Cu-MOF derived CuO@g-C₃N₄ nanozyme for cascade catalytic colorimetric sensing

The use of peroxidase mimics has great potential for various real applications due to their strong catalytic activity. Herein, a facile strategy was proposed to directly prepare CuO@g-C₃N₄ by Cu-MOF derivatization and demonstrated its efficacy in constructing a multiple enzymatic cascade system by loading protein enzymes onto it. The resulting CuO@g-C₃N₄ possessed high peroxidase-like activity, with a Michaelis constant (K_m) of 0.25 and 0.16 mM for H₂O₂ and 3,3',5,5'-tetramethylbenzidine (TMB), respectively. Additionally, the high surface area of CuO@g-C₃N₄ facilitated the loading of protein enzymes and maintained their activity over an extended period, expanding the potential applications of CuO@g-C₃N₄. To test its feasibility, CuO@g-C₃N₄/protein oxidase complex was prepared and used to sense the ripeness and freshness of fruits and meat, respectively. The mechanism relied on the fact that the ripeness of fruits increased and freshness of food decreased with the release of marked targets, such as glucose and xanthine, which could produce H₂O₂ when digested by the corresponding oxidase. The peroxidase mimics of CuO@g-C₃N₄ could then sensitively colorimetric detect H₂O₂ in present of TMB. The obtained CuO@g-C₃N₄/oxidase complex exhibited an excellent linear response to glucose or xanthine in the range of 1.0-120 μmol/L or 8.0-350 μmol/L, respectively. Furthermore, accurate quantification of glucose and xanthine in real samples is achieved with spiked recoveries ranging from 80.2% to 120.0% and from 94.2% to 112.0%, respectively. Overall, this work demonstrates the potential of CuO@g-C₃N₄ in various practical applications, such as food freshness detection.