Response Of Ion-selective Electrodes Research Articles

Near-Infrared Laser Irradiation-Modulated High-Temperature Solid-Contact Ion-Selective Electrodes: Potentiometric Detection of Ca2+ in Seawater.

The high-temperature potentiometry operated by nonisothermal heating is a promising way to break through the traditional potentiometric responses of ion-selective electrodes (ISEs) at room temperature. Herein, a locally heated strategy through near-infrared region (NIR) laser irradiation upon the photothermal mesoporous carbon material placed between the ion-selective membrane and the glassy carbon substrate is introduced to obtain the high-temperature potentiometric performance of a solid-contact Ca2+-ISE for detection of Ca2+ in seawater. Based on the light-to-heat conversion of the mesoporous carbon-based solid contact, the temperature of the solid-contact Ca2+-ISE upon continuous NIR laser irradiation can be increased from room temperature to 60-70 °C, and the slope of the electrode is promoted up to about 30% according to the thermodynamic steady-state potentiometric response. The pulsed potentiometric response of the solid-contact Ca2+-ISE upon a pulsed NIR laser irradiation of 5 s also shows a linear change as a function of Ca2+ activities, and the improved slope from 27.1 ± 0.6 to 38.1 ± 0.9 mV/dec can be obtained under dual control of the temperature of the electrode and the transient current induced by the pulsed NIR laser irradiation. As compared to the traditional potentiometric measurement under zero-current conditions at room temperature, the NIR laser-modulated high-temperature potentiometric response provides an alternative way for measurement of the solid-contact ISEs.

Amperometric response of solid-contact ion-selective electrodes utilizing a two-compartment cell and a redox couple in solution

• The amperometric response of ion-selective electrodes (ISEs) was characterized utilizing a two-compartment cell. • ISEs served as potentiometric transducer when connected as reference electrode. • The amperometric response was reversible and reproducible. • The amperometric signal was optimized by applying a potential equal to the half-peak potential (E p/2 ) of the redox couple. An alternative transduction method for ion-selective electrodes (ISEs) is studied by utilizing oxidation/reduction of a Fe(CN) 6 3-/4- redox couple dissolved in solution. Two compartments are employed as electrochemical cell, connected by an Ag/AgCl bridge. A K + -selective solid-contact ISE (K + -SCISE) with reduced graphene oxide as solid contact works as reference electrode (RE) and is placed in the sample solution compartment. A gold electrode acting as working electrode (WE) and a platinum as counter electrode (CE) are placed in the detection compartment with a solution containing the Fe(CN) 6 3-/4- redox couple. As the primary ion (K + ) concentration changes, the potential of the K + -SCISE will change accordingly. Since the potential between WE and RE is kept constant by using a potentiostat, any change in the concentration of primary ion K + will result in an oxidation/reduction current from the Fe(CN) 6 3-/4- redox couple at the surface of the Au WE, causing a current flowing between the Au WE and Pt CE. The diffusion-limited current from oxidation/reduction of the Fe(CN) 6 3-/4- redox couple is linearly proportional to the logarithm of the activity of the primary ion within a certain concentration range of the primary ion. The reversibility and reproducibility of the amperometric response from oxidation/reduction of Fe(CN) 6 3-/4- is proven in this work. A unique feature of this method is that current signal can be enlarged by increasing the geometric surface area of the WE. The amperometric response of the Fe(CN) 6 3-/4- redox couple is highly dependent on the applied starting potential. The sensitivity of the amperometric response can be optimized by applying a potential equal to the half-peak potential (E p/2 ). Furthermore, the amperometric response is applied for determination of K + ion concentration in such a demanding matrix as human blood serum.

