Enormous Enhancement in Single-Particle Surface-Enhanced Raman Scattering with Size-Controllable Au Double Nanorings

To maximize near-field focusing around nanoscale metal particles, we designed large-sized (214 nm diameter) gold (Au) double nanorings comprising two concentric rings with nanoscale gaps between them (intra-nanogaps). The circular intra-nanogaps resembled the hot halos with which single-particle surface-enhanced Raman scattering (SERS) could occur when analytes are present in the gaps. Herein, we called the single-particle SERS-active Au double nanorings “Au nanohalos”. The synthesis of Au nanohalos was based on a novel strategy employing hexagonal Au nanoplates as a template, the lateral size of which can be controlled by the ratio of Au ions to iodide ions while maintaining the thickness. As is often implemented in organic chemistry synthesis, the initially triangular Au nanoplates went through multiple sequential on-demand steps to transform them into Au nanohalos that exhibited near-field focusing, confirmed by both single-particle SERS measurements and theoretical simulation. Importantly, the correlation between the distances of intragaps and particle sizes of Au nanohalos on single-particle SERS was systematically investigated. The estimated enhancement factor was very high, between 1.1 × 109 and 3.6 × 109, and was highly reproducible.