Synthesis of silica-core gold nanoshells and some modifications/variations

Gold nanoshells are particles usually composed of a spherical silica core coated with a thin gold layer. Their chemical and optical properties make them suitable and attractive for medical applications, namely cancer treatment and diagnosis, as they have been studied for biosensing, imaging, and photothermal ablation. For their synthesis, most of the reported methods are based on the first reported by Oldenburg et al. In this method, silica nanoparticles are first produced and then modified to incorporate amino groups aimed to adsorb small gold clusters, which in turn act as the nucleation sites for the reduction of additional gold until complete gold shells are formed. In this review, we examine some common conditions to synthesize gold nanoshells based on this process, along with important aspects that need to be followed to ensure the production of gold nanoshells having homogeneous size and shape that render suspensions with consistent properties in the near-infrared region of the electromagnetic spectrum. Since the customary method is laborious and time-consuming, three additional processes intended to simplify or reduce some steps are described as well. Finally, fundamental aspects on the chemistry of the synthesis and their variations involved in all the revised processes are also presented. Figures from our own findings are included to support these descriptions. Please notice that this review focuses on the synthesis; other reviews focus in optical properties and applications.