Cerium Phosphate–Cerium Oxide Heterogeneous Composite Nanozymes with Enhanced Peroxidase-Like Biomimetic Activity for Glucose and Hydrogen Peroxide Sensing

In the present work, we focus on the development of CePO₄-CeO₂ composite nanorods with peroxidase mimetic activity for the sensitive detection of hydrogen peroxide and glucose. The Ce³⁺/PO₄^3- molar ratio (CP10:1, CP5:1, CP2:1) in the hydrothermal reaction controlled the formation of pure CePO₄, CePO₄-CeO₂ composite nanozymes with different percentages of CeO₂, and its crystal structure. A higher Ce³⁺/PO₄^3- molar ratio (CP10:1 or CP5:1) was required to obtain CePO₄-CeO₂ composite nanostructure, while a lower Ce³⁺/PO₄^3- molar (CP2:1) ratio was sufficient to fabricate pure CePO₄ nanorods. In the presence of hydrogen peroxide, the prepared nanozymes catalyze the oxidation of chromogenic substrate 3,3',5,5'-tetramethylbenzidine (TMB). Steady state kinetic analysis based on the Michaelis-Menten model revealed that CP10:1 showed excellent affinity toward the TMB ( K_m = 0.236 mM and V_max = 8.78 × 10^-8 M s^-1) in comparison to the catalytic activity of CP5:1 and CP2:1 and horseradish peroxidase ( K_m = 0.434 mM and V_max = 10.0 × 10^-8 M s^-1). The superior peroxidase activity of CePO₄-CeO₂ composite nanozymes can be ascribed to the enhanced redox switching between Ce³⁺ ↔ Ce⁴⁺ sites from the CePO₄ and CeO₂ lattice, respectively. The colorimetric detection of hydrogen peroxide and glucose showed a linear response around 150 μM concentration with the limits of detection (LOD) of 2.9 and 4.1 μM, respectively.