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)-NH₂) nanoparticles were synthesized by simple solvothermal methods and used to assay Cr(Ⅵ). The MIL-101(Fe)-NH₂ performs dual functions: the 2-aminoterephthalic acid (NH₂-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)-NH₂ because of the reduction of Fe³⁺ by AA, resulting in release of NH₂-BDC. In the presence of Cr(Ⅵ), the fluorescence intensity of the MIL-101(Fe)-NH₂ + AA system will be decreased due to the competitive reduction of Fe³⁺ and Cr(Ⅵ). Nevertheless, Cr(Ⅵ) can significantly accelerate the oxidation of TMB by MIL-101(Fe)-NH₂ as it boosts the electron transfer rate between Fe³⁺ and Fe²⁺. 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).