Two-Photon-Excited Single-Molecule Fluorescence Enhanced by Gold Nanorod Dimers

We demonstrate two-photon-excited single-mole-culefluorescence enhancement by single end-to-end self-assembledgold nanorod dimers. We employed biotinylated streptavidin asthe molecular linker, which connected two gold nanorods in end-to-end fashion. The typical size of streptavidin of around 5 nmseparates the gold nanorods with gaps suitable for the access offresh dyes in aqueous solution, yet small enough to give very hightwo-photonfluorescence enhancement. Simulations show thatenhancements of more than 7 orders of magnitude can be achievedfor two-photon-excitedfluorescence in the plasmonic hot spots.With such high enhancements, we successfully detect two-photon-excitedfluorescence for a common organic dye (ATTO 610) at the single-molecule, single-nanoparticle level