A gold-silver bimetallic nanocluster-based fluorescent probe for cysteine detection in milk and apple

Noble metal nanoclusters have attracted much attention due to their excellent optical properties. In the present work, a silver-doped gold-based bimetallic nanoclusters (Au/Ag NCs) were reasonably designed and prepared through a one-pot method by using 5-mercapto-1-tetrazolea-acetic acid sodium salt (MTAS) as a protector and capping agent. In comparison with the monometallic nanoclusters, Ag-doped metallic nanoclusters show better performance. The particle size of the MTAS-Au/Ag NCs is slightly larger than that of the undoped Au NCs by about 1.86 ± 0.5 nm, and the MTAS-Au/Ag NCs exhibit an emission peak at 635 nm with a quantum yield (QY) of 3.05%. The presence of cysteine (Cys) induces efficient quenching of the photoluminescence of the obtained Au/Ag NCs, achieving the sensitive detection of Cys with a detection limit of 16 nM. The fluorescence quenching rate of the nano fluorescent probe has a linear relationship with the cysteine concentration. Under the best detection conditions, the linear range for Cys detection with MTAS-Au/Ag NCs as a probe is 0.05–25.0 μM. Moreover, this probe has been successfully applied to the analysis of Cys in milk and apples, and a satisfactory recovery rate has been obtained, indicating the effectiveness and reliability of the sensor system for the detection of actual samples.