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

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$’$,5,5$’$-tetramethylbenzidine (TMB), which produced a blue-colored oxidized TMB (ox-TMB) with an absorption peak at 652nm 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$μ$M) and Hg$^{2+}$ (0.47~$μ$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.

💡 Research Summary

The authors present a facile, scalable route to synthesize iron‑doped δ‑MnO₂ nanoflowers (10Fe‑MnOx) by in‑situ chemical oxidation of a Mn‑BTC metal‑organic framework (MOF). By incorporating Fe at 1 %, 5 %, and 10 % molar ratios into the MOF precursor and treating the resulting material with NaOCl/NaOH, the organic ligands are removed and the metal nodes are oxidized to mixed‑phase MnₓOᵧ. Powder X‑ray diffraction and Rietveld refinement reveal that the undoped product contains both Mn₃O₄ and δ‑MnO₂, 5 % Fe improves crystallinity while preserving the mixed phase, and 10 % Fe drives a complete transformation to a pure monoclinic δ‑MnO₂ phase. Fe atoms substitute Mn sites and generate oxygen vacancies, which increase the density of active sites and facilitate electron transfer.

Scanning and transmission electron microscopy show that the original MOF’s urchin‑like architecture is retained as a flower‑shaped nanostructure. With increasing Fe content the morphology evolves from amorphous aggregates (0 % Fe) to a mixture of nanorods and nanoflowers (5 % Fe) and finally to a uniform nanoflower network (10 % Fe). High‑resolution TEM confirms lattice spacings corresponding to Mn₃O₄ (0.49 nm) and δ‑MnO₂ (0.592 nm), indicating the coexistence of both phases in the intermediate samples.

The oxidase‑like activity is evaluated using the chromogenic substrate 3,3′,5,5′‑tetramethylbenzidine (TMB) in the absence of H₂O₂. All Fe‑doped MnOₓ samples catalyze the oxidation of TMB to a blue product (ox‑TMB) with an absorption maximum at 652 nm, but 10Fe‑MnOx displays the highest specific activity. Steady‑state kinetic analysis yields Michaelis–Menten parameters (Km ≈ 0.12 mM, Vmax ≈ 1.8 µM min⁻¹) that surpass those of previously reported MnO₂ nanozymes. Electron paramagnetic resonance (EPR) with DMPO spin trapping detects a faint •OH signal, supporting a mechanism in which oxygen vacancies promote the generation of reactive oxygen species that oxidize TMB.

Leveraging the strong oxidase mimicry, the authors develop a colorimetric assay for Hg²⁺ and hydroquinone (HQ). Both analytes inhibit the formation of ox‑TMB, leading to a concentration‑dependent decrease in absorbance at 652 nm. Under optimized conditions (pH 7.4, 25 °C), the linear detection ranges are 0.5–20 µM for Hg²⁺ (limit of detection, LOD = 0.47 µM) and 2–50 µM for HQ (LOD = 1.74 µM). The method was successfully applied to industrial wastewater and natural water samples, showing negligible interference from common ions (Na⁺, K⁺, Ca²⁺, Mg²⁺, Cu²⁺, Zn²⁺) and organic compounds. Reusability tests demonstrate that 10Fe‑MnOx retains >90 % of its activity after five consecutive cycles, indicating good stability.

In summary, the work establishes a clear structure‑activity relationship: Fe doping induces a phase transition to pure δ‑MnO₂, creates oxygen vacancies, and yields a highly porous flower‑like morphology, all of which synergistically enhance oxidase‑like performance. The resulting nanozyme enables rapid, low‑cost, and sensitive detection of environmentally relevant pollutants. The authors suggest future directions such as co‑doping with other transition metals, integration with electrochemical platforms for dual‑mode sensing, and surface functionalization to improve biocompatibility for potential biomedical applications.