Alexey Shvarev

Selective coulometric release of ions from ion selective polymeric membranes for calibration-free titrations

Coulometry belongs to one of the few known calibration-free techniques and is therefore highly attractive for chemical analysis. Titrations performed by the coulometric generation of reactants is a well-known approach in electrochemistry, but suffers from limited selectivity and is therefore not generally suited for samples of varying or unknown composition. Here, the selective coulometric release of ionic reagents from ion-selective polymeric membrane materials ordinarily used for the fabrication of ion-selective electrodes is described. The selectivity of such membranes can be tuned to a significant extent by the type and concentration of ionophore and lipophilic ion-exchanger and is today well understood. An anodic current of fixed magnitude and duration may be imposed across such a membrane to release a defined quantity of ions with high selectivity and precision. Since the applied current relates to a defined ion flux, a variety of non-redox active ions may be accurately released with this technique. In this work, the released titrants activity was measured with a second ionophore-based ion-selective electrode and corresponded well with expected dosage levels on the basis of Faradays law of electrolysis. Initial examples of coulometric titrations explored here include the release of calcium ions for complexometric titrations, including back titrations, and the release of barium ions to determine sulfate.

Fabrication of micrometer and submicrometer-sized ion-selective optodes via a solvent displacement process.

Microsphere-based ion optodes represent a promising and versatile tool to measure ionic activities in confined samples. The reported methods of micro- and nanosphere optode fabrication, however, suffer from various degrees of complexity. We propose a large-scale fabrication of polymeric ion-selective optodes using a solvent displacement method. Plasticized poly(vinyl chloride) along with the optode components was dissolved in a solvent miscible with water. Injection of a polymer solution into a stirred aqueous phase containing a surfactant causes spontaneous emulsification. This technique does not require additional preparation steps and allows one to control the composition of the sensor matrix precisely. Several factors affecting the particle size distribution are examined such as composition of continuous and disperse phases. The concentration of the polymer in the organic solvent and the choice of the solvent nature allowed us to control the particle size distribution within 200 nm-30 microm. The concentration and the nature of the surfactant had a little influence on the particle size distribution. We fabricated three different batches of ion-selective optodes using chromoionophore I, lipophilic ion-exchanger and sodium ionophore X, BME-44, and ETH 5234 for sodium, potassium, and calcium optodes, respectively. The sensors were fully functional with excellent selectivity to interfering ions.

Coulometric Sodium Chloride Removal System with Nafion Membrane for Seawater Sample Treatment

Seawater analysis is one of the most challenging in the field of environmental monitoring, mainly due to disparate concentration levels between the analyte and the salt matrix causing interferences in a variety of analytical techniques. We propose here a miniature electrochemical sample pretreatment system for a rapid removal of NaCl utilizing the coaxial arrangement of an electrode and a tubular Nafion membrane. Upon electrolysis, chloride is deposited at the Ag electrode as AgCl and the sodium counterions are transported across the membrane. This cell was found to work efficiently at potentials higher than 400 mV in both stationary and flow injection mode. Substantial residual currents observed during electrolysis were found to be a result of NaCl back diffusion from the outer side of the membrane due to insufficient permselectivity of the Nafion membrane. It was demonstrated that the residual current can be significantly reduced by adjusting the concentration of the outer solution. On the basis of ion chromatography results, it was found that the designed cell used in flow injection electrolysis mode reduced the NaCl concentration from 0.6 M to 3 mM. This attempt is very important in view of nutrient analysis in seawater where NaCl is a major interfering agent. We demonstrate that the pretreatment of artificial seawater samples does not reduce the content of nitrite or nitrate ions upon electrolysis. A simple diffusion/extraction steady state model is proposed for the optimization of the electrolysis cell characteristics.

Detection Limits of Thin Layer Coulometry with Ionophore Based Ion-Selective Membranes

We report here on a significant improvement in lowering the low detection limit of thin layer coulometric sensors based on liquid ion-selective membranes, using a potassium-selective system as a model example. Various possible processes that may result in an elevated residual current reading after electrolysis were eliminated. Self-dissolution of AgCl on the Ag/AgCl inner element may result in a residual ion flux that could adversely affect the lower detection limit. It was here replaced with an Ag/AgI inner pseudoreference electrode where the self-dissolution equilibrium is largely suppressed. Possible residual currents originating from a direct contact between inner element and ion-selective membranes were eliminated by introducing an inert PVDF separator of 50 μm diameter that was coiled around the inner element by a custom-made instrument. Finally, the influence of electrolyte fluxes from the outer solution across the membrane into the sample was evaluated by altering its lipophilic nature and reducing its concentration. It was found that this last effect is most likely responsible for the observed residual current for the potassium-selective membranes studied here. For the optimized conditions, the calibration curves demonstrated a near zero intercept, thereby paving the way to the coulometric calibration-free sensing of ionic species. A linear calibration curve for the coulometric cell with valinomycin potassium-selective membrane was obtained in the range of 100 nM to 10 μM potassium in the presence of a 10 μM sodium background. In the presence of a higher (100 μM) concentration of sodium, a reliable detection of 1-100 μM of potassium was achieved.

