We report the development of formaldehyde gas sensors based on surface-driven alignment transitions in films of liquid crystals (LC). Formaldehyde is an indoor pollutant that is emitted from building materials, including glues, fiberboard, plywood, and paints. Despite its prevalence, formaldehyde is a known carcinogen and OSHA stipulates that the permissible exposure limit for formaldehyde should not exceed 0.75 parts per million (ppm) in air measured as an 8-hour time-weighted average. We developed novel interfaces for detection of formaldehyde consisting of films of LC mixtures supported on a metal surface. The LC mixtures consisted of nitrile-containing aromatic compounds, and a ketone-containing aromatic compound. The alignment of the LC molecules at the interface with the metal surface, and thus the optical properties of the LC films, was found to depend on the composition of the LC mixtures and the identity of the supporting surface. LC mixtures containing &gt;2 vol.% of the ketone-containing aromatic compound exhibited alignment perpendicular to the surface (homeotropic) when supported on gold, appearing dark when observed with cross-polarized light. Upon exposure to formaldehyde, these LC mixtures exhibited an optical transition from dark to bright, consistent with a change in surface orientation from homeotropic to planar (parallel to the surface). Using a mixture containing 3 vol.% of the ketone-containing compound, we observed an optical response in the LC films at 0.75 ppm of formaldehyde, which is the concentration relevant for monitoring human exposure. We hypothesize that the observed formaldehyde-induced optical response is a consequence of competitive adsorption at the gold surface between formaldehyde and the ketone-containing compound in the LC mixture. These studies provide fundamental insights on the bonding and orientation of molecules at solid interfaces that could give rise to the development of new functional LC materials.

Surface alignment transitions in liquid crystals induced by exposure to formaldehyde gas

2019 Spring National Meeting

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