Thermochromic Microcapsules Containing Chiral Mesogens Enclosed by Hydrogel Shell for Colorimetric Temperature Reporters

Abstract

Chiral mesogens spontaneously form a cholesteric phase and show structural colors through wavelength-selective reflection. Although chiral mesogens are pragmatic materials to build cholesteric phase due to their low toxicity, low cost, and robustness, an elaborate encapsulation technique is not reported yet. In this work, cholesteryl esters are encapsulated with a calcium-alginate hydrogel shell using oil-in-water-in-oil double-emulsion droplets. With capillary microfluidic devices, monodisperse double-emulsion droplets are prepared to have a mixture of cholesteryl ester and toluene in the innermost droplets and an aqueous solution of sodium alginate and calcium-carrying complex in the outer droplets. As toluene diffuses out from the core, cholesteryl ester forms the cholesteric phase. At the same time, calcium ions are dissociated from the complex upon chemical cues, which causes ionic crosslinking of alginate. The microcapsules show the thermochromic property as the structural color of the cholesteric core blue-shifts along with the temperature. Therefore, the microcapsules report local temperatures with colors or reflectance spectra. As the range and sensitivity of the colorimetric temperature measurement are controllable by adjusting the composition of cholesteryl esters, a wide range of temperature can be covered by employing a proper set of compositions while maintaining high sensitivity.