa be and potentiallyco-workers usedused reported forfor thethe delivery delivery the preparation of of photodynamic photodynamic of porphyrin photosensitizer photosensitizer wires and the agents agents assembly in photodynamicin photodynamic of gold nanoparticles therapy. therapy. An onto π Aninteresting interesting-conjugated example example wires of anion of of porphyrins anion sensors sensors based [185]. based onFirst, porphyrins on the porphyrins polymer was of recentlywas porphyrins recently reported reportedwas indeposited the in literature the onliterature a modified [184]. [184].Theglass anchorage The surface anchorage onto by theonto NPsLangmuir-Blodgett the was NPs realized was realized in technique. this in case thisby caseThen, use by ofthe use four glass of pendantfour substrate pendant groups was groups derivedsoaked derived fromwith the fromthioticparticles thiotic acid. Theacid.capped nanocomposite, The with nanocomposite, 4-pyridineethanethiol, when testedwhen tested with resulting six with diff erentsix in different anions,the grafting anions, exhibited of exhibitedthe a significantparticles a significant on increase the glass increase in anion-binding affinities, when compared to the free metalloporphyrin. This improved affinity for anions resulted from the pre-organization of the porphyrins at the surface of the particles. Modified electrodes were also prepared with porphyrins. Based on a layer-by-layer deposition process of an electron acceptor, Au-NPs covalently linked to an ITO electrode could be prepared. Examination of these electrodes in photochemical experiments revealed the resulting photoelectrochemical cells to be of low efficiency with this photosensitizer [173]. In 2007, Ozawa and co-workers reported the preparation of porphyrin wires and the assembly of gold nanoparticles onto π-conjugated wires of porphyrins [185]. First, the polymer of porphyrins was deposited on a modified glass surface by the Langmuir-Blodgett technique. Then, the glass substrate was soaked with the particles capped with 4-pyridineethanethiol, resulting in the grafting of the particles on the glass

Nanomaterials 2020, 10, 548 20 of 36 in anion-binding affinities, when compared to the free metalloporphyrin. This improved affinity for anions resulted from the pre-organization of the porphyrins at the surface of the particles. Modified electrodes were also prepared with porphyrins. Based on a layer-by-layer deposition process of an electron acceptor, Au-NPs covalently linked to an ITO electrode could be prepared. Examination of these electrodes in photochemical experiments revealed the resulting photoelectrochemical cells to be of low efficiency with this photosensitizer [173]. In 2007, Ozawa and co-workers reported the preparation of porphyrin wires and the assembly of gold nanoparticles onto π-conjugated wires of porphyrins [185]. First, the polymer of porphyrins was deposited on a modified glass surface by the Langmuir-Blodgett technique. Then, the glass substrate was soaked with the particles capped with 4-pyridineethanethiol,Nanomaterials 2020, 10, x FOR resulting PEER REVIEW in the grafting of the particles on the glass substrate by interaction 19 of of 34 the pyridine of the NPs with the porphyrin (See Scheme 25). The interest of that technique is that the graftingsubstrate of by Au-NPs interaction on the of glass the pyridine substrate of could the NPs only with be observed the porphyrin where (See the polymerScheme 25). was The previously interest deposited.of that technique As final is systems that the based grafting on porphyrins,of Au-NPs on biomolecules the glass substrate were grafted could ontoonly goldbe observed nanoparticles. where Heminthe polymer chloride was (Hem) previously and cytochrome deposited. c (Cyt As c)—whosefinal systems structures based on are porphyri derived fromns, biomolecules porphyrin—were were immobilizedgrafted onto ongold NPs nanoparticles. and binding ofHemin a metabolic chloride inhibitor (Hem) and (azide cytochrome anion) was c investigated(Cyt c)—whose [186 structures]. Due to aare reduced derived accessibility from porphyrin—were resulting of the immobilized grafting of on the NPs biomolecules, and binding a thermalof a metabolic activation inhibitor to bind (azide the azideanion) anion was was investigated necessary. [186]. Due to a reduced accessibility resulting of the grafting of the biomolecules, a thermal activation to bind the azide anion was necessary.

SchemeScheme 25. 25.1D- 1D-Assembly Assemblyof ofAu-NPs Au-NPschemically chemically linked linked to top-conjugated p-conjugated porphyrin, porphyrin, reproduced reproduced with with permissionpermission from from [ 185[185],], ACS ACS 2007. 2007.

