Reverse microemulsion-mediated synthesis of silica-coated gold and silver nanoparticles

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 × 10¹⁰ 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 × 10¹³ 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.