Theory and applications of ion-selective electrodes Part III

This review of ion-selective electrodes is arranged in the same way as Parts I and II. The continuous growth of the whole subject should be noted. Theory has particularly progressed through mechanistic studies. Among new types of electrodes, ISFET systems have become important. Among new applications of both solid and liquid membrane electrodes, the fluoride electrode still predominates, closely followed by potassium electrodes. In the field of biological and medical applications, the steady growth is significant. More than 800 papers published between mid-1976 and the end of 1978 are mentioned in the review.     

Theory and applications of ion-selective electrodes: Part 5

This review of ion-selective electrodes is arranged in the same way as earlier reviews in this series. The whole subject is steadily growing. More attention has been given to automatic methods of analysis with ion-selective electrodes, electrolysis at the interface of two immiscible electrolyte solutions, and to ion-selective field-effect transistors. The number of publications on traditional subjects like fluoride ion-selective electrodes has dropped. About 700 pagpers published between Spring 1981 and Spring 1983 are mentioned.     

Photoresponsive ion-selective optical sensor

A novel concept of photoresponsive ion-selective optical sensor is described. Photochemical reactions can be utilized to generate and control ion fluxes in an ion-selective optode in the same manner as nonequilibrium electrochemical methods have been used in ion-selective electrodes. In contrast to their equilibrium counterparts, the photoresponsive pH-selective ion optodes are sensitive to both the buffer capacity of the sample and activity of hydrogen ions. Active optical probes are especially attractive for intracellular applications because they can be fabricated as submicron-sized beads. Common optical techniques, such as fluorescence microscopy and flow cytometry, can be combined with active ion probes with only minor modification of the existing experimental setup.     

Beyond potentiometry: robust electrochemical ion sensor concepts in view of remote chemical sensing

For about 100 years, potentiometry with ion-selective electrodes has been one of the dominating electroanalytical techniques. While great advances in terms of selective chemistries and materials have been achieved in recent years, the basic manner in which ion-selective membranes are used has not fundamentally changed. The potential readings are directly co-dependent on the potential at the reference electrode, which requires maintenance and for which very few accepted alternatives have been proposed. Fouling or clogging of the exposed electrode surfaces will lead to changes in the observed potential. At the same time, the Nernst equation predicts quite small potential changes, on the order of millivolts for concentration changes on the order of a factor two, making frequent recalibration, accurate temperature control and electrode maintenance key requirements of routine analytical measurements. While the relatively advanced selective materials developed for ion-selective sensors would be highly attractive for low power remote sensing application, one should consider solutions beyond classical potentiometry to make this technology practically feasible. This paper evaluates some recent examples that may be attractive solutions to the stated problems that face potentiometric measurements. These include high-amplitude sensing approaches, with sensitivities that are an order of magnitude larger than predicted by the Nernst equation; backside calibration potentiometry, where knowledge of the magnitude of the potential is irrelevant and the system is evaluated from the backside of the membrane; controlled current coulometry with ion-selective membranes, an attractive technique for calibration-free reagent delivery without the need for standards or volumetry; localized electrochemical titrations at ion-selective membranes, making it possible to design sensors that directly monitor parameters such as total acidity for which volumetric techniques were traditionally used; and controlled potential coulometry, where all ions of interest are selectively transferred into the ion-selective organic phase, forming a calibration-free technique that would be exquisitely suitable for remote sensing applications.     

Neutral-carrier-type potassium ion-selective electrodes based on polymer-supported liquid-crystal membranes for practical use.

Polymer-supported liquid-crystal membranes have been designed for neutral-carrier-type potassium ion-selective electrodes, aiming for practical applications of high-performance liquid-crystalline membrane ion sensors. Two types of polymer-supported liquid-crystal membranes were tested for their usefulness; one is microporous poly(tetra fluoroethylene) (PTFE) membranes impregnated by thermotropic liquid-crystalline compounds, and another is poly(methyl methacrylate) (PMMA) membrane dispersing the same liquid-crystalline compounds. Both of the polymer-supported liquid-crystal membranes containing a liquid-crystalline benzo-15-crown-5 neutral carrier as well as a lipophilic anion excluder work well as ion-sensing membranes for potassium ion-selective electrodes, the ion selectivities of which can be switched by the measurement temperatures. Specifically, PTFE-impregnated liquid-crystal membranes are better than the PMMA-dispersed ones in the sensitivity and selectivity of the resulting ion electrodes. A potassium ion assay in blood sera has proved that neutral-carrier-type ion-selective electrodes based on the polymer-supported liquid-crystal membranes are reliable for practical uses.     

