Liquid–liquid extraction: a universal method to synthesize liquid colloidal photonic crystals

The liquid colloidal photonic crystal (PC) is a new kind of photonic structure with attractive properties, such as fluidity, structural metastability, and reversibility in colloidal assembly. However, its controllable synthesis is a big challenge, because it is very difficult to keep all the particles well-dispersed in a highly concentrated colloidal system, and then assemble them into colloidal crystals. In this work, a liquid–liquid extraction (LLE) method has been developed to produce liquid PCs via simple mixing and liquid phase separation at room temperature. Single- and two-component extractants are used to extract part of the solvent from a diluted colloidal solution, which effectively concentrates the colloidal solution without inducing any aggregation and produce highly crystalline liquid PCs by spontaneous precipitation. Compared to the conventional methods, this new LLE process provides a quick, convenient, scalable and universal synthesis of liquid PCs composed of different colloidal particles and solvents, which lays a solid basis for its application in structural-color displays, sensors, and coatings.