It plays a critical role in various physiological activities and pathological states of organisms, including gene expression, cell division and inflammatory responses. [2] Monitoring the temperature on the micro/nano-scale is of major fundamental interest for a wide range of disciplines, ranging from biological processes, [3] chemical reactions, [4] micro/nano-electronics, [5] and micro/ nano-fluidics. [6] Currently, temperature microsensors are mainly thermocouples, [7] thermistors [8] and luminescence thermometers. [9] Thermocouples and thermistors are realized based on electrical properties with great stability and reliability; however, their structural features and uncertain material cytotoxicity may cause damage to cells. [7,8] Luminescence thermometer with small size and high sensitivity is an excellent candidate for measuring temperature. However, the fabrication of such systems depends on special chemical reagents and miscellaneous steps, leading to poor biocompatibility. [9c] Therefore, it is still a challenge to fabricate a temperature sensor with good stability and biocompatibility.Colorimetric temperature microsensors hold advantages of simple usage and direct read. In the class of colorimetric materials, cholesteric liquid crystal (CLC) possesses the advantages of quick response, and reversible and adjustable color range, [10] which have shown great potential for As alternatives to electronic devices, optically active structures from responsive nanomaterials offer great opportunity for building up smart functional sensors. This work reports on the construction and application of photonic microcapsules (PMCs) for colorimetric temperature microsensors, enabling miniaturization for injectable local micro-area sensing and integration for large-area sensing. Monodispersed PMCs are produced by in situ photopolymerization of hydrogel shells of cholesteric liquid crystal (CLC)-in-waterin-oil double emulsion droplets prepared using microfluidic devices, with controllable physical structures and chemical compositions. Constructed PMCs exhibit thermal responsive structural color according to the selective Bragg reflection of CLC's periodical helical structures within the microdroplet's spherical confinement. CLC mixtures with phase transition temperature of 56.0 and 37.8 °C have been prepared and employed to construct PMCs exhibiting obviously visible and quantifiable optical response in 52-56 and 33-38 °C, respectively. The PMCs have been successfully applied for monitoring the living cell extracellular temperature via co-incubation with cell suspension, and for sensing human forehead temperature via a flexible device from assembled PMCs. These PMCs can be flexibly applied in either micro-environment or large-area surface, enabling wide applications for a precision temperature monitoring of biological activities (e.g., cells or organs), optoelectronic devices working conditions (e.g., temperature indicators under extreme conditions), etc.
