Carbon-Coated Cu/Cu_<i>x</i>O Nanoparticles for Real-Time Colorimetric Monitoring of Nitrite Levels in Stored Cooked Food

Nitrite (NO2–) accumulation in stored and processed foods poses serious health risks due to its potential conversion into carcinogenic nitrosamines. Traditional detection methods, while accurate, are often complex and ill-suited for rapid, on-site applications. In this study, we developed a colorimetric platform using a carbon-coated Cu/CuxO nanoparticles derived from HKUST-1(Cu) metal–organic framework, designed for real-time monitoring of nitrite levels in cooked cabbage and chicken during storage. The nanoparticles exhibit high peroxidase-like catalytic activity, promoting the oxidation of colorless 3,3′,5,5′-tetramethylbenzidine (TMB) to blue TMB+ in the presence of H2O2. Upon reaction with NO2–, a diazotization process shifts the color from blue to green, enabling direct visual detection. A quantitative model correlating RGB color values with nitrite concentration was established, allowing smartphone-based estimation of nitrite levels without specialized equipment. This platform provides a simple, cost-effective approach for monitoring nitrite concentrations in foods, enhancing food safety and addressing public concerns about the safety of stored cooked foods, such as “overnight leftovers”. This colorimetric platform shows strong potential in food quality assurance.