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Herein, a label-free on–off–on fluorescence sensing method was developed for the dual detection of chromium(VI) (Cr(VI)) and sulfite using porous g-C3N4 nanosheets (PCNs) as a fluorescent nanoprobe. The highly luminescent PCNs prepared by a template-free approach possessed more outstanding water dispersibility and fluorescence quantum yield compared to bulk g-C3N4. As the absorption bands of Cr(VI) overlapped with the emission and excitation spectra of PCNs, the fluorescence of PCNs could be effectively quenched in the presence of Cr(VI) based on the inner filter effect, resulting in an on–off fluorescent mode for detecting Cr(VI). Under optimum conditions, a wide linear concentration range from 0.63 to 300 μM was observed for the determination of Cr(VI) with a limit of detection (LOD) of 0.11 μM. Moreover, the fluorescence of the PCNs-Cr(VI) mixture could be sensitively recovered in the presence of sulfite via the redox reaction, leading to an off–on fluorescence response for detecting sulfite. The calibration plots were linear in the sulfite concentration range of 2–150 μM with an LOD of 0.30 μM. Due to its low cost, simple operation, high sensitivity, and selectivity, this fluorometric approach showed potential application for the detection of Cr(VI) in environmental waters and sulfite in food samples. In addition, the oxidation–reduction reaction could change highly toxic Cr(VI) and sulfite into low toxic Cr(III) and sulfate, which realized the determination and elimination of hazardous substances simultaneously.
Xu et al. (Mon,) studied this question.