Adding Plasticizer Dispersible Conducting Polymer to the Ion-selective Membrane Composition – Revitalizing Single Piece Ion-selective Electrodes Concept

Typically, ion-selective electrodes used in current triggered electrochemical sensing apply a conducting polymer layer covered with an ion-selective membrane. In this work we propose an ion-selective membrane containing a dispersed conducting polymer. Thus obtained system allows elimination of the Achilles hell of heterogeneous ion-selective membranes containing solid particulates dispersed within the ion-selective polymeric membrane. The herein proposed system, even for high conducting polymer loading equal to 5 % w/w, is characterized with insensitivity towards redox interferences, as well as potentiometric detection limits, selectivity well comparable with that for other ion-selective electrodes constructions. Under voltammetric conditions, with increasing loading of the conducting polymer in the membrane cathodic peak potentials are shifted towards more negative values, yet the linear dependence of the peak potential on logarithm of concentration of the analyte in the solution is preserved.     

Nitrate and ammonium ion-selective electrodes as sensors. I. In bacterial growth curves for isolation of nitrate and nitrite reductases from Escherichia coli

Ammonium and nitrate ion-selective electrodes were applied as sensors in bacterial growth curves for the isolation of nitrate and nitrite reductase. A nitrate ion-selective electrode was used as a successful new technique for the assay of nitrate reductase activity: the decrease in nitrate concentration was followed continuously. Likewise, an ammonium ion-selective electrode was used to follow the increase in ammonium ion concentration. Excellent agreement with spectrophotometric procedures ( < 5%) was obtained.     

Theory and applications of ion-selective electrodes: Part 6

This review of ion-selective electrodes is arranged in the same way as earlier reviews of this series. The whole subject continues to grow steadily. Considerable attention has been given to clinical analysis of body electrolytes, particularly by means of automatic devices. About 800 papers published between Spring 1983 and Spring 1985 (including several omitted in Part 5) are mentioned, covering the theory of membrane phenomena, technology, fixed-state eletrodes, liquid-membranes electrodes, potentiometric biosensors and miscellaneous systems.     

Theory and applications of ion-selective electrodes: Part 7

This review on ion-selective is arranged in the same way as earlier reviews of this series. The whole subject continues to grow steadily. The most discussed field is that of analysis of body-fluid cations. About 850 papers published between Spring 1985 and Spring 1987 are mentioned, covering the theory of membrane processes, technology, electrolysis at liquid/liquid interfaces, fixed-site electrodes, liquid-membrane electrodes, potentiometric biosensors and miscellaneous systems.     

Evaluation of an overdetermined system based on multiple ion-selective electrodes of the same type

Possible advantages of using multiple ion-selective electrodes of the same type for the determination of analyte ions in multicomponent mixtures are investigated. Specifically, potassium and ammonium ions are determined in model multicomponent mixtures with a combination of four valinomycin and four nonactin polymeric membrane electrodes. The electrode potentials are monitored by a specially designed data-acquisition system, and conventional matrix operations are used to calculate the ion concentrations. The results indicate that greater confidence in the analyses can be obtained from an overdetermined system of this type.     