7.5.7.5. CarboxylatesCarboxylates andand BaseBase ShiShiffff Coordination Coordination Complexes Complexes ComplexesComplexes withwith variousvarious chargedcharged ligandsligands havehave alsoalso beenbeenused usedto to encase encasegold gold nanoparticles nanoparticlesin in monolayermonolayer organometallic organometallic metal-complex metal-complex shells shells [187 [187]. Hence,]. Hence, chelation chelation of metalof meta ionsl ions can can be realizedbe realized by carboxylateby carboxylate groups. groups. Interest Interest in such in such complexes complexes relies relies in the in weakness the weakness of the coordinationof the coordination whichcan which be destroyedcan be destroyed “on demand”, “on demand”, simply by simply using by a stronger using a chelatorstronger that chelator will dissociate that will thedissociate complex. the The complex. main applicationsThe main applications of such systems of such are thesystems detection are the of heavy detection metal of ions heavy [188 metal,189], ions based [188,189 on the aggregation], based on the of theaggregation nanoparticles of the in the nanoparticles presence of cations,in the thepresence preparation of cations, of self-assembled the preparation monolayer of self-assembled [190–193], or nanocompositesmonolayer [190–193 further], or valuable nanocomposites in catalytic further systems valuable [194]. For in catalytic the firsttwo systems applications, [194]. For nanoparticles the first two wereapplications, conveniently nanoparticles functionalized were byconveniently mercapto-alkyl functionalized acid, the carboxylate by mercapto-alkyl group acting acid, asthe the carboxylate pendant groupgroup able acting to generateas the pendant interparticle group interactionsable to generate under interparticle chelation with interactions a metal ion under (See chelation Scheme 26 with). a metal ion (See Scheme 26).

Scheme 26. Illustration of the oxidizing ability of a dioxygen-activating from nonheme Iron(II)- Benzilate complex immobilized on Au-NPs, reproduced with permission from [187]. ACS 2019.

For the last application, a multistep synthesis was developed to graft Ru-dodecacarbonyl complex, the carboxylic group known to react with the complex to form Ru-carbonyl-carboxylate complexes (See Scheme 27). By this strategy, a dimer or an oligomer was anchored to the particles. Other functions can be used to graft metal complex on particles. Hence, a bis-hydroxamate ligand has been used to generate coordination-based nanoparticle monolayers or multilayers with Zr4+ [195]. By use of a bifunctional molecule (gallic acid) bearing a carboxylate group as the capping agent and a hydroxy group to coordinate Pb (II) cation, a naked-eye detector was prepared. Due to the unique

Nanomaterials 2020, 10, x FOR PEER REVIEW 19 of 34 substrate by interaction of the pyridine of the NPs with the porphyrin (See Scheme 25). The interest of that technique is that the grafting of Au-NPs on the glass substrate could only be observed where the polymer was previously deposited. As final systems based on porphyrins, biomolecules were grafted onto gold nanoparticles. Hemin chloride (Hem) and cytochrome c (Cyt c)—whose structures are derived from porphyrin—were immobilized on NPs and binding of a metabolic inhibitor (azide anion) was investigated [186]. Due to a reduced accessibility resulting of the grafting of the biomolecules, a thermal activation to bind the azide anion was necessary.

Scheme 25. 1D- Assembly of Au-NPs chemically linked to p-conjugated porphyrin, reproduced with permission from [185], ACS 2007.

7.5. Carboxylates and Base Shiff Coordination Complexes Complexes with various charged ligands have also been used to encase gold nanoparticles in monolayer organometallic metal-complex shells [187]. Hence, chelation of metal ions can be realized by carboxylate groups. Interest in such complexes relies in the weakness of the coordination which can be destroyed “on demand”, simply by using a stronger chelator that will dissociate the complex. The main applications of such systems are the detection of heavy metal ions [188,189], based on the aggregation of the nanoparticles in the presence of cations, the preparation of self-assembled monolayer [190–193], or nanocomposites further valuable in catalytic systems [194]. For the first two applications, nanoparticles were conveniently functionalized by mercapto-alkyl acid, the carboxylate groupNanomaterials acting2020 as, 10the, 548 pendant group able to generate interparticle interactions under chelation with21 of 36a metal ion (See Scheme 26).

SchemeScheme 26.26. IllustrationIllustration of of the the oxidizing oxidizing ability ability of a dioxygen-activatingof a dioxygen-activating from nonheme from nonheme Iron(II)-Benzilate Iron(II)- Benzilatecomplex immobilizedcomplex immobilized on Au-NPs, on reproducedAu-NPs, reproduced with permission with permission from [187 ].from ACS [187 2019.]. ACS 2019.

ForFor thethe lastlast application, application, a multistepa multistep synthesis synthe wassis developedwas developed to graft to Ru-dodecacarbonyl graft Ru-dodecacarbonyl complex, complex,the carboxylic the carboxylic group known group to react known with to the react complex with tothe form complex Ru-carbonyl-carboxylate to form Ru-carbonyl-carboxylate complexes (See complexesScheme 27 ).(See By Scheme this strategy, 27). By a dimerthis strategy, or an oligomer a dimer wasor an anchored oligomer to was the anchored particles. Otherto the functionsparticles. Othercan be functions used to graft can be metal used complex to graft metal on particles. complex Hence, on particles. a bis-hydroxamate Hence, a bis-hydroxamate ligand has been ligand used has to 4+ beengenerate used coordination-based to generate coordination-based nanoparticle na monolayersnoparticle monolayers or multilayers or multilayers with Zr with[195]. Zr By4+ [195 use]. of By a usebifunctional of a bifunctional molecule molecule (gallic acid) (gallic bearing acid) abearing carboxylate a carboxylate group as group the capping as the capping agent and agent a hydroxy and a Nanomaterials 2020, 10, x FOR PEER REVIEW 20 of 34 hydroxygroup to group coordinate to coordinate Pb (II) cation, Pb (II) a naked-eye cation, a detectornaked-eye was detector prepared. was Due prepared. to the unique Due to coordination the unique behavior of Pb (II) cation, whose coordination number can be extended until 12, aggregates were coordination behavior of Pb (II) cation, whose coordination number can be extended until 12, obtained with Pb(II) cation. aggregates were obtained with Pb(II) cation.

