Radiotracer studies on ion-selective electrode membranes containing metal complexes of a nonylphenoxypoly(ethyleneoxy)ethanol in poly(vinyl chloride) matrices

Radiotracer studies with (133)Ba, (45)Ca and (36)Cl are reported for PVC matrix membranes containing 2-nitrophenyl phenyl ether and the tetraphenylborate of a barium (or calcium) complex with a nonylphenoxypoly(ethyleneoxy)ethanol (NP), Antarox CO880. The results show that there is very limited permeation of radioactive barium and calcium ions through the membrane systems. However, the continued uptake (with time) of radioactive barium ions by the membranes (the uptake of calcium ion is less) suggests that in relation to selective electrode response the stabilization of ions by the NP ligand within the membrane is important in addition to the simple availability of membrane pathways for primary-ion transport through the membrane. Permselectivity to counter-ions is indicated by the non-permeation of radioactive chloride ions.     

Radiotracer studies on ion-selective membranes based on poly(vinyl chloride) matrices

Radiotracer studies with (45)Ca, (89)Sr and (133)Ba have provided evidence that the permeation of magnesium, strontium and barium ions through PVC membranes containing Orion 92-20-02 liquid ion-exchanger is inhibited by their low affinity for the liquid ion-exchanger sites. Experiments with (7)Be indicate a strong affinity of the membrane for beryllium ions with corresponding inhibition of permeation. When acid is present in the solution on one side of the membrane, preferential permeation by protons may lead to transport of ions against their concentration gradient in order to maintain the balance of charge.     

Ion-sensitive field effect transistor as a monovalent cation detector for ion chromatography and its application to the measurement of Na+ and K+ concentrations in serum

An ion-sensitive field effect transistor, which is a small potentiometric ion-sensing probe, was applied as a detector in the ion chromatography of alkali-metal cations. The cation-sensing transistor was prepared by coating the gate part of the probe to form a poly(vinyl chloride) matrix membrane containing tetranactin, which enables detection of alkali-metal ions such as Na+, K+, Rb+, and Cs+. To be able to analyse amounts less than 1 microliter and make full use of the small ion-sensing probe, we constructed a miniature cation-exchange separation column (support 10 microns polystyrene) with a PTFE tube (20 mm x 1.5 mm I.D. x 2.1 mm O.D). As an application of the system, Na+ and K+ concentrations in serum were determined. The analytical results for these two cations were good agreement with those obtained by flame photometry.     

Solid-State Thin-Film Sensors Based on Chalcogenide Materials Obtained by Planar Technology and Pulsed Laser Deposition

Methods of silicon planar technology and pulsed laser deposition were applied to fabricate fully solid-state chemical sensors for determining ions of copper, lead, cadmium, thallium, and also sulfide and chloride ions on the basis of thin chalcogenide films as ion-sensitive membranes.     

Potential analytical applications of lysenin channels for detection of multivalent ions

Transmembrane protein transporters possessing binding sites for ions, toxins, pharmaceutical drugs, and other molecules constitute excellent candidates for developing sensitive and selective biosensing devices. Their attractiveness for analytical purposes is enhanced by the intrinsic amplification capabilities shown when the binding event leads to major changes in the transportation of ions or molecules other than the analyte itself. The large-scale implementation of such transmembrane proteins in biosensing devices is limited by the difficulties encountered in inserting functional transporters into artificial bilayer lipid membranes and by the limitations in understanding and exploiting the changes induced by the interaction with the analyte for sensing purposes. Here, we show that lysenin, a pore-forming toxin extracted from earthworm Eisenia foetida, which inserts stable and large conductance channels into artificial bilayer lipid membranes, functions as a multivalent ion-sensing device. The analytical response consists of concentration and ionic-species-dependent macroscopic conductance inhibition most probably linked to a ligand-induced gating mechanism. Multivalent ion removal by chelation or precipitation restores, in most cases, the initial conductance and demonstrates reversibility. Changes in lipid bilayer membrane compositions leading to the absence of voltage-induced gating do not affect the analytical response to multivalent ions. Microscopic current analysis performed on individual lysenin channels in the presence of Cu²⁺ revealed complex open–closed transitions characterized by unstable intermediate sub-conducting states. Lysenin channels provide an analytical tool with a built-in sensing mechanism for inorganic and organic multivalent ions, and the excellent stability in an artificial environment recommend lysenin as a potential candidate for single-molecule detection and analysis.     