Resolution increase of ion-selective electrodes response by using a reversed amperometric setup

Typical ion-selective electrodes (ISEs) used in potentiometric mode suffer from low sensitivity, determined by Nernst (Nikolsky-Eisenman) equation. This limits resolution of potentiometric analysis, ultimately leading to significant problems in determination of relatively small changes of analyte ions concentration. A novel method was proposed using a ‘reversed’ amperometric setup with Ag|AgCl electrode as the working electrode and an ion-selective electrode as the reference one. Advantageously, the proposed approach does not require addition of other chemicals to the sample. A change of primary ions concentration results in change of the potential of the ISE used as the reference electrode and thus in the same change of Ag|AgCl electrode potential in amperometric (potentiostatic) mode. Due to high slope of the current vs. potential dependence for silver / silver chloride electrode small changes of ISE potential will result in significant changes of the recorded current, leading to high amplification of the analytical signal towards primary ions of the ISE. Additional unique advantage of herein proposed approach is a linear dependence of the current on analyte concentration, as tested on model examples of potassium- and calcium-selective ISEs.

Emission Intensity Readout of Ion-Selective Electrodes Operating under an Electrochemical Trigger.

We report for the first time on in situ transduction of electrochemical responses of ion-selective electrodes, operating under non-zero-current conditions, to emission change signals. The proposed novel-type PVC-based membrane comprises a dispersed redox and emission active ion-to-electron transducer. The electrochemical trigger applied induces a redox process of the transducer, inducing ion exchange between the membrane and the solution, resulting also in change of its emission spectrum. It is shown that electrochemical signals recorded for ion-selective electrodes operating under voltammetric/coulometric conditions correlate with emission intensity changes recorded in the same experiments. Moreover, the proposed optical readout offers extended linear response range compared to electrical signals recorded in voltammetric or coulometric mode.

Ionic liquid-based reference electrodes for miniaturized ion sensors: What can go wrong?

Ionic liquid-based reference electrodes, especially those containing 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (MeOctIm TFSI), are increasingly used in potentiometric measurements. They replace the conventional salt bridges and establish the reference potential through local partitioning of the ions of the ionic liquid between the reference membrane and the sample solution. Unless the electrochemical cell and the measurement protocol are designed appropriately, these ions can interfere with the response of ion-selective electrodes (ISEs) with polymeric membranes. This work characterizes the effect of MeOctIm TFSI on the response of K+, Na+, and Cl− ISEs with polymeric membranes. The leaching of MeOctIm TFSI from the reference membrane to the sample solution was monitored with UV–vis spectroscopy. The concentration of MeOctIm TFSI in the aqueous phase increased gradually and plateaued at approximately 1.0 mM. Concentrations as low as 100 μM of MeOctIm TFSI caused large changes to the emf (50–150 mV) of K+, Na+, and Cl− ISEs. The presence of 10 μM of TFSI− was enough to cause Donnan failure of K+ ISEs with valinomycin as ionophore (that is, co-extraction of K+ and TFSI− into the sensing membrane). Use of less lipophilic anions such as tetrafluoroborate (BF4-) or triflate (OTf-) postponed the onset of Donnan failure of cation-selective ISEs to higher concentrations of the anion, but decreased the stability in the reference potential and lifetime of the reference electrode. These results imply that although MeOctIm TFSI-based reference electrodes provide sample-independent and stable electrical potentials, they should be used with caution for measurements with polymeric-membrane ISEs, due to strong interference of both MeOctIm+ and TFSI− with measured values of emf.

Overcoming of One More Pitfall in Boundary Element Calculations with Computer Simulations of Ion-Selective Electrode Response.

Computer simulations of ion-selective membrane electrodes using diffusion layer models based on finite-differences principle for calculating diffusion processes in both phases and taking into account the local ion exchange equilibrium at the interface are successfully used for clarifying and even predicting the influence of different diffusion factors on several time-dependent characteristics of electrodes. It is shown here that a well-established approach based on the assumption of the constant concentration of the interfering ion in the sample solution fails for solutions containing strongly interfering ions where the concentration of the interfering ion in the boundary layer of the solution can be far lower in comparison with its concentration in the bulk. The limitation is demonstrated by a drastic discrepancy between experimental and calculated curves for the dependence of potential on time. This limitation can be overcome by taking into account the change of the interfering ion concentration in the boundary layer in accordance with the electroneutrality condition. A good agreement between simulation results and experimental data is demonstrated.

Electrochemical-to-Optical Signal Transduction for Ion-Selective Electrodes with Light-Emitting Diodes.

