Thermal Modulation of Cerium-Based Nanozyme Activity for Colorimetric Detection of Ascorbic Acid

Metal-organic framework (MOF) nanozyme materials have multiple application prospects, and the calcination process is an effective strategy for regulating and enhancing their enzyme-like catalytic performance. In this work, a series of Ce-ATPA-X materials were prepared by calcining the Ce-based framework precursor at 200-800 °C temperatures, and their structural characteristics and enzyme-like activities were systematically investigated. Among them, Ce-ATPA-400 exhibited the strongest oxidase-like (OXD-like) and peroxidase-like (POD-like) activities, approximately 2.4 and 2.6 times those of Ce-ATPA-0, respectively. Various characterization analyses indicated that a heat treatment temperature of 400 °C can roughly maintain the framework structure while transforming the Ce3+ component into a mixed-valence state of Ce3+/Ce4+. The presence of the porous structure and the mixed-valence state may have a structural-activity correlation with the enhanced 3,3',5,5'-tetramethylbenzidine (TMB) oxidation. Based on its catalytic chromogenic activity, a colorimetric sensing platform for ascorbic acid (AA) was developed using 3,3',5,5'-tetramethylbenzidine (TMB) as the chromogenic substrate, which showed a good linear response in the range of 2.5-30 μM and a detection limit of 1.26 μM. Through smartphone-assisted RGB analysis, visualization and portable readout were further achieved, and it exhibited good selectivity, anti-interference capability, and satisfactory recoveries in orange, lemon, and strawberry samples. This study analyzed the possible reasons for the enhancement of enzyme activity by MOF derivatives through the strategy of comparing different calcination temperatures, and constructed a colorimetric sensor for food detection based on the enhanced enzyme-like activity, which has great reference and application value for developing nanomaterials with high enzyme-like activity and effective applications.