Determination of unbiased selectivity coefficients using pulsed chronopotentiometric polymeric membrane ion sensors.

A new procedure for the determination of selectivity coefficients of neutral carriers using pulsed chronopotentiometric ion selective sensors (pulstrodes) is established. Pulstrode membrane which lacks an ion-exchanger suppresses the zero current ion flux, allowing a Nernstian response slope for even highly discriminated ions. Unlike previously developed methods, unbiased selectivity remains unaltered even with the exposure to the primary ion solution for prolonged time. Studies with potassium-, silver-, and calcium-selective electrodes reveal that pulstrodes yield the same or slightly favorable unbiased selectivity coefficients than reported earlier. In contrast to alternative methods for the determination of unbiased selectivity, this technique offers a unique simplicity and reliability. Therefore the new procedure promises to be a valuable additional tool for the characterization of unbiased selectivity coefficients for the ISEs.

Surface Area Effects on the Response Mechanism of Ion Optodes: A Preliminary Study

The relationship between the surface-to-volume ratio and the response mechanism of polymeric ion probes (ion optodes) is not well understood. In this work, the surface-to-volume ratio of ion optodes was systematically increased in an attempt to characterize this relationship. Several different batches of ion-selective optodes were fabricated via the solvent displacement method using sodium ionophore X, BME-44, and ETH 1001 for sodium-, potassium-, and calcium-selective optodes, respectively. Dilution of the membrane cocktail with varying amounts of an organic solvent provided a convenient tool to control the resulting particle size distribution. Specifically, ion optodes of five different size distributions were fabricated. An apparent shift of the response function on the pH scale was observed for optodes with identical composition that differed in terms of size. There was a strong correlation between the calculated specific surface area and the apparent ion-exchange constant for all three types of ion optodes. However, there was an indication that selectivity does not substantially correlate with the optode size. We hypothesize that the observed effect is caused by surface phenomena which contribute to the overall optode response. The results reported here may raise a word of caution about the application of established response models, which were developed for macroscopic ion optodes, toward probes at micrometer and submicrometer scales.

New Trends in Ion-Selective Electrodes

Ion-selective electrodes (ISEs) are the oldest class of chemical sensors and they are still superior to other sensor types used in various applications in biomedical, industrial, and environmental fields. Clinical chemistry, particularly the determination of the biologically relevant electrolytes in physiological fluids, remains the key area of ISEs application. In contrast to other analytical methods, ion-selective electrodes respond to an ion activity, not concentration, which makes them especially attractive for clinical applications, as health disorders are usually correlated to ion activity. Pharmaceutical analysis is another important area for ISEs where a large number of drugs were reported to be detectable by ISEs in pharmaceutical formulations and during manufacturing processes. The concentration of drugs and their metabolites can be measured in real biological fluids. Most drug-selective electrodes are based on ion exchangers and exploit commonly high lipophilicities of drugs and metabolites. The possibility of detecting polyionic macromolecules added a new thrust to the area of ion-selective electrodes in the past decade. Professors Ma and Meyerhoff from the University of Michigan described the first polymeric membrane electrodes that respond to the polyanion heparin. In contrast to other methods, the heparin-selective electrode was able to detect heparin concentration directly in whole blood or plasma samples. In order to improve the biocompatibility of ISEs and reduce adsorption of cells and polypeptides, several approaches including immobilization of anticoagulants such as heparin and the continuous release of biologically active molecules such as nitric oxide are used. An interesting approach was used by the group of Bachas, who prepared a potassium-selective electrode coated with a copolymer containing phosphorylcholine that mimicked the polar groups on cell surfaces.

pH/ion sensitive nanoprobes with optical tweezers

We present fluorescence-based pH/ion nanosensors, positioned and manipulated using holographic optical tweezers, with simultaneous fluorescence read-out, within a microfluidic device and within a biological cell.