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 Fe 3+ (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 –NH 2 on N,S-CDs surface. UV–vis and FTIR revealed that the coordination of –COOH and –NH 2 on N,S-CDs surface with Fe 3+ accounted for FL quenching, and the release of –NH 2 from coordination complexes due to the reduction of Fe 3+ 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.