Monitoring fish freshness by colorimetric detection of hypoxanthine using a Fe3O4@ZIF-8@HRP@SiO2 nanostructure

A colorimetric method for fish freshness monitoring was developed. After coating magnetic Fe 3 O 4 nanoparticle with zeolitic imidazolate framework-8 (ZIF-8), the horseradish peroxidase (HRP) was adsorbed onto it with high efficiency due to the large surface area of ZIF-8, the mesoporous SiO 2 was further decorated onto the Fe 3 O 4 @ZIF-8@HRP to form Fe 3 O 4 @ZIF-8@HRP@SiO 2 nanostructure, in order to minimize the release of HRP, and enhance the stability and catalytic activity of HRP. The morphology, elemental composition, surface functional groups of Fe 3 O 4 @ZIF-8@HRP@SiO 2 were characterized by transmission electron microscopy, X-ray diffraction, energy dispersive spectroscopy, Fourier transform infrared spectroscopy. Hypoxanthine (HX), which formed due to the degradation of adenosine triphosphate in fish, can be oxidized by xanthine oxidase and generated H 2 O 2 intermediate. Thereafter, in the presence of H 2 O 2 , HRP within Fe 3 O 4 @ZIF-8@HRP@SiO 2 nanostructure would catalyze the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) to a blue product of TMBox, allowing an indirect colorimetric sensing of HX content. Under optimal conditions, HX was selectively and sensitively detected in the range of 0–100 μM, with a detection limit of 0.133 μM. Finally, the proposed method was applied to determine the HX content in grass carp continuously stored at 4 °C and 30 °C for 96 h.