Irreversible Bilayer Thermochromic Label Integrated With 3D Printing for Wide‑Range Temperature Monitoring in Food Logistics.

Temperature abuse during logistics is a major cause of quality deterioration in food, pharmaceuticals, and biological products. Therefore, the development of low-cost, irreversible, and tamper‑proof temperature indicators is of great practical significance. Based on the acid-catalyzed irreversible ring-opening coloration of crystal violet lactone (CVL) under acidic conditions, a bilayer temperature monitoring label was prepared in this study, and the label structure was further optimized by introducing three-dimensional (3D) printing technology. Upon temperature excursions, the acidic components in the upper layer diffused into the lower layer, triggering an irreversible color change that remained stable after re-cooling. Quantitative color analysis using a smartphone application showed that at 15°C the hue angle increased linearly with exposure time (correlation coefficient r = 0.9854), demonstrating the feasibility of real-time digital temperature monitoring. To broaden the practical temperature range, an integrated label mold was fabricated via 3D printing, and the monitoring threshold was tuned by adjusting the outer-ring solvent, achieving activation from ambient conditions to ultra-low temperatures. Finally, based on the discolored area of the 3D-printed labels under temperature-excursion conditions, intuitive visual warning levels were established, providing a reliable and tamper-proof indication of temperature abuse.