Developing a self-adhesive and antibacterial intelligent hydrogel for real-time tracking of fish freshness

The practical utilization of existing anthocyanin-based intelligent hydrogels is limited by unreliable attachment, bacteria-induced response failure and complicated pre-extraction of anthocyanins. To address those limitations, we developed a self-adhesive and antibacterial intelligent hydrogel for real-time fish freshness monitoring via a pre-extraction-free one-pot strategy in which κ-carrageenan (κC), potassium gallate (KGA), and clitoria ternatea powder (CP) were blended in hot water, allowing anthocyanins self-release during stirring. Characterizations confirmed the hydrogel exhibited porous structures with CP embedded in the pore walls. Moreover, hydrogen bonding was identified as the dominant interaction in the smart hydrogel. As the amount of KGA and CP reached 0.4 g and 0.5 g, respectively, the hydrogel showed a high self-adhesive strength (>200 kPa) for secure attachment, significant antibacterial activity against Staphylococcus aureus (inhibition zone diameter of 34.06 mm), and adequate compressive strength (52.054 kPa). The hydrogel displayed distinct and rapid colorimetric responses to pH and ammonia, undergoing a visible color transition from blue to green as Total Volatile Basic Nitrogen (TVB-N) levels in fish increased. A critical color change (ΔE = 8.665) was observed as the TVB-N value approached the threshold of 20 mg/100 g at around 19.87 h of storage. Additionally, the hydrogel underwent complete biodegradation in soil within 9 days. This work successfully creates an efficient, robust, and eco-friendly intelligent hydrogel, providing a practical and sustainable strategy for real-time, in-package freshness monitoring of protein-rich foods.