Surface-Enhanced Resonance Raman Scattering on Gold Concentric Rings: Polarization Dependence and Intensity Fluctuations

The spatial and temporal dependence of the surface enhanced resonance Raman scattering (SERRS) from Nile Blue A (NBA) adsorbed on concentric rings on a gold film was probed. The SERRS intensity mapping of the concentric rings, obtained using linearly polarized excitation, was well-fitted by a sin(2) theta function. This spatial variation of SERRS intensity from the nanostructure was in good qualitative agreement with finite-difference time-domain numerical calculations. Histograms of the time-dependent SERRS intensities showed a normal-distribution for concentrations of NBA in the micromolar range and a tailed distribution for diluted solutions (nanomolar range). Time-dependent fluctuations of SERRS intensities were observed when the concentrations of NBA reached 200 nM. The maximum relative SEARS intensity and its absolute variation, obtained from the experiments with diluted solutions, was much lower than previously observed from random substrates that support SERRS, such as colloids immobilized on glass. This suggests that organized nanostructures, such as the concentric nanorings, present a more uniform distribution of regions of localized enhanced electric field.