Bifunctional Carbon Dot Nanozymes for Ratiometric Optical, Photothermal, and Smartphone-Assisted Multimodal Detection of Nitrite

Accurate and reliable nitrite (NO₂^-) detection is crucial for food safety but remains challenging. Herein, we develop a triple-signal sensing strategy utilizing bifunctional carbon dot (CD) nanozymes for NO₂^- analysis in diverse food matrices. CDs with blue fluorescence and photoresponsive oxidase-mimicking activity are successfully synthesized by a simple one-step hydrothermal treatment with citric acid monohydrate (CA) and triethanolamine (TEA) as precursors. The oxidase-mimicking property enables efficient catalytic oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) from colorless to blue ox-TMB. Capitalizing on the specific diazotization reaction between NO₂^- and <i>o</i>-phenylenediamine (OPD)/ox-TMB, the engineered sensor delivers ratiometric fluorescence, ratiometric colorimetric, and photothermal triple-signal outputs, displaying high performance in selectivity and anti-interference capability. The linear detection ranges for NO₂^- are 0.5-400 μM in ratiometric fluorescence sensing, 0.5-100 μM in ratiometric colorimetric sensing, and 5-100 μM in photothermal sensing, with corresponding limits of detection of 0.23 μM, 0.19 μM, and 1.43 μM, respectively. Furthermore, by leveraging the distinct color transitions from ratiometric fluorescence and colorimetric responses, a dual-modality sensing platform assisted by smartphones is engineered to achieve convenient, visual, and on-site NO₂^- detection in food samples. Notably, the integrated photothermal detection specifically overcomes the limitations of conventional optical methods for analyzing colored or autofluorescent samples, while the cross-validation capability of the multimodal strategy ensures reliable and stable results. This synergy provides a comprehensive and promising solution for the accurate and robust monitoring of NO₂^- in multiple types of food matrices.