Co–Mn Mixed Metal Oxide Nanorods for On-Site Colorimetric Detection of SO32– in Food Samples

Excessive sulfite (SO32–) causes serious human health problems, making its detection highly valuable. In this work, a colorimetric sensor was presented for the detection of SO32– on the basis of Co–Mn mixed metal oxide (CoMn MMO) nanorods. CoMn MMOs composed of CoMn2O4 were uniformly dispersed among Co3O4 by calcining Co6(CO3)2(OH)8·H2O@CoMn2O4 nanorods in air, which had intrinsic oxidase-like activity. The synergistic introduction of CoMn2O4 promoted electron transfer in Co3O4, which not only prevented the aggregation problem but also increased the active sites, and the catalytic activity of the resulting CoMn MMO nanoenzyme was significantly enhanced. The obtained nanocomposites could catalyze the oxidation of the chromogenic substrate 3,3′,5,5′-tetramethylbenzidine (TMB) to blue oxTMB with a quite low Michaelis–Menten constant (Km) of 0.042 mM and a quick Vmax of 0.85 × 10–8 M s–1 by generating a superoxide radical (·O2–). Based on the inhibitory effect of SO32– on the CoMn MMOs-TMB system, the proposed method allowed for SO32– detection with the linear concentration range from 1.56 to 15.58 μM and a limit of detection of 62.38 nM under optimal conditions. Combined with a smartphone chromaticity analysis app, we designed a portable and disposable agarose hydrogel test kit using the proposed system, which realized the on-site semiquantitative and visual detection of SO32– in food samples.