Effects of crystal structure on the activity of MnO₂ nanorods oxidase mimics

The crystal structures would directly affect the physical and chemical properties of the surface of the material, and would thus influence the catalytic activity of the material. α-MnO₂, β-MnO₂ and γ-MnO₂ nanorods with the same morphology yet different crystal structures were prepared and tested as oxidase mimics using 3,3',5,5'-tetramethylbenzidine (TMB) as the substrate. β-MnO₂ that exhibited the highest activity had a catalytic constant of 83.75 μmol·m^-2·s^-1, 2.7 and 19.0 times of those of α-MnO₂ and γ-MnO₂ (30.91 and 4.41 μmol·m^-2·s^-1), respectively. The characterization results showed that there were more surface hydroxyls as well as more Mn⁴⁺ on the surface of the β-MnO₂ nanorods. The surface hydroxyls were conducive to the oxidation reaction, while Mn⁴⁺ was conducive to the regeneration of surface hydroxyls. The synergistic effect of the two factors significantly improved the activity of β-MnO₂ oxidase mimic. Using β-MnO₂, a β-MnO₂-TMB-GSH system was established to detect the content of glutathione (GSH) rapidly and sensitively by colorimetry. This method had a wide detection range (0.11-45 μM) and a low detection limit (0.1 μM), and had been successfully applied to GSH quantification in human serum samples.