MOF-derived Fe-doped $\hat{I}´$-MnO${}_2$ nanoflowers as oxidase mimics: Chromogenic sensing of Hg${}^{2+}$ and hydroquinone in aqueous media

Abstract

Structure and morphology play a crucial role in enhancing the biomimetic oxidase activity of nanozymes. In this study, a facile \emph{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 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${}^{\prime }$,5,5${}^{\prime }$-tetramethylbenzidine (TMB), which produced a blue-colored oxidized TMB (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 structure, morphology, and the presence of 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 the detection of Hg${}^{2+}$ and hydroquinone (HQ) in real water samples collected from industrial and natural sources. The calculated detection limits of the 10Fe-MnO${}_x$ colorimetric probe for HQ (1.74~$\hat{I}¼$M) and Hg${}^{2+}$ (0.47~$\hat{I}¼$M) outperformed those of conventional metal oxide-based nanozymes. These findings pave the way for the development of easily synthesizable, scalable, and highly sensitive oxidase-based MOF-derived metal oxide nanomaterials with significant potential in biological and environmental applications.

10 pages, 9 figures