Ionophore-based Ion-selective Electrodes Research Articles

Voltammetric Ion Sensing with Ionophore-Based Ion-Selective Electrodes Containing Internal Aqueous Solution, Improving Lifetime of Sensors.

The possibility of voltammetric ion sensing is demonstrated, for the first time, for ion-selective electrodes (ISEs) containing an internal aqueous solution. ISEs selective to calcium, lithium and potassium ions are used as model systems. The internal solution of the ISEs contains a chloride salt of the respective cation and a ferrocenemethanol or ferrocyanide/ferricyanide redox couple. A platinum wire is used as the internal reference electrode. It is shown, theoretically and experimentally, that the dependence of oxidation and reduction peak potentials on the sample composition obeys the Nernst law, while the peak currents virtually do not depend on the sample composition. Thus, the electrode behavior is similar to that reported by Bakker's group for solid contact ISEs with ultra-thin membranes (200-300 nm). It is shown that the use of classical ISEs with relatively thick membranes (100-300 µm) and internal aqueous solution allows for the sensor lifetime of about one month. It is also shown that use of a suitable background electrolyte allows for improvement of the detection limits in voltammetric measurements with ISEs.

Ionophore-Based Ion-Selective Electrodes in Non-Zero Current Modes: Mechanistic Studies and the Possibilities of the Analytical Application

This mini review briefly describes (i) literature data on the non-zero current measurements with ionophore-based ion-selective electrodes (ISEs) aimed at fundamental studies of the mechanism of their potentiometric response, and (ii) the data on the possibilities of analytical applications of ISEs in voltametric and constant potential chronoamperometric/coulometric modes, in particular the K+ ion assay in blood serum with the sensitivity of 0.1%. A special attention is paid to the basics of voltammetry and chronoamperometry/coulometry with the ionophore-based ISEs, and to how and why these methods differ from the classical voltammetry and coulometry.

Ionophore-based ion-selective electrodes: signal transduction and amplification from potentiometry

Focusing on ionophore-based ion-selective electrodes (ISEs), this work highlights recent advances in increasing sensor sensitivity by converting potentiometric response to a variety of electrical and optical signals including charge, current, luminescence, and color change.

The Origin of the Non-Constancy of the Bulk Resistance of Ion-Selective Electrode Membranes within the Nernstian Response Range.

The dependence of the bulk resistance of membranes of ionophore-based ion-selective electrodes (ISEs) on the composition of mixed electrolyte solutions, within the range of the Nernstian potentiometric response, is studied by chronopotentiometric and impedance measurements. In parallel to the resistance, water uptake by the membranes is also studied gravimetrically. The similarity of the respective curves is registered and explained in terms of heterogeneity of the membranes due to the presence of dispersed aqueous phase (water droplets). It is concluded that the electrochemical equilibrium is established between aqueous solution and the continuous organic phase, while the resistance refers to the membrane as whole, and water droplets hamper the charge transfer across the membranes. In this way, it is explained why the membrane bulk resistance is not constant within the range of the Nernstian potentiometric response of ISEs.

Chronoamperometric and coulometric analysis with ionophore-based ion-selective electrodes: A modified theory and the potassium ion assay in serum samples

The behavior of the same ion-selective electrode (ISE) under three measurement modes: potentiometric, chronoamperometric and coulometric is compared using valinomycin-based K+-ISE as model system. The ISE response to decrease and to increase of the K+ ion activity is studied systematically. It is shown that the theoretical description of the time dependencies of current and of charge must take the concentration polarization in consideration, and the respective equations are presented. The concentration of K+ ion in blood serum samples is measured in the potentiometric, in the chronoamperometric and in the coulometric mode, and satisfactory results are obtained in all three cases.

Non-constancy of the bulk resistance of ionophore-based ion-selective electrode: A result of electrolyte co-extraction or of something else?

The bulk resistance of ionophore-based ion-selective electrodes (ISEs) is studied by means of chronopotentiometry and electrochemical impedance using Ca2+-ISEs with PVC membranes containing ETH 1001 as a model system. It is shown that the membrane bulk resistance is roughly the same within the range of CaCl2 concentrations from 0.3 to 10−3 M, and increases significantly when the concentration is decreased to 10−4 and 10−5 M, although the potentiometric response of the ISEs is Nernstian in all these solutions. This increase of the resistance when 10−3 M CaCl2 is further deleted is larger than the decrease of the resistance when CaCl2 is replaced with Ca(SCN)2 and Ca(ClO4)2. The results suggest that the non-constancy of the ISE bulk resistance is not caused by co-extraction of CaCl2 and therefore is not in conflict with the Nernstian behavior of the IESs potentials. The effect is tentatively ascribed to the regularities of water uptake by ISE membranes.

