Design and pitfalls of ion selective electrodes

Electrodiffusion Phenomena in Neuroscience and the Nernst–Planck–Poisson Equations

This work is aimed to give an electrochemical insight into the ionic transport phenomena in the cellular environment of organized brain tissue. The Nernst–Planck–Poisson (NPP) model is presented, and its applications in the description of electrodiffusion phenomena relevant in nanoscale neurophysiology are reviewed. These phenomena include: the signal propagation in neurons, the liquid junction potential in extracellular space, electrochemical transport in ion channels, the electrical potential distortions invisible to patch-clamp technique, and calcium transport through mitochondrial membrane. The limitations, as well as the extensions of the NPP model that allow us to overcome these limitations, are also discussed.

Determination of unbiased selectivity coefficients using pulsed chronopotentiometric polymeric membrane ion sensors

A new procedure for the determination of selectivity coefficients of neutral carriers using pulsed chronopotentiometric ion selective sensors (pulstrodes) is established. Pulstrode membrane which lacks an ion-exchanger suppresses the zero current ion flux, allowing a Nernstian response slope for even highly discriminated ions. Unlike previously developed methods, unbiased selectivity remains unaltered even with the exposure to the primary ion solution for prolonged time. Studies with potassium-, silver-, and calcium-selective electrodes reveal that pulstrodes yield the same or slightly favorable unbiased selectivity coefficients than reported earlier. In contrast to alternative methods for the determination of unbiased selectivity, this technique offers a unique simplicity and reliability. Therefore the new procedure promises to be a valuable additional tool for the characterization of unbiased selectivity coefficients for the ISEs.

Junction-less reference electrode for potentiometric measurements obtained by buffering pH in a conducting polymer matrix

A reference electrode for potentiometric measurements based on conducting polymers (CP) doped with pH buffering ligands is described. Both the CPs and doping ligands are selected and adjusted in such a way that possible ionic and redox sensitivity is hampered, while the pH buffering property of the CP film is exposed. In this way, the electric potential drop at the conducting polymer|solution interface is stabilized and close to constant over a certain pH range. The electrode behaves as a pseudo-reference electrode in amphiprotic solvents or their mixtures, e.g. water-alcohol mixtures. For the first time titration of sulfates with lead(ii) in water-methanol solution using two "plastic" electrodes, CP-based Pb(2+)-sensitive indicator and CP-based reference electrode, is shown. Because the electrode is junction-less it may easily be miniaturized and maintained and thus may serve in frontier applications of sensors.

Improving the Detection Limit of Anion-Selective Electrodes: An Iodide-Selective Membrane with a Nanomolar Detection Limit

The lower detection limit and the selectivity behavior of anion-selective electrodes (ISEs) are improved by using optimized inner solutions and membrane compositions. With a membrane based on the recently described ionophore [9]mercuracarborand-3, a detection limit of 2 x 10(-9) M has been achieved for iodide. Nevertheless, the improvements are less pronounced than in the case of cation ISEs. This is mainly due to the fact that so far no anion ISE is known with the extremely high selectivities of cation ISEs. If the membrane does not contain an ionophore, leaching of the ion exchanger from the membrane into the sample is also a relevant limiting factor except for ion exchangers of very high lipophilicity.

Thiocyanate in smokers interferes with the Nova magnesium ion-selective electrode

Thiocyanate found in serum ordinarily is the metabolite of cyanide that is inhaled with tobacco smoke and ingested with cyanogenic foods. We investigated the effect of the thiocyanate ion (SCN−) on the ionized magnesium (iMg) and ionized calcium (iCa) results determined with the AVL and Nova magnesium and calcium ion-selective electrodes (ISEs). We analyzed saline and pooled serum with added SCN−, and serum from apparently healthy nonsmokers (n = 20) and smokers (n = 20). The mean (and range) of the measured serum SCN− concentration was 0.019 (0.008–0.046) mmol/L for nonsmokers and 0.077 (0.020–0.138) mmol/L for smokers. Only the Nova iMg results decreased with increasing SCN−concentration, and the change was dependent on the baseline iMg concentration. In the absence of Mg, SCN− decreased the voltage response of the Nova Mg ISE to calcium ions. At apparently normal serum iMg and iCa concentrations, the interference by SCN− appeared to be equimolar (iMg = −1.04 × SCN− + 0.52). Thus, the serum SCN− commonly found in smokers causes a significant (P <0.0001) decrease in the Nova iMg results.

Electric potential measured, concentration reported: how to get mmols from mV

The interdependence between the electric potential of a galvanic cell and concentration of electrolytes will be analyzed in order to show how a clinically relevant report can be obtained. The use of SI units is emphasized.