Cationic Metal-Organic Framework-Based Mixed-Matrix Membranes for Fast Sensing and Removal of Cr₂O₇ ^2- Within Water

Considering that metal-organic framework (MOF)-polymer mixed-matrix membranes (MMMs) can overcome the drawbacks of intrinsic fragility and poor processability of pure-MOF membranes, we designed MOF-based MMMs for efficient removal and fast fluorescence sensing of heavily toxic ions within water systems simultaneously. In this work, a series of MOF-based MMMs are prepared by mixing a hydrolytically stable cationic [Eu₇ (mtb)₅(H₂O)₁₆]·NO₃ 8DMA·18H₂O (denoted as Eu-mtb) MOF material into poly (vinylidene fluoride) with high loadings up to 70%. The free volume at the interface between the polymer and Eu-mtb particles, combined with the permanent porosity and uniform distribution of Eu-mtb particles, enables these MMMs to show fast enrichment of Cr₂O₇ ^2- from solutions and consequently have a full contact between the analyte and MOFs. The developed Eu-mtb MMM (70wt% loading) thus shows both efficient removal and exceptional fluorescence sensing of Cr₂O₇ ^2- in aqueous media. The overall adsorption capacity of the Eu-mtb MMM (70 wt% loading) for Cr₂O₇ ^2- reaches up to 33.34 mg/g, which is 3.4 times that of powder-form Eu-mtb. The detection limit of the Eu-mtb MMM (70 wt% loading) for Cr₂O₇ ^2- is around 5.73 nM, which is lower than that of the reported powder-form Eu-mtb. This work demonstrates that it is feasible to develop flexible luminescent MOF-based MMMs as a significant platform for efficient removal and sensitive sensing of pollutants from water systems simultaneously.