Scheme 27. ExamplesExamples of carboxylate, amidate, and phen phenolateolate based complexes used to coat Au-NPs.

On On the the contrary, contrary, addition addition of other of metalother cationsmetal leavescations the leaves nanoparticles the nanoparticles isolated, as isolated, a consequence as a ofconsequence the coordination of the coordination of these cations of these with cations less ligands with less and ligands the ability and ofthe Pb ability (II) cation of Pb to(II) overcome cation to interparticleovercome interparticle electrostatic electrostatic repulsions. repulsions. Schiff base Schiff ligands base ligands have also have been also usedbeen toused coordinate to coordinate iron (III)iron cations.(III) cations. In thatIn that work, work, two two complementary complementar strategiesy strategies have have been been developed developed toto stabilizestabilize gold nanoparticles. The The first first work work is is based based on on the use of a neutral complex stabilized the NPs by steric repulsion (alkyl(alkyl chain).chain). TheThe secondsecond approach approach is is based based on on the the use use of of a chargeda charged complex. complex. Stabilization Stabilization of theof the particles particles is doneis done by by electrostatic electrostatic repulsion repulsion [196 [196]. ].

7.6. Bioinorganic Complexes The final final type type of of ligands ligands able able to to coordinate coordinate me metaltal ions ions are are all all ligands ligands based based on onbiomolecules. biomolecules. A Agrowing growing interest interest concerns concerns the theelaboration elaboration of nanoco of nanocompositesmposites modified modified with with biomolecules biomolecules due to due their to theirpotential potential use in use electronic, in electronic, optical, optical, and and biosensor applications applications [136,197,198 [136,197,198]. ].Proteins Proteins can can thus thus be covalently attached to gold nanoparticles by generation of a Co (II)(II) complex.complex. The , obtained by genetic engineering, bears bears a a histidine histidine tag tag that that can can coordinate coordinate Co Co (II) (II) ions ions by by ligand ligand exchange exchange on on a grafted Co (II) complex (See Scheme 28) [199]. Several examples of glyconanoparticles have been prepared to study carbohydrate interactions with Ca (II) cation in water [200,201]. Complexation of Ca (II) ions resulted in aggregation of the particles due to calcium cation-carbohydrate-carbohydrate interactions. These interactions are reversible, and addition of a strong chelator such as EDTA can redisperse the particles. Influence of the length of the spacer on complexation was also studied and the lowest detection level was obtained with the shortest linker.

Scheme 28. Biomolecules grafted on gold nanoparticles by immobilization of fully functional proteins onto the surface of Au-NPs through a thioctic acid function, reproduced with permission from [199]. ACS 2005.

Peptide-functionalized gold nanoparticles have also been prepared for Hg (II) ion detection [202]. Gold particles were first functionalized by a exhibiting at both ends an amino and a carboxylic group. Anchorage of the peptide was done by the amino group. Due to the strong affinity of Hg (II) cations for amino groups, addition of the cation resulted in the detachment of the peptide

Nanomaterials 2020, 10, x FOR PEER REVIEW 20 of 34 coordination behavior of Pb (II) cation, whose coordination number can be extended until 12, aggregates were obtained with Pb(II) cation.

Scheme 27. Examples of carboxylate, amidate, and phenolate based complexes used to coat Au-NPs.

On the contrary, addition of other metal cations leaves the nanoparticles isolated, as a consequence of the coordination of these cations with less ligands and the ability of Pb (II) cation to overcome interparticle electrostatic repulsions. Schiff base ligands have also been used to coordinate iron (III) cations. In that work, two complementary strategies have been developed to stabilize gold nanoparticles. The first work is based on the use of a neutral complex stabilized the NPs by steric repulsion (alkyl chain). The second approach is based on the use of a charged complex. Stabilization of the particles is done by electrostatic repulsion [196].

