Optical sensors. Part 23. Effect of Langmuir-Blodgett layer composition on the response of ion-selective optrodes for potassium, based on the fluorimetric measurement of membrane potential

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Abstract
An ion-selective optrode (ISO) for the continuous determination of potassium is presented. The ISO is based on the optical measurement of the membrane potential between an aqueous sample solution and a lipid phase incorporating a potential-sensitive dye. The lipid phase is composed of a chemically modified rhodamine B dye together with either octadecan-1-ol or arachidic acid, with valinomycin as a neutral ion carrier. In order to obtain reproducible results, the Langmuir-Blodgett technique is used to form molecular lipid layers on a glass support. The fluorescence intensity of the potential-sensitive rhodamine dye depends on the potassium concentration in the sample solution and decreases with increasing potassium concentrations. The over-all response is the net result of at least two effects, the strongest of which is the K+ concentration, but interferences were observed with other metal ions. It is shown that these interferences can be compensated for by using a reference optrode. Specifically, membranes made of octadecan-1-ol and valinomycin were studied. To achieve the maximum response, the optimum number of lipid layers was determined. Also, composite membranes (i.e., those consisting of two different lipid layer types) were investigated, but were found to be unsuitable. The response of membranes made of octadecan-1-ol and valinomycin is characterised by a decrease in the fluorescence intensity of up to 17% towards 100 mM potassium ion solutions. The ratios of 100 ×ΔI/I(the reduction in intensity relative to the total signal) to the logarithm of the analyte concentration were found to be linear over a range of 4 decades, typically 0.01–100 mM. The selectivity factor for potassium over sodium is 104 and this can be greatly increased by using a reference optrode. Also, temperature effects on ion-selective lipid membranes are investigated.

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