A paper-based fluorescent test for determination and visualization of cysteine and glutathione by using gold-silver nanoclusters

A method is described for fluorometric determination of cysteine (Cys) and glutathione (GSH), either in solution by using a fluorescence spectrometer, or by using a paper-based test with visual observation. The method is based on the finding that the yellow fluorescence of the gold/silver nanoclusters (AuAg NCs), with excitation/emission maxima of 358/574 nm, is quenched by Cys and GSH via NCs aggregation. The method is selective for Cys and GSH. The limits of detection for both Cys and GSH are 0.74 nM in solution state and 5.0 μM on paper-based test. Response is linear in the range from 2.3 nM to 110 μM and from 2.3 nM to 63 μM in solution for Cys and GSH, respectively. The method is not interfered by other amino acids even if their concentrations are 10 times higher. It was successfully applied to the determination of Cys in spiked serum samples.