7.6. Bioinorganic Complexes The final type of ligands able to coordinate metal ions are all ligands based on biomolecules. A growing interest concerns the elaboration of nanocomposites modified with biomolecules due to their potential use in electronic, optical, and biosensor applications [136,197,198]. Proteins can thus be Nanomaterials 2020, 10, 548 22 of 36 covalently attached to gold nanoparticles by generation of a Co (II) complex. The protein, obtained by genetic engineering, bears a histidine tag that can coordinate Co (II) ions by ligand exchange on a grafteda grafted Co Co (II) (II) complex complex (See (See Scheme Scheme 28) 28 )[[199199]].. Several Several examples examples of of glyconanoparticles glyconanoparticles have have been prepared to study carbohydrate interact interactionsions with Ca (II) cation in water [200200,201,201].]. Complexation of of CaCa (II) (II) ions ions resulted resulted in in aggregation aggregation of of the the particles particles due due to to calcium calcium cation-carbohydrate-carbohydrate cation-carbohydrate-carbohydrate interactions.interactions. These These interactions interactions are are reversible, reversible, and and addition addition of of a a strong strong chelator chelator such such as as EDTA EDTA can redisperseredisperse the particles.particles. InfluenceInfluence ofof the the length length of of the the spacer spacer on on complexation complexation was was also also studied studied and and the thelowest lowest detection detection level level was was obtained obtained with with the the shortest shortest linker. linker.

Nanomaterials 2020, 10, x FOR PEER REVIEW 21 of 34

from the surface of the particle. A 1D-linear assembly of colloidal particles was then observed. Scheme 28. Biomolecules grafted on gold nanoparticles by immobilization of fully functional proteins Addition of an alkali solution of EDTA decomplexed the peptide from the Hg (II) ion and the free Schemeonto the 28. surface Biomolecules of Au-NPs grafted through on gold a thioctic nanoparticles acid function, by immobilization reproduced with of fully permission functional from proteins [199]. peptideonto can the surfaceabsorb ofon Au-NPs the gold through surface. a thioctic DNA-functionalized acid function, reproduced gold nano withparticles permission have fromalso [been199]. used ACS 2005. [ ] [ ] [ ] for theACS colorimetric 2005. detection of various cations such as Hg (II) 203 , Pb (II) 204–208 , Cu (II) 209 . A rutheniumPeptide-functionalized complex has also gold been nanoparticles used to graft have DNA also to been gold prepared nanoparticles, for Hg the (II) ruthenium ion detection complex [202]. [ ] GoldactingPeptide-functionalized particles as the DNA-intercalating were first functionalized gold unitnanopa and byrticles athe peptide linker have exhibiting with also the been gold at bothprepared nanoparticles ends anforamino Hg 177 (II) and . ionAnchorage a carboxylicdetection of [group.the202 ]DNA. Gold Anchorage template particles containing of were the peptidefirst 80functionalized base was pairs done was by by the donea peptide amino according group.exhibiting to Duean at original toboth the endsstrong strategy. ana aminoffi Asnity the ofand first Hg a carboxylic(II)step, cations streptavidin-coated group. for amino Anchorage groups, gold of addition thenanoparticles peptide of the was cationwere done resultedprepared. by the amino in Then, the group. detachment the Dueruthenium to of the the strong peptidecomplex affinity fromwas ofthedesigned Hg surface (II) cationsin of such the fora particle. way amino that A groups, the 1D-linear biotin-phenanthrol addition assembly of th ofe cationine colloidal ligand resulted particles was ablein the was to detachment bind then streptavidin, observed. of the Addition peptide and the ofphenazine an alkali ligand solution was of able EDTA to intercalat decomplexede into the DNA peptide duplex from (See the Scheme Hg (II) ion29). and the free peptide

can absorbSeveral on metal the goldcomplexes surface. have DNA-functionalized been covalently linked gold nanoparticlesto gold nanoparticles, have also exclusively been used forfor thecatalytic colorimetric and structural detection studies. of various Hence, cations chiral suchrhodium-diphosphine as Hg (II) [203], Pb complexes (II) [204– on208 gold], Cu colloids (II) [209 have]. A [ ] rutheniumbeen used complexfor enantioselective has also been hydrogenation used to graft DNA of α to-acetamidocinnamate gold nanoparticles, the146 ruthenium, and Ti-binolate complex actingcomplexes as the on DNA-intercalating particles have been unit synthetized and the linker and with applied the gold to the nanoparticles catalytic asymmetric [177]. Anchorage alkylation of the of [ ] DNAbenzaldehyde template containing210 . A dimer 80 base of ruthenium pairs was complex done according has also to been an original graftedstrategy. on particles Asthe as a first catalyst step, [ ] streptavidin-coatedfor ring-opening metathesis gold nanoparticles polymerization were prepared. 144 . Other Then, Ruthenium the ruthenium complexes complex have was been designed grafted in suchonto large a way nanoparticles that the biotin-phenanthroline for cell luminescence ligand imaging, was ablewhich to revealed bind streptavidin, their biomolecular and the association phenazine [ ] ligandwith chromatin was able toin intercalatethe nucleus into of cancer the DNA cells duplex 211 . (SeeRecently, Scheme ruthenium 29). complexes surrounding gold nanoparticles have been used to photocrosslink collagen [212].

