Fabrication of Monodispersed Au@SiO₂ Nanoparticles with Highly Stable Silica Layers by Ultrasound-Assisted Stöber Method

Abstract

Metal@dielectric composite nanostructures are of high demand for their vast technological applications. The Stöber method, either in its original form or by modification, has been utilized for the fabrication of silica shells over metal, semiconductor, or even dielectric nanostructures, with the aim to protect them from degradation, enhance their biocompatibility, or use them for molecular anchoring. However, the stability of silica shells and the dispersion of core–shell nanostructures remain the main limitations for their efficient applications. Here we demonstrate that utilization of ultrasound during hydrolysis and condensation of the metal–organic silicon precursor in Stöber process can produce stable and uniform silica shell layers around gold nanoparticles, enhancing both their stability and dispersion. Through transmission electron microscopy and infrared spectroscopy techniques, we demonstrate that the Au@SiO2 nanoparticles fabricated with ultrasound treatment during silica shell growth contain a lower content of silanol (Si–OH) groups, which are principally responsible for the instability of silica-coated metal nanostructures. The core–shell structures fabricated by ultrasound-assisted hydrolysis using prefabricated Au nanoparticles are well-dispersed, uniform in size, and protected from further hydrolysis in aqueous media, including simulated body fluid. The method applied to fabricate silica-coated Au nanoparticles can be utilized to fabricate other silica-coated metal nanoparticles to enhance their chemical and thermal stability.