MnO2 nanozyme induced the chromogenic reactions of ABTS and TMB to visual detection of Fe2+ and Pb2+ ions in water

In this study, we used a simple and rapid colourimetric reaction for visual sensing of Fe2+ and Pb2+ ions in water by employing nano-MnO2 as a natural oxidase mimic to respectively catalyse ABTS and TMB in citrate-phosphate buffer solution (C-PBS) at 25°C and pH 3.8. It was found that nano-MnO2 possessed highly oxidase-mimicking activity with the Km values of 0.030 and 0.027 toward ABTS and TMB, respectively, indicating TMB had a stronger affinity on nano-MnO2 than ABTS. Interestingly, the presence of 0.01 mmol·L−1 Fe2+/Pb2+ ion was able to significantly down-regulate the activity of MnO2 nanozyme in nano-MnO2-mediated ABTS reaction processes (P < 0.01), which mainly due to the strong adsorption of metal ion toward nano-MnO2 surface via the electrostatic attractions, thus leading to the passivation and inactivation of MnO2 nanozyme catalytic activity. Thereinto, Fe2+ reacted with multivalent manganese by oxidation-reduction, while Pb2+ was specifically adsorbed onto the surface of MnO2 nanozyme and formed complexes. Notably, only Fe2+ ion inhibited the activity of MnO2 nanozyme-TMB with a detection limit as low as 1.0 μmol·L−1. In MnO2 nanozyme-ABTS sensing systems, Fe2+ and Pb2+ ions detection limit of 0.5 and 2.0 μmol·L−1 were, respectively, achieved with a linear response range of 0–0.02 and 0–0.8 mmol·L−1, implying the developed MnO2 nanozyme-ABTS sensor was potentially applicable for the visual determination of Fe2+ and Pb2+ ions in water. In the real water samples, MnO2 nanozyme-ABTS achieved high accuracy (relative errors: 3.4‒10.5%) and recovery (96‒110%) for respective detection of Fe2+ and Pb2+ ions. The simple and rapid MnO2 nanozyme-ABTS sensing systems might provide a practical assay for visual detection of Fe2+ and Pb2+ ions in the environmental water samples.