Optical ion sensors normally have a relative narrow sensitive detection window. Here, based on multicolor light-emitting diodes (LEDs), we report on an electrochemical-to-optical signal transduction scheme under chronoamperometry control to convert the potentiometric response of ion-selective electrodes (ISEs) to optical output with tunable sensitivity and much wider response range. The sensing principle was demonstrated on K+, Ca2+, and Pb2+. LED light intensity was found to depend linearly on the concentration of monovalent ions. Optical signals could be captured with photomultiplier tubes or digital cameras, and a visual alarming system to monitor abnormal ion concentration was also developed from super-Nernstian electrodes.

Application of the interface equilibria-triggered dynamic diffusion model of the boundary potential for the numerical simulation of neutral carrier-based ion-selective electrodes response

It is shown that a simple dynamic diffusion model of the boundary potential based on a separate, step-by-step, account of ion transfer across the membrane/aqueous solution interface and the diffusion processes within both phases which was proposed earlier for describing the response of ionophore-free membranes, can be successfully used for ionophore-based membranes as well. The model makes it possible to carry out both separate and joint account of the effects of co-extraction, transmembrane transport and ion exchange on the boundary potential and retains robustness in all the variants studied. The model adequately describes the ionophore-based electrode response over the entire range of concentrations and allows one to clearly demonstrate the dependence of lower detection limit on such parameters as the diffusion coefficients and the concentration of electroactive substances in the membrane phase, the thickness of the diffusion layer in the sample solution, the duration of the measurement, and the composition of the internal reference solution. The results of numerical simulation are in good agreement with the experimental data presented in the literature. As all the factors of influence considered above can easily be regulated in more or less wide limits, but at the same time, an estimation of their cumulative effect is not always possible on an intuitive level, the present model can be of practical interest for justifying the ways of optimizing the design of the ISE and the algorithm for performing measurements in solving specific analytical problems.

Fluorimetric readout of ion-selective electrode potential changes

A novel method of quantitative optical readout of potentiometric responses of ion-selective electrodes is proposed. This idea was realized in a simple self-powered bipolar electrode system, with an ion-selective pole directly connected with the second pole, where a fluorimetric signal is generated. In this way, space and time separated optical readout and sampling can be achieved.This concept was tested on a model example of a system containing potassium-selective electrode (ISE-K) with polypyrrole solid contact, connected to a zinc wire immersed in the solution. In this arrangement increase of potassium ions activity in the solution results in change of the membrane potential of ISE-K, inducing reduction of polypyrrole solid contact and corresponding oxidation of the other component - zinc. Increase of zinc ions concentration is read as increase of emission intensity. Thus, this simple system allows quantitative in situ potassium ions determination, with the fluorescence intensity being a linear function of logarithm of analyte activity in solution in turn-on mode.Moreover, due to simplicity of the system and presence of reactants in the solution phase, a quantitative correlation between electrochemical and optical behaviour was confirmed and indications for the system optimization could be proposed.

An Interface Equilibria-Triggered Time-Dependent Diffusion Model of the Boundary Potential and Its Application for the Numerical Simulation of the Ion-Selective Electrode Response in Real Systems.

A simple dynamic model of the phase boundary potential of ion-selective electrodes is presented. The model is based on the calculations of the concentration profiles of the components in membrane and sample solution phases by means of the finite difference method. The fundamental idea behind the discussed model is that the concentration gradients in both membrane and sample solution phases determine only the diffusion of the components inside the corresponding phases but not the transfer across the interface. The transfer of the components across the interface at any time is determined by the corresponding local interphase equilibria. According to the presented model, each new calculation cycle begins with the correction of the components' concentrations in the near-boundary (first) layers of the membrane and solution, based on the constants of the interphase equilibria and the concentrations established at a given time as a result of diffusion. The corrected concentrations of the components in the boundary layers indicate the start of a new cycle every time with respect to the calculations of diffusion processes inside each phase from the first layer to the second one, and so on. In contrast to the well-known Morf's model, the above-mentioned layers do not comprise an imaginary part and are entirely localized in the corresponding phases, and this allows performing the calculations of the equilibrium concentrations by taking into account material balance for each component. The model remains operational for any realistic scenarios of the electrode functioning. The efficiency and predictive ability of the proposed model are confirmed by comparing the results of calculations with the experimental data on the dynamics of the potential change of a picrate-selective electrode in nitrate solutions when determining the selectivity coefficients using the methods recommended by IUPAC.

