Polymeric Membrane Ion-selective Electrodes Research Articles

Quality control criteria for solid-contact, solvent polymeric membrane ion-selective electrodes

After a long history and conflicting views, solid-contact (SC) solvent polymeric membrane ion-selective electrodes (ISEs) emerged as reliable potentometric-sensing devices with unique advantages. From the large variety of proposed SCs inherently conductive polymers emerged as the materials of choice. In our view, the most attractive feature of SC ISEs is their compatibility with thin- and thick-film microfabrication technologies that can provide cheap, mass-produced sensors and sensor arrays that can be integrated with the measuring, data acquisition, and control electronics in a straightforward way. However, despite the impressive properties of certain SC electrodes and their potential advantages, they remained primarily in the research laboratories. To make the jump from the research laboratories into commercial devices, it would be essential to prove that miniaturized SC ISEs can indeed match or surpass the performance characteristics of the conventional, liquid-contact macroelectrodes. In addition, it would be important to settle on the quality control criteria and testing protocols for assessing the performance characteristics of SC electrodes. It could help in interpreting the sometimes-inconsistent experimental data. Once cheap, miniaturized, SC ISEs will mach the performance characteristics of macroscopic-size electrodes, it is expected to have an important impact in a variety of applications requiring robust, maintenance-free, or single-use ISEs, e.g., in homecare or bedside diagnostics, environmental analysis, and quality control assessment. In addition, reliable SC ISEs are expected to revitalize the field of ion-selective field effect transistors and open new possibilities in combination with nanowire-based devices.

Direct sensing of total acidity by chronopotentiometric flash titrations at polymer membrane ion-selective electrodes.

Polymer membrane ion-selective electrodes containing lipophilic ionophores are traditionally interrogated by zero current potentiometry, which, ideally, gives information on the sample activity of ionic species. It is shown here that a discrete cathodic current pulse across an H (+)-selective polymeric membrane doped with the ionophore ETH 5294 may be used for the chronopotentiometric detection of pH in well-buffered samples. However, a reduction in the buffer capacity leads to large deviations from the expected Nernstian response slope. This is explained by the local depletion of hydrogen ions at the sample-membrane interface as a result of the galvanostatically imposed ion flux in direction of the membrane. This depletion is found to be a function of the total acidity of the sample and can be directly monitored chronopotentiometrically in a flash titration experiment. The subsequent application of a baseline potential pulse reverses the extraction process of the current pulse, allowing one to interrogate the sample with minimal perturbation. In one protocol, total acidity is found to be proportional to the magnitude of applied current at the flash titration end point. More conveniently, the square root of the flash titration end point time observed at a fixed applied current is a linear function of the total acid concentration. This suggests that it is possible to perform rapid localized pH titrations at ion-selective electrodes without the need for volumetric titrimetry. The technique is explored here for acetic acid, MES and citric acid with promising results. Polymeric membrane electrodes based on poly(vinyl chloride) plasticized with o-nitrophenyl octyl ether in a 1:2 mass ratio may be used for the detection of acids of up to ca. 1 mM concentration, with flash titration times on the order of a few seconds. Possible limitations of the technique are discussed, including variations of the acid diffusion coefficients and influence of electrical migration.

New membrane electrodes based on a functionalized tetraphenylborate covalently bound to the polymeric backbone

Abstract In this study new liquid contact polymeric membrane ion-selective electrodes (ISEs) have been realized with the aim of reducing leaching phenomena of active component and so improving the shelf life of the electrode. A new tetraphenylborate ion exchanger functionalized with allylic moieties has been synthesized and linked to a copolymer acrylonitrile–butadiene, alternative to PVC. The linking reaction takes place by radicalic mechanism and is catalysed by dicumyl peroxide. The leaching of the so obtained polymers has been evaluated by spectrophotometric and potentiometric measurements to check the effectiveness of the reaction. In order to test the performances of the new polymeric material, ISEs with and without ionophore have been prepared using the functionalized matrix as well as traditional PVC as polymeric membrane. Analysis of variance was carried out on the detection limit and on the slope of the calibration curve obtained for every kind of electrode, in order to identify the effects of the covalently bound ion exchanger on the lifetime and performance of the sensors.