Screening highly selective ionophores for heavy metal ion-selective electrodes and potentiometric sensors

Screening highly selective ionophores from a large amount of ionophores is the principle and most realizable method to discover new ionophores used in ion-selective electrodes (ISEs). In this paper a screening method is described for ionophore-based polymeric membrane heavy metal ion-selective electrodes (IBPMHM ISEs) to determine heavy metal ions (Cu2+, Pb2+, Zn2+, Cd2+, Hg2+ and Ag+). The protocol uses the averaged negative logarithmic potential selectivity coefficients (pK5¯) of five major interfering ions as screening parameters. The specified five major interfering ion group (Group A) can enhance the screening ability of the parameter pK5¯. The molecular categories of preferred ionophores (pK5¯>2.00) for these ISEs were as follows: Schiff bases for Cu2+-ISE; Schiff bases and macrocycles for Zn2+-ISE; calixarenes, crown ethers and macrocycles for Ag+-ISE; calixarenes, Schiff bases and crown ethers for Pb2+-ISE, and another group for both Cd2+- and Hg2+-ISEs. Strong interaction between the donor atoms of ionophores and the heavy metal ions follows the hard soft acids and bases rule (HSAB). This approach efficiently screens the best ionophores for these IBPMHM ISEs, and provides a new route to explore new ionophores for other ionophore-based ISEs and potentiometric sensors.

Characterization of a new ionophore-based ion-selective electrode for the potentiometric determination of creatinine in urine

The optimization, analytical characterization and validation of a novel ion-selective electrode for the highly sensitive and selective determination of creatinine in urine is presented. A newly synthesized calix[4]pyrrole-based molecule is used as an ionophore for the enhanced recognition of creatininium cations. The calculation of the complex formation constants in the polymeric membrane with creatininium, potassium and sodium confirms the strong selective interactions between the ionophore and the target. The optimization of the potentiometric sensor presented here yields an outstanding analytical performance, with a linear range that spans from 1µM to 10mM and limit of detection of 10−6.2M. The calculation of the selectivity coefficients against most commonly found interferences also show significant improvements when compared to other sensors already reported. The performance of this novel sensor is tested by measuring creatinine in real urine samples (N=50) and comparing the values against the standard colorimetric approach (Jaffé's reaction). The results show that this sensor allows the fast and accurate determination of creatinine in real samples with minimal sample manipulation.

Impact of the Electrolyte Co-Extraction to the Response of the Ionophore-based Ion-Selective Electrodes

The impact of the aqueous electrolyte co-extraction to the potentiometric response of ionophore-based ion-selective electrodes (ISEs) is studied theoretically and experimentally. A simple theoretical model is developed to describe quantitatively how co-extraction of electrolytes influences the lower and the upper detection limits of ISEs. The theory is successfully verified with valinomycin-based K+-ISE as a model system, using potentiometric, chronopotentiometric, impedance and UV–vis measurements. A special (symmetric) setup of the galvanic cell is proposed which clearly demonstrates how co-extraction from the internal solution determines the lower detection limit of ISEs. The values of the partition coefficients of potassium salts used in the study are consistent with the respective Gibbs energies of anion transfer from water to organic phase. The model also gives a hint why the slope of real ISEs is typically slightly sub-Nernstian.

(Invited) Ionophore-Doped Ion-Selective Electrode Membranes

Ion-selective electrodes with ionophore-doped sensing membranes are are highly sensitive and selective analytical tools that offer a variety of advantages, such as simplicity of measurement, high analysis throughput, rapid detection, and low cost of analysis. While such sensors are used in clinical laboratories for billions of measurements every year, applications in biomedical sciences, the food industry, and environmental monitoring are hindered by biofouling and the frequent need for recalibration. With prolonged exposure to biological samples, solid contact polymeric-membrane ISEs exhibit a breakdown of selectivity and response due to biofouling and changes in the phase boundary potential at the interface of the ion-selective membrane and the underlying electron conductor. Therefore, frequent recalibrations are needed for many clinical and biological applications. This talk will discuss experimental techniques such as impedance spectroscopy and hydrodynamic methods to characterize sensor aging, and it will address the use of hydrophobic redox buffers incorporated into the sensing membranes to stabilize the phase boundary potential at the interface to the underlying electron conductor. These redox buffers permit a high electrode-to-electrode reproducibility of the emf response, which is needed for the development of ion-selective potentiometric devices that can be used in a calibration-free manner [1]. [1] Calibration-Free Ionophore-Based Ion-Selective Electrodes With a Co(II)/Co(III) Redox Couple-Based Solid Contact, Zou, X. U.; Zhen, X. V.; Cheong, J. H.; Bühlmann, P. Anal. Chem., 2014, 86, 8687–8692.

