Old dogs, new trick: Classical starch˗sodium iodide reagent as the chromogenic substrate of peroxidase-like nanozymes

2, 2′-azinobis-3-ethylbenzthiazoline-6-sulfonate (ABTS), 3, 3′, 5, 5′-tetramethylbenzidine (TMB), o-phenylenediamine (OPD) are typical chromogenic substrates to evaluate the activity of peroxidase-like nanozymes. However, these substrates are toxic, unstable, and the coloration reactions are slow. Herein, the classical starch-sodium iodide reagent was utilized as the chromogenic substrate to evaluate peroxidase-like activity of nanozymes. Starch-sodium iodine is less toxic, more stable and faster color reaction than ABTS, TMB and OPD. Under the catalysis of peroxidase-like nanozymes such as Mo5N6 nanosheets, graphene oxide nanosheets, Fe3O4 nanoparticles, CeO2 nanoparticles, iodide ion (I–) is oxidized by H2O2 to produce blue color complex (starch-I2). Taking Mo5N6 nanosheets as the peroxidase-like nanozyme, the color reaction completed within 5 min at room temperature (25 ºC), and the chromogenic substrate remained stable within 30 days. The maximum reaction rate (Vmax) using starch-sodium iodine as the chromogenic substrate was more than 100-fold higher than that of TMB. Based on this, a sensitive colorimetric sensor and facile portable agarose gel test kits were proposed and applied for the detection of H2O2 residue in liquid milk samples with fast response. •Classical chromogenic reagent starch-I2 was utilized as the substrate of peroxidase-like nanozymes.•Starch-sodium iodine is less toxic, more stable and faster color reaction than ABTS, TMB and OPD.•A sensitive colorimetric sensor and portable agarose gel test kits were proposed for H2O2 in liquid milk.