A facile and rapid detection of Cr(VI) and ascorbic acid using N,S-co-functionalized carbon dots: A dual-mode 'on-off-on' fluorescent sensor

• Microwave-assisted hydrothermal method mediated N,S-CDs act as a fluorescent probe. • The excellent water-soluble N,S-CDs exhibit excitation-independent and pH-responsive fluorescence. • Cr(VI) and ascorbic acid were detected using a unique "on-off-on" fluorescence mechanism. • Inner filter effect and static interactions synergistically caused fluorescence quenching in the Cr(VI)-CDs system. • The developed fluorescent probe was successfully exploited for real-sample analysis. This study reports the successful synthesis of nitrogen- and sulfur-co-functionalized carbon dots (N,S-CDs) using citric acid and L-cysteine through a microwave-assisted hydrothermal process. These N,S-CDs were employed for the quantification of dichromate ions (Cr(VI)) and ascorbic acid (AA) through a fluorescence “on-off-on” detection method. The N,S-CDs displayed deep blue fluorescence, with an emission peak at 420 nm upon excitation at 349 nm. The presence of Cr(VI) significantly quenched the fluorescence intensity, enabling its selective detection over a linear range of 1–40 μM with a limit of detection of 58 nM. The addition of AA restored the fluorescence intensity by reducing Cr(VI) to Cr(III), allowing for AA detection within a dynamic range of 5–90 μM with a detection limit of 180 nM. The fluorescence quenching by Cr(VI) involved both the inner filter effect (IFE) and static interactions and the IFE was reversed upon the introduction of AA, which restored the fluorescence. This method demonstrated excellent sensitivity and selectivity for detecting Cr(VI) and AA, with successful application in the analysis of real samples.