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

Abstract 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.