The iron-manganese bimetal-MOF with double mimic enzyme: DFT verification and colorimetric detection of Cr(vi)

Abstract

Chromate (Cr(VI)) is a highly toxic heavy metal ion that not only endangers human health but also causes environmental pollution. To control and remove Cr(VI) from water more effectively, we report a simple, novel, and rapid detection technique for Cr(VI) in water based on an iron-manganese bimetal-metal organic framework (Fe_xMn_y-MOF). Firstly, the synthesized Fe_xMn_y-MOF has a large specific surface area and porosity; for the first time, an MOF with dual mimic enzyme properties is reported, which can catalyze the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) to the blue product in the presence or absence of H₂O₂. Interestingly, the mimic enzyme properties of Fe_xMn_y-MOF are enhanced than those of Fe-MOF and Mn-MOF. The free energies of elementary reactions and adsorption energies (E_ads) of H₂O₂ and O₂ adsorbed on different materials were calculated by density functional theory (DFT) to verify the mechanism of action of Fe_xMn_y-MOF. The Fe₁Mn₁-MOF mimics oxidase-like oxidation of TMB; 8-hydroxyquinoline (8-HQ) is used as the inhibitor of TMB oxidation, and the specific interaction between 8-HQ and Cr(VI) weakens the inhibition of 8-HQ on TMB oxidation to achieve the detection of Cr(VI). The established method has high sensitivity and selectivity with a detection limit of 51 nM and a linear range of 80–3500 nM. This method has been successfully applied to the determination of Cr(VI) in tap water and the Yellow River, showing its great potential in environmental monitoring.