Materials for the ionophore-based membranes for ion-selective electrodes: Problems and achievements (review paper)

An overview is presented on the materials for the membranes of the ionophore-based ion-selective electrodes (ISEs). The basics of these membranes are briefly discussed. Bearing in mind that polyvinylchloride (PVC) is so far predominating as the ionophore-based ISE membrane matrix, and the limited life-time of these electrodes is due to an insufficient lipophilicity of the membrane components, the plasticizers in the first place, a discussion is presented on polymers alternative to PVC and ionophores immobilized on the polymer backbone. Data are presented on ISEs with membranes based on perfluorocompounds, which are especially important for the medical and biological applications. Studies are briefly described where instead of the replacement of PVC with self-plasticized polymers, the low-molecular plasticizers are replaced with polymeric analogs, retaining PVC as the polymeric matrix. Use of ionic liquids as the ISE membrane components is discussed, together with the interpretation of the literature data on the regularities in the functioning of such electrodes. Issues, studied mostly from the material science view are discussed throughout the review in their relation to electrochemistry.

Calibration-free ionophore-based ion-selective electrodes with a Co(II)/Co(III) redox couple-based solid contact.

A high electrode-to-electrode reproducibility of the emf response of solid contact ion-selective electrodes (SC-ISEs) requires a precise control of the phase boundary potential between the ion-selective membrane (ISM) and the underlying electron conductor. To achieve this, we introduced previously ionophore-free ion exchanger membranes doped with a well controlled ratio of oxidized and reduced species of a redox couple as redox buffer and used them to make SC-ISEs that exhibited highly reproducible electrode-to-electrode potentials. Unfortunately, ionophores were found to promote the loss of insufficiently lipophilic species from the ionophore-doped ISMs into aqueous samples. Here we report on an improved redox buffer platform based on equimolar amounts of the much less hydrophilic Co(III) and Co(II) complexes of 4,4'-dinonyl-2,2'-bipyridyl, which makes it possible to extend the redox buffer approach to ionophore-based ISEs. For example, K(+)-selective electrodes based on the ionophore valinomycin exhibit electrode-to-electrode standard deviations as low as 0.7 mV after exposure of freshly prepared electrodes for 1 h to aqueous solutions. Exposure of freshly prepared ISE membranes to humidity prior to their first contact to electrolyte solution minimizes the initial (reproducible) emf drift. This redox buffer has also been successfully applied to sodium, potassium, calcium, hydrogen, and carbonate ion-selective electrodes, which all exhibit the high selectivity over interfering ions as expected for ionophore-doped ISE membranes.

Electrochemistry of Ionophore-Based Ion-Selective Electrodes: Response Limits, Slope and Potential Stability

The contribution is devoted to the electrochemical issues related to the response span, slope, and potential stability of ionophore-based ion-selective electrodes (ISEs). The discussion is primarily focused on those research topics in this area which have been studied by the late R.P. Buck.Theoretical and experimental efforts aimed at the improvement of the lower detection limit of the ISE response constitute the current mainstream in the ISE theory and practice. This is mostly due to high environmental relevance of the trace level analysis. The galvanostatic polarization of ISEs, as the most flexible approach to the low detection problem, was proposed first by R.P. Buck et al [1]. Since then, the method was largely improved and modified in different ways. We recently proposed tuned galvanostatic polarization [2, 3] which proved to be suitable also for ISEs with crystalline membranes [4]. The idea of the method will be discussed and illustrated with several applications.It will be shown that the upper detection limit of ISEs (which is mostly of industrial relevance) can also be improved in a very similar way. Thus the so-called anion interference (or cation interference in the case of anion-sensitive ISEs): the effect studied originally by R.P. Buck in 1970s [5], can be overpassed successfully. Furthermore, simple theoretical modeling of the trans-membrane fluxes effect in the ISE potentiometric response allows for a unified description of both lower and upper limits of the ISEs response, together with the response slope. Theoretical conclusions on the origin of slightly sub-Nernstian slopes typically observed for real-world ISEs, and on the relation between the slope and the response span, will be supported with the direct experimental evidence.When studying the so-called solid contact ISEs (those without internal solution), R.P. Buck suggested that a reversible and fast RedOx reaction at the interface between ionically-conducting membrane and electronically-conducting substrate is a prerequisite for the electrode potential stability over time [6]. This idea is widely recognized among the ISE researchers community. However, our recent studies showed that this RedOx reaction is the necessary, but not the sufficient condition, and low diffusional polarizability at the interface is also needed for the solid contact ISEs stability [7, 8].Besides the experimental facts which can be treated positively in the frames of the existing concepts of the ISE response mechanism, some peculiar observations will be presented. These evidences obtained by impedance and chronopotentiometric studies appear challenging and require further thorough studies. E. Lindner, R.E. Gyurcsanyi, R.P. Buck, Electroanalysis, 1999, 11 695.M.A. Peshkova, T. Sokalski, K.N. Mikhelson, A. Lewenstam, Anal. Chem., 2008, 80 9181.M.A. Peshkova, K. N. Mikhelson, Electrochimica Acta, 2013, 110, 829.G. Lisak, T. Sokalski, J. Bobacka, L. Harju, K. Mikhelson, A. Lewenstam, Anal. Chim. Acta, 2011, 707, 1.J.H. Boles, R.P. Buck Anal. Chem. 1973, 45, 2057.R.P. Buck, V.R. Shepard, Anal. Chem. 1974, 46, 2097.E.N. Samsonova, V.M. Lutov, K.N. Mikhelson, J Solid State Electrochem. 2009, 13, 69.N.M. Ivanova, I.V. Podeshvo, M.Ya. Goikhman, A.V. Yakimanskii, K.N. Mikhelson, Sensors & Actuators B, 2013, 186, 589.

