ZnS quantum dots surface-loaded with zinc(II) ions as a viable fluorescent probe for glutathione

The fluorescence of ZnS quantum dots in colloidal water solution changes very slightly on addition of glutathione (GSH) but is strongly enhanced in the presence of Zn(II) even in concentrations as low as 10 μM. The resulting Zn(II)-enhanced fluorescence is found to be quenched by GSH. In contrast to GSH, cysteine does not cause an effect. Response surface methodology was applied to optimize the experimental parameters. The best data can be obtained at 305/427 nm as excitation/emission wavelengths. These findings were used to design an indirect method for the fluorometric determination of GSH that has a 0.9 μM detection limit and a response that is linear in the 2.0-104.0 μM GSH concentration range. The relative standard deviation at a level of 65 μM of GSH (for n = 5) is 1.9%. Graphical abstract Schematic presentation of the detection strategy for glutathione and the influence of the mediator (Zn²⁺). Direct pathway shows that the glutathione cannot cause a change in the blue fluorescence of ZnS QDs. The presence of Zn²⁺ causes the enhancement of the fluorescence intensity, and this generates the indirect pathway to glutathione detection.