For purposes of understanding accumulation and toxicity from environmental exposure, it is necessary to be able to selectively detect a variety of synthetic organic chemicals.Whereas highly sensitive methods of detection of specific chemical compounds do exist, these methods require laboratory-based instrumentation and thus are not suited for measurement of personal exposure.We are investigating an alternative where surface-driven, orientational transitions in supported films of liquid crystals (LCs) are triggered by exposure to parts-per-billion vapor concentrations of targeted low molecular weight molecules, including organoamine and organophosphorous compounds. The recognition-driven change is used to visually detect instantaneous or cumulative exposure to a targeted compound (Figure 1). Fabrication of the detection system is sufficiently simple that it may find use as the basis of wearable personal sensors for exposure to environmental pollutants.

Three principles underlie the design of our detection system. First, the solid surface presents immobilized chemical receptors that weakly bind molecules that form a LC phase to orient it in a well-defined direction. Second, the receptors bind targeted analytes more strongly than they bind the molecules forming the LC. We exploit knowledge of the relative strengths of hydrogen bond, acid-base and metal-ligand coordination interactions to guide selection of appropriate molecular receptors and LC for each targeted analyte. The competitive displacement of the LC from the molecular receptor reversibly releases the LC from its receptor-enforced orientation upon exposure to analyte. Because non-targeted species that bind the receptors more weakly than the LC will not change the orientation of the LC, this competitive interaction provides a level of specificity for the targeted analyte that is substantially greater than schemes based on non-competitive binding of analytes to receptors. Third, the surface possesses a nanometer-scale topography that directs the liquid crystal to assume a predictable and visually distinct orientation in the absence of a receptor-mediated-anchoring of the mesogen at the surface.