Optimized Silica Shell Synthesis Surrounding Gold Nanorods for Enhanced Spectroscopies

We present a precise study for coating gold nanorods (GNRs) by a silica shell layer. Different parameters like pH, reaction time, and, most importantly, the ratio between the selected surfactant and the tetraethyl orthosilicate (TEOS) are studied and discussed. This latter is the key parameter to increase reproducibility, stability, reaction time, and the control of the targeted silica shell thickness. Moreover, the silica shell growth induces an optical shift of the localized surface plasmon resonance (LSPR) bands, which is explained by the change of the local refractive index. Both extinction spectroscopy and scanning electronic microscopy (SEM) are used to characterize the growth of the silica shell in time. Based on these results, it is thus possible to follow the growth process just by extinction spectroscopy, which ensures a rapid control of the reaction and makes it possible to stop it on time depending on the targeted shell thickness. Controlling the thickness is mandatory for further GNRs functionalization and surface plasmon enhanced spectroscopies.