Measurement of total calcium by flash chronopotentiometry at polymer membrane ion-selective electrodes

Ionophore-based ion-selective electrodes are widely used for potentiometric electrolyte measurements, in which case they are known to detect the free ion activity. Total ion concentrations cannot be directly assessed by this methodology if the ion is predominantly present in a complexed form. We present here the direct measurement of total calcium using a calcium ion-selective electrode interrogated in a flash chronopotentiometric transduction mode. A high magnitude of cathodic current pulse is applied across a calcium ion-selective membrane containing the ionophore ETH 5234 but void of ion-exchanger to prevent spontaneous extraction. This induces a defined flux of calcium ions from the sample side to the membrane and results in the release of labile bound calcium and a concomitant depletion at the membrane surface at a critical current or time. This is observed as an inflection point on the potential–time curve and the square root of the transition time is linearly related to the total concentration in the sample. It is shown that the responses to solutions of labile calcium complexes of nitrilotriacetic acid (NTA) are in a good agreement with that of the same concentration of calcium chloride in saline solution with this protocol. Initial applications are aimed towards assaying extracellular calcium. Calcium binding to albumin is shown to be inconsequential with sample dilutions typical for clinical assays. Calcium calibration curves in real and artificial dilute serum are finally shown to correspond to that of calcium chloride, suggesting that the methodology is indeed capable of detecting total calcium under these conditions. The present membrane materials allow detection of up to over 0.5 mM total calcium in serum, currently requiring such samples to be diluted about 5-fold. The slopes of the square root of time–concentration dependence for the calibrations of free calcium in a background of NaCl and total serum calcium were found to be 3.857 and 3.717 s 1/2 mM −1, respectively, deviating by just 3.6%. The lower detection limit (3× SD) was calculated as 12 μM.

Electrochemical sensors based on ionophores: Current state, trends, and prospects

The review considers ionophore-based electrochemical sensors (ion-selective electrodes). In general the progress in the field is subdivided in two distinctly different trends: (1) an extensive development (enhancing of the list of ions whose concentration can be selectively measured with the corresponding new sensors) and (2) an intensive development (improvement of the analytical capabilities of already known sensors). Recent articles (mostly published after the year 2000) describing the ionophore-based ion-selective electrodes are classified and briefly characterized from this viewpoint.

Ion imprinted polymer based sensor for monitoring toxic uranium in environmental samples

In view of the extreme toxicity of uranium and consequent stringent limits fixed by WHO and various national governments, it is essential to monitor the uranium content in the environment which is at ultratrace levels. Conventional ionophore based ion selective electrodes, barring a few, have limitations in terms of sensitivity and selectivity for the above mentioned purpose. We now propose an ion imprinted polymer (biomimetic) based potentiometric sensor by dispersing the uranyl ion imprinted polymer particles in 2-nitrophenyloctyl ether (plasticizer), which is embedded in polyvinyl chloride matrix. The sensor responds to uranyl ion over a wide concentration range of 2.0 × 10 −8 to 1.0 × 10 −2 M. The limit of detection was 2.0 × 10 −8 M. It showed a good selectivity for uranyl ion over alkali, alkaline earth, transition and heavy metal cations. The sensor is successfully tested for the monitoring of toxic uranium in tap and sea water samples.

