Insight into the oxidase-mimicking activity enhancement of MnO ₂ nanozymes by Co ion-doping for colorimetric sensor assays

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 (<i>K</i> _m = 1.17 mM) and an elevated maximum velocity (<i>V</i> _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.