Ionophore-Based\nVoltammetric Ion Activity Sensing with Thin Layer Membranes

Abstract

As shown in recent work, thin layer\nion-selective multi-ionophore\nmembranes can be interrogated by cyclic voltammetry to detect the\nion activity of multiple species simultaneously and selectively. Additional\nfundamental evidence is put forward on ion discrimination with thin\nmulti-ionophore-based membranes with thicknesses of 200 ± 25\nnm and backside contacted with poly-3-octylthiophene (POT). An anodic\npotential scan partially oxidizes the POT film (to POT⁺), thereby initiating the release of hydrophilic cations from the\nmembrane phase to the sample solution at a characteristic potential.\nVarying concentration of added cation-exhanger demonstrates that it\nlimits the ion transfer charge and not the deposited POT film. Voltammograms\nwith multiple peaks are observed with each associated with the transfer\nof one type of ion (lithium, potassium, and sodium). Experimental\nconditions (thickness and composition of the membrane and concentration\nof the sample) are chosen that allow one to describe the system by\na thermodynamic rather than kinetic model. As a consequence, apparent\nstability constants for sodium, potassium, and lithium (assuming 1:1\nstoichiometry) with their respective ionophores are calculated and\nagree well with the values obtained by the potentiometric sandwich\nmembrane technique. As an analytical application, a membrane containing\nthree ionophores was used to determine lithium, sodium, and potassium\nin artificial samples at the same location and within a single voltammetric\nscan. Lithium and potassium were also determined in undiluted human\nplasma in the therapeutic concentration range.