Plasmonic Nanocavities with Metallic Spacer Layers for Ultrasensitive Detection of Cr⁶⁺Ions

Plasmonic nanocavities are ideally suited for optical sensing due to their unique ability to localize and focus light on the nanoscale. However, plasmonic nanocavities formed between metallic nanoparticles on a metal film are either hydrophobic or entirely filled with nonmetallic spacer layers, hindering molecular diffusion into the hotspot regions. In this study, we designed and fabricated a plasmonic structure of multitipped Au@Ag–Au particle in nanocuboids (PCs), integrated with a metal film to create open plasmonic nanocavities. By adjusting the pH of the growth solution to control the reduction rate and the etching rate of Au³⁺, we precisely regulated the selective growth of metallic nanoparticles on the surface of Au@Ag core–shell nanocuboids. This plasmonic nanocavity is spaced by metal nanoparticles and forms 5 nm cavities open to the surroundings, which results in significantly enhanced Raman performance compared to traditional plasmonic nanocavities. Finally, functionalization of this plasmonic nanocavity enabled ultrasensitive detection of Cr⁶⁺Ions.