Selective coulometric release of ions from ion selective polymeric membranes for calibration-free titrations

Coulometry belongs to one of the few known calibration-free techniques and is therefore highly attractive for chemical analysis. Titrations performed by the coulometric generation of reactants is a well-known approach in electrochemistry, but suffers from limited selectivity and is therefore not generally suited for samples of varying or unknown composition. Here, the selective coulometric release of ionic reagents from ion-selective polymeric membrane materials ordinarily used for the fabrication of ion-selective electrodes is described. The selectivity of such membranes can be tuned to a significant extent by the type and concentration of ionophore and lipophilic ion-exchanger and is today well understood. An anodic current of fixed magnitude and duration may be imposed across such a membrane to release a defined quantity of ions with high selectivity and precision. Since the applied current relates to a defined ion flux, a variety of non-redox active ions may be accurately released with this technique. In this work, the released titrant's activity was measured with a second ionophore-based ion-selective electrode and corresponded well with expected dosage levels on the basis of Faraday's law of electrolysis. Initial examples of coulometric titrations explored here include the release of calcium ions for complexometric titrations, including back titrations, and the release of barium ions to determine sulfate.

Glass and polymeric membrane electrodes for the measurement of pH in milk and cheese

While there is a considerable interest in the food industry in determining various analytes using ion-selective electrodes (ISEs), only few reports describe their use for direct measurements in food. In this study, the suitability of glass electrodes and ionophore-based solvent polymeric ISEs for the determination of pH in Process cheese, Cheddar cheese and milk was investigated. The liquid junction potential between a 3 M KCl bridge electrolyte and diluted as well as undiluted Process cheese was found to be negligible. Reference electrodes with ceramic plug and sleeve-type junctions performed well, although precautions needed to be taken to prevent plugging at the junctions. While the protein rennet casein posed no problems in pH measurements, the extraction of neutral lipophilic compounds or hydrophobic peptides into solvent polymeric membranes was evident, resulting in some loss of selectivity for monovalent cations upon exposure to cheese. However, it was found that ISEs based on tridodecylamine (R 3N) as ionophore and o-nitrophenyl octyl ether (oNPOE) as plasticizer can be used to accurately measure the pH of milk and, after desensitization of the electrodes in a cheese emulsion, of diluted Process cheese. Since pH measurements with a glass electrode showed that emulsions of cheese moderately diluted to a cheese content of 70% have the same pH as undiluted cheeses, it is possible to determine the pH in cheese with ionophore-based ISEs. R 3N membranes also performed well in undiluted milk.

Current Response of Ionophore-Based Ion-Selective Electrode Membranes at Controlled Potential

From electrochemical measurements at the interface of two immiscible electrolytes, the current at controlled potential is usually a linear function of the ion concentration in the aqueous phase. Surprisingly, a linear relationship between the current and the logarithm of the sample ion activity is found here for corresponding measurements on ion-selective electrode membranes. Here we document these new findings and give a theoretical explanation for the apparent contradiction between the results obtained with the two kinds of systems.

A generalized model for apparently "non-Nernstian" equilibrium responses of ionophore-based ion-selective electrodes. 1. Independent complexation of the ionophore with primary and secondary ions.

A generalized model that describes apparently "non-Nernstian" equilibrium responses of ionophore-based ion-selective electrodes (ISEs) is presented. It is formulated for primary and secondary ions of any charges that enter the membrane phase and independently form complexes with the ionophore, respectively. Equations for the phase boundary potential model were solved numerically to obtain whole response curves as a function of the sample activity of the primary ion, and analytical solutions could be obtained for apparently non-Nernstian response sections in these response curves. Ionophore-based ISEs can give three types of apparently non-Nernstian equilibrium responses, i.e., apparently "super-Nernstian", "inverted-Nernstian", and "sub-Nernstian" responses. The values of the response slopes depend on the charge numbers of the primary and secondary ions and on the stoichiometries of their complexes with the ionophore. The theoretical predictions for super-Nernstian responses agree well with the experimental results obtained with ISEs based on acidic ionophores or metalloporphyrin ionophores. Also, theoretical response curves with inverted-Nernstian slopes were found to be similar in character to the pH responses of Ca2+-selective electrodes based on organophosphate ionophores, which have been known to exhibit a so-called "potential dip". The quantitative understanding of apparently non-Nernstian response slopes presented here provides an insight into ionophore-analyte complexation processes in ISE membranes and should be helpful for the design of new ionophores.