Dependence of Ion Selectivity on Ordered Orientation of Neutral Carriers in Ion-Sensing Membranes Based on Thermotropic Liquid Crystals

Abstract

Thermotropic liquid-crystalline compounds were applied as membrane materials (membrane solvent and neutral carrier) for neutral carrier-type ion sensors to investigate how the ordered arrangement of neutral carriers affects the property of the resulting ion sensors. Nematic, smectic, and cholesteric liquid-crystalline compounds were used as the membrane solvents and crown ether derivatives with a molecular structure similar to the liquid-crystalline solvent as the K+ neutral carriers. Polarized IR spectroscopy and X-ray diffraction experiments confirmed that the highly ordered arrangement of membrane components was retained in the liquid-crystal-based ion-sensing membranes containing a neutral carrier and a lipophilic salt. The ordered arrangement of neutral carriers in the liquid-crystalline membranes enhanced the ion selectivity significantly, probably due to the efficient cooperation of two adjacent crown ether moieties in the highly ordered and aggregated state.

Neutral-Carrier-Type Potassium Ion-Selective Electrodes Based on Polymer-supported Liquid-Crystal Membranes for Practical Use

“…In the membrane systems using NPOE, which is a conventional solvent for ion-sensing PVC membranes, the electrode also showed good sensor properties. Although the linear ranges for the PTFE-impregnated membranes are slightly narrow compared with those of the ISEs based on their corresponding pure liquid-crystalline membranes, 3,4 there seems to be no problem in the analysis of K + for real samples, such as biomedical samples or environmental samples. These results indicate that liquid-crystalline membrane ISEs, in which a PTFE porous membrane was used as the supporting material, respond to K + activity changes with a Nernstian response in any of the nematic, smectic, and isotropic liquid states.…”

“…We have previously reported that thermotropic liquid-crystalline compounds are very promising as functional membrane solvents for ISEs. 3,4 The ion-sensor property could be controlled by the orientation of the membrane components. In other words, liquid-crystalline compounds can induce an ordered molecular orientation of other organic components in the liquid-crystal phase.…”

Strong Molecular Aggregation of Neutral Carriers Bearing Perfluoroalkyl Chains in Liquid-Crystalline Ion-Sensor Membranes

“…The pure liquid membrane was incorporated into a plastic tip of an adjustable pipette (Q-110, Quality Science Plastics), which has an inner diameter of 0.5 mm, for the capillary-type ion-selective electrode. 3,4 The electrodes were built up as follows. To a capillary, 10 μL of 1 × 10 -3 M KCl aqueous solution as an internal solution was injected.…”

“…In the case of compound 3, which is able to form a nematic liquid crystal phase, it was not possible to observe the surface of membranes due to the high fluidicity. Therefore, compound 4, which is able to form a less fluidic phase (smectic liquid crystal), was employed here as the membrane solvent, because it possesses similar structural and sensor properties 4,5 to those of compound 3. The content of compound 2 in the membranes was 5.0 wt%, because no clear AFM images were observed when the content of compound 2 was 1.0 wt%.…”

“…We have previously reported that thermotropic liquidcrystalline compounds for membrane solvents of ISEs have conferred a high functionality on membrane ion sensors. [3][4][5][6] Liquid crystals are functional materials, because their molecular orientation varies in response to changes in temperature, light, electric field, etc. Thus, the ion-sensor property could be controlled by the orientation of the membrane components in liquid-crystalline compounds.…”