Abstract: A visual, non-destructive method for monitoring fish and shrimp freshness was established using an amine-responsive agarose hydrogel loaded with purple sweet potato anthocyanin (PSPA). Trimethylamine (TMA) was selected as an indicator of fish and shrimp spoilage. The redshift of the absorption peak of PSPA hydrogels after reaction with different concentrations of TMA (0–194.22 × 10-9 mol/L) was investigated. The red (R), green (G), and blue (B) channel values of the hydrogels were analyzed using smartphone imaging combined with the Image J software. The concentration of TMA was quantified using G/R as the output signal. The limit of detection (LOD) was 0.84 × 10-9 mol/L and the linear range was 8.78 × 10-9–19.386 × 10-9 mol/L (R2 = 0.992 1). When used to detect the freshness of fish and shrimp, the proposed method showed a good correlation with the national standard method for measuring total volatile basic nitrogen (TVB-N) content with a Pearson correlation coefficient of 0.998 4 and 0.996 6, respectively. Due to the application of functional hydrogel and the introduction of smartphones, the method had the advantages of simplicity, non-destructiveness, and portability. This study provides a new method for on-site monitoring the real-time freshness of aquatic products, which has a broad application prospect in the field of aquatic food safety detection.

Key words: purple sweet potato anthocyanins; hydrogel; aquatic products; freshness; red, green and blue method