Membrane-based Ion-selective Electrodes Research Articles

Development of Machine Learning Models for Ion-Selective Electrode Cation Sensor Design.

Polyvinyl chloride (PVC) membrane-based ion-selective electrode (ISE) sensors are common tools for water assessments, but their development relies on time-consuming and costly experimental investigations. To address this challenge, this study combines machine learning (ML), Morgan fingerprint, and Bayesian optimization technologies with experimental results to develop high-performance PVC-based ISE cation sensors. By using 1745 data sets collected from 20 years of literature, appropriate ML models are trained to enable accurate prediction and a deep understanding of the relationship between ISE components and sensor performance (R 2 = 0.75). Rapid ionophore screening is achieved using the Morgan fingerprint based on atomic groups derived from ML model interpretation. Bayesian optimization is then applied to identify optimal combinations of ISE materials with the potential to deliver desirable ISE sensor performance. Na+, Mg2+, and Al3+ sensors fabricated from Bayesian optimization results exhibit excellent Nernst slopes with less than 8.2% deviation from the ideal value and superb detection limits at 10-7 M level based on experimental validation results. This approach can potentially transform sensor development into a more time-efficient, cost-effective, and rational design process, guided by ML-based techniques.

Determination of forced convection effects on the response of membrane-based ion-selective electrodes via numerical solution to the Navier-Stokes-Nernst-Plank-Poisson equations

Ion selective electrodes (ISEs) enable measurements via the build-up of a phase boundary potential at the surface of a sensing membrane. While a framework exists to understand the performance of ISEs in stagnant samples, the influences of fluid flow on ISEs are less studied. We model the transport of ions in solution occurring near interfaces between ISE membranes and aqueous samples when subject to an external flow. We developed a numerical model extending the Pressure-Implicit with Splitting of Operators (PISO) algorithm to incorporate the Navier-Stokes-Nernst-Plank-Poisson system of equations. We find that external flow distorts the aqueous side of the formed double layer at the ISE membrane and aqueous sample interface, leading to an increase in the phase boundary potential. The change in potential is shown to be a function of a novel set of dimensionless numbers, most notably the Debye Length Reynolds number, i.e., the Reynolds number with the Debye Length as the system dimension.

Membrane-Based Electrochemical Detection of Uranium: A Review

The determination of uranium in environmental samples has always been a crucial environmental issue due to its adverse impacts on human life. Electrochemical detection is one of the most suitable methods for directly determining uranium because of its portable instrument and quick response characteristics. The ion-selective membrane in the working electrodes is selectively responsible for uranium transport and separation. This mini-review provides a general overview of the membrane-based ion-selective electrodes in detecting uranium ions reported in the literature. The ion-selective membranes are classified according to their ionophore categories. Furthermore, the limits and outlook are also discussed to provide a reference for further developing membrane-based electrochemical uranium sensors.

Potentiometric pH Nanosensor for Intracellular Measurements: Real-Time and Continuous Assessment of Local Gradients.

We present a pH nanosensor conceived for single intracellular measurements. The sensing architecture consisted of a two-electrode system evaluated in the potentiometric mode. We used solid-contact carbon nanopipette electrodes tailored to produce both the indicator (pH nanosensor) and reference electrodes. The indicator electrode was a membrane-based ion-selective electrode containing a receptor for hydrogen ions that provided a favorable selectivity for intracellular measurements. The analytical features of the pH nanosensor revealed a Nernstian response (slope of −59.5 mV/pH unit) with appropriate repeatability and reproducibility (variation coefficients of <2% for the calibration parameters), a fast response time (<5 s), adequate medium-term drift (0.7 mV h–1), and a linear range of response including physiological and abnormal cell pH levels (6.0–8.5). In addition, the position and configuration of the reference electrode were investigated in cell-based experiments to provide unbiased pH measurements, in which both the indicator and reference electrodes were located inside the same cell, each of them inside two neighboring cells, or the indicator electrode inside the cell and the reference electrode outside of (but nearby) the studied cell. Finally, the pH nanosensor was applied to two cases: (i) the tracing of the pH gradient from extra-to intracellular media over insertion into a single PC12 cell and (ii) the monitoring of variations in intracellular pH in response to exogenous administration of pharmaceuticals. It is anticipated that the developed pH nanosensor, which is a label-free analytical tool, has high potential to aid in the investigation of pathological states that manifest in cell pH misregulation, with no restriction in the type of targeted cells.

Anion-Selective Electrodes Based On a CH-Hydrogen Bonding Bis-macrocyclic Ionophore with a Clamshell Architecture.

