Transport Properties of H+-Selective Membranes Containing Amine-Derivative Ionophores and Mobile Sites

Insights into Primary Ion Exchange between Ion-Selective Membranes and Solution. From Altering Natural Isotope Ratios to Isotope Dilution Inductively Coupled Plasma Mass Spectrometry Studies

Although ion-selective electrodes have been routinely used for decades now, there are still gaps in experimental evidence regarding how these sensors operate. This especially applies to the exchange of primary ions occurring for systems already containing analyte ions from the pretreatment step. Herein, for the first time, we present an insight into this process looking at the effect of altered ratios of naturally occurring analyte isotopes and achieving isotopic equilibrium. Benefiting from the same chemical properties of all isotopes of analyte ions and spatial resolution offered by laser ablation and inductively coupled plasma mass spectrometry, obtaining insights into primary ion diffusion in the preconditioned membrane is possible. For systems that have reached isotopic equilibrium in the membrane through ion exchange and between the membrane phase and the sample, quantification of primary ions in the membrane is possible using an isotope dilution approach for a heterogeneous system (membrane-liquid sample). Experimental results obtained for silver-selective membrane show that the primary ion diffusion coefficient in the preconditioned membrane is close to (6 ± 1) × 10^-9 cm²/s, being somewhat lower compared to the previously reported values for other cations. Diffusion of ions in the membrane is the rate limiting step in achieving isotopic exchange equilibrium between the ion-selective membrane phase and sample solution. On the contrary to previous reports, quantification of silver present in the membrane clearly shows that contact of the membrane with silver nitrate solution of concentration 10^-3 M leads to pronounced accumulation of silver ions in the membrane, reaching almost 150% of ion exchanger amount. The magnitude of this effect increases for higher concentration of the electrolyte in the solution.

Interpretation of chronopotentiometric transients of ion-selective membranes with two transition times

Passing currents through ion-selective membranes has contributed to the development of a variety of novel methods. In this work, chronopotentiometric (CP) transients with two transition times (breakpoints) are presented for the first time, with the theoretical interpretation of such voltage transients. The validity of our theory has been confirmed in experiments utilizing ETH 5294 chromoionophore-based pH sensitive membranes with and without lipophilic background electrolyte and ETH 5234 ionophore-based calcium selective membranes in which the ionophore forms 3:1 complexes with Ca(2+) ions. The conditions under which two breakpoints can be identified in the chronopotentiometric voltage transients are discussed.Spectroelectrochemical microscopy (SpECM) is used to show that the two breakpoints in the CP curves emerge approximately when the free ionophore and ion-ionophore complex concentrations approach zero at the opposite membrane-solution interfaces. The two breakpoint times can be utilized to follow simultaneously the concentration changes of the free ionophore, the ion-ionophore complex, and the mobile anionic sites in cation-selective membranes. In membranes with known composition, the time instances where breakpoints occur can be used to estimate the free ionophore and the ion-ionophore complex diffusion coefficients.

Spectroelectrochemical microscopy: spatially resolved spectroelectrochemistry of carrier-based ion-selective membranes

High-resolution spectroscopic imaging of the cross section of ion-selective membranes and the adjoining solution phases during real-time electrochemical measurement is termed as spectroelectrochemical microscopy (SpECM). The novel SpECM instrument utilizes wavelength-dispersive multispectral imaging of a thin membrane strip separating the two sides of a four-electrode thin-layer electrochemical cell. SpECM is aimed as a tool for optimizing the experimental conditions in mass transport-controlled ion-selective electrode membranes for improved detection limit. Some of the capabilities of the new technique are demonstrated using fix site, chromoionophore-based, pH-sensitive membranes as model systems. The experimental results are discussed in the light of the existing theory of fixed-site membranes. The quantitative expression for the time-dependent change of the free ionophore concentration across the ion-selective membrane showed close correlation to the recorded concentration profiles.

Laser Ablation Inductively Coupled Plasma Mass Spectrometry Assisted Insight into Ion-Selective Membranes

Laser ablation inductively coupled plasma mass spectrometry was used to evaluate ion depth profiles across ion-selective membranes. Advantageously, this approach does not require incorporation of additional components (e.g., chromoionophore) in the membrane composition, as compared to that used in typical potentiometric applications. Moreover, comparison of the distribution of ions in differently pretreated membranes is possible. Concentration profiles of primary and interfering agent (Na+) ions were recorded, for example, of Pb2+-selective poly(vinyl chloride)-based membranes. It was found that the contents and the distribution of Pb(2+) and Na+ ions across the membrane is strongly dependent on the composition of the solutions to which both sides of the membrane are exposed during preconditioning and on the plasticizer included in the membrane formulation. Typical plasticizers, bis(2-ethylhexyl sebacate) (DOS) and the more polar 2-nitrophenyl octyl ether (o-NPOE), were used. It was found that faster ion transport occurs for o-NPOE, and the membrane saturation with Pb2+ ions was achieved within less than 20 h for a 400-microm-thick membrane. In the case of the less polar plasticizer DOS, due to slower rate of ion transport, even after 20 h, the Pb2+ concentration gradients were still visible within the membrane. On the basis of concentration profiles, primary ion diffusion coefficients in both membranes were calculated, and the value obtained for o-NPOE containing membrane was found to be approximately 2 times higher than for its DOS-plasticized counterpart.

