A smartphone-assisted portable sensing hydrogel modules based on UCNPs and CoO NPs for fluorescence quantitation of hypoxanthine in aquatic products.

Hypoxanthine is a promising index for evaluating the freshness of various aquatic products. Combined the hydrogels containing upconversion nanoparticles (UCNPs), CoO NPs, and N-ethyl-N-(3-sulfopropyl)-3-methylaniline sodium salt/4-amino-antipyrine (TOPS/4-AAP) with a smartphone, a portable sensor was developed for the convenient, sensitive detection of hypoxanthine. With the HO from xanthine oxidase (XOD)-catalyzed reactions of hypoxanthine, the fluorescence of UCNPs was effectively quenched by the purple product produced from the oxidization of TOPS/4-AAP catalyzed by CoO NPs exhibiting peroxidase activity, among which the color change could be transformed into digital signals for quantification of hypoxanthine. The Green value in the RGB analysis of the fluorescence image was negatively proportional to hypoxanthine concentration in the range of 2.5-20 mg/L with a detection limit of 0.69 mg/L and a quantitation limit of 2.30 mg/L. Finally, this sensor was applied for hypoxanthine detection in real aquatic products, showing potential application for freshness evaluation of aquatic products.