Surface-Enhanced Raman Spectroscopy: from the Few-Analyte Limit to Hot-Spot Saturation

Surface-enhanced Raman spectroscopy (SERS) gained much attention following initial claims and subsequent verifications of single-molecule sensitivity. SERS substrates based on plasmonic nanoparticles in close proximity create "hot spots" when illuminated, which, in the single-molecule limit, follow log–normal statistics for molecular occupancy. Here, we rigorously examine the transition from the single-molecule limit to the limit of hot spot saturation, a regime that follows Gaussian statistics, by varying a 1:1 bianalyte concentration over 3 orders of magnitude. The bianalyte model is extended here to follow this transition, and the electromagnetic "hot spots" of both Au nanoparticle and Au nanoshells-based SERS substrates are described theoretically. This combined experimental-theoretical study provides a rigorous foundation for quantifying trace analyte detection over a wider and highly practical concentration range.