Carbon dot nanozymes driven ratiometric colorimetric and smartphone-assisted hydrogel for detection of nitrite in...

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 nm significantly decreased, while the characteristic peak at 445 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 °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.