The enhanced oxidase-like activity of modified nanoceria/ZIF-67 for fluorescence and smartphone-assisted visual detection of tannic acid

As a substitute for natural enzymes, nanozymes have unique characteristics of high catalytic activity, excellent stability and low cost, making them pivotal in sensing applications. Among them, nanozymes with oxidase-like activity garner significant attention for antioxidant detection, as they efficiently catalyze the oxidation of antioxidants in the presence of oxygen. Herein, a nanozyme with an enhanced oxidase-like activity was synthesized through a 3-step strategy, that is, ceria nanoparticles were firstly modified with amino acid and oligopeptides, then combined with zeolitic imidazolate framework-67 (ZIF-67), and finally the composite was etched in neutral water. The as-synthesized nanozyme can oxidize the substrate 3,3 ' ,5,5 '-tetramethylbenzidine (TMB) or Amplex Red (AR) in the presence of oxygen, inducing a change in their color or fluorescent signals, verifying its oxidaselike activity. Tannic acid (TA), an antioxidant, competes with TMB/AR for oxidation by the nanozyme. Leveraging this competitive relationship, a fluorescent and portable trichromatic sensor was developed for the detection of tannic acid (TA) in food samples. The fluorescent sensor shows high sensitivity with detection limits (S/N = 3) of 0.011 mu M, and the trichromatic sensor presents comparable performance to colorimetric sensor, exhibiting a linear range of 30 - 270 mu M and detection limit of 1.03 mu M. By using a smartphone platform, the trichromatic sensor enables convenient, rapid, inexpensive, and quantitative assessment of TA. Both two sensors were successfully applied to TA detection in 9 different foods. This study not only provides an effective strategy to enhance enzyme performance but also paves the way for their applications in food testing.