<p>Manganese-doped carbon dots (Mn-CDs) nanozymes were synthesized, which exhibited strong oxidase activity, excellent stability, and good water solubility. Compared to other nanozymes, Mn-CDs could effectively catalyze 3,3’,5,5’-tetramethylbenzidine (TMB) to generate blue oxidized TMB (oxTMB) without the use of strong oxidant H₂O₂. With this superior property, an efficient ratiometric colorimetric method was developed. In the presence of nitrite (NO₂⁻), oxTMB was reacted with NO₂⁻ to produce diazotized TMB, resulting in a significant visual change from blue to green to yellow. Therefore, the characteristic peak at 652&#xa0;nm significantly decreased, while the characteristic peak at 445&#xa0;nm significantly increased, enabling sensitive detection of NO₂⁻ in various samples. It was noteworthy that a new detection method based on Mn-CDs was constructed by using hydrogel as the carrier and protective agent (Mn-CDs@hydrogels). The results were collected and analyzed through a smartphone, and then converted into digital signals, thus achieving real-time on-site detection of NO₂⁻. The shelf life of Mn-CDs@hydrogels could reach 6 months at 4&#xa0;°C, and its effectiveness and reliability in various food samples had also been evaluated. The strategy proposed will promote advanced applications of Mn-CDs in the field of sensing in the future.</p> Graphical Abstract <p></p>