Time–Temperature Integrating Optical Sensors Based on Gradient Colloidal Crystals

Abstract Manipulation‐free and autonomous recording of temperature states for extended periods of time is of increasing importance for food spoilage and battery safety assessment. An optical readout is preferred for low‐tech visual inspection. Here, a concept for time–temperature integrators based on colloidal crystals is introduced. Two unique features in this class of advanced materials are combined: 1) the film‐formation kinetics can be controlled by orders of magnitude based on mixtures of particles with distinct glass transition temperatures. 2) A gradual variation of the particle mixture along a linear gradient of the colloidal crystal enables local readout. Tailor‐made latex particles of identical size but different glass transition temperatures provide a homogenous photonic stopband. The disappearance of this opalescence is directly related to the local particle ratio and the exposure to a time and temperature combination. This sensing material can be adjusted to report extended intermediate and short excessive temperature events, which makes it specifically suitable for long‐term tracing and threshold applications.