“Switch-On” Fluorescent Sensing of Ascorbic Acid in Food Samples Based on Carbon Quantum Dots–MnO2 Probe

This work describes a “switch-on” fluorescence approach for sensing of ascorbic acid (AA) in food samples. In the present method, the fluorescence intensity (FL) of carbon quantum dots (CQDs) was first quenched by addition of MnO2 nanosheets through an inner filter effect to form a CQDs-MnO2 probe. When reductive AA was introduced into the quenched CQDs solution, the added MnO2 was destroyed due to the redox reaction between AA and MnO2 nanosheets, and the FL of the system was recovered. Under the optimal conditions, the limit of detection for AA was 42 nM, with a wide concentration linear range of 0.18–90 μM. Furthermore, the as-fabricated fluorescent sensing system was successfully applied to the analysis of AA in fresh fruits, vegetables, and commercial fruit juices samples with satisfactory results.