Integration of multiple-ion-sensing on a capillary-assembled microchip

Multiple-ion-sensing functions are integrated on a capillary-assembled microchip (CAs-CHIP). Since the CAs-CHIPs are fabricated by embedding various chemically functionalized square capillaries onto a lattice PDMS channel plate having same channel dimensions as outer dimensions of square capillaries, integration of parallel multiple-ion-sensing is easily realized. Here, three ion-sensing capillaries are prepared and used for integrating these functions onto a single microchip. Ion-sensing square capillaries (sodium, potassium, calcium) are prepared by attaching ion-selective optode membranes to inner wall of capillaries, and are characterized in terms of response time, response range, and ion selectivity. Finally, fully characterized ion-sensing capillaries are embedded into PDMS channel plate in parallel to fabricate a multiple-ion-sensing chip. The CAs-CHIP-based strategy is promising for integrating multiple chemical sensing functions onto a single microchip.     

Calibrant delivery for mass spectrometry

This article describes a means of sampling ions that are created at a location remote from the primary ion source used for mass spectral analysis. Such a source can be used for delivery of calibrant ions on demand. Calibrant ions are sprayed into an atmospheric pressure chamber, at a position substantially removed from the sampling inlet. A gas flow sweeps the calibrants towards the sampling inlet, and a new means for toggling the second ion beam into the instrument can be achieved with the use of a repelling field established by an electrode in front of the sampling inlet. The physical separation of two or more sources of ions eliminates detrimental interactions due to gas flows or fields. When using a nanoflow electrospray tip as the primary ion source, the potential applied to the tip completely repels calibrant ions and there is no compromise in terms of electrospray performance. When calibrant ions are desired, the potential applied to the nanoflow electrospray tip is lowered for a short period of time to allow calibrant ions to be sampled into the instrument, thus providing a means for external calibration that avoids the typical complications and compromises associated with dual spray sources. It is also possible to simultaneously sample ions from multiple ion beams if necessary for internal mass calibration purposes. This method of transporting additional ion beams to a sampling inlet can also be used with different types of atmospheric pressure sources such as AP MALDI, as well as sources configured to deliver ions of different polarity.     

Distinguishing free and total calcium with a single pulsed galvanostatic ion-selective electrode

A pulsed galvanostatic technique is presented to distinguish free and total levels of calcium with a single membrane electrode by varying the magnitude of the applied current. Pulsed chronopotentiometry creates the possibility of accurate control of ion fluxes across ion-selective plasticized polymeric membranes without ion-exchanger properties (note, however, that a low concentration of ion-exchanger impurities is always present in such membranes). During a constant current pulse ions are forced to extract from the aqueous sample into the contacting membrane phase. Each current pulse is followed by a constant potential pulse to remove the extracted ions from the membrane. The induced concentration gradients are reproducible from pulse to pulse. At relatively small applied currents and in the presence of labile complexes in the sample, the sensor responds to the ion activity, in analogy to a conventional ISE. If a larger current is applied, the flux of complexed ions dominates the sensor response because of its increased magnitude, and the observed potential is now a function of the total ionic concentration. This approach allows one to detect, with the same sensor, the levels of free and total ionic concentration by varying the magnitude of the applied current. The technique utilized here gives much more stable signals than with earlier work demonstrating the principle with zero-current potentiometry. This concept is illustrated with calcium selective membranes based on the ionophore ETH 5234.     

Discrimination between the permeation of H+ and OH− ions in precipitation membranes

The unique property of precipitation membranes, which allows determination of permeation rates of single ion species by measurement of current—voltage relationships, has been used to measure the transport rates of H⁺ and Oil ions. The permeation of OH^? was found to be approximately 4-5 times higher than that of H⁺ in both BaSO₄ and calcium oxalate precipitation membranes.     