Detecting Biorecognition Events at Blocked Interface Polymeric Membrane Ion‐Selective Electrodes Using Electrochemical Impedance Spectroscopy and Atomic Force Microscopy

AbstractImmobilization of a biorecognition element onto a polymeric membrane ion‐selective electrode (ISE) using a self‐assembly approach may provide scope for a novel biosensor technology platform based on the altered potentiometric response at the blocked ISE interface. In this paper, the authors have investigated the influence of solution adsorption of the model biorecognition element, avidin‐biotin, on the electrode kinetics of a conventional polymeric membrane Ca2+ ISE using atomic force microscopy (AFM) coupled with electrochemical impedance spectroscopy (EIS). It is demonstrated that solution adsorption of avidin followed by biotin incorporation leads to a demonstrable biorecognition event characterized by an impediment in the Ca2+ ion transfer kinetics of the modified ISE surface. This kinetic principle is amenable to biosensing using pulsed chronopotentiometric polymeric ISEs, which is an established dynamic electrode technique for use with polymeric membrane ISEs.

Modern Directions for Potentiometric Sensors.

This paper gives an overview of the newest developments of polymeric membrane ion-selective electrodes. A short essence of the underlying theory is given, emphasizing how the electromotive force may be used to assess binding constants of the ionophore, and how the selectivity and detection limit are related to the underlying membrane processes. The recent developments in lowering the detection limits of ISEs are described, including recent approaches of developing all solid state ISEs, and breakthroughs in detecting ultra-small quantities of ions at low concentrations. These developments have paved the way to use potentiometric sensors as in ultra-sensitive affinity bioanalysis in conjunction with nanoparticle labels. Recent results establish that potentiometry compares favorably to electrochemical stripping analysis. Other new developments with ion-selective electrodes are also described, including the concept of backside calibration potentiometry, controlled current coulometry, pulsed chronopotentiometry, and localized flash titration with ion-selective membranes to design sensors for the direct detection of total acidity without net sample perturbation. These developments have further opened the field for exciting new possibilities and applications.

Potentiometric measurement of ascorbate by using a solvent polymeric membrane electrode

A novel potentiometric method for the determination of ascorbate is described in this communication. It is based on ascorbate oxidation with permanganate which is continuously released from the inner reference solution of a ligand-free tridodecylmethylammonium chloride (TDMAC)-based polymeric membrane ion selective electrode (ISE). The ISE potential determined by the activity of permanganate ions released at the sample–membrane phase boundary is increased with the consumption of permanganate. The proposed membrane electrode is useful for continuous and reversible detection of ascorbate at concentrations in 0.1 M NaCl ranging from 1.0 × 10 −6 to 1.0 × 10 −3 M with a detection limit of 2.2 × 10 −7 M.

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.

Kinetic Modulation of Pulsed Chronopotentiometric Polymeric Membrane Ion Sensors by Polyelectrolyte Multilayers

Polymeric membrane ion-selective electrodes are normally interrogated by zero current potentiometry, and their selectivity is understood to be primarily dependent on an extraction/ion-exchange equilibrium between the aqueous sample and polymeric membrane. If concentration gradients in the contacting diffusion layers are insubstantial, the membrane response is thought to be rather independent of kinetic processes such as surface blocking effects. In this work, the surface of calcium-selective polymeric ion-selective electrodes is coated with polyelectrolyte multilayers as evidenced by zeta potential measurements, atomic force microscopy, and electrochemical impedance spectroscopy. Indeed, such multilayers have no effect on their potentiometric response if the membranes are formulated in a traditional manner, containing a lipophilic ion exchanger and a calcium-selective ionophore. However, drastic changes in the potential response are observed if the membranes are operated in a recently introduced kinetic mode using pulsed chronopotentiometry. The results suggest that the assembled nanostructured multilayers drastically alter the kinetics of ion transport to the sensing membrane, making use of the effect that polyelectrolyte multilayers have different permeabilities toward ions with different valences. The results have implications to the design of chemically selective ion sensors since surface-localized kinetic limitations can now be used as an additional dimension to tune the operational ion selectivity.

