Reversible Photobleaching of Gold Nanoclusters: A Mechanistic Investigation

Abstract

Fluorescent gold nanoclusters (AuNCs) have found widespread applications for designing of novel sensing elements for diagnostics and in vivo and in vitro imaging. Herein, we investigated the effect of ultraviolet (UV) irradiation on the fluorescence characteristics of bovine-serum-albumin-capped AuNCs. When the aqueous solution of these nanoclusters was exposed to UV irradiation, a rapid decrease in the emission intensity at about 620 nm accompanied by an increase in emission at about 420 nm was observed. After turning off the UV light and keeping the solution in the dark, a gradual recovery of the quenched fluorescence was observed such that after about 400 min it reached its original emission value. The mechanism of these fluorescence changes was investigated by transmission electron microscopy, X-ray photoelectron spectroscopy, and monitoring of the effect of pH. The observed on–off fluorescence characteristics can be used as the basis for development of novel sensors. In this work, we used this system for an assay of antioxidant activity. The antioxidant inhibited the photobleaching and thus could turn on the photoquenched fluorescence of the BSA-AuNCs.