Colorimetric Detection of Gallic Acid in Tea Based on the Catalase-like Activity of Co₃O₄-NC Nanozyme

Objective: The present study was aimed to develop a rapid colorimetric method for the detection of gallic acid in tea based on the catalase-like activity of Co₃O₄-NC nanozyme. Methods: The Co₃O₄-NC was prepared by calcination of the composite material of zeolitic imidazolate frameworks-67 (ZIF-67) and polydopamine. The letramethylbenzidine (TMB) could be oxidized to blue oxTMB in the presence of H₂O₂ by Co₃O₄-NC nanozyme, while the addition of gallic acid would restrain the catalase-like activity and the color of the solution will be lighter. Based on the mechanism, a colorimetric sensing platform for the detection of gallic acid was established. Results: The linear curve between ΔA and GA concentration was observed within the range of 0.25~20.00 μmol/L, the linear equation was ΔA=0.0310C+0.0384 (R²=0.995), the limit of detection (LOD, 3σ/S) was 0.077 μmol/L and the limit of quantitation (LOQ, 10σ/S) was 0.26 μmol/L. Conclusion: The method could be applied to detect gallic acid equivalent (GAE) in tea, and the GAE in green tea, black tea and oolong tea were tested as 2.33%, 1.82%, and 0.46%, respectively. The recovery rates ranged from 90.97% to 105.22% with RSD<5%, indicating that the colorimetric method had a good application prospect.