Single-molecule and single-nanoparticle SERS: Examining the roles of surface active sites and chemical enhancement

Recent research in several groups has identified a new class of metal colloidal nanoparticles that is able to enhance the efficiencies of surface-enhanced Raman scattering (SERS) by as much as 10(14)-10(15) fold. This enormous enhancement allows single-molecule detection and spectroscopy at room temperature. Previous single-molecule and single-particle studies have yielded important insights into the mechanism of electromagnetic field enhancement, but little is known about the contributions of surface active sites and chemical enhancement. Here we report a direct examination of chemical enhancement by using an integrated flow injection and ultrasensitive optical imaging/spectroscopy system. A key feature is that colloidal silver nanoparticles are immobilized on a glass surface inside a microflow device and that single-particle SERS signals are observed in real time while the immobilized particles are treated by chemical reagents in the flow cell. In situ surface plasmon scattering studies of spatially isolated particles indicate that their electromagnetic properties do not change after chemical treatment. Thus, the observed SERS spectral changes should primarily come from chemical enhancement at surface active sites. Our experimental data reveal that three halide ions (Cl-, Br-, and I-) have a substantial activating effect, whereas other ions such as citrate, sulfate, and fluoride have little or no effect on single-particle SERS. A "quenching" effect is observed for thiosulfate ions, which completely destroys the SERS activity. The chemical enhancement factors are estimated to be about 10(2)-10(3) for rhodamine 6G molecules adsorbed on single Ag nanoparticles.