Quantification of Resonance Raman Enhancement Factors for Rhodamine 6G (R6G) in Water and on Gold and Silver Nanoparticles: Implications for Single-Molecule R6G SERS

Abstract

The resonance Raman (RR) enhancement factors of Rhodamine 6G (R6G) in water and on gold and silver nanoparticles (AuNPs and AgNPs) were determined using a double ratiometric method where adenine is used as the internal reference. The RR enhancement factor for R6G on AgNPs upon laser excitation at 532 nm is 537.6 ± 214.8. This is ∼5 times lower than the experimental (2.7 ± 0.3) × 103 RR enhancement factor for R6G in water. These experimental RR enhancement factors for R6G in water and on AgNPs are 104 smaller than the 107 RR enhancement proposed in literature for R6G in water and on SERS substrates. In addition, a simple back-of-the-envelope calculation showed that even with this damped RR for R6G on AgNPs in comparison to R6G in water, a SERS enhancement factor of 106 is sufficient to explain the single-molecule resonance SERS activities reported for R6G located in nanoparticle junctions. This conclusion is deduced from fact that normal Raman spectrum could be readily obtained with 24 fmol of adenine at laser focal volume of ∼150 fL at 532 nm excitation. This work provides the first direct experimental evidence for the recent theoretical predication that plasmonic nanoparticles quench the resonance Raman signal. In addition, the double ratiometric method reported in this work represents a significant technique development in Raman and SERS, which should pave the way for quantitative investigations of the RR for dye molecules dissolved in solution or adsorbed on plasmonic nanoparticles.