Hierarchical-Structured Thermochromic Microcapsules With Second‑Scale Response and High‑Contrast for Low‑Temperature Visual Indicators

Reliable temperature monitoring in cold chain logistics and food storage necessitates rapid detection of temperature excursions to ensure safety and quality. However, most existing approaches depend on powered sensors and complex data systems, limiting their suitability as passive, intuitive, and large area indicators for low temperature anomalies. Herein, raspberry-like low-temperature thermochromic microcapsules (RLTMs) were developed via a two-step interfacial polymerization approach as passive, large area visual indicators for low temperature monitoring. Structural analysis confirmed a hierarchical architecture with strong SiO 2 confinement, lowering the phase change onset from 12.1°C to −6°C and enabling graded thermal transitions. The RLTMs exhibited rapid, reversible color switching from −10°C to 37°C, achieving a visually distinct color difference (ΔE > 50) within 2 s at −10°C and full transition within 10 s. This high sensitivity originated from multi scale surface curvature that enhances diffuse scattering while minimizing glare. After 50 thermal cycles, the microcapsules retained fast response (<5 s) and high chromatic contrast, demonstrating excellent cycling stability. With a uniform size of 46.33 ± 5 µm, they can be directly screen printed as durable visual labels for food packaging and cold chain monitoring applications.