Single-Particle Analysis on Plasmonic Nanogap Systems for Quantitative SERS

With advances in the design, synthesis and analysis of various metallic nanoparticles and substrates, surface-enhanced Raman scattering (SERS) with plasmonic nanostructures has been extensively studied, and numerous SERS applications have been demonstrated in various applications including biomedical applications; however, the mechanism of SERS is not completely understood yet, and many challenges, including structural and spectral reproducibility, exist to achieve quantitative SERS analysis for practical and reliable use of SERS. Since SERS signal reproducibility mainly stems from structural reproducibility of targeted nanostructures, single-particle SERS analysis is highly beneficial in understanding SERS signals generated from different plasmonic nanostructures and provides analytical insights that cannot be obtained with ensemble-average spectrum-based analysis. Single-particle analysis is typically composed of single-particle images and spectra, and the statistical results show the single-particle SERS enhancement factor distribution of SERS signals and precise structure-spectrum relationship. In particular, studying and evaluating single-molecule SERS results require single-particle analysis to fully understand how single-particle images and spectra are correlated with how the position, orientation and resonance of a Raman dye affect single-molecule SERS signals from individual nanoparticles, and this is often correlated with computational simulation results. In this mini-review, we introduce key issues for quantitative SERS and present the fundamental SERS features obtained by single-particle analysis, focused on plasmonic nanogap structures since these structures offer the very strong electromagnetic field-based SERS signals with high controllability in structure and signal. We categorized the nanogap particle-based SERS platforms into two different classes - plasmonic nanogap strctures with an intergap (the gap between two structures; intergap nanoparticles) and plasmonic nanogap structures with an intragap (the gap formed inside a single particle; intragap nanoparticles). Finally, we discuss the challenges and perspectives in designing and synthesizing nanogap structures that deliver strong, reproducible, and reliable SERS signals for the quantitative SERS analysis.