Hierarchical Fe₃O₄@C@MnO₂ nanozymes: bridging structural engineering to on-site biosensing of tannic acid

Manganese dioxide (MnO₂)-based heterogeneous catalysts have gained significant attention in nanozyme research due to their exceptional oxidase-mimicking capabilities. This work reports the rational design of MnO₂ functionalized magnetic carbon nanotubes (Fe₃O₄@C@MnO₂) through a multistep synthesis strategy. The magnetic N-doped hollow carbon substrate (Fe₃O₄@C) was first synthesized via controlled pyrolysis of FeOOH-coated polydopamine precursors. Subsequent interfacial redox reaction with KMnO₄ enabled the in situ deposition of ultrathin MnO₂ nanosheets on the nanotube substrate, forming a triple-component coaxial architecture. This hollow structure not only facilitates dense MnO₂ deposition but also maintains robust magnetic responsiveness. The obtained Fe₃O₄@C@MnO₂ exhibits remarkable enzyme-mimetic capability, catalyzing TMB chromogenic conversion with 0.92-fold enhanced catalytic performance relative to pristine MnO₂. Systematic mechanistic studies reveal that tannic acid (TA) competitively inhibits ·OH radical generation via MnO₂-TA complexation, while the carbon interlayer enhances electron transfer efficiency. Owing to the superior enzyme-mimetic performance of Fe₃O₄@C@MnO₂ and their inhibition effect by TA, a sensitive colorimetric biosensing assay was fabricated, achieving a TA detection limit of 0.029 μM with a 0.096-4.82 μM linear concentration range. This work exemplifies the synergy of structural engineering and component optimization in designing multifunctional nanozymes, showing potential applications in food quality monitoring, environmental analysis, and point-of-care diagnostics.