Ratiometric fluorescent sensors based on molecule@support hybrids for specific and sensitive detection of chromium(VI)

Chromium(VI) contamination poses severe threats to ecosystems and human health. Therefore, it is urgently needed to develop efficient, sensitive, and reliable detection methods. In this work, a nanozyme-based ratiometric fluorescence platform was developed for high-performance Cr(VI) sensing by using an engineered composite (A@FeTImp@ZIF-8). This composite, classified as a molecule@support material, was fabricated by grafting 5, 10, 15, 20-Tetrakis(4-(imidazol-1-yl) phenyl) iron porphyrin (FeTImp) onto zeolitic imidazolate framework-8 (ZIF-8) surfaces via a simple ligand exchange reaction. The resulting A@FeTImp@ZIF-8 composite exhibited enhanced peroxidase-mimicking activity alongside intrinsic fluorescence properties. Under H 2 O 2 activation, A@FeTImp@ZIF-8 catalyzes the oxidation of o-phenylenediamine (OPD) to fluorescent 2,3-diaminophenazine (DAP, emission at 565 nm). The generated DAP subsequently quenches acridine (A) fluorescence signal (435 nm) via an inner filter effect (IFE). The addition of 8-hydroxyquinoline (8-HQ) competitively inhibits OPD oxidation, leading to a decrease in the DAP signal (565 nm) and a recovery of the A fluorescence (435 nm). Upon introduction of Cr(VI), it complexes with 8-HQ, reactivating the OPD oxidation process. By utilizing these synergistic effects, a highly sensitive and selective detection of Cr(VI) was achieved over a wide linear range (0.01–180 μM), with an ultra-low detection limit of 3.74 nM (S/N = 3). The sensor was also successfully applied to monitor Cr(VI) in environmental water samples. • An IFE-based ratiometric fluorescent biosensor was developed for sensitive Cr(VI) detection. • A@FeTImp@ZIF-8 was fabricated via ligand exchange, ensuring precise catalyst immobilization. • The composite integrates FeTImp as a peroxidase mimic and acridine as a fluorophore. • A stable dual-emission signal enables reliable and sensitive sensing.