We investigated the enhancement and quenching of dye fluorescence by single gold nanoparticles using an aperture-type scanning near-field optical microscope to evaluate the shape dependence. Enhancement or reduction of fluorescence was found to be strongly dependent on the shape of the particle. Gold nanoplates showed significant fluorescence enhancement, as did aggregated nanoparticles for the most part. On the other hand, gold nanorods with high aspect ratios showed quenching effects. The results suggest that the electromagnetic near-field in the vicinity of the surface of nanoparticles, not just near the apex or the tip but the whole surface, is important for understanding fluorescence enhancement and quenching.
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