UV fluorescence enhancement from nanostructured aluminum materials

Abstract

Interest in label-free detection of biomolecules has given rise to the need for UV plasmonic materials. DNA bases and amino acid residues have electronic resonances in the UV which allow for sensitive detection of these species by surface-enhanced UV fluorescence spectroscopy. Electrochemical roughening has been used extensively to generate plasmonically-active metal surfaces that produce localized enhancement of excitation and emission of electromagnetic radiation from surface-bound molecules. Electrochemically roughened gold and silver surfaces produce enhancement in the visible and near-IR regions, but to the best of our knowledge, application of this technique for producing UV-enhancing substrates has not been reported. Using electropolishing of aluminum, we are able to generate nanostructured surfaces that produce enhanced spectroscopic detection of molecules in the UV. Aluminum is a natural choice for substrate composition as it exhibits a relatively large quality factor in the UV. We have fabricated electropolished aluminum films with nanometer scale roughness and have studied UV-excited fluorescence enhancement from submonolayer coverage of tryptophan on these substrates using a UV-laser based spectrometer. Quantitative dosing by dip-coating was used to deposit known surface concentrations of the aromatic amino acid tryptophan, so that fluorescence enhancement could be evaluated. Compared to a dielectric substrate (surface-oxidized silicon), we observe a 180-fold enhancement in the total fluorescence emitted by tryptophan on electropolished aluminum under photobleaching conditions, allowing detection of sub-monolayer coverages of molecules essential for development of biosensor technologies.