Fluorescent/colorimetric probe for the detection of Cr(Ⅵ) based on MIL-101(Fe)–NH2 with peroxidase-like activity

In the present research, Fe-based metal-organic frameworks (MIL-101(Fe)–NH 2 ) nanoparticles were synthesized by simple solvothermal methods and used to assay Cr(Ⅵ). The MIL-101(Fe)–NH 2 performs dual functions: the 2-aminoterephthalic acid (NH 2 -BDC) ligand endows a strong fluorescence emission, and the Fe metal nodes are able to facilitate the oxidation of 3,3′,5,5′- tetramethylbenzidine (TMB) directly, resulting in the generation of oxidized-TMB (ox-TMB). Our research results showed that reducing agents such as ascorbic acid (AA) can collapse the structures of MIL-101(Fe)–NH 2 because of the reduction of Fe 3+ by AA, resulting in release of NH 2 -BDC. In the presence of Cr(Ⅵ), the fluorescence intensity of the MIL-101(Fe)–NH 2  + AA system will be decreased due to the competitive reduction of Fe 3+ and Cr(Ⅵ). Nevertheless, Cr(Ⅵ) can significantly accelerate the oxidation of TMB by MIL-101(Fe)–NH 2 as it boosts the electron transfer rate between Fe 3+ and Fe 2+ . Therefore, a fluorescent/colorimetric dual-mode platform was developed for the detection of Cr(Ⅵ) with an extensive linear range (7.5–750 μg/L and 13.3–1000 μg/L) as well as a remarkably low detection limit (0.99 μg/L and 1.98 μg/L). This MOF with the ability to release ligands not only provides inspiration for the design of new luminescent materials, but also offers a novel and reliable solution for the detection of Cr(Ⅵ).