Nematic Liquid Crystal Reorientation at Aqueous-LC Interface for Monitoring Biochemical Interactions by Specific Ions Effects

• Published in: Journal of advanced optics & photonics Vol. 1; no. 1; p. 35
• Main Authors: He, Fengjie, Liu, Huilong, Xiong, Xingliang, Zhai, Shengjie
• Format: Journal Article
• Language: English
• Published: Henderson Tech Science Press 01.01.2018
• Subjects: Alignment; Aqueous solutions; Dipole interactions; Electric contacts; Electric fields; Interfaces; Liquid crystals; Nematic crystals; Photomicrographs; Polarized light; Thin films
• ISSN: 2578-2274
• DOI: 10.3970/jaop.2018.001.035

In this paper, we attempt to show that contacting aqueous solution made of either kosmotropic or chaotropic ions within a certain concentration range can trigger concentrationdependent transitions of the nematic liquid crystal (LC) at the aqueous-LC interface, going from planar to homeotropic. The effects of different ions (both anions and cations) with different concentrations on the LC orientation have thus been investigated. Nearly every ion, irrespective of being chaotrope or kosmotrope, is found to have its critical concentration, and upon reaching this concentration, homeotropic alignment in LC can be induced. Most chaotropic ions, however, induce homeotropic alignment in LC with a much lower critical concentration than their kosmotropic counterparts. Furthermore, the LC ordering transitions are related to the ion types, concentrations, and pH values, as revealed by the polarized light micrographs of LC thin films in contact with various ionic solutions. The homeotropic ordering of LC at the aqueous-LC interfaces upon the addition of kosmotropic ions is attributed to an internal electric field that results from the concentration difference between the interface and the bulk solutions. As a sharp contrast, this internal electric field, together with the ion-induced dipole interaction between the nitrile group of 4-cyano-4’-pentylbiphenyl (5CB) and the chaotropic ions, determines the orientations of LC upon the addition of chaotropic ions. Understanding the underlying mechanism of the ionic phenomena at aqueous-LC interface is of great importance for the design of aqueous-LC interface for detecting interactions involving ions, as the case in biosensing and many important applications.