Insight into the oxidase-mimicking activity enhancement of MnO2 nanozymes by Co ion-doping for colorimetric sensor assays

Abstract

Manganese dioxide (MnO₂) nanomaterials have garnered significant attention as enzyme mimics due to their excellent catalytic properties. However, when applied to catalytic oxidation in biological systems, pure MnO₂ has shown limited performance. To address this limitation, we synthesized Co-doped MnO₂ nanoparticles with enhanced oxidase-mimicking activity through a facile chemical method. These Co–MnO₂ nanoparticles are characterized by the structural incorporation of Co ions into the MnO₂​ lattice, likely substituting Mn sites, which crucially induces a high density of surface oxygen vacancies (OVs), a key factor for enhanced catalysis. The presence of these OVs, confirmed by XPS, alongside the introduction of Co, leads to modulated electronic configurations and promotes the active participation of Mn³⁺/Mn⁴⁺ and Co²⁺/Co³⁺ redox couples. This combination is believed to facilitate synergistic catalytic reactions and accelerated electron transfer during catalytic reactions. Co–MnO₂ nanoparticles demonstrated notable kinetic parameters toward the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB), exhibiting a low Michaelis–Menten constant (K_m = 1.17 mM) and an elevated maximum velocity (V_max = 4.69 × 10⁻⁶ M s⁻¹). UV-Vis spectroscopy and colorimetric analysis revealed a notably intensified chromogenic response at 650 nm for Co–MnO₂ systems. Leveraging these properties, a sensitive and reliable colorimetric platform was developed for ascorbic acid (AA) detection, achieving a broad linear range (2–100 μM) and a low detection limit (1.23 μM). The practical application of this colorimetric sensing platform was validated through successful testing with real samples, highlighting its potential for various colorimetric sensing applications.