Detection of low quantum yield fluorophores and improved imaging times using metallic nanoparticles.

The behavior of a fluorophore near a gold nanoparticle is rationalized by a theoretical description of the parameters that modify the fluorescence emission: nanoparticle-fluorophore distance, fluorescence quantum yield (φ(0)), and fluorophore absorption and emission spectra, to find optimum conditions for designing fluorophore-nanoparticle probes. The theoretical maximum gain in brightness of the nanoparticle-fluorophore system with respect to the isolated molecule increases almost inversely proportional to φ(0). The brightness enhancement in imaging experiments in vitro was assessed by using Au-SiO(2) core-shell nanoparticles deposited on glass. A ~13-fold emission brightness enhancement for weakly fluorescent molecules was observed. A significant increase in fluorophore photostability, rendering longer imaging times, was obtained for fluorophores interacting with gold nanoparticles incorporated by endocytosis in cells. Our results illustrate a way to increase imaging times and to study molecules in the vicinity of a metallic nanoparticle after photobleaching of background fluorescence.