Orientational behaviour of ultraviolet-tailored 4-cyano-4'-pentylbiphenyl at the aqueous/liquid crystal interface

Abstract

In this study, we examined the anchoring behaviour of the ultraviolet (UV)-tailored nematic liquid crystal (LC) 4-cyano-4'-pentylbiphenyl (5CB) at the aqueous/LC interface. A range of photochemical reaction products are generated when 5CB is exposed to UV light, including the main product 4-cyano-4’-biphenylcarboxylic acid (CBA). When UV-treated 5CB is in contact with an aqueous sodium hydroxide or sodium chloride solution, the optical appearance of the LC undergoes a bright-to-dark shift, which indicates an orientational transition in the 5CB from a planar to homeotropic (perpendicular to the surface) state. Moreover, when the photochemically degraded 5CB was immersed in an aqueous mixture of hydroxide ions and electrolytes, the optical response was even more sensitive, which was associated with the deprotonation of CBA and the screening of electrostatic repulsion between the carboxylate head groups at the interface. In this study, the competitive orientational behaviour of LC in relation to biomolecular conjugation and ionic interaction at the interface was examined. When biomacromolecules, such as urease, were conjugated to a mixed monolayer of tetra(ethylene glycol)-terminated lipid and carboxylic acid-terminated lipid at the interface, UV-treated 5CB exhibited an ordering transition from a homeotropic to planar state. However, when a sodium hydroxide solution was in contact to the urease-decorated interface, the LC reverted back to its homeotropic state. By exploiting the orientational behaviour of the UV-treated 5CB at the aqueous/LC interface, we concluded that the acid-doped LC could potentially be used for the sensitive screening of pathogens and biomolecules.