CH-hydrogen bonding provides access to new building blocks for making macrocyclic ionophores with high degrees of preorganization and selective anion recognition. In this study, an anion-binding ionophore in the shape of a clamshell (ClS) was employed that is composed of two cyanostar (CNstar) macrocycles with preorganized cavities linked with a 12-carbon chain. This ionophore allows for anion complexation by CH-hydrogen bonding. The potentiometric performance of membrane-based ion-selective electrodes incorporating this ionophore was evaluated. Different membrane compositions were prepared to determine the optimum concentrations of the ionophore and lipophilic additive in the membrane. The optimized electrode had a slope of -58.2 mV/decade and demonstrated an anti-Hofmeister selectivity pattern toward iodide with a nanomolar detection limit. Electrospray ionization mass spectrometry was employed to study the relative association strengths of ClS with various anions. The observed mass peaks of the ion-ionophore complexes were found to be consistent with the potentiometric selectivity pattern of the corresponding electrodes. Overall, the selectivity of the electrode could be altered by using an ionophore in which the two CNstar macrocycles are linked together with a flexible 12-carbon chain to control the molecularity of the binding event.

Development of membrane electrodes for selective determination of lisinopril in pharmaceuticals

BackgroundLisinopril (LNP) is an angiotensin-converting enzyme inhibitor used as anti-hypertensive, cardiovascular, in anti-prophylactic and anti-diabetic nephropathy drug. Development of two new, simple, low cost, and selective membrane-based ion-selective electrodes has been proposed for the determination of LNP in pharmaceuticals.MethodsThe electrodes are based on poly(vinyl)chloride membrane doped with LNP-phosphotungstic acid (LNP-PTA) and LNP-phosphomolybdic acid (LNP-PMA) ion-pairs as molecular recognition materials.ResultsThe developed LNP-PTA and LNP-PMA electrodes are applicable for the determination of LNP over the linear range of 5 × 10−5–2.4 × 10−3 mol l−1. The working pH ranges to measure potentials were 2.5 to 6.4 and 2.3 to 6.0 for LNP-PTA and LNP-PMA ISEs, respectively. The electrodes displayed the rapid Nernstian responses as revealed by the values of slopes 55.06 and 52.39 mV/decade, with limit of detection (LOD) values of 1.2 × 10−5 and 1.18 × 10−5 mol l−1 for LNP-PTA and LNP-PMA electrodes, respectively. The limits of quantitation (LOQ) values have also been calculated for both the electrodes. The developed electrodes have potential stability for up to 1 month and emerged as highly selective for the determination of LNP over other spiked ions and compounds.ConclusionsThe proposed electrodes have been validated and found that they are suitable for the determination of LNP in pharmaceuticals in pure form and in dosage forms. The results obtained in the analysis of LNP using proposed electrodes have been compared statistically with reference method’s results to assess the accuracy and precision. Robustness and ruggedness of the developed electrodes have also been checked and found satisfactory. The recovery studies have been performed by standard addition procedure to assess the role of excipients in tablets containing LNP and the results obtained are satisfactory.

Development of New Cs+ Ion‐Selective Electrode with Alkyl‐Bridged Calix[4]arene Crown‐6 Compounds for the Determination of Cs+ in Nuclear Waste Streams

AbstractA new liquid membrane based Ion Selective Electrodes (ISE) for Cs+ employing three 1,3 cycloalkyloxy bridged calix[4]arene crown 6 ether compounds (L1: cyclooctyloxy, L2: cyclodecyloxy, L3: cyclotetradecyloxy) as ionophores were developed. The best response was observed when L3 was used as an ionophore. The cation exchange resin DOWEX‐50 W was used to maintain low activity of Cs+ in inner filling solution (IFS) to improve the performance. With optimized IFS, Nernstian response (56.3 mV/decade of Cs+) over the concentration range 10−7 to 10−2 M of Cs+ was observed. Studies were also carried out to develop all solid contact Cs+ ISE employing gold‐ polyaniline composites as a transducer. Superior response for Cs+ was observed with IFS based ISE. The developed ISE showed detection limit of 3.7x10−8 M Cs+ with response time less than 20 seconds. Separate solution method was applied to determine selectivity for Cs+ over alkali, alkaline earth and transition metal ions. The response of ISE for Cs+ was fairly constant over the pH range 4–11. The concentration of Cs+ was determined in two simulated high level active waste solutions and spiked tap water samples and results agreed well with atomic absorption spectroscopy (AAS) values and expected values, respectively.

Manual and Flow-Injection Detection/Quantification of Polyquaterniums via Fully Reversible Polyion-Sensitive Polymeric Membrane-Based Ion-Selective Electrodes.

