Analyte-induced ordering transitions of liquid crystals at chemically functionalized interfaces

This research explores the proposition that thin films of liquid crystals (LCs) supported on chemically functionalized surfaces can be utilized to detect gas-phase analytes. The orientational ordering of the molecules that form liquid crystals are influenced by molecular interactions at interfaces. Thus, a change in the molecular-level structure of a surface supporting a LC thin film, such as caused by the binding of an analyte, can lead to an orientational ordering transition within the LC thin film that is transduced into an optical signal. In particular, nitrile-containing LCs such as 4'-pentyl-4-biphenyl-carbonitrile (5CB), 4'-octyl-4-biphenyl-carbonitrile (8CB), and the mixture E7 (a proprietary mixture of four nitrile-containing molecules) supported on metal perchlorate-decorated surfaces are used to report the presence of organophosphorous gas-phase chemical warfare agents and simulants. Detection of the simulant dimethyl methylphosphonate (DMMP) is reported at the parts-per-billion level within tens of seconds. Potentially interfering agents such as ethanol, acetone, and hexanes at parts-per-thousand concentrations, as well as 75% relative humidity do not lead to a response of the LC thin film. Thin LC films are also shown to report the presence of organophosphorous nerve agents GB, GD, GA and VX.; The orientation of nitrile-containing LCs supported on metal perchlorate salt surfaces is shown to reflect the strength of coordination of the nitrile group of the LC molecules to the metal ion present. Metal ions that coordinate strongly to the LC nitrile groups lead to a homeotropic (perpendicular) orientation of the LC molecules at the surface whereas metal ions that do not coordinate strongly to the LC molecules lead to planar or tilted anchoring at the metal salt interface. Exposure to vapor-phase DMMP, an organophosphorous nerve agent simulant, is shown to lead to an orientational transition from homeotropic to a planar or tilted orientation of LC thin films supported on some metal perchlorate salts. The presence of this orientational transition depends upon the competition between LC molecules and DMMP for metal ion coordination and thus the relative strengths of these coordination interactions.