(De)constructing the ryanodine receptor: modeling ion permeation and selectivity of the calcium release channel

Biological ion channels are proteins that passively conduct ions across membranes that are otherwise impermeable to ions. Here, we present a model of ion permeation and selectivity through a single, open ryanodine receptor (RyR) ion channel. Combining recent mutation data with electrodiffusion of finite-sized ions, the model reproduces the current/voltage curves of cardiac RyR (RyR2) in KCl, LiCl, NaCl, RbCl, CsCl, CaCl(2), MgCl(2), and their mixtures over large concentrations and applied voltage ranges. It also reproduces the reduced K(+) conductances and Ca(2+) selectivity of two skeletal muscle RyR (RyR1) mutants (D4899N and E4900Q). The model suggests that the selectivity filter of RyR contains the negatively charged residue D4899 that dominates the permeation and selectivity properties and gives RyR a DDDD locus similar to the EEEE locus of the L-type calcium channel. In contrast to previously applied barrier models, the current model describes RyR as a multi-ion channel with approximately three monovalent cations in the selectivity filter at all times. Reasons for the contradicting occupancy predictions are discussed. In addition, the model predicted an anomalous mole fraction effect for Na(+)/Cs(+) mixtures, which was later verified by experiment. Combining these results, the binding selectivity of RyR appears to be driven by the same charge/space competition mechanism of other highly charged channels.     

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 titrant's activity was measured with a second ionophore-based ion-selective electrode and corresponded well with expected dosage levels on the basis of Faraday's 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.     

Surface Electrical Properties of Barium Sulfate Modified by Adsorption of Poly alpha, beta Aspartic Acid

The surface electrical properties of precipitated barium sulfate in aqueous solution have been probed in the presence and absence of synthetic poly alpha, beta aspartic acid. The effect of cation type, ion concentration, and pH on the electrokinetic properties of barite with and without adsorbed polyaspartic acid has been analyzed to afford insights into the mechanism by which polyaspartic acid acts as an inhibitor of barium sulfate precipitation. In addition, vibrational spectroscopy has been employed to monitor bond changes within polyaspartic acid molecules adsorbed on precipitated barium sulfate. These studies have indicated that the surface electrical charge of "pure" barite differs significantly from that of barite precipitated in the presence of calcium ions. Moreover, the presence of the cations Ca2+ and Mg2+ in the suspending liquor has a marked influence of the measured electrokinetic properties. The infrared spectral studies strongly suggest that polyaspartic acid is bound to the barite via the backbone nitrogen atoms. This proposal in conjunction with the electrokinetic and adsorption data has allowed a tentative model of barium sulfate inhibition to be proposed; viz, calcium ions are needed to complex with the polyaspartic acid, first, to reduce the molecules electrical charge in order to promote adsorption onto a charged surface; second, to induce a conformational change in the molecule in order to allow optimal anchoring via the backbone nitrogen atoms; and, third, to reduce the solubility of the molecule in order to increase its surface activity. Copyright 1999 Academic Press.     

Potassium ion-selective electrodes based on valinomycin/PVC overlayered solid substrates

In this work, permutations of plasticized PVC membranes with incorporated valinomycin are coated over various substrate metallizations. Characterization of the resulting potassium electrodes includes measurement of sensitivity, short- and long-term potential drift, dissolved oxygen induced potential transients and probe lifetime. Results using the best performing metallizations compare favorably with those obtained using identical membranes in the symmetrical solution contact configuration. Prospects for use of this approach as the ion-sensing layer of ISFET (Ion-Sensitive Field-Effect Transistor) devices are considered.     

Radiotracer studies on calcium ion-selective electrode membranes based on poly(vinyl chloride) matrices

Radiotracer studies with ⁴⁵Ca and ³⁶Cl demonstrate that PVC matrix membranes containing Orion 92-20-02 liquid calcium ion-exchanger are permselective to counter-cations. Diffusion coefficients are quoted for the migration of ⁴⁵Ca between pairs of calcium solutions and are discussed in terms of solution concentration, membrane thickness and concentration level of sensor in the membrane. Migration of calcium ions from calcium chloride solution to a Group (II) metal chloride solution through a PVC membrane containing calcium liquid ion-exchanger is discussed in terms of solvent extraction and electrode selectivity coefficient parameters. Thus, magnesium, strontium and barium ions appear to inhibit migration through the membrane by their low affinity for the membrane liquid ion-exchanger sites, while the inhibition by beryllium ions is attributed to site blockage by the strong affinity of dialkylphosphate sites for beryllium.     

