Synthesis of a new Ag+-decorated Prussian blue analog with high peroxidase-like activity and its application in measuring the content of the antioxidant substances in Lycium ruthenicum Murr.

A new Prussian blue analog (PBA) that contains three metal elements and has peroxidase-like activity was synthesized by a simple method. Then, AgNO3 solution was added slowly to the PBA solution under continuous stirring. We found that this synthesis method could be used to prepare other PBAs, and that the anchoring of Ag+ on the surface of PBA could enhance the peroxidase-like activity of the material, suggesting potential applications for the Ag+-decorated Prussian blue analog (Ag-PBA) in traditional Chinese medicine. Ag-PBA is a new type of multi-metal cubic nano-enzyme that exhibits good stability and excellent peroxidase-like activity; as such, it could catalyze the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) in the presence of H2O2 and Ag-PBA. We then developed a new method to measure the content of antioxidant substances in Chinese herbs by using the excellent peroxidase-like activity of Ag-PBA. Using the Chinese herb Lycium ruthenicum Murr. as a model compound, we measured the content of the antioxidant substances in Lycium ruthenicum Murr. by this new method. After optimization of reaction temperature, concentrations of TMB and H2O2, and reaction time, the content of the antioxidant substances was measured and calculated in comparison with anthocyanidin standards. The results of the Ag-PBA method and the classical DPPH method were compared by a paired t-test, with no statistically significant difference found between the methods. Hence, these two methods can be used interchangeably, although the Ag-PBA method had the advantages of simplicity, rapidness, and good stability. Moreover, the Ag-PBA method has a low limit of quantification and a shorter reaction time, which are improvements on the DPPH method, and it is not necessary to avoid light. Therefore, we anticipate that the Ag-PBA method may be used widely for the measurement of the content of antioxidant substances in Chinese herbs.