Plasmonic Coupling and Raman Enhancement in Two-Dimensional Arrays of Octahedral Au Nanoparticles

Surface-enhanced Raman scattering (SERS) has been widely investigated as a sensitive technique for chemical and biological sensing. For applications such as intracellular sensing, nanoparticles used as SERS probes must be of an appropriate size to be introduced into cells while maintaining strong Raman enhancement. In this study, two-dimensional assemblies of octahedral gold nanoparticles (AuNPs) were investigated as a plasmonic sensing platform. The morphology and assembly structure of the octahedral AuNPs were characterized using scanning electron microscopy and scanning transmission electron microscopy. Localized plasmon modes in the nanoparticle assemblies were analyzed using electron energy loss spectroscopy. The results show that strong plasmon localization occurs in the gaps between neighboring octahedral AuNPs, indicating the formation of plasmonic hotspots. Raman measurements demonstrate that the assembled octahedral AuNPs provide clear SERS signals at analyte concentrations down to 100 nM, with a signal-to-noise ratio of 17.5 for the characteristic peak. Compared with conventional spherical nanoparticle systems, the faceted geometry of octahedral AuNPs enables smaller interparticle gaps and efficient plasmon coupling without requiring additional shell structures, or complex gap-engineering processes. These results suggest that shell-free assemblies of octahedral AuNPs provide a simple platform for correlating nanoscale LSPR localization with Raman enhancement and may serve as a basis for compact SERS probes toward future cellular sensing applications.