The liquid crystalline state of matter arises from orientation-dependent, non-covalent interactions between molecules within condensed phases. Because the balance of intermolecular forces that underlies the formation of liquid crystals (LCs) is delicate, this state of matter can, in general, be easily perturbed by external stimuli (such as an electric field in a display). In this review, we present an overview of recent efforts that have focused on exploiting the responsiveness of LCs as the basis of chemical and biological sensors. In this application of LCs, the challenge is to design liquid crystalline systems that undergo changes in organization when perturbed by targeted chemical and biological species of interest. The approaches described below revolve around the design of interfaces that selectively bind targeted species, thus leading to surface-driven changes in the organization of the LCs. Because LCs possess anisotropic optical and dielectric properties, a range of different methods can be used to read out the changes in organization of LCs that are caused by targeted chemical and biological species. This review focuses on principles for LC-based sensors that provide an optical output.

The ordering of liquid crystals (LCs) is known to be influenced by surfaces and contaminants. Here, we report that picogram per milliliter concentrations of endotoxin in water trigger ordering transitions in micrometer-size LC droplets. The ordering transitions, which occur at surface concentrations of endotoxin that are less than 10⁻⁵ Langmuir, are not due to adsorbate-induced changes in the interfacial energy of the LC. The sensitivity of the LC to endotoxin was measured to change by six orders of magnitude with the geometry of the LC (droplet versus slab), supporting the hypothesis that interactions of endotoxin with topological defects in the LC mediate the response of the droplets. The LC ordering transitions depend strongly on glycophospholipid structure and provide new designs for responsive soft matter.

We report an experimental system based on rigid, monodisperse, microparticles with tunable surface properties for the study of topological defects that form about dispersed solid phases in anisotropic media. We clearly observe equatorial Saturn-ring defects to form a distance $(0.09\pm 0.01)R$ from the surface of isolated microspheres (with strong homeotropic anchoring) dispersed in nematic phases. These defects, some of which are observed to be stable for up to a month, are similar in size to past theoretical predictions (with strong homeotropic anchoring) and can be reversibly expanded in an electric field.