Facile preparation of S, N co-doped carbon dots and the application as an "on-off-on" fluorescent sensor for detecting Cr(VI) and ascorbic acid/sulfide

Recently, syntheses of various kinds of carbon dots (CDs) and their application as ions sensing probes, as well as the relative mechanism, have attracted great attention of scientists. In this paper, sulfur and nitrogen co-doped carbon dots (SN-CDs) were synthesized by a facile one-step hydrothermal method using citric acid and thiourea as precursors with uniform size and excellent fluorescence properties. Importantly, based on the inner filter effect (IFE), the fluorescence of the SN-CDs was quenched by Cr(VI), and thus can be applied as an "on-off" fluorescent sensor with high selectivity and high sensitivity. The detection limit was 0.428 mu M, much lower than that from the conventional detection methods. Interestingly, the SN-CDs/Cr(VI) system can also act as an "off-on" fluorescent sensor for detecting ascorbic acid (AA) or sulfide by converting Cr(VI) to Cr(III) and thus weakening the IFE, with the detection limits of 0.686 mu M and 0.872 mu M, respectively. Consequently, an "on-off-on" fluorescent sensor based on the SN-CDs was established, showing the great potential for detecting Cr(VI), AA and sulfide effectively. Moreover, the proposed sensor has been successfully applied to quantitatively detect in actual samples.