Smartphone-assisted colorimetric sensor based on Au/Fe3O4 nanozyme for total antioxidant capacity evaluation in fruits.

A smartphone-based colorimetric sensor has been developed using Au/Fe3O4 nanozymes for rapid, visual, and on-site total antioxidant capacity (TAC) determination. The Au/Fe3O4 nanozymes were synthesized via a two-step post-doping method and exhibited spherical morphology with an average diameter of 282 nm. Compared with pristine Fe3O4, the Au/ Fe3O4 nanozymes demonstrated significantly enhanced peroxidase-like activity, attributed to an increased Fe2+/Fe3+ ratio (from 0.97 to 1.03) and a higher oxygen vacancy content (from 24.4% to 32.1%). Electron paramagnetic resonance and cyclic voltammetry confirmed that the catalytic mechanism involves hydroxyl radical (·OH) generation and electron transfer via the Fe2+/Fe3+ redox cycle. Au/Fe3O4 nanozyme catalyzed the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) to blue oxidized TMB (oxTMB). Based on the inhibition effect of antioxidants on TMB oxidation, the platform realized sensitive determination of ascorbic acid (AA), L-cysteine and glutathione with limits of detection (0.2, 0.8, 1.2 µM, respectively). Smartphone software extracted RGB and grayscale values to build quantitative standard curves of AA. Five fruit juices were detected with AA as TAC equivalent, and the results matched well with the classic DPPH method. Spike recovery tests presented recoveries of 96.33%-105.80% and RSD below 2.71%. This portable, low-cost and visualized smartphone-assisted colorimetric sensor provides a facile alternative for on-site rapid TAC evaluation of food.