On-off-on fluorescent sensor for glutathione based on bifunctional vanadium oxide quantum dots induced spontaneous formation of MnO₂ nanosheets

Fluorescence sensing of glutathione by tailor-made chemical sensors is a prospective technique, which could provide simple, fast, and visual detection. Herein, a fluorescence sensor based on vanadium oxide quantum dots (VO_x QDs) and permanganate (MnO₄⁻) has been designed for monitoring glutathione. The bifunctional VO_x QDs, possessing rich redox chemistry and robust fluorescence (exhibiting fluorescence near 505 nm upon excitation at 450 nm), were synthesized via cryogenic-mediated liquid-phase exfoliation. In the presence of MnO₄⁻, VO_x QDs induced the spontaneous formation of MnO₂ nanosheets which caused the fluorescence quenching. However, the subsequent introduction of glutathione could trigger MnO₂ reduction to Mn²⁺, and the fluorescence was recovered. Based on this phenomenon, an "on-off-on" fluorescence sensor for glutathione detection was established. Under the optimal conditions, this sensor allowed detection of glutathione in the linear range of 0.5–100 μM with a detection limit of 0.254 μM. Additionally, the proposed strategy revealed the selectivity toward glutathione and the potential of practical application in the analysis of human serum, vegetable, and fruit samples. Graphical abstract: