Solid-Contact Electrode with Composite PVC-Based 3D-Printed Membrane. Optimization of Fabrication and Performance.

Intense interest in reference electrode design and fabrication has recently been enriched with the application of 3D printing of electrodes with salt-loaded PVC membranes. This type of material is attractive in sensor technology and is challenging to implement in 3D. In this report, several improvements and simplifications in the technology were focused on and supported by a fundamental electrochemical characterization.

A Breakthrough Application of a Cross-Linked Polystyrene Anion-Exchange Membrane for a Hydrogencarbonate Ion-Selective Electrode.

Polystyrene cross-linked with divinylbenzene and functionalized by a quaternary ammonium cation anion site is used as the membrane of a hydrogencarbonate (i.e., bicarbonate) ion-selective electrode. The polystyrene matrix membrane improves the selectivity towards interfering lipophilic ions in comparison to previously described polyvinyl chloride membranes. The reason for this behaviour is sought in coupled ion-exchange and pore-diffusion processes in the membrane and the resulting kinetic discrimination of interfering ions. The electrode is successfully used for determination of bicarbonates in mineral drinking waters. The simplex method is employed to refine the analytical outcome.

New ISE-Based Apparatus for Na+, K+, Cl-, pH and Transepithelial Potential Difference Real-Time Simultaneous Measurements of Ion Transport across Epithelial Cells Monolayer⁻Advantages and Pitfalls.

Cystic Fibrosis (CF) is the most common fatal human genetic disease, which is caused by a defect in an anion channel protein (CFTR) that affects ion and water transport across the epithelium. We devised an apparatus to enable the measurement of concentration changes of sodium, potassium, chloride, pH, and transepithelial potential difference by means of ion-selective electrodes that were placed on both sides of a 16HBE14σ human bronchial epithelial cell line that was grown on a porous support. Using flat miniaturized ISE electrodes allows for reducing the medium volume adjacent to cells to approximately 20 µL and detecting changes in ion concentrations that are caused by transport through the cell layer. In contrast to classic electrochemical measurements, in our experiments neither the calibration of electrodes nor the interpretation of results is simple. The calibration solutions might affect cell physiology, the medium composition might change the direction of actions of the membrane channels and transporters, and water flow that might trigger or cut off the transport pathways accompanies the transport of ions. We found that there is an electroneutral transport of sodium chloride in both directions of the cell monolayer in the isosmotic transepithelial concentration gradient of sodium or chloride ions. The ions and water are transported as an isosmotic solution of 145 mM of NaCl.

Sensitivity and Selectivity of Ion-Selective Electrodes Interpreted Using the Nernst-Planck-Poisson Model.

The Nernst-Planck-Poisson model is used for modeling the sensitivity and selectivity of ion-selective electrodes (ISEs) with plastic membranes. Two pivotal parameters characterizing ISE response are in focus: sensitivity and selectivity. An interpretation of sensitivity, which considers the concurrent influence of anions and cations on the ISE slope, is presented. The interpretation of selectivity shows the validity and limits of approaches hitherto taken to measure the true (unbiased) selectivity coefficient. The validity of more idealized interpretations by the diffusion-layer model is conceived.

All-Solid-State Reference Electrode with Heterogeneous Membrane.

Novel reference electrodes with a solid contact coated by a heterogeneous polymer membrane are described. The electrodes are obtained using Ag nanoparticles, AgBr, KBr suspended in tetrahydrofuran solution of PVC and DOS and deposited on Ag substrate, or another substrate covered with Ag, by drop casting. After a short period of soaking in a KBr solution, stable and reproducible formal potentials of -157 ± 2 mV (vs Ag/AgCl/3 M KCl) were observed, and the solid-contact reference electrodes were ready to use. It is shown that the described reference electrodes are relatively insensitive to the changes in the sample matrix, the concentrations of ions, the pH and the redox potential. These electrodes can also be fabricated in miniaturized form, and thus used to produce miniaturized multielectrode probes.

In Situ Potentiometry and Ellipsometry: A Promising Tool to Study Biofouling of Potentiometric Sensors.

In situ potentiometry and null ellipsometry was combined and used as a tool to follow the kinetics of biofouling of ion-selective electrodes (ISEs). The study was performed using custom-made solid-contact K(+)-ISEs consisting of a gold surface with immobilized 6-(ferrocenyl)hexanethiol as ion-to-electron transducer that was coated with a potassium-selective plasticized polymer membrane. The electrode potential and the ellipsometric signal (corresponding to the amount of adsorbed protein) were recorded simultaneously during adsorption of bovine serum albumin (BSA) at the surface of the K(+)-ISEs. This in situ method may become useful in developing sensors with minimized biofouling.

Neutral-carrier ion-selective electrodes assessed by the Nernst-Planck-Poisson model.

Ion-selective electrodes (ISEs) containing neutral ionophores are used in clinical, industrial, and environmental analysis. The wide range of applications requires deep theoretical description. This work concentrates on the development of the general approach to the description of electro-diffusion processes, namely, Nernst-Planck-Poisson (NPP) model to allow the description of the time-dependent responses in the case of complexation reactions occurring in the ion-selective membranes. The impact of the chemical reaction on the calibration curves and apparent selectivity of ISE is discussed. Results obtained using NPP model with time-dependent reaction are compared with those obtained with the Phase Boundary Model (PBM), as well as with the previous solutions of NPP model, using the infinite reaction rates and constant ligand concentration assumption. The validity of these assumptions is investigated and the limitations of PBM in the description of neutral-carrier ISE are discussed.

Multielectrode bisensor system for time-resolved monitoring of ion transport across an epithelial cell layer.

An ion-selective multielectrode bisensor system is designed to ensure reliable real-time concentration measurements of sodium, potassium, chloride, and pH in a small volume of biological liquid bathing a living human bronchial epithelial cell monolayer. The bisensor system allows the monitoring of major ions, which are simultaneously transported through the epithelia in both directions.

Solid-contact ion-selective electrodes with highly selective thioamide derivatives of p-tert-butylcalix[4]arene for the determination of lead(II) in environmental samples.

Thioamide derivatives of p-tert-butylcalix[4]arene were used as ionophores in the development of solid-contact ion-selective electrodes based on conducting polymer poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate (PEDOT/PSS) which was synthesized by electrodeposition on the glassy carbon electrodes. The typical ion-selective membranes with optionally two different plasticizers [bis(2-ethylhexyl)sebacate (DOS) and 2-nitrophenyl octyl ether (NPOE)] were investigated. The potentiometric selectivity coefficients were determined by separate solution method (SSM) for Pb(2+) over Cu(2+), Cd(2+), Ca(2+), Na(+), and K(+). High selectivity toward Pb(2+) was obtained. By applying two conditioning protocols, a low detection limit log(a(DL)) ≈ -9 was achieved. The fabricated ion-selective electrodes were used to determine Pb(2+) concentration in environmental samples. The obtained results were compared to analysis done by inductively coupled plasma mass spectrometry (ICPMS).

An analytical quality solid-state composite reference electrode.

A new type of all-solid-state reference electrode was designed and characterized. The electrodes are based on a polymer/inorganic salt composite and a silver/silver chloride reference element. A rigorous testing procedure was used to reveal the possible influence of pH, solution composition, as well as the concentration and mobility of ions. The tests demonstrated the insensitivity of the electrodes to the matrix effects, excellent stability of the potential readings, and significantly reduced leakage of inorganic salt. In the performed tests the composite reference electrodes were on a par with or better than the high-quality commercial reference electrodes. The reference electrodes described here are of analytical quality allowing for continuous, prolonged, and intensive usage.

The Influence of Chemical Activity Models on the Description of Ion Transport through Micro-Structured Cementitious Materials.

The significance of ion activity in transport through a porous concrete material sample with steel rebar in its center and bathing solution is presented. For the first time, different conventions and models of ion activity are compared in their significance and influence on the ion fluxes. The study closes an interpretational gap between ion activity in a stand-alone (stagnant) electrolyte solution and ion transport (dynamic) through concrete pores. Ionic activity models developed in stationary systems, namely, the Debye-Hückel (DH), extended DH, Davies, Truesdell-Jones, and Pitzer models, were used for modeling the transport of ions driven through the activity gradient. The activities of ions are incorporated into a frame of the Nernst-Planck-Poisson (NPP) equations. Calculations were done with COMSOL software for a real concrete microstructure determined by X-ray computed tomography. The concentration profiles of four ions (Na+, Cl-, K+, OH-), the ionic strength, and the electric potential in mortar (with pores) and concrete samples (with aggregates and pores) are presented and compared. The Pitzer equation gave the most reliable results for all systems studied. The difference between the concentration profiles calculated with this equation and with the assumption of the ideality of the solution is negligible while the potential profiles are clearly distinguishable.

Electrically Enhanced Sensitivity (EES) of Ion-Selective Membrane Electrodes and Membrane-Based Ion Sensors.

The use of external electronic enforcement in ion-sensor measurements is described. The objective is to improve the open-circuit (potentiometric) sensitivity of ion sensors. The sensitivity determines the precision of analyte determination and has been of interest since the beginning of ion-sensor technology. Owing to the theoretical interpretation founded by W.E. Nernst, the sensitivity is characterized by the slope and numerically predicted. It is empirically determined and validated during calibration by measuring an electromotive force between the ion sensor and the reference electrode. In practice, this measurement is made with commercial potentiometers that function as unaltered "black boxes". This report demonstrates that by gaining access to a meter's electrical systems and allowing for versatile signal summations, the empirical slope can be increased favorably. To prove the validity of the approach presented, flow-through ion-sensor blocks used in routine measurements of blood electrolytes (Na+, K+, Li+, Cl-) and multielectrode probes with flat surfaces, similar to those applied previously for monitoring transmembrane fluxes of Na+, K+, Cl- through living biological cells, are used. Several options to serve real-life electroanalytical challenges, including linear calibration for sensors with high-resistance membranes, responses with non-Nernstian slopes, non-linear calibration, and discrimination of nonfunctional sensors, are shown.

Long-Time Evaluation of Solid-State Composite Reference Electrodes.

In this study, the performance and long-time evaluation of solid-state composite (SSC) reference electrodes were investigated. The stability of all the SSC reference electrodes was continuously monitored by using potentiometry and electrochemical impedance spectroscopy methods over a period of several months. A multi-solution protocol was used to study the influence of the ionic strength of the sample solution, ion charge, and mobility, and the sample pH values on the performance of the reference electrodes. The SSC reference electrodes were used in the calibration of commercial indicator electrodes for different ions at different temperatures. The concentrations of K+, Na+, Ca2+, and Cl- ions and pH values were measured in river water samples at different temperatures using the SSC reference electrodes. The obtained results for the same samples were compared with the results given by an independent laboratory specialized in routine water analyses. The agreement between the results was very good and even better than the case where commercial reference electrodes were used. Our study showed that the SSC reference electrodes exhibit good long-term stability and excellent performance, both in the calibrations and analyses of environmental samples.

Special Issue "Advances in Artificial and Biological Membranes: Mechanisms of Ionic Sensitivity, Ion-Sensor Designs, and Applications for Ion Measurement".

Ion sensors, conventionally known as ion-selective membrane electrodes, were devised 100 years ago with the invention of a pH electrode with a glass membrane (in 1906 Cremer, in 1909 Haber and Klemensiewicz) [...].

Measurement of Multi Ion Transport through Human Bronchial Epithelial Cell Line Provides an Insight into the Mechanism of Defective Water Transport in Cystic Fibrosis.

We measured concentration changes of sodium, potassium, chloride ions, pH and the transepithelial potential difference by means of ion-selective electrodes, which were placed on both sides of a human bronchial epithelial 16HBE14σ cell line grown on a porous support in the presence of ion channel blockers. We found that, in the isosmotic transepithelial concentration gradient of either sodium or chloride ions, there is an electroneutral transport of the isosmotic solution of sodium chloride in both directions across the cell monolayer. The transepithelial potential difference is below 3 mV. Potassium and pH change plays a minor role in ion transport. Based on our measurements, we hypothesize that in a healthy bronchial epithelium, there is a dynamic balance between water absorption and secretion. Water absorption is caused by the action of two exchangers, Na/H and Cl/HCO3, secreting weakly dissociated carbonic acid in exchange for well dissociated NaCl and water. The water secretion phase is triggered by an apical low volume-dependent factor opening the Cystic Fibrosis Transmembrane Regulator CFTR channel and secreting anions that are accompanied by paracellular sodium and water transport.

Precipitation of Inorganic Salts in Mitochondrial Matrix.

In the mitochondrial matrix there are insoluble, osmotically inactive complexes that maintain constant pH and calcium concentration. In the present paper we examine the properties of insoluble calcium and magnesium salts, namely phosphates, carbonates and polyphosphates which might play this role. We find that non-stoichiometric, magnesium-rich carbonated apatite, with very low crystallinity, precipitates in the matrix under physiological conditions. Precipitated salt acts as pH buffer, and hence can contribute in maintaining ATP production in ischemic conditions, delaying irreversible damages to heart and brain cells after stroke.

Effective and Apparent Diffusion Coefficients of Chloride Ions and Chloride Binding Kinetics Parameters in Mortars: Non-Stationary Diffusion-Reaction Model and the Inverse Problem.

A non-equilibrium diffusion-reaction model is proposed to describe chloride transport and binding in cementitious materials. A numerical solution for this non-linear transport with reaction problem is obtained using the finite element method. The effective chloride diffusion coefficients and parameters of the chloride binding are determined using the inverse method based on a diffusion-reaction model and experimentally measured chloride concentrations. The investigations are performed for two significantly different cements: ordinary Portland and blast furnace cements. The results are compared with the classical diffusion model and appropriate apparent diffusion coefficients. The role of chloride binding, with respect to the different binding isotherms applied, in the overall transport of chlorides is discussed, along with the applicability of the two models. The proposed work allows the determination of important parameters that influence the longevity of concrete structures. The developed methodology can be extended to include more ions, electrostatic interactions, and activity coefficients for even more accurate estimation of the longevity.

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.

Biomimetic study of the Ca(2+)-Mg2+ and K(+)-Li+ antagonism on biologically active sites: new methodology to study potential dependent ion exchange.

Competitive divalent (magnesium and calcium) or monovalent (potassium, lithium and sodium) ion exchange and its influence on a membrane potential formation was studied at biological ligands (BL) such as adenosine triphosphate (ATP), asparagine (Asn) and glutamine (Gln) sites. The sites are dispersed electrochemically in membranes made of the conducting polymers (CPs)--poly(N-methylpyrrole) (PMPy) and poly(pyrrole) (PPy). The membranes are made sensitive to calcium and magnesium or to potassium, sodium and lithium by optimized electrodeposition and soaking procedures supported by the study of membrane topography and morphology. Distinctively different electrochemical responses, i.e. electrical potential transients or currents, are observed in the case of "antagonistic" calcium and magnesium or potassium and sodium/lithium ion pairs. Dissimilarity in the responses is ascribed to a difference between on site vs. bulk concentrations of ions, and is dictated by different transport properties of the ions, as shown by using the Nernst-Planck-Poisson (NPP) model and the diffusion-layer model (DLM). The method described allows inspecting potential-dependent competitive ion-exchange processes at the biologically active sites. It is suggested that this approach could be used as an auxiliary tool in study of potential dependent block in realistic membrane channels, such as Mg block in the N-methyl D-aspartate receptor channel (NMDA).

Reference Electrodes with Polymer-Based Membranes-Comprehensive Performance Characteristics.

Several types of liquid membrane and solid-state reference electrodes based on different plastics were fabricated. In the membranes studied, equitransferent organic (QB) and inorganic salts (KCl) are dispersed in polyvinyl chloride (PVC), polyurethane (PU), urea-formaldehyde resin (UF), polyvinyl acetate (PVA), as well as remelted KCl in order to show the matrix impact on the reference membranes' behavior. The comparison of potentiometic performance was made using specially designed standardized testing protocols. A problem in the reference electrode research and literature has been a lack of standardized testing, which leads to difficulties in comparing different types, qualities, and properties of reference electrodes. Herein, several protocols were developed to test the electrodes' performance with respect to stability over time, pH sensitivity, ionic strength, and various ionic species. All of the prepared reference electrodes performed well in at least some respect and would be suitable for certain applications as described in the text. Most of the reference types, however, demonstrated some weakness that had not been previously highlighted in the literature, due in large part to the lack of exhaustive and/or consistent testing protocols.