The detection of four different polyquaterniums (PQs) using a fully reversible potentiometric polyion sensor in three different detection modes is described. The polyion sensing "pulstrodes" serve as the detector for direct dose-response experiments, beaker titrations, and in a flow-injection analysis (FIA) system. Direct polycation response toward PQ-2, PQ-6, PQ-10, and poly(2-methacryloxyethyltrimethylammonium) chloride (PMETAC) yields characteristic information about each PQ species (e.g., relative charge densities, etc.) via syringe pump addition of each PQ species to a background electrolyte solution. Quantitative titrations are performed using a syringe pump to deliver heparin as the polyanion titrant to quantify all four PQs at μg/mL levels. Both the direct and indirect methods incorporate the use of a three-electrode system including counter, double junction reference, and working electrodes. The working electrode possesses a plasticized poly(vinyl chloride) (PVC) membrane containing the neutral lipophilic salt of dinonylnaphthalenesulfonate (DNNS-) tridodecylmethylammonium (TDMA+). Further, the titration method is shown to be useful to quantify PQ-6 levels in recreational swimming pool water collected in Ann Arbor, MI. Finally, a FIA system equipped with a pulstrode detector is used to demonstrate the ability to potentially quantify PQ levels via a more streamlined and semiautomated testing platform.

PVC membrane-based portable ion analyzer for hydroponic and water monitoring

Rapid on-site measurement of chemical ions in hydroponic and water solutions would allow efficient management of nutrients used for plant production as well as early monitoring of chemical pollutants. This paper reports the development of an embedded portable analyzer incorporated with a sensor array of polyvinyl chloride (PVC) membrane-based ion-selective electrodes (ISEs) to directly measure the concentrations of NO3, K, and H ions in hydroponic solutions and water. The developed ion analyzer consisted of an AVR microcontroller, five channels of sensor inputs, a 16-bit analog-to-digital converter, a 7-inch LCD touch panel, and SD memory card-based data storage. The use of a two-point normalization method consisting of sensitivity adjustment followed by offset compensation was effective in minimizing signal drifts resulting from a series of sample measurements while reducing variability in response among multiple ISEs during replicate measurements in practical manners. The sensitivity and predictive capability of the PVC membrane-based H ISEs, in conjunction with a three-point calibration method, were satisfactory, showing sub-Nernstian slopes of 51.9–56.1mV/decade over a pH range of 4–10 (R2>0.96) and providing results in close agreement with the results of a conventional pH meter (a nearly 1:1 regression slope and a y-intercept near 0). A response time of 50s and a wide sensitivity range of 11–884mgL−1 measured with the prototype portable ion analyzer equipped with PVC membrane-based NO3 and K ISEs, each with a lifetime of 60days, enabled direct analysis of hydroponic and water samples without the need to dilute samples. Strong linear relationships between the prototype ion analyzer and the standard instrument methods (R2>0.97, slopes ranging from 0.89 to 1.24) were exhibited.

Circumventing Traditional Conditioning Protocols in Polymer Membrane-Based Ion-Selective Electrodes.

Preparation of ion-selective electrodes (ISEs) often requires long and complicated conditioning protocols limiting their application as tools for in-field measurements. Herein, we eliminated the need for electrode conditioning by loading the membrane cocktail directly with primary ion solution. This proof of concept experiment was performed with iodide, silver, and sodium selective electrodes. The proposed methodology significantly shortened the preparation time of ISEs, yielding functional electrodes with submicromolar detection limits. Moreover, it is anticipated that this approach may form the basis for the development of miniaturized all-solid-state ion-selective electrodes for in situ measurements.

An Automated Water Nitrate Monitoring System based on Ion-Selective Electrodes

th , 2016; Revised: May 20 th , 2016; Accepted: May 27 th , 2016 Purpose: In-situ water quality monitoring based on ion-selective electrodes (ISEs) is a promising technique because ISEs can be used directly in the medium to be tested, have a compact size, and are inexpensive. However, signal drift can be a major concern with on-line management systems because continuous immersion of the ISEs in water causes electrode degradation, affecting the stability, repeatability, and selectivity over time. In this study, a computer-based nitrate monitoring system including automatic electrode rinsing and calibration was developed to measure the nitrate concentration in water samples in real-time. Methods: The capabilities of two different types of poly(vinyl chloride) membrane-based ISEs, an electrode with a liquid filling and a carbon paste-based solid state electrode, were used in the monitoring system and evaluated on their sensitivities, selectivities, and durabilities. A feasibility test for the continuous detection of nitrate ions in water using the developed system was conducted using water samples obtained from various water sources. Results: Both prepared ISEs were capable of detecting low concentrations of nitrate in solution, i.e., 0.7 mg/L NO3-N. Furthermore, the electrodes have the same order of selectivity for nitrate: NO3 >> HCO3 > Cl > H2PO4 > SO4 2, and maintain their sensitivity by > 40 mV/decade over a period of 90 days. Conclusions: The use of an automated ISE-based nitrate measurement system that includes automatic electrode rinsing and two-point normalization proved to be feasible in measuring NO3-N in water samples obtained from different water sources. A one-to-one relationship between the levels of NO3-N measured with the ISEs and standard analytical instruments was obtained.

