N,S co-doped carbon dots based fluorescent "on-off-on" sensor for determination of ascorbic acid in common fruits

A switchable sensor based on N and S co-doped carbon dots (N,S-CDs) was developed to determine ascorbic acid (AA) content in common fruits via fluorescence quenching of N,S-CDs by Fe³⁺ (off) and recovery by AA (on). TEM showed that the particle size was 2.35-7.95 nm with an average diameter of 5.12 nm. XPS and FTIR confirmed the presence of -OH, -COOH and -NH₂ on N,S-CDs surface. UV-vis and FTIR revealed that the coordination of -COOH and -NH₂ on N,S-CDs surface with Fe³⁺ accounted for FL quenching, and the release of -NH₂ from coordination complexes due to the reduction of Fe³⁺ by AA led to partly FL recovery. The sensor allowed detecting AA in the linear range of 10-200 μmol/L with a LOD of 4.69 μmol/L. AA contents in common fruits detected by the sensor were in good agreement with the reference method (P > 0.05), indicating a practical fluorescent sensor for AA detection in common fruits.