SchemeScheme 29. CatalystsCatalysts or ormetallic metallic complex complex for forcell cellluminescence luminescence imaging imaging used used to surround to surround gold goldnanoparticles. nanoparticles.

7.7. CrownSeveral Ethers metal Devices complexes have been covalently linked to gold nanoparticles, exclusively for catalytic and structural studies. Hence, chiral rhodium-diphosphine complexes on gold colloids Crown ethers were also used to chelate metal cations of the d-element. Two examples are have been used for enantioselective hydrogenation of α-acetamidocinnamate [146], and Ti-binolate reported in the literature. The first one concerns the covalent linkage of 2-(12-mercaptododecyloxy) complexes on particles have been synthetized and applied to the catalytic asymmetric alkylation of methyl-15-crown-5 to gold nanoparticles, the final goal being the preparation of monolayer for the benzaldehyde [210]. A dimer of ruthenium complex has also been grafted on particles as a catalyst capture of Pb (II) cations [213]. A trapping capacity of 2.8 10-10 mol Pb (II) per cm² was obtained. The for ring-opening metathesis polymerization [144]. Other Ruthenium complexes have been grafted second example deals with bifunctionalized gold nanoparticles prepared for optical Pb (II) sensing onto large nanoparticles for cell luminescence imaging, which revealed their biomolecular association [214,215]. Two different ligands (2-(12-mercaptododecyl-oxy) methyl-15-crown-5 and thiotic acid) were grafted to gold nanoparticles (See Scheme 30). Without addition of Pb (II) cations, aggregation of nanoparticles was observed, due to interparticle hydrogen bonding. By addition of Pb (II) cations, a significant change in color was observed, indicative of the dispersion of the nanoparticles by breaking interparticle hydrogen bonding. A high selectivity of the nanocomposite for Pb (II) cation was also evidenced.

Scheme 30. Two examples of crown ethers grafted onto Au-NPs.

Nanomaterials 2020, 10, x FOR PEER REVIEW 21 of 34 from the surface of the particle. A 1D-linear assembly of colloidal particles was then observed. Addition of an alkali solution of EDTA decomplexed the peptide from the Hg (II) ion and the free peptide can absorb on the gold surface. DNA-functionalized gold nanoparticles have also been used for the colorimetric detection of various cations such as Hg (II) [203], Pb (II) [204–208], Cu (II) [209]. A ruthenium complex has also been used to graft DNA to gold nanoparticles, the ruthenium complex acting as the DNA-intercalating unit and the linker with the gold nanoparticles [177]. Anchorage of the DNA template containing 80 base pairs was done according to an original strategy. As the first step, streptavidin-coated gold nanoparticles were prepared. Then, the ruthenium complex was designed in such a way that the biotin-phenanthroline ligand was able to bind streptavidin, and the phenazine ligand was able to intercalate into the DNA duplex (See Scheme 29). Several metal complexes have been covalently linked to gold nanoparticles, exclusively for catalytic and structural studies. Hence, chiral rhodium-diphosphine complexes on gold colloids have been used for enantioselective hydrogenation of α-acetamidocinnamate [146], and Ti-binolate complexes on particles have been synthetized and applied to the catalytic asymmetric alkylation of benzaldehyde [210]. A dimer of ruthenium complex has also been grafted on particles as a catalyst for ring-opening metathesis polymerization [144]. Other Ruthenium complexes have been grafted onto large nanoparticles for cell luminescence imaging, which revealed their biomolecular association with chromatin in the nucleus of cancer cells [211]. Recently, ruthenium complexes surrounding gold nanoparticles have been used to photocrosslink collagen [212].

Nanomaterials 2020, 10, 548 23 of 36

Scheme 29. Catalysts or metallic complex for cell luminescence imaging used to surround gold withnanoparticles. chromatin in the nucleus of cancer cells [211]. Recently, ruthenium complexes surrounding gold nanoparticles have been used to photocrosslink collagen [212]. 7.7. Crown Ethers Devices 7.7. Crown Ethers Devices Crown ethers were also used to chelate metal cations of the d-element. Two examples are reportedCrown in the ethers literature. were alsoThe usedfirst one to chelateconcerns metal the covalent cations oflinkage the d-element. of 2-(12-mercaptododecyloxy) Two examples are reportedmethyl-15-crown-5 in the literature. to gold The nanoparticles, first one concerns the fina thel goal covalent being linkage the preparation of 2-(12-mercaptododecyloxy) of monolayer for the methyl-15-crown-5capture of Pb (II) cations to gold [213 nanoparticles,]. A trapping capacity the final of goal 2.8 10 being-10 mol the Pb preparation(II) per cm² was of monolayer obtained. The for 10 2 thesecond capture example of Pb deals (II) with cations bifunctionalized [213]. A trapping gold nanoparticles capacity of 2.8prepared 10− molfor optical Pb (II) Pb per (II) cm sensingwas obtained.[214,215]. TheTwo seconddifferent example ligands deals (2-(12-mercaptododec with bifunctionalizedyl-oxy) gold methyl-15-crown-5 nanoparticles prepared and thiotic for optical acid) Pbwere (II) grafted sensing to [ 214gold,215 nanoparticles]. Two different (See ligands Scheme (2-(12-mercaptododecyl-oxy) 30). Without addition of Pb (II) methyl-15-crown-5 cations, aggregation and thioticof nanoparticles acid) were was grafted observed, to gold due nanoparticles to interparticle (See hy Schemedrogen 30 bonding.). Without By additionaddition of Pb (II) cations, aggregationa significant of change nanoparticles in color was was observed, observed, due indicative to interparticle of the hydrogen dispersion bonding. of the Bynanoparticles addition of Pbby (II)breaking cations, interparticle a significant hydrogen change in bonding. color was A observed, high selectivity indicative of the of the nanocomposite dispersion of thefor nanoparticlesPb (II) cation bywas breaking also evidenced. interparticle hydrogen bonding. A high selectivity of the nanocomposite for Pb (II) cation was also evidenced.

