Robust and Reproducible Quantification of SERS Enhancement Factors Using a Combination of Time-Resolved Raman Spectroscopy and Solvent Internal Reference Method

Recent research has demonstrated that the nanoparticle (NP) surface enhanced Raman spectroscopy (SERS) substrate modifies an analyte's Raman signal through two competitive mechanisms, SERS enhancement and NP inner filter effect, instead of SERS enhancement alone as commonly believed. Using a combination of time-resolved Raman spectroscopy and a solvent internal reference method, reported herein is a quantitative determination of the SERS enhancement factors (EFs) of mercaptobenzimidazole (MBI), a model organothiol, adsorbed onto gold and silver nanoparticles (AuNPs and AgNPs). The peak MBI SERS EF depends only on the type and size of NPs, but not analyte and NP concentrations, or the type (KF, KCl, KBr, and K2SO4) and concentrations of the electrolytic aggregation agents. The experimental SERS EFs of MBI on both AuNPs and AgNPs can be fully explained by the electromagnetic mechanism alone. This result, combined with our recent findings that a series of structurally diverse organothiols have similar SERS EFs, argues quite strongly against the possibility of large chemical enhancement (e.g., >10 times) for organothiols adsorbed onto colloidal AuNPs and AgNPs.