Fabrication of PVC Membrane Based Ion Selective Electrode by Using the Newly Synthesised Copper Schiff Base Complex

Fabrication of PVC Membrane Based Ion Selective Electrode by Using the Newly Synthesised Copper Schiff Base Complex

Capacitive mixing for the extraction of energy from salinity differences: Survey of experimental results and electrochemical models

The “capacitive mixing” (CAPMIX) technique is an emerging technology aimed at the extraction of energy from salinity differences, e.g. between sea and river waters. CAPMIX benefits from the voltage rise that takes place between two electrodes dipped in a saline solution when its salt concentration is changed. Several kinds of electrodes have been proposed so far: activated carbon materials (Brogioli, 2009), membrane-based ion-selective electrodes (Sales et al., 2010), and battery electrodes (Biesheuvel and van der Wal, 2010). The power production mainly depends on two properties of each single electrode: the amplitude of the potential rise upon salinity change, and the potential in the high-salinity solution. The various electrode materials that have been used returned different values of the two parameters, and hence to different power productions. In this paper, we apply electrokinetic and electrochemical models to qualitatively explain the experimentally observed behaviors of various materials under different experimental conditions. The analysis allows to devise techniques for tailoring new materials, particularly suited for the CAPMIX technique.

A New Device for Simultaneous Measurements of Dynamic Interfacial Tension and Electrical Potential Difference

Abstract A new device for simultaneous time-resolved measurements of electromotive force (EMF) and interfacial tension (IT) at liquid–liquid interfaces is described. Two examples of applications are presented: 1) measurement of the variations in IT during simulated EMF measurements in model membrane-based ion-selective electrodes, and 2) measurements of transient changes in EMF in a system displaying IT oscillations without any chemical reaction.

A novel potentiometric sensor for l-ascorbic acid based on molecularly imprinted polypyrrole

A polymeric membrane-based ion-selective electrode (ISE) for the determination of l-ascorbic acid ( l-AA) was developed. The sensor was fabricated by modifying glassy carbon electrodes with molecularly imprinted polypyrrole (PPy) synthesized by electropolymerization of the monomer in the presence of ascorbate. A comparison between the non-imprinted modified electrode (PPy-NIP) and the molecularly imprinted polymer (PPy-MIP) is reported. The molecularly imprinted polymer improves the performances of the sensor, especially if the PPy film has been submitted to an oxidation treatment. The PPy-MIPox electrode shows near-Nernstian response (approx. −57 mV/decade) to the ascorbate over the concentration range between 5 × 10 −6 M and 2 × 10 −3 M and it can be used for more than 3 weeks without any considerable change in the measured response. The potentiometric sensor has been successfully applied to the determination of ascorbate in food and pharmaceutical samples (recoveries of about 100%).

Potentiometric Response Characteristics of Membrane-BasedCs+-Selective Electrodes Containing Ionophore-Functionalized Polymeric Microspheres

Cs+-selective solvent polymeric membrane-based ion-selective electrodes (ISEs) were developed by doping ethylene glycol-functionalized cross-linked polystyrene microspheres (P-EG) into a plasticized poly(vinyl chloride) (PVC) matrix containing sodium tetrakis-(3,5-bis(trifluoromethyl)phenyl) borate (TFPB) as the ion exchanger. A systematic study examining the effects of the membrane plasticizers bis(2-ethylhexyl) sebacate (DOS), 2-nitrophenyl octyl ether (NPOE), and 2-fluorophenyl nitrophenyl ether (FPNPE) on the potentiometric response and selectivity of the corresponding electrodes was performed. Under certain conditions, P-EG-based ion-selective electrodes (ISEs) containing TFPB and plasticized with NPOE exhibited a super-Nernstian response between1×10−3and1×10−4 M Cs+, a response characteristic not observed in analogous membranes plasticized with either DOS or FPNPE. Additionally, the performance of P-EG-based ISEs was compared to electrodes based on two mobile ionophores, a neutral lipophilic ethylene glycol derivative (ethylene glycol monooctadecyl ether (U-EG)) and a charged metallacarborane ionophore, sodium bis(dicarbollyl)cobaltate(III) (CC). In general, P-EG-based electrodes plasticized with FPNPE yielded the best performance, with a linear range from 10-1–10-5 M Cs+, a conventional lower detection limit of8.1×10−6 M Cs+, and a response slope of 57.7 mV/decade. The pH response of P-EG ISEs containing TFPB was evaluated for membranes plasticized with either NPOE or FPNPE. In both cases, the electrodes remained stable throughout the pH range 3–12, with only slight proton interference observed below pH 3.