Scheme 30. Two examples of crown ethers graftedgrafted ontoonto Au-NPs.Au-NPs. Nanomaterials 2020, 10, x FOR PEER REVIEW 22 of 34 Sensing of alkali metal ions has attracted considerable interest owing to the significance of these metalSensing ions in of biology. alkali metal ions has attracted considerable interest owing to the significance of these metalAmong ions in biology. recent approaches to the design and the fabrication of alkali sensors, gold nanoparticles-basedAmong recent approaches sensors have to the focused design much and the attention fabrication as a of promising alkali sensors, operating gold nanoparticles- principle. By additionbased sensors of alkali have cations, focused one much simple attention way of signalingas a promising recognition operating events principle. is to generate By addition a modification of alkali incations, the fluorescence one simple intensity way of or signaling the color ofrecognition the solution. events Chelation is to of generate alkali cations a modification is based in all in cases the onfluorescence the use of intensity crown ethers, or the which color are of knownthe solution. for their Chelation unusual of properties alkali cations of forming is based stable in all complexes cases on withthe use alkali of crown cations. ethers, Among which all crown are known ethers, for 15-crown-5 their unusual is known properties to generate of forming highly stable stable complexes with Naalkali+ cation cations. whereas Among 18-crown-6 all crown ethers, prefers 15-crown-5 K+ as cation is (Seeknown Scheme to generate 31)[216 highly,217]. stable complexes with Na+ cation whereas 18-crown-6 prefers K+ as cation (See Scheme 31) [216,217].

Scheme 31. Crown alkali ether used to functionalizefunctionalize AuAu°◦ nanoparticles.

The firstfirst example example of colorimetricof colorimetric sensing sensing is based is on 15-crown-5-functionalizedbased on 15-crown-5-functionalized gold nanoparticles, gold wherenanoparticles, the linkage where between the linkage the crown between ether andthe crown the particles ether isand insured the particles by a dodecyl is insured chain by [218 a dodecyl]. Stable particleschain [218]. were Stable thus particles obtained were in water thus due obtained to steric in repulsionswater due betweento steric particles.repulsions Stable between colloids particles. were alsoStable obtained colloids upon were addition also obtained of Na+, whereasupon addition addition of of Na K++,resulted whereas in addition the aggregation of K+ resulted of the particles in the withaggregation a noticeable of the color particles change. with The a noticeable stabilization color of K change.+ cation The being stabilization ensured by of two K+ crowncation ethers,being ensured by two crown ethers, the chelation of K+ cations by crown ethers involved into two different complexations was observed, resulting in their aggregations. Another group of research took advantage of aggregation by the addition of cations to prepare assembled nanoparticle films [219]. A modification of that first system published by Li was brought by the introduction of a second functionality on the nanocomposite [220]. A cooperative effect was observed when thiotic acid was grafted on the nanoparticles, whereas replacement of thiotic acid by thioctic amine altered the properties. Bifunctionalized nanoparticles were prepared according a two-step procedure consisting first of a ligand exchange of citrate ligand by thioctic acid, followed by a second partial exchange of thiotic ligands by thiolated crown ether [221,222]. Influence of the spacer on the kinetic of complexation was also studied, and a similar study was realized with 12-crown-4 and Na+ as a cation [220].

7.8. Functionalisation by Coordination Complexes of F-Block Elements To date, few examples of lanthanide complexes on gold nanoparticles are reported in the literature [223,224]. Several complexes were covalently grafted on gold nanoparticles due to their luminescent properties. In most of the cases, the goal was to prepare ion sensors. As a first example— and to ensure a strong binding of the complex to the surface of the nanoparticle—a diethylene triamine pentaacetic acid (H5DTPA) substituted with two thiophenol groups was used [225]. A water- soluble Eu (III) nanocomposite and luminescent nanobeads were thus obtained (See Scheme 32) [225].

