3D printed Ca2+ crosslinked sodium alginate gel-anthocyanin natural sensor with enhanced structural stability for tracking fruit and vegetable quality.

A calcium ion cross-linked sodium alginate-anthocyanin pH-responsive label was fabricated via 3D printing and evaluated for monitoring fruit and vegetable freshness during storage. Gradient sensors constructed with purple sweet potato anthocyanins for operation across a pH range of 9-12 exhibited dense microstructures, good structural integrity, and high environmental stability. During cold storage of five representative fruits and vegetables, the sensors displayed distinct ΔE color responses to variations in CO₂ concentration and relative humidity (RH). The gradient design enabled graded and intuitive visual color transitions, allowing stepwise freshness indication. High-respiration produce (mushrooms and broccoli) induced pronounced acidification responses in sensors conditioned at pH 9 and 10, whereas sensors conditioned at pH 12 exhibited fading due to pigment instability under alkaline and high-RH conditions. For moderate- and low-respiration crops (peppers, blueberries, and grapes), sensor responses reflected combined effects of CO₂ release and moisture-induced degradation. Across all samples, ΔE inflection points or stabilization typically occurred at days 4-6 (CO₂-dominated stage) or days 10-12 (humidity-dominated stage), demonstrating the sensor's adaptability and effectiveness in tracking freshness deterioration. This work addresses the stability limitations of anthocyanin-based sensors under alkaline conditions and provides a scalable, visual, and real-time freshness monitoring platform for intelligent packaging applications under realistic high-humidity storage conditions.