Pulsed Galvanostatic Control of Solid-State Polymeric Ion-Selective Electrodes

We report on galvanostatically controlled solid-state reversible ion-selective sensors for cationic analytes utilizing a conducting polymer as a transduction layer between the polymeric membrane and electron-conductive substrate. The instrumental control of polymeric membrane ion-selective electrodes based on electrochemically induced periodic ion extraction in alternating galvanostatic/potentiostatic mode was introduced recently creating exciting possibilities to detect clinically relevant polyions such as heparin and protamine and drastically improve the sensitivity of ion-selective sensors limited by the Nernst equation. The present study forms the basis for development of reliable, robust, and possibly maintenance-free sensors that can be fabricated using screen-printing technology. Various aspects of the development of solid-contact galvanostatically controlled ion-selective electrodes with a conducting polymer as a transduction layer are considered in the present work on the example of a model system based on a sodium-selective membrane. The protamine-selective solid-contact sensor was fabricated and characterized, which represents the next step toward commercially viable polyion sensing technology. A substantial improvement of a low detection limit (0.03 mg L-1) was achieved. A simplified diffusion-based theoretical model is discussed predicting the polarization at the interface of the conducting polymer and the membrane, which can cause the disruption of the sensor response function at relatively small current densities.

Polymeric membrane ion-selective electrode for copper(II) ions based on CuII-cyclohexaneone thiosemicarbazone complex

A PVC-based membrane of copper(ll)-cyclohexaneone thiosemicarbazone complex shows a Nernstain poteniometric response (with slope of 29.2 mV/decade) for copper(II) ions over a wide concentration range (1 x 10 -1 to 1 x 10 -9 M). The potential of the electrode is independent of pH in the range 2.0-7.0. It has a fast response of about 15 s and was used for a period of 2 months with good reproducibility.

Nitrate-selective electrode based on a cyclic bis-thiourea ionophore

Compounds containing urea and thiourea groups can hydrogen bond oxoanions and, as such, represent a class of potential ionophores for anion-selective electrodes. We have evaluated cyclic and linear bis-thiourea compounds as ionophores in polymer membrane ion-selective electrodes. An improvement in the selectivity coefficient of nitrate against chloride of greater than one order of magnitude was observed with the cyclic ionophore in comparison to the Hofmeister lipophilicity series ( log K N O 3 − , C l − pot of −3.50 with ionophore versus −2.40 with no bis-thiourea ionophore), indicating the preference of the cyclic ionophore for nitrate. This preference for nitrate over chloride may be attributed to the spatial arrangement of the thiourea hydrogen atoms, which are capable of forming hydrogen bonds with nitrate, as well as the size of the ionophore binding cavity. In the lowest-energy conformation, all thiourea hydrogen atoms point toward a putative nitrate binding site, likely contributing to enhanced coordination.

Approaches to Improving the Lower Detection Limit of Polymeric Membrane Ion-Selective Electrodes.

More than ten different approaches for improving the lower detection limit of polymeric membrane ion-selective electrodes have been suggested during the recent years. In this contribution, their principles are briefly summarized with a focus to their general practical applicability. The methods that are the most rugged and the easiest to implement in a routine laboratory will be highlighted.

ISE-based sensor array system for classification of foodstuffs

A system composed of an array of polymeric membrane ion-selective electrodes and a pattern recognition block—a so-called ‘electronic tongue’—was used for the classification of liquid samples: milk, fruit juice and tonic. The task of this system was to automatically recognize a brand of the product. To analyze the measurement set-up responses various non-parametric classifiers such as k-nearest neighbours, a feedforward neural network and a probabilistic neural network were used. In order to enhance the classification ability of the system, standard model solutions of salts were measured (in order to take into account any variation in time of the working parameters of the sensors). This system was capable of recognizing the brand of the products with accuracy ranging from 68% to 100% (in the case of the best classifier).

Polymeric membrane ion-selective and cross-sensitive electrode-based electronic tongue for qualitative analysis of beverages.

An electronic tongue based on the sensor array of polymeric membrane ion-selective electrodes combined with pattern recognition tools was applied to qualitative analysis of various brands of orange juice, tonic, and milk. The capability of this device to reliably discriminate between different brands of those products was presented. The tests of the system were performed using products of the same brand, but with different manufacture dates (and thus comparable by the term of taste). The fusion of two types of sensors-classical selective ones and partially selective in one versatile array, and working out the sensor array's response by means of principal component analysis and back propagation neural network methods allowed the discrimination between different brands of various beverages with very high accuracy (90-100%). The real performance of the electronic tongue was evaluated applying testing samples from another manufacture lot, than the samples used in the learning set.