Multispectral imaging of ion transport in neutral carrier-based cation-selective membranes

BACKGROUND

High-resolution spectroscopic imaging of the cross section of ion-selective membranes during real-time electrochemical measurements is termed spectroelectrochemical microscopy (SpECM). SpECM is aimed for optimizing the experimental conditions in mass transport controlled ion-selective electrode (ISE) membranes for improved detection limit.

METHODS

The SpECM measurements are performed in a thin layer electrochemical cell. The key element of the cell is a membrane strip spacer ring assembly which forms a two compartment electrochemical cell. The cell is placed onto the stage of a microscope and the membrane strip is positioned in the center of the field of view. A slice of the image is focused onto the entrance slit of the imaging spectrometer.

RESULTS

SpECM has been used for the determination of the diffusion coefficients of different membrane ingredients and for the quantitative assessment of the charged site concentrations in ISE membranes and membrane plasticizers. In addition, changes in the concentration profiles of the ionophore (free and complexed) and charged mobile sites inside the ISE membranes are documented upon the application of large external voltages.

CONCLUSIONS

This account demonstrates the power and advantages of SpECM, a multispectral imaging method for investigations of mass transport processes in ISE membranes during electrochemical measurements.

Effect of Polymer Concentration on Partitioning and Molecular Recognition in Plasticized Poly(vinyl chloride)

Mixtures of poly(vinyl chloride) (PVC) with plasticizers have been used in ion-selective electrodes for many years. The same material has proven useful in solid-phase microextraction (SPME), both with and without artificial receptors. We hypothesized that by changing the polymer concentration in plasticized PVC membranes containing artificial receptor from the standard 33 wt %, the selectivity of the extraction of barbiturates over similar molecules could be improved. Partition coefficients and receptor-substrate formation constants of a target species, phenobarbital, in membranes with various polymer concentrations were determined. Diffusion coefficients of the solute phenobarbital in receptor-free membranes were also determined. Kamlet-Taft solvatochromic properties beta and pi* were measured for the PVC/dioctyl sebacate materials. Cohesive energy densities were calculated for the same materials. Partition coefficients for phenobarbital (from aqueous solution to membrane) decrease as [PVC] increases, while the formation constants for the complex of the solute with its receptor increase. Diffusion coefficients decrease as the polymer concentration increases as well. The increase in polymer concentration brings about a decrease in hydrogen-bonding basicity and an increase in dipolarity and cohesive energy density. The values of the solvatochromic parameters determined at various compositions are highly correlated; thus, it is impossible to calculate how much each factor contributes to the changes associated with partition and complex formation. The solvatochromic "polarizability correction factor" has been determined to be 0 for PVC. In SPME experiments at 30%, 40%, and 50% (w/w) PVC, as polymer concentration increases, selectivity for barbiturate extraction over other cyclic imides becomes better in the presence of barbiturate receptor and worse without receptor.

Steady-state concentration distribution of artificial receptor and target analyte in plasticized PVC membrane between solutions differing in target analyte concentration

A barbiturate receptor has proven effective in improving selectivity in solid-phase microextraction of barbiturates when doped into plasticized poly(vinyl chloride) (PVC). It would be beneficial to have selective extractions for any given organic species; however, the receptors do not exist. They will be found by screening of libraries of potential receptors; thus, a screening method is needed. It is important to screen the receptors in the medium in which they will work: plasticized PVC. We hypothesize that we can make receptors move in solution in response to the presence of a solute to which they bind. This work examines whether we can establish a sufficient free energy gradient for a good receptor to move to a predetermined place in space. A difference in the barbiturate solute (substrate or guest) concentration in solutions bathing the two sides of a plasticized PVC membrane containing the barbiturate receptor (or host) creates a spatial concentration gradient of the substrate in the membrane. This causes the receptor's chemical potential to vary across the membrane. Upon binding to the analyte, the receptor undergoes a local activity drop, which decreases its free energy. This process produces a flux of receptor to accumulate at place where there is a high substrate concentration. A concentration gradient of substrate can be maintained across the membrane at steady state. In membranes for which the formation of the complex is favored, the receptor responds to the gradient of substrate. In membranes for which binding is not favored, a gradient of substrate is completely ignored by the receptor. Thus, the receptor does respond to the gradient but only if the concentration gradient of guest corresponds to a chemical potential gradient.