Åbo Akademi University, Process Chemistry Centre, c/o Laboratory of Analytical Chemistry, Biskopsgatan 8, FI-20500 Turku-Åbo, Finland; Faculty of Material Science and Ceramics, AGH-University of Science and Technology, Al. Mickiewicza 30, PL-30059 Cracow, Poland; and Åbo Akademi University, Process Chemistry Centre, c/o Center for Process Analytical Chemistry and Sensor Technology (ProSens), Biskopsgatan 8, FI-20500 Turku-Åbo, Finland

Potentiometric Ion Sensors

Selectivities of solvent polymeric membrane ion-selective electrodes (ISEs) are quantitatively related to equilibria at the interface between the sample and the electrode membrane. However, only correctly determined selectivity coefficients allow accurate predictions of ISE responses to real-world samples. Moreover, they are also required for the optimization of ionophore structures and membrane compositions. Best suited for such purposes are potentiometric selectivity coefficients as defined already in the 1960s. This paper briefly reviews the basic relationships and focuses on possible biases in the determination of selectivity coefficients. The traditional methods to determine selectivity coefficients (separate solution method, fixed interference method) are still the same as those originally proposed by IUPAC in 1976. However, several precautions are needed to obtain meaningful data. For example, errors arise when the response to a weakly interfering ion is also influenced by the primary ion leaching from the membrane. Wrong selectivity coefficients may be also obtained when the interfering agent is highly preferred and the electrode shows counterion interference. Recent advances show how such pitfalls can be avoided. A detailed recipe to determine correct potentiometric selectivity coefficients unaffected by such biases is presented.

Selectivity of potentiometric ion sensors.

Artificial Receptors for the Recognition of Phosphorylated Molecules Amanda E. Hargrove, Sonia Nieto, Tianzhi Zhang, Jonathan L. Sessler,* and Eric V. Anslyn* Department of Chemistry and Biochemistry, University of Texas at Austin, 1 University Station A5300, Austin, Texas 78712-0165, United States Universidad de Zaragoza, Zaragoza, Spain Henkel Corporation, Rocky Hill, Connecticut 06067 Department of Chemistry, Yonsei University, Seoul, 120-749 Korea

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Artificial receptors for the recognition of phosphorylated molecules

Potentiometric sensing, which requires the use of ion-selective electrodes (ISEs) and reference electrodes, is used to determine electrochemically the concentration of target ions in a variety of chemical environments. In view of the need for more affordable and portable analytical devices with small sample volumes, all-solid-state ISEs and reference electrodes, in which a solid contact is used as ion-to-electron transducer, are highly desirable. This review describes how all-solid-state ISEs and reference electrodes function and presents important aspects that should be considered when designing such sensors for specific applications. Approaches to improving reproducibility, the stability of the emf response, lowering detection limits, and novel sensor designs are discussed along with specific examples from the recent literature. Emphasis is placed on the ion-to-electron transduction mechanism and the development of new solid contact materials, with a particular view to miniaturized ion-sensing devices with low cost and calibration-free sensing.

Rational design of all-solid-state ion-selective electrodes and reference electrodes

Conducting polymers, i.e., electroactive conjugated polymers, are useful both as ion-to-electron transducers and as sensing membranes in solid-state ion-selective electrodes. Recent achievements over the last few years have resulted in significant improvements of the analytical performance of solid-contact ion-selective electrodes (solid-contact ISEs) based on conducting polymers as ion-to-electron transducer combined with polymeric ion-selective membranes. A significant amount of research has also been devoted to solid-state ISEs based on conducting polymers as the sensing membrane. This review gives a brief summary of the progress in the area in recent years.

Conducting Polymer‐Based Solid‐State Ion‐Selective Electrodes

The lower detection limits of ion-selective polymeric membrane electrodes (ISEs) are in the micromolar range except when sample ion activities are adjusted by using ion buffers, which maintain low and constant activity via superimposed complexation or solubility equilibria. 1 Therefore, they are unsuitable for many important applications. In contrast, detection limits in the picomolar range have been achieved with optical sensors on the basis of similar technology and the same ionophores, 2

Large Improvement of the Lower Detection Limit of Ion-Selective Polymer Membrane Electrodes

The influence of the composition of the internal electrolyte solution on the response of Pb2+- and Ca2+-selective membrane electrodes is investigated. It is shown that the lower detection limit is improved by generating, in the membrane, ionic gradients that lead to a flux of primary ions toward the inner reference electrolyte solution. If the ion flux is too strong, it may cause analyte depletion at the membrane surface and, as a consequence, apparent super-Nernstian response. Such electrodes are not adequate to measure low analyte activities but can be used to determine unbiased selectivity factors. The results are interpreted in terms of a steady-state model, introduced in the companion paper, that describes the influence of concentration gradients generated by ion-exchange and coextraction processes on both sides of the membrane.

Lowering the Detection Limit of Solvent Polymeric Ion-Selective Membrane Electrodes. 2. Influence of Composition of Sample and Internal Electrolyte Solution

The processes determining the lower detection limit of carrier-based ion-selective electrodes (ISEs) are described by a steady-state ion flux model under zero-current conditions. Ion-exchange and coextraction equilibria on both sides of the membrane induce concentration gradients within the organic phase and, through the resulting ion fluxes, influence the lower detection limit. The latter is shown to improve considerably when very small gradients of decreasing primary ion concentration toward the inner electrolyte solution are created. By merely altering the concentration of the inner electrolyte, detection limits may vary by more than 5 orders of magnitude. Very large gradients, however, are predicted to lead to significant depletion of analyte ions in the outer membrane surface layer and thus to apparent super-Nernstian response. The currently recommended IUPAC definition of the lower detection limit leads to nonrealistic values in such cases. Small changes in the concentration profiles within the memb...

Lowering the detection limit of solvent polymeric ion-selective electrodes. 1. Modeling the influence of steady-state ion fluxes

The K+ complex of 2-(4-dipropylaminophenylazo)benzoic acid octadecyl ester (ETH 2418), a new lipophilic chromoionophore, is used to monitor the water uptake of dry solvent polymeric membranes based on 2-nitrophenyl octyl ether/poly(vinyl chloride) (2:1). Upon contact with an alkaline solution, its λmax changes from 535 to 435 nm. This process can be reversed by drying the membranes. From the time-dependent recordings of absorbance changes, an apparent water diffusion coefficient, DH2O*, of 2×10-8 cm2 s-1 is determined. In contrast to results obtained with hydrophilic water indicators, no light scattering due to water droplets is observed in the bulk of the membrane.

Tomasz Sokalski

Papers

Large Improvement of the Lower Detection Limit of Ion-Selective Polymer Membrane Electrodes

Lowering the Detection Limit of Solvent Polymeric Ion-Selective Membrane Electrodes. 2. Influence of Composition of Sample and Internal Electrolyte Solution

Lowering the detection limit of solvent polymeric ion-selective electrodes. 1. Modeling the influence of steady-state ion fluxes

Chromoionophore-Mediated Imaging of Water Transport in Ion-Selective Membranes