Controlled spontaneous generation of gold nanoparticles assisted by dual reducing and capping agents

During the last decades, noble-metal nanoparticles have attracted a great deal of interest by their unique optical, electronic, magnetic, and catalytic properties and intense research efforts are still devoted to develop new synthetic and functionalizing strategies [ 1 – 5 ]. This extremely active research field was supported by the amazing chemical and physical properties displayed by the metal particles of nanometric size that are markedly different from those of the corresponding bulk materials [ 6 ]. Especially, optical and electronic properties of metal nanoparticles can be easily tuned by modifying their size and shape [ 7 ]. Regarding noble metals nanoparticles, gold nanoparticles (Au-NPs) are without contest at the forefront in this research area. Academic interest for Au-NPs, which showed fast growth over the past years, is motivated by the strong surface plasmon resonance displayed by gold nanoparticles. In addition, gold nanoparticles gained a renewal of interest by finding potential uses in medical diagnostics, imaging, and therapeutic treatments. In these last fields, preparation of Au-NPs with benign reactants is often favored to remove all potential contamination of the colloidal solutions [ 8 – 14 ]. To date, four different classes of biological applications for Au-NPs have been identified: labeling, delivering, heating, and sensing [ 15 ]. However, the use of gold nanoparticles was not limited to biological applications and Au-NPs were also successfully employed as scaffolds for molecular recognition of elements and molecules [ 16 ], in optoelectronics and data storage [ 17 ], in nanotechnology with molecular switches [ 18 ] and motors [ 19 ], or in light-harvesting assemblies [ 20 , 21 ]. Typically, gold nanoparticles are obtained by chemical reduction of tetrachloroauric acid [ 22 , 23 ]. However, this conventional approach is based on the use of external chemical reductants that often produce undesired side-products. Therefore, a series of functionalizing agents for Au-NPs has recently been developed that display a dual role of effective reducing agents of gold salts and of stabilizers, by providing a robust coating to gold nanoparticles, within a unique reaction step. Seven different types of these reducing/capping agents were investigated to date: microorganisms and bacteria, plants extracts and physiological molecules, inorganic reagents and metal complexes, organic molecules, organic acids and salts, liposomes, and polymers (Table 1 ). In this review, we propose to focus on these exciting functionalizing agents exerting the dual role of reducing and coating agents and to discuss the precise size-controlled synthesis of Au-NPs using this approach.