Nanostructure Engineering of Ce(III) Species in Cerium Oxide/Polymeric Carbon Nitride Nanozymes for Simulated Oxidase Activities

Abstract Nanostructure engineering significantly influences the simulated enzyme catalysis and activity of nanozymes. Herein, using urea and cerium(III) nitrate as precursors, nonstoichiometric cerium oxide species were in situ deposited simultaneously with the growth of polymeric carbon nitride by a one-step thermal polycondensation method, resulting in cerium oxide/polymeric carbon nitride composite nanozymes with excellent oxidase activity. The nitrogen-rich coordination configuration of carbon nitride stabilizes cerium oxide species, and the matrix of the polymer plays a role in isolating species. These effects create the features of high dispersion and high exposure of cerium oxide active species. More importantly, the enriched ammonia atmosphere with strong reducing effect produced during urea polycondensation fundamentally affects the ratio of Ce(IV) and Ce(III) shuttle pairs in cerium oxide, leading to the dominant existence of Ce(III). As a result, an extremely rich surface oxygen species emerged in the oxygen vacancies associated with Ce(III). This unique architecture endows the composite nanozyme with a strong capacity to store reactive oxygen species, thereby conferring it a distinctive oxidase-like activity. The corresponding Michaelis–Menten kinetic constant is as low as 0.12 mmol L–1, while the maximum reaction rate can reach 8.26 × 10–8 mol L–1 s–1. Based on this composite nanozymes, an ascorbic acid colorimetric detection system was constructed, achieving a detection limit as low as 102 nmol L–1. This study highlights the great potential of polymeric carbon nitride loaded with cerium-based active species as composite nanozymes, offering the insights into the structure–activity relationships of highly active cerium oxide nanozymes.