Reverse Microemulsion-Mediated Synthesis of Silica-Coated Gold and Silver Nanoparticles Yu Han, Jiang Jiang, Su Seong Lee, and Jackie Y. Ying* Institute of Bioengineering and Nanotechnology, 31 Biopolis Way, The Nanos, Singapore 138669 ReceiVed NoVember 6, 2007. ReVised Manuscript ReceiVed March 2, 2008 A reverse microemulsion method is reported for preparing monodispersed silica-coated gold (or silver) nanoparticles without the use of a silane coupling agent or polymer as the surface primer. This method enables a fine control of the silica shell thickness with nanometer precision. As compared to the Stöber method reported for direct silica coating, which can only coat large gold particles (∼50 nm in diameter) at low concentrations (<1.5 × 10 10 particles/mL), this new approach is capable of coating gold particles of a wide range of sizes (from 10 to 50 nm) at a much higher concentration (∼1.5 × 10 13 particles/mL). Moreover, it enables straightforward surface functionalization via co- condensation between tetraethyl orthosilicate and another silane with the desired functional groups. The functional groups introduced by this method are readily accessible and thus useful for various applications. Introduction Gold and silver colloidal nanoparticles have been intensively studied due to their unique size- and shape-dependent optical properties, which can be further tailored by their surrounding dielectric medium (e.g., silica shell). 1–8 Silica coating can also enhance the stability of the nanoparticles against aggregation and provide tunable solubility in various solvents. 9 Monodispersed silica-coated particles readily self-assemble into photonic crystals with an optical band gap in the visible region. 3,5 Moreover, the silica shells can be easily functionalized, allowing the nano- particles to be conjugated to other substrates or molecules. 10 Therefore, it is of great research interest to develop a simple and versatile procedure for the silica coating of gold and silver nanoparticles. Unlike metal oxide nanoparticles or semiconductor quantum dots, which can be easily coated with silica, 11–15 gold and silver were considered not to be suitable for direct silica coating due to their low chemical affinity to silica. It was believed that a surface primer was needed to initiate the growth of a silica shell on a gold surface. 16,17 Mulvaney and co-workers first reported a three-step procedure for coating gold nanoparticles with silica using a silane coupling agent, aminopropyltrimethoxysilane (APTMS), as the surface primer. 16 A precoating step with sodium silicate was also required before further growing the silica shell via the Stöber process. This method is effective but time- consuming, and it is not very easily controlled due to the complicated process involved. Alternatively, using a nonionic polymer, poly(vinylpyrrolidone) (PVP), instead of a silane coupling agent as the surface primer, Graf et al. developed a general method to coat colloidal particles (including gold and silver) with silica. 17 Although simpler and faster, the polymer- mediated route requires the proper choice of polymer, whose length strongly influences the homogeneity and smoothness of the silica coating. 9,17 It was found later that surface primers were not necessary for the silica coating of gold or silver particles. Several research groups have reported independently that a slightly modified Stöber process could be applied directly for the silica coating of gold or silver nanoparticles without the use of any surface primers. 5,10,18 However, this straightforward scheme can only be used for coating large gold particles (∼50 nm in diameter) because small particles are unstable and tend to aggregate in the alcoholic solution used for the Stöber process. 5,10 Second, the method is only effective when the concentration of gold nanoparticles is below 1.5 × 10 10 particles/mL. 5,10 A higher concentration would give rise to the formation of irregular silica- coated particles with multiple gold cores, as is illustrated later in this study. As a result, this process would require a significant use of solvent. Although much effort has been put into the synthesis of silica- coated gold nanoparticles, 1–8,19 surface functionalization of such particles has seldom been investigated. 9,10 Surface functional- ization would allow the desired molecules to be immobilized on or conjugated to the particles, enabling targeted biological applications such as site-specific markers, probes, and sensors. Herein, we present a simple reverse microemulsion method for preparing silica-coated gold and silver nanoparticles. Without using a silane coupling agent or bulky polymer stabilizer, this method enables direct silica coating on gold and silver particles. * Corresponding author. E-mail: jyying@ibn.a-star.edu.sg.; fax: (+65) 6478-9020. (1) Carotenuto, G.; Pepe, G. P.; Nicolais, L. Eur. Phys. J. B 2000, 16, 11. (2) Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2001, 105, 5599. (3) Wei, A.; Kim, B.; Sadtler, B.; Tripp, S. L. Chem. Phys. Chem. 2001, 2, 743. (4) Wang, W.; Asher, S. A. J. Am. Chem. 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