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 (phi(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 phi(0). The brightness enhancement in imaging experiments in vitro was assessed by using Au-SiO2 core shell nanoparticles deposited on glass. A similar to 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.