Co–Mn Mixed Metal Oxide Nanorods for On-Site Colorimetric Detection of SO₃^2– in Food Samples

Excessive sulfite (SO 3 2– ) causes serious human health problems, making its detection highly valuable. In this work, a colorimetric sensor was presented for the detection of SO 3 2– on the basis of Co–Mn mixed metal oxide (CoMn MMO) nanorods. CoMn MMOs composed of CoMn 2 O 4 were uniformly dispersed among Co 3 O 4 by calcining Co 6 (CO 3 ) 2 (OH) 8 ·H 2 [email protected] 2 O 4 nanorods in air, which had intrinsic oxidase-like activity. The synergistic introduction of CoMn 2 O 4 promoted electron transfer in Co 3 O 4, 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 ( K m ) of 0.042 mM and a quick V max of 0.85 × 10 –8 M s –1 by generating a superoxide radical (·O 2 – ). Based on the inhibitory effect of SO 3 2– on the CoMn MMOs-TMB system, the proposed method allowed for SO 3 2– 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 SO 3 2– in food samples.