Modeling of the effect of diffusion processes on the response of ion-selective electrodes by the finite difference technique: Comparison of theory with experiment and critical evaluation

Experimental data are compared with the results of calculations by the finite difference technique within the dynamic diffusion model of the interphase potential on an example of a picrate-selective electrode in real scenarios corresponding to the conditions of the determination of selectivity coefficients by the methods recommended by IUPAC. It was found that, in the majority of the considered cases, the calculated values of the potential and selectivity coefficients and also the dynamics of potential change at particular steps well agree with the experimental data. The model has principal restrictions, leading the failure of calculations, when the concentration of potential-determining ions in the near-electrode layer of the solution performed is low according to the algorithm of measurements, whereas the instant increase in its concentration in the surface membrane layer due to the replacement of the sample solution induces a flux of these ions from the surface deep into of the membrane.

Potentiometric responses of ion-selective electrodes after galvanostatically controlled incorporation of primary ions

A new method of quantitative incorporation of primary cations into ion-selective membrane by means of galvanostatic cathodic polarization/conditioning, before measurement step, was proposed and tested on the example of potassium-selective electrode with ionophore – valinomycin in poly(vinyl chloride) based membrane and with polypyrrole solid contact. Open circuit potential values recorded after polarization can be quantitatively explained by changes of primary cations and ionophore concentration in the surface part of the membrane. The influence of potassium ions concentration in the membrane (in relation to ion exchange sites amount) on the shape of potentiometric calibration plots was also observed. Improved characteristics, with extended linear range, can be obtained for membrane of minor loading with primary cations (around 25%), the responses are relatively stable in course of following calibrations.

High-Temperature Potentiometry: Modulated Response of Ion-Selective Electrodes During Heat Pulses

The concept of locally heated polymeric membrane potentiometric sensors is introduced here for the first time. This is accomplished in an all solid state sensor configuration, utilizing poly(3-octylthiophene) as the intermediate layer between the ion-selective membrane and underlying substrate that integrates the heating circuitry. Temperature pulse potentiometry (TPP) gives convenient peak-shaped analytical signals and affords an additional dimension with these sensors. Numerous advances are envisioned that will benefit the field. The heating step is shown to give an increase in the slope of the copper-selective electrode from 31 to 43 mV per 10-fold activity change, with a reproducibility of the heated potential pulses of 1% at 10 microM copper levels and a potential drift of 0.2 mV/h. Importantly, the magnitude of the potential pulse upon heating the electrode changes as a function of the copper activity, suggesting an attractive way for differential measurement of these devices. The heat pulse is also shown to decrease the detection limit by half an order of magnitude.

Room Temperature Ionic Liquids (RTILs) and Multiwalled Carbon Nanotubes (MWCNTs) as Modifiers for Improvement of Carbon Paste Ion Selective Electrode Response; A Comparison Study with PVC Membrane

AbstractRoom temperature ionic liquids (RTILs), 1‐n‐butyl‐3‐methylimidazolium tetrafluoroborate, [bmim]BF4, and multiwalled carbon nanotubes (MWCNTs) were used for improvement of a praseodymium carbon paste ion selective sensor response. [bmim]BF4 can be a better binder than mineral oils. MWCNTs have a good conductivity which helps the transduction of the signal in carbon paste electrode. The characteristics of these electrodes as potentiometric sensors were evaluated and compared with PVC membrane sensor. The results indicate that potentiometric sensor constructed with ionic liquid shows an increase in performance in terms of Nernstian slope, selectivity, response time, and response stability compared to Pr(III) PVC membrane sensor.

