CuCo-N-C nanozyme-mediated hydrogel colorimetric platform: dual-reading modes of ratiometry and smartphone for nitrite detection.

As a potentially hazardous food additive, nitrite acts as a double-edged sword, since excessive intake can trigger severe health risks. Developing an efficient and convenient on-site detection method for nitrite in food is therefore of great significance, yet remains challenging. To address this issue, a dual-mode colorimetric platform integrating a ratiometric assay with a hydrogel-based sensor has been constructed on the basis of the high oxidase-like activity of CuCo-N-C nanozymes. These nanozymes catalyze dissolved oxygen to oxidize colorless 3,3',5,5'-tetramethylbenzidine (TMB) into blue ox-TMB. The addition of nitrite triggers a diazotization reaction with ox-TMB, leading to decreased absorbance at 652 nm and increased absorbance at 450 nm. Furthermore, a smartphone-assisted portable kit was fabricated by embedding CuCo-N-C and TMB within an agarose hydrogel, enabling on-site visual quantification of nitrite in traditional foods including ham sausage, preserved eggs, salted duck eggs, mustard and fermented bean curd. This dual-mode colorimetric system is simple and reliable, with portable hardware and easy operation, and holds promise as a sensing platform for food safety monitoring.