Conducting polymers in modelling transient potential of biological membranes

The possibility of using conducting polymer (CP) films doped with biological ligands as artificial biological membranes to study potential formation mechanisms is presented. Calcium and magnesium ion-binding anionic sites--asparagine, glutamine, adenosinotriphosphate and heparin are incorporated into the poly(pyrrole) film during electrochemical polymerization. This approach allows the competitive calcium-magnesium ion-exchange to be inspected by open circuit measurements. After a close-to-Nernstian sensitivity of the CP membranes was induced by soaking in alkaline solutions of calcium or magnesium, dynamic experiments were performed by a change in the bulk concentration of magnesium or calcium ions. A characteristic transitory potential response, though distinctively different for the calcium and magnesium ions, was observed and explained using the diffusion layer model (DML).     

Some alkylphosphoric acid esters for use in coated-wire calcium ion-selective electrodes: Part II. Selectivities and use in potentiometric titrations

The interferences of a range of ions (including the hydrogen ion) on the potential responses of a series of coated-wire calcium-selective electrodes are reported. The extent of the interferences is discussed as a function of the composition of the PVC membranes. The use of these electrodes as end-point sensors in potentiometric titrations of calcium with EDTA is also reported.     

Towards enhanced ligand-centred photoluminescence in inorganic-organic frameworks for solid state lighting

Three novel inorganic-organic framework compounds containing the organic chromophore ligand 9-fluorenone-2,7-dicarboxylic acid (abbreviated H(2)FDC) and barium (BaFDC), cadmium (CdFDC) and manganese (MnFDC), respectively, have been synthesized and evaluated for their use as phosphor materials for solid state lighting and other applications. The results are compared with two earlier reported structures containing the same ligand with calcium (CaFDC) and strontium (SrFDC). The barium- and cadmium-containing compounds both show blue excited, yellow photoluminescence, while the manganese structure does not. The trends in luminescent efficiency for the Ba, Cd, Ca, and Sr derivatives are discussed in relation to crystallographic, optical, and low-temperature specific heat considerations.     

Miniaturization in analytical,chemistry—a combination of flow injection analysis and ion-sensitive field effect transistors for determination of ph,and potassium and calcium ions

The combination of flow injection analysis and ion-sensitive field effect transistors is described for determinations of pH, potassium and calcium ions. The reference electrode is placed in the bypass of the sample injector, thereby avoiding the detrimental effect of serum proteins on the liquid junction potential for the reference electrode. The reagent consumption is halved compared to the use of the conventional electrodes without adversely affecting the sampling rate and the standard deviation of the measurement. An attractive feature of this combination is the possibility of multi-ion analysis.     

Calcium-selective electrodes based on calcium tetra(4-chlorophenyl)borate complexes of macrocyclic polyether diamides

The preparation of macrocyclic 7,19-dibenzyl-7,19-diaza-1,4,10,13,16-pentaoxa-cycloheneicosane-6, 20-diones substituted in positions 2 and 3 with methyl groups, and their properties in PVC membranes as calcium sensors are described. Complexes of these polyether diamides (PEDA) with calcium and tetra(4-chlorophenyl)borate (TClPB) ions having the composition 2PEDA·Ca·2TClPB were prepared. Calcium electrodes based on these complexes have selectivity coefficients for calcium over barium up to 10³, and over alkali metals up to 3 × 10⁴.     

Photocontrol of ion-sensor performances in neutral-carrier-type ion sensors based on liquid-crystalline membranes

Neutral-carrier-type ion-selective electrodes based on liquid-crystalline ion-sensing membranes containing an azobenzene derivative as the photosensitive chromophore show remarkable changes in their ion selectivity on photoirradiation, which is induced by the phase transition of the membranes.