Hollow nanozyme with improved peroxidase-like activity for nitrite detection

Nitrite (NO₂^-) is a well-known pollutant and carcinogen, making point-of-care (POC) detection an urgent concern. In this study, Fe₃C/Mo₂C hollow spheres (Fe₃C/Mo₂CHS) were successfully synthesized. As a nanozyme, Fe₃C/Mo₂CHS exhibits superior peroxidase-like (POD-like) activity compared with Fe₃C and Mo₂C. For H₂O₂, K_m is 0.0683 mM with V_max 3.87 × 10^- 5 M·min^- 1. Within Fe₃C/Mo₂CHS, strong d-d coupling between Mo and Fe induces charge redistribution at the Fe₃C/Mo₂C heterointerface. In the presence of Fe₃C/Mo₂CHS and H₂O₂, colorless 3,3,5,5-tetramethylbenzidine (TMB) is oxidized to blue ox-TMB, which subsequently reacts with NO₂^- to produce a yellow diazotization product. Based on this reaction, a POC digital image colorimetric method was developed for NO₂^- detection using a smartphone. The method demonstrates an excellent linear response over the 2-300 µM range. The limit of detection (LOD) for Fe₃C/Mo₂CHS is 2.09 µM. Interference studies with various ions confirm the high selectivity of this approach for NO₂^-. Moreover, the method exhibits reliable performance and reproducibility for practical NO₂^- detection in food and water samples.