Nanomaterials 2020, 10, 548 24 of 36 the chelation of K+ cations by crown ethers involved into two different complexations was observed, resulting in their aggregations. Another group of research took advantage of aggregation by the addition of cations to prepare assembled nanoparticle films [219]. A modification of that first system published by Li was brought by the introduction of a second functionality on the nanocomposite [220]. A cooperative effect was observed when thiotic acid was grafted on the nanoparticles, whereas replacement of thiotic acid by thioctic amine altered the properties. Bifunctionalized nanoparticles were prepared according a two-step procedure consisting first of a ligand exchange of citrate ligand by thioctic acid, followed by a second partial exchange of thiotic ligands by thiolated crown ether [221,222]. Influence of the spacer on the kinetic of complexation was also studied, and a similar study was realized with 12-crown-4 and Na+ as a cation [220].

7.8. Functionalisation by Coordination Complexes of F-Block Elements To date, few examples of lanthanide complexes on gold nanoparticles are reported in the literature [223,224]. Several complexes were covalently grafted on gold nanoparticles due to their luminescent properties. In most of the cases, the goal was to prepare ion sensors. As a first example—and to ensure a strong binding of the complex to the surface of the nanoparticle—a diethylene triamine pentaacetic acid (H5DTPA) substituted with two thiophenol groups was used [225]. A water-soluble Eu (III) nanocomposite and luminescent nanobeads were thus obtained (See Scheme 32)[225]. Nanomaterials 2020, 10, x FOR PEER REVIEW 23 of 34

Scheme 32. TEMTEM Pictures Pictures of of Eu(III) Eu(III) complexes complexes grafted grafted onto onto Au-NPs Au-NPs as as described described in in ref. ref. [204]. [204]. Reproduced from [225],], RSC 2006.

Another europium complex complex grafted grafted via via a along long alky alkyll chain chain to togold gold nanoparticles nanoparticles was was used used as an as anefficient efficient sensor sensor for for phosphate-anion phosphate-anion sensing sensing in inaque aqueousous solution solution [226]. [226]. In In this this case, case, detection of phosphate anionsanions was was done done in threein three steps, steps, the first the one first consisting one consisting in the grafting in theof grafting a none-luminescent of a none- Euluminescent (III) complex. Eu Then,(III) complex. addition ofThen, a β-diketone addition gave of a rise β-diketone to a highly gave luminescent rise to a complex, highly luminescent by exchange ofcomplex, two water by exchange molecules, of withtwo water one molecule molecules, of β with-diketone one molecule on the complex. of β-diketone The laston the step complex. consisted The of thelast additionstep consisted of a phosphate of the additi anionon thatof a switchedphosphate off anionthe luminescence. that switched Eu off (III) the andluminescence. Tb (III) complexes Eu (III) wereand Tb also (III) used complexes as sensors were for metalalso used cations as [sensors227]. These for metal nanocomposites—based cations [227]. These on nanocomposites— bipyridine capped nanoparticles—provedbased on bipyridine capped to be nanoparticles—proved highly phosphorescent. to Especially be highly forphosphoresce Eu (III)-basednt. Especially nanocomposites, for Eu addition(III)-based of nanocomposites, earth metal ionsand addition transition of earth metal metal ions resultedions and in transition the decrease metal of luminescence,ions resulted duein the to andecrease isomorphous of luminescence, substitution due of Euto (III)an isomorphous ions by these substitution cations. Gd (III)of Eu complexes (III) ions on by particles these cations. were used Gd in(III) vivo complexesas contrast on particles agents, combining were used both in vivo X-ray-computed as contrast agents, tomography combining and both Magnetic X-ray-computed Resonance Imagingtomography (MRI) and (See Magnetic Scheme 33Resonance)[228,229 ].Imaging These particles, (MRI) (See suitable Scheme for dual 33) imaging,[228,229]. exhibited These particles, a strong contrastsuitable for enhancement dual imaging, in MRI exhibited stems. a strong Other andcontrast recent enhancement works described in MRI Gd stems.3+ complexes Other and for recent IRM applicationsworks described [230 –Gd2323+]. complexes for IRM applications [230–232].

Scheme 33. Gd3+ complexes used to coat Au-NPs for MRI applications as described in [230]. Reproduced with permission from [230], ACS, 2006.

8. Conclusions To conclude, the diversity of Au°-based nanomaterials comprising inorganic entities as functionalizing agent was clearly evidenced. This unique combination between Au-NPs and these inorganic entities opened the way for Au nanoparticles for practical applications ranging from catalysis, opto-electronic, or biomedical applications. In these different examples, the stability of the resulting nanocomposites is a major issue for further applications and this point is still an active research field. Depending on the applications, the toxicity of the functionalizing agent must also be considered; this point has only been scarcely examined in these different works. Considering that the stability and the toxicity of the nanocomposites will govern their future applications, this point should be carefully examined in the coming years.

Author Contributions: Writing-Original Draft Preparation, F.D., E.D. and C.R.M.; Writing-Review & Editing, F.D., E.D. and C.R.M.

