Oxygen-Vacancy-Enhanced Peroxidase-like Activity of Reduced Co 3 O 4 Nanocomposites for the Colorimetric Detection of H 2 O 2 and Glucose.

Colorimetric assays have drawn increasing research interest with respect to the quantitative detection of hydrogen peroxide (H ₂ O ₂ ) based on artificial enzymes because of their advantages with respect to natural enzymes, including design flexibility, low cost, and high stability. Regardless, the majority of the artificial enzymes exhibit low affinity to H ₂ O ₂ with large Michaelis-Menten constants ( K _m ). This indicates that the catalytic oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) to blue-colored oxTMB requires a high H ₂ O ₂ concentration, hindering the sensitivity of the colorimetric assay. To address this problem, novel reduced Co ₃ O ₄ nanoparticles (R-Co ₃ O ₄ ) have been synthesized in this study via a step-by-step procedure using ZIF-67 as the precursor. R-Co ₃ O ₄ exhibits a considerably enhanced peroxidase-like activity when compared with that exhibited by pristine Co ₃ O ₄ (P-Co ₃ O ₄ ). The catalytic process in the case of R-Co ₃ O ₄ occurs in accordance with the typical Michaelis-Menten equation, and the affinity of R-Co ₃ O ₄ to H ₂ O ₂ is apparently higher than that of P-Co ₃ O ₄ . Furthermore, the density functional theory calculations revealed that the introduction of oxygen vacancies to R-Co ₃ O ₄ enhances its H ₂ O ₂ adsorption ability and facilitates the decomposition of H ₂ O ₂ to produce ·OH radicals, resulting in improved peroxidase-like activity. A simple and convenient colorimetric assay has been established based on the excellent peroxidase-like activity of R-Co ₃ O ₄ for detecting H ₂ O ₂ in concentrations of 1-30 μM with a detection limit of 4.3 × 10 ^-7 mol/L (S/N = 3). Furthermore, the R-Co ₃ O ₄ -based colorimetric method was successfully applied to glucose detection in human serum samples, demonstrating its potential for application in complex biological systems.