Synthesis of optically tunable bumpy silver nanoshells by changing the silica core size and their SERS activities

The fabrication of hollow metal nanostructures on a silica core template has been widely studied by taking advantage of the chemical stabilities of silica cores. When the size of the silica core reduces, however, this benefit is no longer effective because there are the synthetic difficulties which often cause dispersion instability, and therefore finally the aggregations of nanoparticles (NPs) are often caused during introduction of metallic nanostructures. This study reports the successful fabrication of silver nanoshells (AgNSs; 119, 152, 165, 186, and 207 nm) on amorphous silica nanoparticles (Si NPs) of different core sizes (59, 82, 103, 124, and 148 nm) by overcoming the increased instabilities during fabrication with reduced core sizes. Improvements related to fabrication were made by changing the reducing agents, controlling the amount of the dispersing agent and the concentration of Si NPs. All the AgNSs showed broad extinction from the visible to the near-infrared (NIR) region regardless of particle sizes, and their size-dependent surface properties were analyzed by introducing a concept of roughness factor, with AgNS of 152 nm exhibiting the highest degree of roughness. High SERS enhancements of AgNSs of all sizes were observed at three laser excitation wavelengths (532, 660, and 785 nm), and these enhancements correlated positively with the surface roughness. Therefore, our results provide a clear understanding of size-dependent SERS activity for AgNS, facilitating proper selection of AgNS with an appropriate size depending on the purpose of the investigation.