Environmentally Responsive Emulsions of Thermotropic Liquid Crystals with Exceptional Long-Term Stability and Enhanced Sensitivity to Aqueous Amphiphiles.

Abstract

We report colloidally stable emulsions of thermotropic liquid crystals (LCs) that can detect the presence of amphiphilic analytes in aqueous environments. Our approach makes use of a Pickering stabilization strategy consisting of surfactant-nanoparticle complexes (SiO2/CnTAB, n = 8, 12, 16) that adsorb to aqueous/LC droplet interfaces. This strategy can stabilize LC emulsions against coalescence for at least 3 months. These stabilized LC emulsions also retain the ability to respond to the presence of model anionic, cationic, and nonionic amphiphiles (e.g., SDS, C12TAB, C12E4) in aqueous solutions by undergoing "bipolar-to-radial" changes in LC droplet configurations that can be readily observed and quantified using polarized light microscopy. Our results reveal these ordering transitions to depend upon the length of the hydrocarbon tail of the CnTAB surfactant used to form the stabilizing complexes. In general, increasing CnTAB surfactant tail length leads to droplets that respond at lower analyte concentrations, demonstrating that this Pickering stabilization strategy can be used to tune the sensitivities of the stabilized LC droplets. Finally, we demonstrate that these colloidally stable LC droplets can report the presence of rhamnolipid, a biosurfactant produced by the bacterial pathogen Pseudomonas aeruginosa. Overall, our results demonstrate that this Pickering stabilization strategy provides a useful tool for the design of LC droplet-based sensors with substantially improved colloidal stability and new strategies to tune their sensitivities. These advances could increase the potential practical utility of these responsive soft materials as platforms for the detection and reporting of chemical and biological analytes.