Ursula E. Spichiger

Plasticizers for liquid polymeric membranes of ion-selective chemical sensors

Abstract A variety of 55 different plasticizers based on various structural elements were investigated in respect to their applicability in liquid polymeric membranes of chemical sensors. The plasticizers and the overall performance of the membrane were judged in respect to the properties of lipophilicity, solubility, exudation and selectivity, respectively. A strategy for the design of plasticizers is derived from theoretical considerations and verified by experiments. The selectivity data were obtained by measurements with a standard Mg 2+ selective electrode optimized for clinical applications. A plasticizer needs several large alkyl and/or aryl residues to exhibit sufficient lipophilicity. In order to avoid crystallization or exudation, the use of branched alkyl chains as well as the link or the substitution of the structural elements by an adequate number of polar or polarizable groups is encouraged. However, to ensure high selectivities, plasticizers should not contain functional groups which may act as coordination sites and therefore compete with the carrier. In contrast to earlier publications it was observed that other properties than the dielectric constant of the plasticizer are more important for the selectivity of the membrane. A correlation between selectivity coefficient and the dielectric constant of the plasticizer could not be found.

Glucose Nanosensor Based on Prussian-Blue Modified Carbon-Fiber Cone Nanoelectrode and an Integrated Reference Electrode

An enzyme nanosensor, based on a carbon fiber cone nanoelectrode modified by codeposition of Prussian blue (PB) and glucose oxidase (GOD), has been constructed. The nanosensor displays a low-potential electrocatalytic detection of the enzymatically liberated hydrogen peroxide, along with good reproducibility and high selectivity. The dependence of the nanosensor response upon parameters of the sensor preparation (e.g., amount of GOD, PB deposition time, Nafion coverage), and upon measurements conditions (e.g., pH, applied potential) was evaluated. An operating potential of –0.1 V (vs. Ag/AgCl) yielded the highest selectivity towards glucose, with no interference from ascorbic acid. The lifetime of the nanosensor is more than two weeks. Such miniaturization and analytical performance offer great promise for measurements of glucose in extremely small volumes.

Reversible optical sensing membrane for the determination of chloride in serum

Abstract An optical sensor membrane for the determination of chloride is described. It incorporated trioctyltin chloride as an ionophore and a chromoinophore in a plasticized PVC matrix. The concentration of chloride is indicated by co-extraction of chloride and hydrogen ions into the bulk of the membrane yielding large absorbance changes which can easily be measured in the visible spectral range. The performance of this sensor and its application to chloride determinations in human plasma are reported.

Reversible chemical reactions as the basis for optical sensors used to detect amines, alcohols and humidity

A new class of indicator dyes for use in analytical chemistry is presented. In contrast to most existing indicator dyes, which change colour upon complexation or protonation/deprotonation, the dyes presented here perform reversible chemical reactions with the analyte, resulting in changes in absorbance or fluorescence. Specifically, azo and stilbene dyes which exhibit a reactive trifluoroacetyl group, reversibly interact with alcohols, amines or water to give the respective diol, hemiaminal, hemiacetal or zwitterion. Indicator dyes that combine the properties of a chemical reagent and a ligand molecule are termed ‘chromo-’ or ‘fluororeactands’. They are embedded in various polymer materials and have been characterised for the optical sensing of aliphatic amines, alcohols and humidity. Furthermore, methacrylate derivatives of the dyes have been tested. They were prepared to link the dyes covalently to the polymer matrix. The optical and physical properties of the dye methacrylates make them promising candidates not only for optical sensing but also for applications in molecularly imprinted polymers and non-linear optics.

Integrated optical sensors based on refractometry of ion-selective membranes

Abstract The properties of optode membranes to be used for realizing miniature integrated optical sensors based on refractometry have been investigated theoretically and experimentally. In situ referencing by using optode and reference membranes on the same chip is a powerful means to discriminate against unspecific effects. The plasticized polymer membranes provide the function of selective extraction and chemical recognition of the analyte. In experiments performed for a pair of such membranes, refractive index differences ≈3 × 10 −3 per decade of molar Ca 2+ concentration in buffered aqueous solutions have been observed. With the characterized type of sensing pad implanted in a miniature integrated optical device, a typical overall resolution of 0.7 to 0.07% in units of Ca 2+ concentration was calculated.

