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

Abstract

Potentiometric ion sensors, such as neutral-carrier-type ionselective electrodes (ISEs), are very useful. 1 The ion selectivities are generally determined by the ion-exchange equilibrium at the interface between the membrane and the aqueous sample phases, which is in turn the metal-ion extraction equilibrium between the two immiscible phases. 2The ion selectivity for liquid-membrane ion sensors based on neutral carriers is often governed by the ion selectivity of the neutral carriers in the membrane. However, the ion selectivity for the neutral-carrier-type ion sensors is also considerably affected by other membrane conditions, such as the property of the membrane solvent. For instance, the polarity of membrane solvents may modify the selectivity in ion sensors based on neutral carriers. 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. However, since we used pure liquid membranes consisting of a liquid-crystalline membrane solvent and a neutral carrier without any polymer support, the ionsensing membrane system may not be very practical. Polymersupported membranes are useful for conventional neutralcarrier-based ISEs to raise their handling performance, and also to prevent effusion of the membrane solvents. We, therefore, attempted to apply polymeric membrane supports for the liquidcrystalline ISEs to improve their practicability. In this study, we report in detail on the ion sensor properties for liquidcrystalline membrane ISEs based on microporous polymer membranes impregnated by liquid-crystalline compounds (polymer-impregnated liquid-crystal membrane) or polymer membranes dispersed by liquid-crystalline compounds [polymer-dispersed liquid-crystal (PDLC) 5-7 membrane] as the ion-sensing membranes. Experimental ReagentsThe liquid-crystalline compounds shown in Fig. 1 Polymer-supported liquid-crystal membranes have been designed for neutral-carrier-type potassium ion-selective electrodes, aiming for practical applications of high-performance liquid-crystalline membrane ion sensors. Two types of polymer-supported liquid-crystal membranes were tested for their usefulness; one is microporous poly(tetra fluoroethylene) (PTFE) membranes impregnated by thermotropic liquid-crystalline compounds, and another is poly(methyl methacrylate) (PMMA) membrane dispersing the same liquid-crystalline compounds. Both of the polymersupported liquid-crystal membranes containing a liquid-crystalline benzo-15-crown-5 neutral carrier as well as a lipophilic anion excluder work well as ion-sensing membranes for potassium ion-selective electrodes, the ion selectivities of which can be switched by the measurement temperatures. Specifically, PTFE-impregnated liquid-crystal membranes are better than the PMMA-disperse...

Triethylene Glycol Ether End-grafted Carbosilane Dendrimer: A Potential Ionophore for Potassium Ion Recognition

“…The proposed K + ISE exhibits good selectivity compared with previously described sensors 8,[22][23][24][25][26] (Table 3). It exhibits a better working concentration range, slope and potentiometric selectivity for K + over many cations.…”

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

“…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.…”

Preparation and Application of Potassium Ion-Selective Membrane Electrode Based on Benzo-15-Crown-5 Ether

“…Oosaki S. and colleagues prepared two types of liquid crystal potassium ion selective electrodes. While for one type microporous PTFE (polytetrafloroethylene) was used, for the second one PMMA (polymethilmethacrylate and a derivative of benzo-15-crown-5-ether as neutral carrier [40] was used. Sungeun Kim and et al stated that PVC membrane electrode for selective potassium ion could be prepared by using 1,3-alternate calix [4] crown-5-azacrown-5 ether [41].…”