Precision-Engineered Nanononamers with Open Internal Hotspots for Single-Particle Surface-Enhanced Raman Scattering.

Plasmonic nanostructures with open intraparticle nanogaps are essential for concentrating the electromagnetic near-field, enabling reliable, label-free surface-enhanced Raman scattering (SERS) detection. However, precise engineering of such nanostructures in a simple and general way for optimized near-field focusing remains challenging. Here, we present a facile synthetic strategy for fabricating frame-like AuAg nanononamers (NNs) with tunable internal hotspots for single-particle SERS applications. The synthesis involves a controlled overgrowth of Ag layers on Au nanoparticles, followed by a galvanic replacement reaction, producing an architecture that encapsulates a central Au nanosphere or nanocube within a network of interconnected AuAg nanosphere octamers. By tuning the core Au nanoparticle shape and Ag layer thickness, the structural parameters and intragap sizes can be precisely adjusted. The optimized AuAg NNs exhibit intense near-field focusing at the nanogaps, resulting in a high single-particle SERS activity. This strategy offers significant potential for ultrasensitive SERS applications.