Funding: This research was funded by Université Paris-Sud, ENS Paris-Saclay, Aix Marseille University, the University of Versailles Saint Quentin en Yvelines and the Centre National de la Recherche Scientifique (CNRS).

Acknowledgments: Authors thanks Université Paris-Sud, ENS Paris-Saclay, Aix Marseille University, the University of Versailles Saint Quentin en Yvelines and the Centre National de la Recherche Scientifique (CNRS) for financial support.

Conflicts of Interest: The authors declare no conflict of interest.

Nanomaterials 2020, 10, x FOR PEER REVIEW 23 of 34

Scheme 32. TEM Pictures of Eu(III) complexes grafted onto Au-NPs as described in ref. [204]. Reproduced from [225], RSC 2006.

Another europium complex grafted via a long alkyl chain to gold nanoparticles was used as an efficient sensor for phosphate-anion sensing in aqueous solution [226]. In this case, detection of phosphate anions was done in three steps, the first one consisting in the grafting of a none- luminescent Eu (III) complex. Then, addition of a β-diketone gave rise to a highly luminescent complex, by exchange of two water molecules, with one molecule of β-diketone on the complex. The last step consisted of the addition of a phosphate anion that switched off the luminescence. Eu (III) and Tb (III) complexes were also used as sensors for metal cations [227]. These nanocomposites— based on bipyridine capped nanoparticles—proved to be highly phosphorescent. Especially for Eu (III)-based nanocomposites, addition of earth metal ions and transition metal ions resulted in the decrease of luminescence, due to an isomorphous substitution of Eu (III) ions by these cations. Gd (III) complexes on particles were used in vivo as contrast agents, combining both X-ray-computed tomography and Magnetic Resonance Imaging (MRI) (See Scheme 33) [228,229]. These particles, Nanomaterials 2020, 10, 548 25 of 36 suitable for dual imaging, exhibited a strong contrast enhancement in MRI stems. Other and recent works described Gd3+ complexes for IRM applications [230–232].

SchemeScheme 33.33. GdGd3+3+ complexescomplexes used used to to coat coat Au-NPs Au-NPs for for MRI MRI applications applications as as described described in in [230]. [230]. [ ] ReproducedReproduced withwith permissionpermission fromfrom [230230],, ACS,ACS, 2006.2006.

8.8. Conclusions

ToTo conclude,conclude, thethe diversitydiversity ofof AuAu°-based◦-based nanomaterialsnanomaterials comprisingcomprising inorganicinorganic entitiesentities asas functionalizingfunctionalizing agentagent waswas clearlyclearly evidenced.evidenced. This uniqueunique combinationcombination betweenbetween Au-NPsAu-NPs andand thesethese inorganicinorganic entitiesentities opened opened the the way way for Aufor nanoparticlesAu nanoparticles for practical for practical applications applications ranging fromranging catalysis, from opto-electronic,catalysis, opto-electronic, or biomedical or biomedical applications. applicatio In thesens. di Inff erentthese examples,different exam the stabilityples, the of stability the resulting of the nanocompositesresulting nanocomposites is a major issueis a major for further issue applicationsfor further applications and this point and is stillthis anpoint active is researchstill an active field. Dependingresearch field. on theDepending applications, on the the applications, toxicity of the the functionalizing toxicity of the agentfunctionalizing must also agent be considered; must also this be pointconsidered; has only this been point scarcely has only examined been scarcely in these examin differented in works. these Consideringdifferent works. that Considering the stability andthat the toxicitystability ofand the the nanocomposites toxicity of the will nanocomposites govern their future will govern applications, their future this point applications, should be this carefully point examinedshould be incarefully the coming examined years. in the coming years.

AuthorAuthor Contributions:Contributions: Writing—OriginalWriting-Original Draft Draft Preparation, Preparation, F.D., E.D.E.D. andand C.R.M.;C.R.M.; Writing—ReviewWriting-Review & Editing, F.D.,F.D., E.D.E.D. andand C.R.M.C.R.M. All authors have read and agree to the published version of the manuscript.

Funding:Funding: This research was funded by UniversitUniversiteé́ Paris-Sud, ENS Paris-Saclay, Aix MarseilleMarseille University, thethe UniversityUniversity ofof Versailles SaintSaint QuentinQuentin enen YvelinesYvelines andand thethe CentreCentre NationalNational dede lala RechercheRecherche ScientifiqueScientifique (CNRS).(CNRS). Acknowledgments: Authors thanks Université Paris-Sud, ENS Paris-Saclay, Aix Marseille University, the UniversityAcknowledgments: of Versailles Authors SaintQuentin thanks enUniversite Yvelineś Paris-Sud, and the Centre ENS National Paris-Saclay, de la RechercheAix Marseille Scientifique University, (CNRS) the forUniversity financial of support. Versailles Saint Quentin en Yvelines and the Centre National de la Recherche Scientifique (CNRS) for financial support. Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest. References

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