The facile synthesis of nitrogen and sulfur co-doped carbon dots for developing a powerful "on-off-on" fluorescence probe to detect glutathione in vegetables

To determine the glutathione (GSH) content in vegetables, an "on-off-on" fluorescence probe was developed by a synthesis of nitrogen and sulfur co-doped carbon dots (N,S-CDs) using the microwave pyrolysis considering citric acid and L-cysteine as precursors. The fluorescence of N,S-CDs was quenched by adding Cu2+ at a concentration of 20-200 mu mol/L due to the inner filter effect. The quenched fluorescence of N,S-CDs@Cu2+ system was recovered by adding the GSH at a concentration of 10-150 mu mol/L due to the sulfhydryl-metal compound mechanism. By observing the GSH concentrations measured by our N,S-CDs@Cu2+ system vs. a traditional fluorescent chelating method, the two measurements provided the GSH data with a good consistence by showing the RSD range of 1.86%-2.27%. This indicates the validation and novelty of our N,S-CDs@Cu2+ system as being a powerful fluorescent probe for effectively and efficiently determining the GSH in vegetables.