Improving the lower detection limit of potentiometric sensors by covalently binding the ionophore to a polymer backbone

The lower detection limit of polymeric membrane ion-selective electrodes (ISEs) is impaired by zero-current ion fluxes through the organic phase. This adverse effect is largely eliminated by covalently attaching the ionophore to a polymer backbone. To this purpose, the Pb 2+-selective ligand, 4- tert-butylcalix[4] arene-tetrakis( N, N′-dimethylthioacetamide) is substituted on its upper rim by a diol derivative which is subsequently copolymerized with poly(tetrahydrofuran)diol and 2,2,4-trimethylhexamethylene diisocyanate to the corresponding polyurethane. By measurements on sandwich membranes, it is shown that through binding the ionophore to the polymer, the mobility of Pb 2+ in the ISE membrane is strongly reduced. As a consequence, the response range of such an ISE is extended by several orders of magnitude. This is the case even when using an internal electrolyte that with an ISE based on a mobile ionophore leads to strong deviations from the linear response because of ion uptake from the sample into the membrane or ion release from the membrane into the sample. With a conventional inner filling solution of 10 −1 M Pb(NO 3) 2, a lower detection limit of 1.7×10 −9 M Pb 2+ has been achieved in the presence of 10 −4 M Na +.

Potassium ion-selective membrane electrodes based on 1,3-alternate calix[4]crown-5-azacrown-5

A series of 1,3-alternate calix[4]azacrown ethers for which the monoaza and unsymmetrical crowned-azacrown with different side arms were examined as an ionophore for ion-selective polymeric membrane electrode toward potassium ion. Among them, the electrode based on calix[4]crown-5-azacrown-5 with a phenyl group exhibited near Nernstian response for K + ions over a wide concentration range (1×10 −5 to 1×10 −1 M) with a limit of detection of 2×10 −6 M. It has a fast response time of approximately 2–3 s and can be used for at least 2 months without observing any major deterioration. Selectivity coefficients indicated that interference from all common alkali, alkaline and transition metal ions was very small.

Lowering the detection limit of calcium selective ISFETs with polymeric membranes

Detection limits of Ca sensitive ISFETs with photocured polyurethane membranes with a neutral carrier and an ion-exchanger as ionophores are studied. It is shown that conditioning of ISFETs in solutions containing only interfering ions results in reduction of the detection limit. Zero-current ion fluxes approach elaborated for polymeric membrane ion-selective electrodes (ISEs) is used to explain the obtained results.

Response and Diffusion Behavior of Mobile and Covalently Immobilized H+-Ionophores in Polymeric Membrane Ion-Selective Electrodes

Response and Diffusion Behavior of Mobile and Covalently Immobilized H+-Ionophores in Polymeric Membrane Ion-Selective Electrodes

Plasticizer-Free Polymer Membrane Ion-Selective Electrodes Containing a Methacrylic Copolymer Matrix

A methyl methacrylate and decyl methacrylate (MMA-DMA) copolymer has been used to fabricate plasticizer-free ion-selective membranes. The copolymer matrix showed good mechanical properties, and can be solvent cast in complete analogy to traditional ISE membranes. The material was evaluated in ionophore-free cation and anion exchanger membranes as well as in ion-selective electrode membranes containing five different neutral ionophores. The selectivity of Na (X) based copolymer membranes was found to be superior when compared to corresponding membranes made from poly(vinyl chloride) plasticized with bis(2-ethylhexyl sebacate) (PVC-DOS) and other plasticizer free membranes. Other ionophore-based MMA-DMA membranes, with the exception of magnesium-selective systems, also showed excellent selectivity similar to the corresponding PVC-DOS membranes. This makes MMA-DMA a potentially superior choice over alternative membrane matrices reported in the literature and a promising platform for the establishment of covalently immobilized membrane components.

Novel sulfate ion-selective polymeric membrane electrode based on a derivative of pyrilium perchlorate

A sulfate ion-selective PVC membrane sensor based on 4-(4-bromophenyl)-2,6-diphenylpyrilium perchlorate (BDPP) as a novel sensing material is successfully developed. The electrode shows a good selectivity for sulfate ion with respect to common organic and inorganic anions. The sensor exhibits a good linear response with slope of −28.9±0.5 mV per decade over the concentration range of 1.0×10 −6–1.0×10 −2 M, and a detection limit of 8.0×10 −7 M of SO 4 2− ions. The electrode response is independent of pH in the range of 4.0–9.0. The proposed sensor was applied as an indicator electrode in potentiometric titration of sulfate and barium ions, and to the determination of zinc in zinc sulfate tablets.