Plasmonic Gold Prism Array for Digital Surface-Enhanced Raman Spectroscopy Sensing.

While surface-enhanced Raman spectroscopy (SERS) has proved itself as a powerful tool in the analytical domain, its capability hinges on, and therefore, is governed by the interaction between molecules and plasmonic hotspots on nanostructured metallic substrates. Yet, the highly dynamic and complex molecule-metal interactions give rise to pronounced SERS signal fluctuations, compromising quantitative analysis at ultralow molecular concentrations. Rather than overcoming SERS signal fluctuations, the recently introduced digital SERS approach takes full advantage of their stochastic nature and allows digitalized visualization of SERS events at the single pixel level based on a predefined signal threshold, significantly pushing down the lowest detectable molecular concentration. Nevertheless, extending digital SERS for routine molecular analysis requires superior two-dimensional plasmonic substrates. Herein, we introduce a plasmonic gold prism array for digital SERS sensing. As opposed to nanostructured metallic arrays with similar geometry where the vertex and edge modes dominate, the gold prism array displays strong scattering as well as pronounced surface roughness and nanoparticle-like surface features, thus supporting additional plasmonic enhancements beyond the vertex and edge modes. Using 4-aminothiophenol as a model molecule, we first verify the counterintuitive distribution of SERS hotspots at the center of a gold prism instead of its vertices, and further demonstrate digital SERS sensing at the femtomolar level. We envision that the gold prism array could serve as a superior plasmonic platform for a wide range of digital SERS applications in diagnostics, environmental monitoring, and food safety.