The target-induced redox and diazotized reaction for colorimetric ratio detection of nitrite using CoOOH nanosheets as mimetic oxidase

Nitrite is a commonly used food additive and water contaminant that has received widespread attention due to its harmful effects on humans. Here, a colorimetric ratio sensing platform for the detection of nitrite in foods as well as aquatic systems was developed via the catalytic oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) by CoOOH nanosheets (CoOOH NSs). Interestingly, in the presence of nitrite, TMB complexes in acidic environments can be oxidized and diazotized to produce yellow oxidized TMB (oxTMB) and diazotized TMB, resulting in the nitrite concentration-dependent ratio variation for the absorbance peaks at 655 and 450 nm (A655/A450). The colorimetric ratio sensing offers higher sensitivity and better selectivity compared to conventional detection methods because of the specific target-induced reduction-oxidation and diazotized reaction, as well as the excellent mimetic oxidase activity of CoOOH NSs. Based on this strategy, a smartphone-assisted portable approach was designed for the in-situ/visual detection of nitrite, which has good application prospects.