Smartphone-assisted multicolor hypoxanthine sensing for on-site freshness assessment of aquatic products

Hypoxanthine (Hx) has been considered an essential indicator for aquatic products' freshness, especially in the early stage of spoilage. It could generate hydrogen peroxide (H2O2) and uric acid (UA) with the catalyst of xanthine oxidase (XOD). Existed Hx biosensors are only based on H2O2-triggered signal changes or are unsuitable for on-site freshness assessment. Here, we design an on-site Hx sensing method via the detection of both H2O2 and UA by taking advantage of the specific properties of manganese dioxide (MnO2) nanosheets. MnO2 nanosheets could oxidize colorless 3,3′,5,5′-tetramethylbenzidine (TMB) into yellow TMB2+, which can etch gold nanorods (Au NRs) with vivid color changes. However, in the presence of Hx, MnO2 nanosheets were reduced to Mn2+ rapidly by H2O2 and UA, inhibiting the etching step and mediating color changes. The H2O2 and UA meditated strategy exhibits excellent sensitivity toward Hx with a limit of detection (LOD) of 0.378 µM. A WeChat mini program was also constructed to achieve the on-site evaluation and satisfy the special population' requirement. The proposed method has been successfully used to monitor actual sample spoilage under different storage conditions. We expect this method to provide another insight into the on-site freshness assessment of aquatic products.