The plasmonic engineering of metal nanoparticles for enhanced fluorescence and Raman scattering

We have investigated the effects of tuning the localized surface plasmon resonances (LSPRs) of silver nanoparticles on the fluorescence intensity, lifetime, and Raman signal from nearby fluorophores. The presence of a metallic structure can alter the optical properties of a molecule by increasing the excitation field, and by modifying radiative and nonradiative decay mechanisms. By careful choice of experimental parameters we have been able to decouple these effects. We observe a fourfold increase in fluorescence enhancement and an almost 30-fold increase in decay rate from arrays of Ag nanoparticles, when the LSPR is tuned to the emission wavelength of a locally situated fluorophore. This is consistent with a greatly increased efficiency for energy transfer from fluorophores to surface plasmons, resulting in a significant increase in quantum yield. Additionally, spatial mapping of the surface enhanced Raman scattering signal from a nanoparticle array reveals highly localized differences in the excitation field.