Studies on the Preparation and Analytical Applications of Various Metal Ion-Selective Membrane Electrodes Based on Polymeric,Inorganic and Composite Materials—A Review

Ion-selective membrane electrodes commonly known as electrochemical sensors are important in view of the ability to make direct or indirect measurement of various metal ions. The fact is that the use of ion-selective electrodes for such type of measurements requires relatively inexpensive equipment, which makes ion-selective electrodes attractive to scientists in many disciplines. Thus, potentometric sensors can offer an inexpensive and convenient method for the analysis of heavy metal ions in solutions providing acceptable sensitivity and selectivity. For this purpose, many organic, inorganic, chelating, intercalating and composite materials were studied as electroactive materials for the preparation of ion-selective membrane electrodes. The present study provides a detailed review of literature for the fabrication, characterization and analytical applications of ion-selective membrane electrode based on different electro active components.     

Novel electroosmotic micromixer configuration based on ion-selective microsphere

In this paper, a micromixer of a new configuration is presented, consisting of a spherical chamber in the center of which an ion-selective microsphere is placed. Stratified liquid is introduced through the chamber via inlet and outlet holes under an external pressure gradient and an external electric field is directed in such a way that the resulting electroosmotic flow is directed against the pressure-driven flow, resulting in mixing. The investigation is carried out by direct numerical simulation on a super-computer. Optimal values of the applied electric field are determined to yield strong mixing. Above this optimal mixing regime, a number of instabilities and bifurcations are realized, which qualitatively coincide with those occurring during electrophoresis of an ion-selective microgranule. As shown by our calculation, these instabilities do not lead to an enhanced mixing. The resulting electroconvective vortices remain confined near the surface of the microgranule, and do not sufficiently perturb the stratified fluid flow further from the granule. On the other hand, another type of instability caused by the salt concentration gradient can generate sufficiently strong oscillations to enhance mixing. However, this only occurs when the external electric field is sufficiently high that the electroosmotic flow is comparable to the pressure-driven flow. This ultimately leads to creation of reverse flows of the liquid and cessation of the device operation. Thus, it was shown that the best mixing occurs in the absence of electrokinetic instability. Based on the data obtained, it is possible to select the necessary geometric characteristics of the micromixer to achieve the optimal mixing mode for a given set of liquids, which may be ten times more effective than passive mixers at the same flow rates. A comparison with the experimental data of the other authors confirms the effectiveness of this device and its other capabilities. Furthermore, the basic device design can be operated in other modes, for example, an electrohydrodynamic pump, a streaming current generator, or even a micro-reactor, depending on the system parameters and choice of an ion-selective granule.     

Determination of selectivity coefficients of ion-selective electrodes by a matched-potential method

A new method is proposed for the determination of selectivity coefficients of ion-selective electrodes. A reference solution of the primary ion is used and potential changes are measured after adding either the primary ion or secondary ion. Increasing concentrations of the secondary ion are added to provide the same potential change as obtained for a fixed added concentration of the primary ion. The ratio of the primary-to-secondary ion concentrations for this potential change represents the selectivity coefficient. This method is illustrated by the use of a sodium glass electrode and a potassium valinomycin electrode. Results obtained are compared with those from conventional determinations of selectivity coefficients. The advantages of this method are discussed.     

A solid-state ion-selective electrode with an ionic-electronic transducer for determining chlordiazepoxide

A solid-state ion-selective electrode for determining chlordiazepoxide is described. A characteristic feature of the electrode is the presence of an intermediate conducting polymeric layer (transducer) of poly(o-aminophenol) between the ion-selective membrane with ionic conduction and a Pt conductor. The properties of ion-selective electrodes with transducers are compared with those of ISEs without transducers (coated-wire electrodes, CWEs). The most important distinctive features of ion-selective electrodes with transducers are the high stability of their potentials in time and their small sizes. The main performance characteristics of the electrode and the scope of its application are found.Translated from Zhurnal Analiticheskoi Khimii, Vol. 60, No. 1, 2005, pp. 74–78.Original Russian Text Copyright © 2005 by Gorelov, Ryasenskii, Kartamyshev, Fedorova.     

