Photobleaching of Fluorophores on the Surface of Nanoantennas

Photobleaching is a widespread problem in any optical study or application involving fluorophores and is particularly acute in the context of single-molecule detection. Fluorophores adsorbed on metallic nanostructures are not immune to this problem, but little effort has been devoted to understanding how photobleaching is modified on metallic surfaces and if any additional mechanisms are involved. Here, we tackle this issue both theoretically and experimentally. We present a model accounting for the modification of the standard triplet-state-mediated photobleaching mechanism on metal nanostructures supporting plasmon resonances. The wide distribution of enhancement factors on the surface is shown to result in an extremely nonexponential decay with a range of decay rates covering several orders of magnitude. This model is verified experimentally by performing time-dependent and power-dependent surface-enhanced Raman and fluorescence measurements on uniform arrays of gold nanodiscs with tunable plasmon resonances. We, moreover, discuss the possibility of extracting the enhancement factor distribution from such experiments. Finally, additional mechanisms of photobleaching associated with photoinduced heating of the metallic nanostructures are shown to play a role in some conditions. This study of photobleaching on nanoantennas paves the way for developing new techniques to either mitigate the problems or further exploit them.