Comparative insights into pristine and mixed oxides of cerium and manganese as polyphenol oxidase mimics

Oxides of cerium and manganese have been extensively employed in catalytic reactions due to their redox versatility. In the present study, pristine CeO₂, Mn₃O₄ and mixed metal oxides (with Ce : Mn in ratios 2 : 1, 1 : 1 and 1 : 2) were synthesized via a facile sol-gel auto-combustion method and evaluated as polyphenol oxidase (PPO) mimics. Comprehensive structural and morphological characterization was performed using XRD, XPS, BET, TEM, SEM-EDX and EIS. Manganese doping modulated the Ce³⁺/Ce⁴⁺ ratio and enhanced electron transfer ability and surface area. Manganese oxide (Mn₃O₄) displayed the highest PPO-mimicking activity, as corroborated by structural studies. The reaction parameters, namely, pH, substrate concentration, contact time, temperature and catalytic dose, were optimized to achieve maximum activity. Michaelis-Menten kinetic studies confirmed the highest V_max and K_m for pristine Mn₃O₄ (K_m = 0.84 mM) and the lowest for pristine CeO₂ (K_m = 3.48 mM), indicating enhanced substrate affinity of Mn₃O₄ NPs. A colorimetric sensing assay employing 3,3',5,5'-tetramethylbenzidine was used for the determination of catechol (LOD = 3.25 μM) and resorcinol (LOD = 6.9 μM) with low detection limits. Furthermore, the study was successfully extended to the detection of polyphenols in green tea and wine samples. The results correlated with the conventional Folin-Ciocalteu assay with the added advantages of fast detection, sensitivity and reusability of NPs. Thus, the present study demonstrates the potential of Mn₃O₄ NPs for the colorimetric detection of polyphenols.