Architecting Plasmonic Hotspots at the Vertices of Octahedral AuAg Hybrid Nanocages for Surface-Enhanced Raman...

Achieving precise control over nanostructure morphology is key to advancing surface-enhanced Raman spectroscopy (SERS) through the creation of highly active plasmonic hotspots. Here, we transform solid Au@Ag core-shell nano-octahedra via galvanic replacement with HAuCl₄ into two distinct architectures: nano-octahedra with Au tipping on the corner at low precursor concentration and hollow Au-vertexed nanocages at higher concentration. Elemental mapping confirms a Ag-dominant octahedral body with Au-rich corners, which serve as localized plasmonic hotspots. The hollow nanocages can be further etched with hydrogen peroxide to enlarge their interior cavity, enabling systematic tuning of their plasmon resonance. These vertex-engineered nanostructures demonstrate a significantly improved SERS response, with octahedral AuAg hybrid nanocages achieving an optimal enhancement factor (EF) of 1.0 × 10⁹ using crystal violet (CV) as a probe molecule at a concentration as low as 10^-9 M. Specifically, for the 1619 cm^-1 peak, the enhancement is ∼1.14× greater at 10^-6 M and ∼1.69× greater at 10^-9 M compared to Au-tipped octahedral Au@Ag nanocrystals. Finite-element simulations attribute this improvement to the synergistic generation of strong electromagnetic field confinement and efficient charge separation at the Au-tipped vertices. This work provides a general strategy for designing high-performance SERS substrates through precise morphological engineering of metallic nanocages.