A New Figure of Merit to Assess the SERS Enhancement Factor of Colloidal Gold Nanoparticle Aggregates

Plasmonic nanoparticle aggregates are one of the most widely employed nanostructures as colloidal surface enhanced Raman spectroscopy (SERS) substrates mainly due to their ability to generate huge near-field enhancements. However, the available definitions to determine the enhancement factor, a key parameter to quantitatively describe the quality of SERS substrates, exhibit important limitations to adequately describe the performance of colloidal plasmonic nanoparticle aggregates as SERS platforms. Herein, we introduce a new figure of merit named active concentration enhancement factor (ACEF) to assess the SERS enhancement factor of colloidal gold nanoparticle aggregates, which shows significant improvements to characterize the SERS performance with respect to formerly reported parameters. The determination of the analyte active concentration, that is, the concentration of analyte molecules that effectively contributes to the SERS signal, was achieved according to a strategy that involves a rigorous modeling of the extinction spectra of the colloidal gold nanoparticle aggregates. Furthermore, the experimentally obtained ACEF values can be directly compared with theoretically calculated electromagnetic field enhancement factors. We believe that the figure of merit introduced might help to strengthen the development and design of SERS substrates.