Principles of detecting vaporous thiols using liquid crystals and metal ion microarrays

In this article, we show that the optical texture of a layer of liquid crystal 4-cyano-4'-pentylbiphenyl (5CB) supported on a thiol-sensitive layer can be applied to detect 1-octanethiol and other vaporous thiols with high specificity. As demonstrated in our ellipsometry and XPS results, a thiol-sensitive layer comprising an array of copper ions is capable of oxidizing thiols to disulfides and immobilizing them on the surface. Because of the hydrophobic hydrocarbon chain of 1-octanethiol, the immobilization of 1-octanethiol lowers the surface energy. Thus, after a thin layer of 5CB is supported on the surface, the lower surface energy causes 5CB to adopt different orientations in regions where copper ions were deposited. Because 5CB is a birefringent material, different orientations of 5CB also result in distinct optical textures, which are visible to the naked eye under a pair of polarizers. Interestingly, 5CB supported on copper ions only responds to longer and less volatile thiols such as 1-octanethiol, 1-hexanethiol and 1-butanethiol, but it did not respond to shorter and more volatile thiols such as ethanethiol. The system also shows no response to water and other volatile organic compounds such as acetone, ethanol, heptanol and heptane.