Single-Molecule Spectroscopy of Semiconductor Nanocrystals on Plasmonic Nanostructures

In the past several years we have demonstrated the metal-enhanced fluorescence (MEF) and the significant changes in the photophysical properties of fluorophores in the presence of metallic nanostructures and nanoparticles using ensemble spectroscopic studies. Here, in the present study, we explored the new insights of these interactions using single-molecule fluorescence spectroscopy. The single molecule study is expected to provide more information, especially on the heterogeneity in the fluorescence enhancement and decrease in lifetimes associated with fluorophore-metal interactions, which is otherwise not possible to observe using ensemble measurements. For the present study, we considered using CdTe nanocrystals (QDots) prepared using modified Weller method as the fluorophores under investigation. QDots having few nanometer sizes, tunable absorption and fluorescence spectral properties, and high photo-stabilities are of important class of fluorescent probes. Because of these unique features Qdots are widely used as probes in various fields, including biological labeling and imaging. These CdTe nanocrystals show characteristic spectral features in solution and on the solid substrate. The CdTe nanocrystals dispersed in PVA and spin-casted on SiFs surface show ∼5-fold increase in fluorescence intensity and ∼3-fold decrease in lifetimes compared to on glass substrate. The data obtained using ensemble and single molecule spectroscopy are complimentary to each other. Additionally as anticipated we have seen increased heterogeneity in the plasmon induced fluorescence modulations. Moreover single molecule spectroscopic study revealed significant reduction in blinking of CdTe nanocrystals on plasmonic nanostructures. Subsequently, we present these important findings on metal-fluorophore interactions of CdTe nanocrystals (QDots) on plasmonic nanostructures.