nanomaterials Review Functionalization of Gold Nanoparticles 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 nanocomposite 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 nanocomposites (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 Nanomaterials 2020, 10, 548; doi:10.3390/nano10030548 www.mdpi.com/journal/nanomaterials Nanomaterials 2020, 10, 548 2 of 36 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. solvent 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. 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