Nitrite-selective microelectrodes

The development of nitrite-selective liquid membrane microelectrodes based on a synthetic charged ionophore is described. The addition of potassium tetrakis(4-chlorophenyl)borate and poly(vinyl chloride) to the membrane phase is essential to lower the ohmic resistance and to prolong the lifetime of the microelectrodes. The detection limits for sodium nitrite solutions without and with an ion background of 0.1M chloride are 10(-5.1) and 10(-4.2)M, respectively. The comparison with macroelectrodes shows that the miniaturization reduces, to some extent, the selectivity and the slopes of the EMF response functions.

Comparison of different methods for determining the selectivity coefficient using a magnesium-selective electrode

A lot of effort has been put into correctly describing the selectivity of ion-selective membrane electrodes (ISEs). The fact that the selectivity coefficients of ions with different charge numbers cannot be exchanged for each other was of major practical and theoretical concern. Therefore the “matched potential method” (MPM) was recommended by IUPAC for reporting the selectivity coefficients Kpoti,j correctly when ions of unequal charge numbers are involved. In our experiments, however, the MPM was found to be inaccurate, inconsistent, and not practical. The MPM was made use of in order to investigate the selectivity pattern of magnesium-selective membrane electrodes incorporating the ionophore ETHT 5506. The drawbacks and problems are discussed. Furthermore, a consistent selectivity coefficient Kconsi,j is defined, and an algorithm is derived mathematically from the Nernst and Nickolsky equations which describe the electrode response, and allows to report consistent selectivity coefficients for a primary and an interfering ion of different charge number and vice versa.

An impedance study of Mg2+-selective membranes

Abstract Mg 2+ -selective polymeric membranes were studied by means of impedance analysis with regard to the bulk membrane resistance, the charge transfer resistance and the membrane selectivity. The dependence of the bulk membrane resistance on the presence of lipophilic anionic sites is clearly shown by impedance spectra. The mechanisms of borate catalysis and how they actively contribute to the membrane’s interfacial charge-transfer process is elucidated. The membrane selectivity was investigated by impedance analysis. This is an alternative method to potentiometric measurements (not orthogonal), and can, unlike calorimetry or transport studies, be used to test ion-selective membranes under real conditions.

Critical parameters and optimization of a magnesium-selective liquid membrane electrode for application to human blood serum

SummaryThe selectivity of a new magnesium ionophore (ETH 7025) induced in membranes of different compositions is experimentally studied in view of the ion activities in human serum. The required selectivity coefficient against calcium for the application of an ion-selective magnesium electrode to human serum is calculated for the worst case. Other critical parameters for the application of a liquid PVC-based ion-selective membrane to undiluted human serum discussed are: the sensor lifetime which is related to the lipophilicity of the carrier as well as the ruggedness of the membrane against interactions with components of the relatively lipophilic sample.

Optodes in clinical chemistry: potential and limitations☆

Abstract Total ion concentrations of sodium, potassium, calcium and chloride have been measured reversibly by ion-selective optode membranes with comparable selectivity to potentiometric systems. The results of quality-control samples analysed by the selective optode membranes show a good agreement with the assigned values and a reasonable reproducibility, though for quantification of sodium and potassium the unknown concentrations are evaluated in a first trial by a standard addition method. All measurements are referred to a pair of blank membranes in a reference cell to compensate for interferences induced by the diluted plasma sample. The efficiency of the evaluated values with regard to the medical interpretation encompasses the performance standard of the analytical procedure as well as the intra- and interindividual variation of the subject studied. The required performance characteristics of a sensor are constituted differently, depending on the mode of application and the medical question to be answered. The type of optodes described may be tailored to a large extent to adapt them to the required performance boundary conditions. This will be discussed with respect to other optical sensor schemes.