Oxidative Etching and Metal Overgrowth of Gold Nanorods within Mesoporous Silica Shells

Composite noble metal-based nanorods for which the surface plasmon resonances can be tuned by composition and geometry are highly interesting for applications in biotechnology, imaging, sensing, optoelectronics, photovoltaics, and catalysis. Here, we present an approach for the oxidative etching and subsequent metal overgrowth of gold nanorods, all taking place while the nanorods are embedded in mesoporous SiO₂ shells (AuNRs@meso-SiO₂). Heating of the AuNRs@meso-SiO₂ in methanol with HCl resulted in reproducible oxidation of the AuNRs by dissolved O₂, specifically at the rod ends, enabling precise control over the aspect ratio of the rods. The etched-AuNRs@meso-SiO₂ were used as a template for the overgrowth of a second metal (Ag, Pd, and Pt), yielding bimetallic, core-shell structured nanorods. By varying the reaction rates of the metal deposition both smooth core-shell structures or gold nanorods covered with a dendritic overlayer could be made. This control over the morphology, including metal composition, and thus the plasmonic properties of the composite rods were measured experimentally and also confirmed by Finite-Difference Time-Domain (FDTD) calculations. The presented synthesis method gives great control over tuning over both plasmonic properties and the particle stability/affinity for specific applications.