Fluorescence decay rate engineering using aluminum nanohole arrays

Abstract

UV plasmonics has drawn increased attention in recent years, holding promise in enabling label-free sensing of biomolecules such as DNA, peptides, and proteins whose intrinsic fluorescence lies in the UV range. However, these biomolecules exhibit relatively small quantum yields (QY) and extinction cross sections. In order to realize label-free detection of biomolecules, significant enhancement needs to be achieved. Several plasmonic structures have been reported to enhance native fluorescence of DNA and amino acids, with <80 x net enhancement for DNA and <15 x net enhancement for amino acids. Orders of magnitude improvement in the net enhancement factor are needed in order to achieve a detection limit comparable to commercial bioassays. In addition, quantitative fluorescence analysis that can differentiate the contribution of radiative and excitation enhancement is needed for UV studies. Here we report fluorescence enhancement of tryptophan on aluminum hole arrays. By optimizing excitation geometry, the hole size and spacings, we are able to achieve <40 x net enhancement factor, the highest ever observed for tryptophan molecules. We conducted photobleaching experiments and observed 2 x reduction in the fluorescence decay rate on the aluminum hole array compared to an aluminum thin film. The enhancement of total photon yield reaches 17 x, which indicates enhanced radiative rate. The studies we conducted will pave a way for label-free biosensing using UV plasmonics.