Wide Scope of Polymeric Membrane Based Ion Selective Electrodes

Wide Scope of Polymeric Membrane Based Ion Selective Electrodes

On the origin of the Hofmeister effect in anion-selective potentiometric electrodes with tetraalkylammonium salts

Abstract A new, adsorption-based mechanism of potentiometric response of the plasticized PVC membranes is proposed. In contrast to previous mechanistic studies of the membrane-based Ion-Selective Electrodes (ISE), the membranes in this study did not contain any purposely-added additives: lipophilic salt or ionophore. Cationic or anionic response of such “blank” ISEs was observed upon contact with ionic surfactants: cationic cetyltrimethylammonium bromide (CTAB) and anionic sodium dodecyl sulfate (SDS), respectively. Such behavior cannot be explained using a classical Interface between Two Immiscible Electrolyte Solutions (ITIES) formalism, based on the Nernst equation for ion partitioning between the aqueous and the membrane phases. Moreover, the “blank” membranes responded anionically (with a negative slope) to the presence of sodium salts of bromide, nitrate and fluoride at constant concentration of CTAB. Also in this case, the theoretical Galvani potential difference vs. added salt concentration, calculated with the ITIES formalism did not fit well the measured potential differences. Better agreement was obtained when assuming that the potentiometric response stems from partitioning between the aqueous phase and the interface (adsorption), and not between the aqueous phase and the membrane phase (bulk partition), as in the classical approach. The Stern layer potentials were calculated from interfacial tension isotherms for CTAB, both alone and in the presence of sodium bromide, nitrate and fluoride using the Surface quasi-Two Dimensional Electrolyte model (STDE). The calculated Stern potentials compare well with those obtained experimentally using ISE.

Preparation, Characterization, and Analytical Application of Ramipril Membrane‐Based Ion‐Selective Electrode

The fabrication and electrochemical evaluation of two PVC membrane-based Ion-Selective electrodes responsive for ramipril drug have been proposed. The sensitive membranes were prepared using ramipril-phosphomolibdate and ramipril-tetraphenylborate ion-pair complexes as electroactive sensing materials in plasticized PVC support. The electrodes based on these materials provide near-Nernestian response (sensitivity of 53 ± 0.5–54 ± 0.5 mV/concentration decade) covering the concentration range of 1.0 × 10−2–1.0 × 10−5 mol L−1 with a detection limit of 3.0 × 10−6–4.0 × 10−6 mol L−1. The suggested electrodes have been successfully used in the determination of ramipril drug in some pharmaceutical formulations using direct potentiometry with average recovery of >96% and mean standard deviation of <3% (n = 5).

Bilayer lipid membrane (BLM) based ion selective electrodes at the meso-, micro-, and nano-scales

This paper presents a novel method for making micron-sized apertures with tapered sidewalls and nano-sized apertures. Their use in bilayer lipid membrane-based ion selective electrode design is demonstrated and compared to mesoscale bilayers and traditional PVC ion selective electrodes. Micron-sized apertures are fabricated in SU-8 photoresist films and vary in diameter from 10 to 40μm. The tapered edges in SU-8 films are desired to enhance bilayer lipid membrane (BLM) formation and are fabricated by UV-light overexposure. Nano-apertures are made in boron diffused silicon film. The membranes are used as septa to separate two potassium chloride solutions of different concentrations. Lecithin BLMs are assembled on the apertures by ejecting lipid solution. Potassium ionophore, dibenzo-18-crown-6, is incorporated into BLMs by dissolving it in the lipid solution before membrane assembly. Voltage changes with increasing potassium ion concentrations are recorded with an A/D converter. Various ionophore concentrations in BLMs are investigated. At least a 1% concentration is needed for consistent slopes. Electrode response curves are linear over the 10−6 to 0.1M range with a sub-Nernstian slope of 20mV per Log concentration change. This system shows high selectivity to potassium ions over potential interfering sodium ions. BLMs on the three different aperture sizes at the meso-, micro-, and nano-scales all show similar linear ranges and limits of detection (LODs) as PVC ion selective membranes.