A mechanically tough and fluorescent double network hydrogel simultaneously featuring advanced information encryption and smart food monitoring

Fluorescent hydrogels show promise for either information encryption or food freshness indication; however, combining both features in a hydrogel system with strong mechanics for addressing information leakage and structural damage during transport remains challenging. Herein, we developed a mechanically tough hydrogel through copolymerization of acrylamide with UV-responsive (spiropyran-linked acrylate) and alkaline-responsive (7-acryloxy-4-methylcoumarin) fluorescent monomers, using a sodium alginate-Ca²⁺ system. The resulting hydrogel exhibited remarkable mechanics, including a 3450 % breaking elongation, 290 kPa fracture stress, and 568 MJ/m³ toughness, capable of lifting 400 g—656 times its own weight—outperforming current fluorescent hydrogels. It also demonstrated anti-fatigue, and excellent tearing resistance, with a tearing energy of 14 kJ/m². Its fluorescence could shift from red to blue under UV light depending on pH (acidic pH = 1, alkaline pH = 13). These properties enabled advanced anti-counterfeiting applications, such as multi-color information encryption, ionic printing, and QR codes. Notably, the hydrogels also effectively monitored beef and milk freshness, with correlation coefficients (R²) exceeding 0.992 between TVB-N and ΔE for beef and 0.995 between acidity and ΔE for milk, surpassing existing hydrogel indicators with R² < 0.90. Overall, this strategy may inspire novel, sustainable fluorescent materials for anti-counterfeiting, smart food tags, and visual sensing systems.