Reducing the interference from CO2 or organic acids in ion-selective polymer membrane sensors having a field-effect transistor as internal reference element

Changes in sample concentrations of CO₂ or organic acids cause potential instabilities when polymer membranes are directly applied to the surface of ion-selective field-effect transistors (ISFETs). Currently used designs avoid this well-documented effect by placing a layer of aqueous buffer between polymeric membrane and ISFET serving as internal reference element. Here, we propose another solution to the problem. In order to compensate for the effect of pH changes on the ISFET threshold voltage, a double membrane is applied whose inner layer is pH-sensitive, while the outer layer exposed to the sample is a conventional ion-selective membrane. It is shown that this approach strongly reduces the earlier-mentioned interference effects.     

Theory and applications of ion-selective electrodes. Part 8

This review on ion-selective electrodes (ISEs) is arranged in the same way as earlier reviews of this series. The whole subject continues to grow. As in the preceding review, the most discussed field is that of analysis for body-fluid cations, particularly with automatic devices. Almost 800 papers published between Spring 1987 and Spring 1989 are mentioned, including some omitted in the preceding review. They cover the theory of membrane processes, technology (including ISFETs and electrolysis at liquid/liquid interfaces), fixed-site electrodes, potentiometric biosensors and miscellaneous systems.     

PVC膜离子选择电极检测下限的优化

价格低廉、携带方便、适用浓度宽、操作简单快捷、能耗低的离子选择电极在医院、分析实验室、野外等领域得到了越来越广泛的应用。尽管如此,由于PVC膜中存在的离子流严重破坏了更低检测下限的获取,限制了离子选择电极的进一步发展。因此,本文从减小甚至消除PVC膜中存在的离子流角度出发,系统论述了优化PVC膜离子选择电极检测下限的原理和优良策略,根据收集归纳的大量数据定量阐述传感膜组成的优化、电极组装和调制、应用旋转电极以及电流极化处理等对检测下限的优化提升作用,进一步总结出各种方法的改善规律,分析它们的优势和面临的问题。提出在PVC铸膜液中要突破传统配方,减小增塑剂和离子交换剂用量,以抑制传感膜两侧的离子流,同时外加电流补偿处理等也是降低电极检测下限的有效方法,对检测下限的改善最好的可降低5个数量级。这一总结为PVC膜离子选择电极的高性能化明确了研究方向。     

Oscillations in the concentration of fluoride ions induced by a pH oscillator

Sustained oscillations in the concentration of free fluoride ions can be generated when the BrO3--SO32--Mn(II) oscillator is coupled either to Al3+-F- complex formation or to the Ca2+-F- precipitation process in a flow reactor. By careful analysis of the relevant equilibria and optimization of the reactant concentrations, one can obtain [F-] oscillations of several orders of magnitude as measured with an ion-selective electrode. The BrO3--SO32--Mn(II)-Al(NO3)3-NaF system also exhibits bistability, that is, simultaneously stable steady states of high and low [F-].     

Experimental probes of silver metal nanoparticle formation kinetics: Comparing indirect versus more direct methods

The kinetics of noble metal nanoparticle formation in bottom-up syntheses are important for controlling and optimizing these methods. Hence, experimental probes that are easily accessible to most laboratories are also of interest. We collected kinetic curves for the formation of silver nanoparticles in a modified Turkevich method with citrate acting as the reducing and stabilizing agent by (i) measuring the change in silver nanoparticle surface plasmon resonance by UV-visible spectroscopy, a somewhat indirect method, and then also by (ii) measuring the change in silver ion concentration by ion-selective electrode potentiometry and/or atomic absorption spectroscopy, two more direct methods. The resulting sigmoidal kinetic curves were curvefitted with the Finke-Watzky two-step kinetic model of slow, continuous nucleation and fast autocatalytic growth to extract average rate constants. We found that the kinetic curves obtained by following the change in silver ion concentration were apparent mirror images of those constructed by following the change in nanoparticle surface plasmon resonance, and that their respective curvefits displayed the same sigmoidal characteristics. The resultant values of the rate constants for nucleation and growth overlapped within experimental error between the methods and showed similar trends over the range of citrate concentrations studied. The use of multiple probes in this work to follow the kinetics of nanoparticle formation helps fill a need for the comparison and evaluation of different methods available to scientists, particularly those considered easily accessible.     

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.