Nanoplasmonic enhancement of single-molecule fluorescence

We demonstrate that the fluorescence rate from a single molecule with near-unity quantum yield can be enhanced by a factor of approximate to 10 by use of a single laser-irradiated noble metal nanoparticle. The increased fluorescence rate is primarily the result of the local field enhancement. However, at particle-molecule distances shorter than 2 nm, nonradiative decay of the excited molecule due to energy transfer to the metal dominates over the local field enhancement giving rise to fluorescence quenching. These counteracting processes depend on the wavelength-dependent dielectric function of the particle antenna. In this study, we quantitatively compare single-molecule fluorescence enhancement near 80 nm gold and silver nanoparticles excited at a fixed wavelength of lambda = 637 nm. In accordance with theory we find similar enhancements for both gold and silver nanoparticles.