Quantitative SERS by hot spot normalization - surface enhanced Rayleigh band intensity as an alternative evaluation parameter for SERS substrate performance

The performance of surface-enhanced Raman spectroscopy (SERS) substrates is typically evaluated by calculating an enhancement factor (EF). However, it is challenging to accurately calculate EF values since the calculation often requires the use of model analytes and requires assumptions about the number of analyte molecules within the laser excitation volume. Furthermore, the measured EF values are target analyte dependent and thus it is challenging to compare substrates with EF values obtained using different analytes. In this study, we propose an alternative evaluation parameter for SERS substrate performance that is based on the intensity of the surface plasmon enhanced Rayleigh band (I-Rayleigh) that originates from the amplified spontaneous emission (ASE) of the laser. Compared to the EF, I-Rayleigh reflects the enhancing capability of the substrate itself, is easy to measure without the use of any analytes, and is universally applicable for the comparison of SERS substrates. Six SERS substrates with different states (solid, suspended in liquid, and hydrogel), different plasmonic nanoparticle identities (silver and gold), as well as different nanoparticle sizes and shapes were used to support our hypothesis. The results show that there are excellent correlations between the measured SERS intensities and I-Rayleigh as well as between the SERS homogeneity and the variation of I-Rayleigh acquired with the six SERS substrates. These results suggest that I-Rayleigh can be used as an evaluation parameter for both SERS substrate efficiency and reproducibility.