Structural Color Liquids with Sol-Gel Irreversibility for Visualized Freeze-Thaw Monitoring

Protein-based bio-products such as vaccines, antibodies, enzymes, and plasma are crucial in public health and life sciences, yet their efficacy is frequently compromised by temperature fluctuations, especially repeated freeze-thaw cycles during storage and transport. While monitoring freeze-thaw damage is critical for the quality control of these bio-products, current methods lack the capability to indicate the exact number of freeze-thaw cycles. Here, structural color liquids enable visualized freeze-thaw monitoring (FT-SCLs) are introduced by harnessing their irreversible sol-gel phase transition under repeated freeze-thaw cycles, which are constructed by assembling periodically structured poly(styrene-acrylic acid) colloidal particles within a poly(vinyl alcohol) suspension. The FT-SCLs undergo irreversible sol-gel transition and therefore unidirectional alteration of their periodic structures during freeze-thaw cycling, imparting stepwise and unrecoverable color change (from red to green) to indicate the exact number of freeze-thaw cycles. Through modulating the sol-gel transition, the FT-SCLs are constructed with adjustable sensitivity across practically relevant temperature ranges (−80–−4 °C) and customizable response thresholds for diverse application scenarios. Leveraging their unique capabilities of freeze-thaw monitoring via non-tampered optical signals, such FT-SCLs exhibit broad applicability in vaccine storage, whole blood preservation, and enzyme stability monitoring, which can further be extended for cell cryopreservation and the food industry.