A pH-responsive Fe,Co,P-CDs hydrogel compound and its application in visual assessment of aquatic product freshness

Hypoxanthine (Hx) and pH are important indicators for evaluating the freshness of aquatic products, and their rapid assessment can maximize public food safety and minimize economic losses. A nanozymes-loaded smart responsive hydrogel compound provides a new strategy for rapidly assessing the freshness of aquatic products. Accordingly, this paper reports the preparation of a hydrogel compound with pH responsiveness and peroxidase (POD) activity using a one-step method. In this method, sodium alginate, chitosan, and gelatin are used as the raw materials and Fe,Co,P-CDs (CDs) with POD activity as well as 3,3′,5,5′-tetramethylbenzidine and H 2 O 2 are simultaneously introduced as substrates. The hydrogel compound transforms from a solid to a sol based on its pH response (pH = 6.5–8.0), and its color changes from blue to colorless because of its Hx response to the inhibition of the POD activity of CDs. The color difference shows a good linear relationship with the Hx concentration in the range of 1.5–1500 μM, with a detection limit of 1.48 μM and an excellent recovery rate of practical samples. Based on the pH enhancement and Hx accumulation during the spoilage of shrimps, naked-eye and smartphone-assisted, real-time monitoring of the freshness of shrimp has been realized through the morphology and color changes of the hydrogel compound. This dual-response hydrogel compound provides a new strategy for monitoring the freshness of aquatic products with great potential in food safety monitoring.