Copper-based metal-organic framework for sensitive sensing of Cr₂O₇^2- in aqueous solution

The dichromate ion (Cr₂O₇^2-) is a highly toxic and carcinogenic pollutant in wastewater, which requires accurate and rapid monitoring techniques. In this study, a water-stable copper-based metal-organic framework (Cu-OCH₃ MOF) is developed as a fluorescent sensor for detecting Cr₂O₇^2- in aqueous media. The copper ions in this MOF exhibit mixed oxidation states of Cu²⁺ and Cu⁺, which is important for fluorescence detection. The MOF shows an excitation maximum at 310 nm and an emission maximum at 393 nm, with a quantum yield of 17.61% and a fluorescence lifetime of 9.3 ns. In an aqueous solution of Cr₂O₇^2- (1 mM, pH ≈ 4.68), the sensor achieves a quenching efficiency of approximately 95%, together with fast response, high sensitivity, a wide linear range (0.0209-1 mM), and a low detection limit of (6.27 ± 0.005) × 10^-6 M. The sensing mechanism is based on the inner filter effect (IFE) and dynamic quenching effect (DQE), where the absorption band of Cr₂O₇^2- overlaps with both the excitation and emission spectra of the MOF, leading to efficient fluorescence quenching.