Calcium alginate reinforced zwitterionic double network hydrogel with mechanical robustness and antimicrobial activity for freshwater shrimp spoilage detection.

Hydrogel indicators promise to monitor food spoilage, but their poor mechanics can cause defects in transport. Herein, a novel zwitterionic double network (DN) hydrogel was developed by polymerizing arylamide and sulfobetaine methacrylate in an alginate-Ca system. This hydrogel exhibited enhanced mechanical properties, including a maximum 2087 % breaking elongation and 135 ± 12 kJ/m toughness, significantly outperforming the current zwitterionic DN hydrogels, which typically exhibit less than 1800 % breaking elongation, capable of supporting 150 g-136 times its own weight. Importantly, it also demonstrated excellent anti-fatigue and tear resistance, with a tearing energy of 2200 ± 180 J/m. The abundant zwitterionic polymers, hydroxyl groups, and amino groups within hydrogel contribute to its strong adhesiveness to various substrates. Additionally, the hydrogel exhibited antimicrobial activity against E. coli and S. aureus. Furthermore, a pH/NH dual-sensitive hydrogel indicator, using phenol red, bromocresol purple, and their mixtures in ratios of 1:1, 1:2, and 2:1, was developed for monitoring shrimp freshness. The results showed that the hydrogel indicator with phenol red, effective at 4 °C and 25 °C, showed clear color changes (ΔE > 80) in response to spoilage and strong correlations between ΔE and TVB-N (R > 0.98) and pH (R > 0.97), surpassing the existing hydrogel indicators used for shrimp freshness monitoring, which generally have a correlation coefficient of only 0.86. This novel dual-responsive hydrogel, with excellent mechanical performance and TVB-N sensitivity, is suitable for real-time freshness monitoring in intelligent packaging.