Single molecule sensitivity in surface enhanced Raman scattering

Abstract

Elastic scattering and emission spectra simultaneously observed with enormous SERS (Surface Enhanced Raman Scattering) signal from dye and adenine on hot or blinking silver nanoparticles are discussed to get insight into the enhancement mechanism. Huge SERS signal from touching Ag nanoparticles is extinguished by duration of measurement possibly due to thermal diffusion or desorption of adsorbed molecules. Simultaneously, elastic scattering peak at ca. 630 nm disappeared. Three-dimensional Finite Difference Time Domain (FDTD-3D) method manifests this scattering peak originates from enhanced coupling of localized surface plasmon (LSP) on adjacent Ag particles through absorption of adspecies located at their junction. Simultaneously with vast SERS signal, distinct emission peaks were observed at 550-600 nm and 600-750 nm. Based on the spectral variations for different surface coverage and for different dye species, the shorter and longer wavelength peaks were attributed to excited electron on metal, and from fluorescence of molecules, respectively. Furthermore, we found the shorter wavelength peak shows invariant Stokes shift irrespective of excitation wavelengths and adsorption species, indicating it arises from inelastic scattering of excited electron by adsorbed molecules. Nanosphere lithography was exploited to fabricate metal nanostructure with single molecule sensitivity in SERS, which results in much higher probability of the blinking compared to continuous Ag films.