The Construction of n‐n Heterojunction in Polyoxometalate Based Nanozyme to Enhance Peroxidase‐Like Activity for Nitrite Detection

Nitrite (NO ₂ ⁻ ) is a notorious pollutant that has attracted increasing concern. This study synthesizes a novel polyoxometalate (POM) nanozyme Fe ₂ V ₄ O ₁₃ /V ₂ O ₅ and leverages its excellent catalytic performance to develop asmartphone‐based platform for the highly sensitive detection of NO ₂ ⁻ . An n–n heterojunction formed between Fe ₂ V ₄ O ₁₃ and V ₂ O ₅ induces electron redistribution, generating a built‐in electric field that improves the interaction between Fe ₂ V ₄ O ₁₃ and substrates in peroxidase‐like (POD‐like) catalysis. Given the excellent POD‐like activity of Fe ₂ V ₄ O ₁₃ /V ₂ O _5, NO ₂ ⁻ can undergo diazotization reaction with the blue oxidized product ox‐TMB generated by the nanozyme catalyzing 3,3,5,5‐tetramethylbenzidine (TMB), resulting in the solution color changing from blue to yellow. Consequently, a visible colorimetry is developed for NO ₂ ⁻ detection with a low limit of detection (LOD). The method has demonstrated excellent performance in the actual detection of NO ₂ ⁻ in water samples. This work not only offers a new construction strategy to enhance the catalytic activity of POM‐based nanozymes, but also provides an efficient and practical platform for the highly sensitive NO ₂ ⁻ detection.