MOF-derived Fe-doped δ-MnO2 nanoflowers as oxidase mimics: chromogenic sensing of Hg(ii) and hydroquinone in aqueous media

Abstract

The structure and morphology play a crucial role in enhancing the biomimetic oxidase activity of nanozymes. In this study, a facile in situ chemical oxidation strategy was employed to synthesize MOF-derived MnO_x, utilizing the structural features of the parent MOF to enhance oxidase-mimicking activity. We have systematically investigated the effects of phase evolution, structural modulation and morphology on the oxidase activity of MnO_x with Fe substitution. The oxidase-like activity was evaluated using the chromogenic substrate 3,3′,5,5′-tetramethylbenzidine (TMB), which produced blue colored ox-TMB with an absorption peak at 652 nm upon oxidation. While all Fe-doped MnO_x nanostructures exhibited oxidase-like activity, the 10% Fe-doped sample (10Fe-MnO_x) demonstrated the highest performance, likely due to a synergistic effect of the structure, morphology and existing oxygen vacancies. The underlying oxidase mechanism was investigated using steady-state kinetics and electron paramagnetic resonance (EPR) analysis. In addition, a colorimetric assay was developed for detecting Hg²⁺ and hydroquinone (HQ) in real water samples collected from industrial and natural sources. The calculated detection limits of the colorimetric probe 10Fe-MnO_x for HQ (1.74 μM) and Hg²⁺ (0.47 μM) outperformed conventional metal oxide-based nanozymes. These findings may pave the way for the development of easily synthesizable, scalable and highly sensitive MOF-derived metal oxide nanozymes with promising applications in biological and environmental settings.