Ultrasensitive detection of Cr(VI) (Cr₂O₇²⁻/CrO₄²⁻) ions in water environment with a fluorescent sensor based on metal-organic frameworks combined with sulfur quantum dots

Accurate, simple and quick detection methods for Cr(VI) detection are urgently needed for water quality monitoring. Herein, a novel and facile method of detecting Cr(VI) (Cr₂O₇²⁻/CrO₄²⁻) ions is developed via the fluorescent detection technology based on metal-organic frameworks (MOFs) doped with sulfur quantum dots (SQDs) (SQDs@UiO-66-NH₂). The blue-light-emitting SQDs@UiO-66-NH₂ composites exhibit excellent fluorescent properties in water environment with high quantum yield (68%) and ideal fluorescent stability, thus demonstrating excellent potential for serving as a chemical sensor. After characterizing the performance and stability of SQDs@UiO-66-NH₂, qualitative and quantitative detection of Cr₂O₇²⁻ and CrO₄²⁻ ions was successfully conducted. The fluorescence of SQDs@UiO-66-NH₂ composites in aqueous solution was quenched effectively with more than 90% quenching efficiency by Cr(VI) via the inner filter effect. The detection system provides considerable advantages such as rapid response (10 s), high sensitivity with a low detection limit of 0.16 μM in a broad linear range of 0–200 μM (R² = 0.99) for Cr₂O₇²⁻ and 0.17 μM for CrO₄²⁻ in a broad linear range of 0–220 μM (R² = 0.99), high selectivity and reproducibility for at least five cycles with simple washing with alcohol. In practical applications, the sensor showed rapid response, high sensitivity and excellent recoveries (96.7%–105.4%) for detecting Cr₂O₇²⁻ in real water samples. Furthermore, a SQDs@UiO-66-NH₂-based fluorescent test paper was successfully developed, providing a simple, reliable and portable method for Cr(VI) (Cr₂O₇²⁻/CrO₄²⁻) detection in water environment.