The Au–CeO2 Nanozyme with Enhanced Peroxidase-like Activity for Highly Sensitive Ratiometric Colorimetric Detection of Nitrite

Nitrite (NO2−) is a common food additive but it poses serious health risks due to its potential to form carcinogenic N-nitrosamines. Rapid and sensitive detection of nitrite in food matrices remains a critical need. Herein, we report a ratiometric assay based on Au-loaded mesoporous CeO2 (Au–CeO2) nanozymes with enhanced peroxidase-like activity for highly sensitive nitrite detection. The Au–CeO2 nanozyme was synthesized via a facile thermal decomposition and reduction method. Au–CeO2 nanozyme exhibited a higher Ce3+/Ce4+ ratio (from 25:75 to 31:69) and increased oxygen vacancy density (from 22.4% to 31.2%) compared to pristine CeO2. Meanwhile, the mesoporous structure effectively enhances the interaction between catalytic substrates and the active sites of nanozymes. The mesoporous structure is favorable for substrate accessibility to the active sites, which may contribute to the enhanced peroxidase-like catalytic performance. The nanozyme efficiently catalyzed the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) to oxTMB, producing a strong absorbance at 652 nm. Upon the addition of nitrite, diazotization of oxTMB occurred, leading to a decreased signal at 652 nm and a new absorbance peak at 445 nm. This ratiometric response (A652/A445) enabled sensitive detection of NO2− in the range of 1–200 μM with a detection limit of 0.43 μM. The ratiometric colorimetric assay demonstrated excellent selectivity and was successfully applied to real food samples with a satisfactory recovery (95.4–106.6%, RSD < 2.73%). The ratiometric colorimetric method provides a novel analytical strategy for environmental and food safety monitoring, offering high sensitivity, high accuracy, and excellent reliability.