With the improvement of living standards and the emphasis on healthy diet, people tend to use dietary intake of antioxidants to balance the excessive production of free radicals in the body, so as to prevent oxidative stress damage to cells and organs [1], [2]. Antioxidants in food are complex and cannot be quantified singly. Therefore, total antioxidant capacity (TAC) is considered as an important indicator to assess the cumulative activity of all antioxidants in food [3], [4]. In conclusion, there is a need for sensitive accurate and simple method to detect TAC in food. A higher TAC value indicates that the system has a strong reducing power and can be used as an antioxidant to balance the ROS levels in the system. Currently, nanozymes with peroxidase activity have been used for colorimetric analysis of TAC [5], [6]. However, there are also problems such as low affinity and low catalytic activity. This is attributed to the aggregation of the low catalytic activity sites of the nanozyme with in adequate contact with the substrate [7], [8], which is usually found in metal clusters or compound nanozyme. Uniformly dispersed metal centers can optimize the catalytic activity of the active site, and when the metal sites are more uniformly dispersed, the catalytic activity is significantly increased [8], [9], [10], [11]. In addition, support materials are also important role in the catalytic activity of nanomaterials [9]. The robust interaction between metal atoms and the substrate can ensure the uniform dispersion of the metal ions on the surface and prevent aggregation [12]. In previous studies, researchers have explored various ways to improve the catalytic activity of nanozyme. Wang and colleagues reported a defection-rich molybdenum disulfide/rGO vertical heterostructures containing surface defects with dual S and Mo vacancies, which provided a large amount of active catalytic site [13]. Huang et al. synthesized oxygen vacancy-rich Cu/Fe3O4 @FeOOH, aiming to enhance the catalytic activity through the synergistic interaction of oxygen vacancies with Cu [14]. Niu et al. designed an Fe-N-C single-atom nanozyme in which the metal atoms are uniformly distributed and can be used as active sites independently. The atomic utilization was improved and the enzyme-like properties were significantly enhanced [15]. In conclusion, catalytic activity can be improved by enhancing the uniform distribution of metal atoms and strengthening the strong interaction with the substrate. Therefore, it’s necessary to find a method to improve metal active sites dispersion and strong interaction with substrate to improve affinity and catalytic activity.

Metal-phenolic networks (MPNs) is a supramolecular network structure composed of coordination with polyphenols and metal ions [16]. MPNs not only have the specific function of metal ions, but also have high affinity for various substrate surfaces, so it is widely used in various functional surface coatings. In addition, MPNs are also used to prepare nanoparticles [17], nanocapsules [18], etc. Tannic acid is a classic phenol, which is widely existed in plants and has been approved by FDA. At present, it is widely used in the construction of MPNs with Fe ions. Under low pH conditions, TA and Fe can form mono-complexes and prevent the self-polymerization of TA, resulting in the formation of homogeneous MPNs on the substrate [19]. So far, Li and colleagues have effectively eliminated the formation of coordination complexes by layer-by-layer deposition of TA-Fe, reducing the agglomeration of nanospheres [15]. The homogeneous TA-Fe coating contributes to the formation of metal nanoparticles with narrow particle size distribution and significantly enhanced catalytic activity. It has been reported that MPNs can introduce homogeneous and easily accessible active sites. Moreover, the active elements can be diffused into the carbon matrix and form Fe-Nx by coordination with nitrogen atoms during the carbonization process [20], [21]. Therefore, it is a promising method to realize uniform distribution of metal atoms and strong interaction of substrate to improve the catalytic activity of nanozymes by MPNs.

In this study, CN-FeC nanozyme with high catalytic activity was synthesized on C3N4 by using the metal-phenolic networks formed by the coordination of tannic acid and Fe3+. The synthesis was shown in Scheme 1. Fe was attached to C3N4 by triazine groups, and then the low coordination formed by TA-Fe networks to make it more homogeneous and dispersed. After calcination, the polyphenolic component was removed and CN-FeC nanozyme was successfully synthesized. With abundant and uniform Fe active sites, interlayer hollow structure and Fe-Nx coordination, CN-FeC showed high peroxidase activity. CN-FeC catalyzed the generation of ·OH, 1O2 and O2·– from H2O2, and the generated reactive oxygen species oxidized TMB to oxTMB, which changed the reaction system from colorless to blue. Based on the competitive effect of antioxidants in the colorimetric system, the practical application of AA detection was also explored. AA can not only react the reactive oxygen in the system, but also reduce oxTMB to TMB. The degree of color change indicates the content of TAC in the system. The determination of actual samples yielded good recoverability, and CN-FeC exhibited excellent sensitivity, stability, and effectiveness. On the basis, CN-FeC nanozyme with peroxidase-like activity was successfully synthesized by MPNs, with a highly sensitive colorimetric method for the determination of TAC, and further applied it to practical samples.