Bright Green Light-Emitting Gold Nanoclusters Confined in Insulin as Selective Fluorescent Switch Probes for Sensing and Imaging of Copper Ions and Glutathione

In this study, an on–off–on fluorescence probe is developed for rapid, sensitive, and selective detection of Cu2+ ions and glutathione (GSH) based on green light-emitting insulin-capped gold nanoclusters (Ins/Au-NCs). Ins/Au-NCs with low toxicity and excellent stability were prepared by utilizing a simple and green strategy. Cu2+ ions can selectively decrease the fluorescence intensity of Ins/Au-NCs by an aggregation-caused quenching mechanism, which can be recovered in the presence of GSH. Under the optimized condition, the detection limits for determination of Cu2+ ions and GSH are estimated as 7.5 and 9.2 nM, with linear ranges of 0.05 to 1.7 μM and 0.05 to 1.5 μM, respectively. Chemical modeling was also employed to investigate the interaction of the Ins/Au-NCs probe with Cu2+, Hg2+, glutathione, and cysteine. The calculated molecular orbital energy well confirmed the experimental selective results obtained. Luminescence cellular imaging exhibited a sensitive response of the Ins/Au-NCs probe toward Cu2+ ions in breast cancer cells MCF-7, and thus, it potentially acted as an intracellular GSH probe in MCF-7 compared to the human fibroblast HFF cells. The as-prepared probe was successfully applied to low levels of Cu2+ ions and GSH in water and human serum samples. The designed Ins/Au-NCs-based probe may also provide a biocompatible platform for label-free GSH detection in biological and clinical implications that comprise staining and early prevention of cancer, AIDS, and Alzheimer's and cardiovascular diseases.