Mixed-Valence Ce-BPyDC Metal-Organic Framework with Dual Enzyme-like Activities for Colorimetric Biosensing.

Enzyme-like metal-organic frameworks (MOFs) are currently one type of starring material in the fields of artificial enzymes and analytical sensing. However, there has been little progress in making use of the MOF structures based on the catalytically active metal center with multiple valences. Herein, we report a mixed-valence Ce-MOF (Ce-BPyDC) that can exhibit both oxidase-like and peroxidase-like activities. Ce-BPyDC was synthesized by a facile hydrothermal method, which preserves the rare coexistence of Ce(III) and Ce(IV) in the MOF structure. The enzymatic studies demonstrated the enzyme-like activities of Ce-BPyDC follow the Michaelis-Menten kinetics and are strongly dependent on temperature, pH, and reaction time. Ce-BPyDC was also revealed to exert high catalytic activity that could transcend horseradish peroxidase and other MOF nanozymes, due to the redox-active Ce(III)/Ce(IV) cycles inside. Furthermore, the simple synthesis, high nanozyme activity, and great stability of Ce-BPyDC motivated us to establish a colorimetric biosensing platform using 3,3',5,5'-tetramethylbenzidine as a color reagent. Adopting this strategy, we established a visual, sensitive, and selective colorimetric method for ascorbic acid (AA) detection, for which the linear interval and limit of detection were 1-20 and 0.28 µM, respectively. The successful AA detection in real juice samples implies the promising use of such mixed-valence MOF nanozymes in food and biomedical samples.