Time-Dependent Phenomena in the Potential Response of Ion-Selective Electrodes Treated by the Nernst−Planck−Poisson Model. Part 2: Transmembrane Processes and Detection Limit

The detection limit of ion-selective electrodes (ISEs) is of great interest because of the many possible practical applications of ISEs in trace analysis. Existing theoretical interpretations of the detection limit of ISEs are restricted by severe assumptions such as steady-state and electroneutrality, which hamper theorizing on this problem. For this reason, the Nernst-Planck-Poisson (NPP) equations are used to predict and visualize the detection limit variability under nonequilibrium conditions. For the first time, the NPP model is applied to the so-called inverse problem: finding the optimal measurement time and inner solution concentration for lowering the detection limit.

Improvement of Carbon Paste Ion Selective Electrode Response by Using Room Temperature Ionic Liquids (RTILs) and Multi-Walled Carbon Nanotubes (MWCNTs)

Improvement of Carbon Paste Ion Selective Electrode Response by Using Room Temperature Ionic Liquids (RTILs) and Multi-Walled Carbon Nanotubes (MWCNTs)

Theory and computer simulation of the time-dependent selectivity behavior of polymeric membrane ion-selective electrodes

A theoretical treatment of the time-dependent potential response of ion-selective electrodes to sample solutions containing primary and interfering ions is presented. The theory accounts for the influence of ion fluxes in the electrode membrane and the contacting aqueous sample layer and describes the variations in the apparent selectivity behavior as a function of the measuring time. The applicability of the theory is demonstrated by comparing predicted response curves with results of virtual experiments based on computer simulation. A close and convincing agreement was achieved for a large series of different examples, which confirms that the new theory can be successfully applied for general cases.

Potentiometric Responses of Ion-Selective Electrodes Doped with Diureidocalix[4]arene towards Un-dissociated Benzoic Acid

Diureidocalix[4]arene have been applied as new ionophore in liquid membraneelectrode (ISE) sensitive towards un-dissociated benzoic acid. The electrode demonstratedresponse towards benzoic acid with the detection limit 2.0 x 10-4 M which is sufficient forthe determination of benzoic acid added to beverages as preservative in milimolarconcentration. The selectivity coefficients measured by the matched potential method(MPM) showed its good selectivity against common anions present in drink samples. Allmeasurements were made in presence of 1.0 x 10-2 M NaHSO4 pH 3.0 in order to reducethe influence of OH-. The applicability of diureidocalix[4]arene incorporated ISE has beenchecked by recovery test of benzoic acid in the presence of artificial drink matrix and bystandard addition method.

Time-Dependent Phenomena in the Potential Response of Ion-selective Electrodes Treated by the Nernst−Planck−Poisson Model. 1. Intramembrane Processes and Selectivity

The variability of selectivity coefficients, resulting from potential changes over time and the concentration ratio of primary to interfering ions, impedes many practical applications of ion-selective electrodes (ISEs). Existing theoretical interpretations of ISE selectivity are restricted by severe assumptions, such as steady state and electroneutrality, which hinder theorizing on this problem. For this reason, for the first time, the Nernst-Planck-Poisson equations are used to predict and visualize the selectivity variability over time and the concentration ratio. Special emphasis is placed on the non-Nernstian response in the measurements with liquid-ion-exchanger- and neutral-carrier-based ISEs. The conditions under which measured selectivity coefficients are true (unbiased) are demonstrated.

Membrane drotaverine-selective electrodes based on tetraphenylborate derivatives: Electrochemical, adsorption, and transport properties and analytical application

The potentiometric response of ion-selective electrodes (ISEs) based on different lipophilic derivatives of tetraphenylborate to drotaverine hydrochloride was studied. The composition of a polymeric membrane was optimized to obtain the best electroanalytical properties of ISEs. The transport properties of selective membranes, the permeability and the flow of ions through the interface, were studied. Linear correlations between the solubility of ionophoric membrane components, membrane transport, and electroanalytical properties were revealed. The kinetic studies of ion-exchange adsorption showed that two limiting stages of transfer, namely, diffusion through the boundary layer and diffusion throug the membrane phase, occurred. Procedures for the potentiometric determination of drotaverine hydrochloride in pharmaceutical forms were proposed.