Manganese oxide (MnOx) as peroxidase-mimicking nanozymes with valence-dependent activity for single-use colorimetric bioassays

Manganese oxide (MnO_x)-based nanozymes have attracted growing interest due to their low cost, high catalytic robustness, and adjustable oxidation states that influence enzyme-mimicking performance. In this study, we report a systematic approach to synthesize MnO_x nanocomposites with tunable Mn valence states via a two-step pyrolysis of Mn-BTC precursors, enabling us to explore their valence-dependent peroxidase-like activities. Among the synthesized forms, Mn₃O₄ exhibited the highest catalytic efficiency by facilitating the decomposition of H₂O₂ to generate hydroxyl radicals (•OH), which oxidize tetramethylbenzidine (TMB) to produce a blue-colored product. The catalytic mechanism was supported by UV-vis spectroscopy and fluorescence assays. Kinetic studies revealed low Km values, indicating high substrate affinity. Leveraging these properties, we further developed a disposable paper-based colorimetric bioassay using Mn₃O₄, which demonstrated excellent selectivity, sensitivity, and a low detection limit (0.25 μM in solution and 6.77 μM on paper). This work not only elucidates the correlation between Mn valence state and catalytic behavior but also provides a promising strategy for designing efficient nanozymes for low-cost, portable biosensing platforms.