Photonic Capsule Sensors with Built‐In Colloidal Crystallites

Abstract Technologies to monitor microenvironmental conditions and its spatial distribution are in high demand, yet remain unmet need. Herein, photonic microsensors are designed in a capsule format that can be injected, suspended, and implanted in any target volume. Colorimetric sensors are loaded in the core of microcapsules by assembling core–shell colloids into crystallites through the depletion attraction. The shells of the colloids are made of a temperature‐responsive hydrogel, which enables the crystallites to rapidly and widely tune the structural color in response to a change in temperature while maintaining close‐packed arrays. The spherical symmetry of the microcapsules renders them optically isotropic, i.e., displaying orientation‐independent color. In addition, as a solid membrane is used to protect the delicate crystallites from external stresses, their high stability is assured. More importantly, each microcapsule reports the temperature of its microenvironment so that a suspension of capsules can provide information on the spatial distribution of the temperature.