Large-Scale Silica Overcoating of Gold Nanorods with Tunable Shell Thicknesses

Gold nanorods (GNRs) overcoated with SiO₂ are of interest for enhancing the shape stability of GNRs during photo-thermal heating, for further functionalization with silanes, and for biomedical applications. While methods have recently been developed for synthesizing GNRs on a large scale, SiO₂ overcoating of GNRs is still conducted on a small reaction scale. Here, we report a method for large-scale synthesis of SiO₂-overcoated GNRs (SiO₂-GNRs), which gives ~190 mg of SiO₂-GNRs. SiO₂ is deposited onto and encapsulates the cetyltrimethylammonium bromide (CTAB) coatings that stabilize GNRs by adding tetraethoxysilane (TEOS) via syringe pump. Control over the CTAB concentration is critically important for obtaining uniform overcoatings. Optical absorbance spectra of SiO₂-GNRs closely resemble uncoated GNRs, which indicates overcoating of single rather than multiple GNRs and confirms that they remain well dispersed. By adjusting the reaction conditions, shells as thick as ~20 nm can be obtained. For thin shells (< 10 nm), addition of poly(ethylene glycol) silane (PEG-silane) at different times during the overcoating reaction allows facile control over the shell thickness, giving shells as thin as ~2 nm. The bulky PEG chain terminates further crosslinking and deposition of SiO₂.