Neutral Ionophores Research Articles

Potentiometric Performance and Interfacial Kinetics of Neutral Ionophore Based ISE Membranes in Interfering Ion Solutions Before and After Contact with Primary Ions

Potentiometric Performance and Interfacial Kinetics of Neutral Ionophore Based ISE Membranes in Interfering Ion Solutions Before and After Contact with Primary Ions

Potentiometric detection of organic acids in liquid chromatography using polymeric liquid membrane electrodes incorporating macrocyclic hexaamines

Potentiometric detection employing coated-wire electrodes was applied to the determination of organic acids in liquid chromatography (LC). Poly(vinyl chloride)-based liquid membranes, incorporating lipophilic macrocyclic hexaamines as neutral ionophores were used as electrode coatings. The selectivity and sensitivity of the macrocycle-based electrodes were found to be superior to an electrode based on a lipophilic anion exchanger (a quaternary ammonium salt). Sensitive detection was obtained for the di- and tricarboxylic acids tartaric, malonic, malic, citric, fumaric, succinic, pyruvic, 2-oxoglutaric and maleic acids after separation in reversed-phase LC. Detection limits (signal/4 σ noise=3) of 6 pmol for malonic acid and 2 pmol for maleic acid were attained. The detection was explained using a molecular recognition model. The hexaamine-based potentiometric electrodes had a 1-s response time at 1 ml min −1 flow-rates. They were stable for at least 4 months, with an intra-electrode variation of 3.2% ( n=5).

Ion Pairing between Cl-or ClO4-and Alkali Metal Complexes of Ionophore Antibiotics in Organic Solvents: A Multinuclear NMR and FT-IR Study

The extent of ion pairing in chloride and perchlorate salts was studied by measurement of the Cl- and ClO4- resonances and the observation of the perchlorate stretching frequency by use of nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared spectroscopy (FT-IR), respectively, for a variety of ionophores in various solutions and in large unilaminar vesicles (LUVs). The NMR line widths of chloride and perchlorate were larger in solutions containing the neutral ionophores valinomycin (Val) and nonactin (Non) than in solutions containing the negatively charged ionophores nigericin (Nig), lasalocid (Las), and monensin (Mon). The viscosity-corrected perchlorate NMR line widths in solutions containing Val and Las were significantly negatively correlated (r2 > or = 0.99) with the dielectric constant of the solvent. Solvents with low dielectric constants favored ion pair formation. From methanolic solutions containing the Li+, Na+, K+, and Cs+ salts of Cl- and ClO4-, it was determined that the cation with the highest selectivity for the ionophore affords the most ion pairing. A decrease in pH from 7 to 3 had no significant effect on the NMR line widths of chloride and perchlorate in methanolic solutions containing Val, whereas a similar decrease in pH in a methanolic solution containing Mon caused a 2-fold increase in the line widths. The FT-IR difference spectrum of KClO4 in a methanolic solution containing Val showed splitting at the perchlorate stretching frequency. No band splitting was observed in the FT-IR difference spectrum of KClO(4) in methanolic solutions containing Las. The efflux of 35Cl in LUVs containing the neutral ionophore Val followed first-order kinetics with an efflux constant of 1.70 x 10(-3) x min(-1), as determined by 35Cl NMR spectroscopy. The induction of increased membrane permeability in LUVs by the ionophore was determined to be negligible for Val and Nig by fluorescence spectroscopy.

On-Chip Integration of Neutral Ionophore-Based Ion Pair Extraction Reaction

Integration of a neutral ionophore-based ion pair extraction reaction onto a glass microchip was performed. Since the liquid microspace provides a short molecular diffusion distance and a large specific interfacial area of the liquid−liquid interface, novel attractive analytical features arise such as extremely fast ion sensing and ultrasmall reagent solution volume. In contrast to the slow response time of a standard ion optode, in which the response time is basically governed by slow diffusion of ionic species in the viscous polymer membrane, that of the on-chip ion-sensing system is clearly faster due to the short molecular diffusion distance and low viscosity of organic solution. In this case, the organic solution containing a neutral ionophore and a lipophilic pH indicator dye and the aqueous solution containing sample ion were independently introduced into microchannel to form an organic−aqueous interface. Then determination of the ion was performed by thermal lens microscopy at the downstream of th...

The synthesis of some pyridyl functionalized calix[4]arenes as the sensor molecule for silver ion-selective electrodes

25,27-Bis(6-hydroxymethyl-2-pyridylmethoxy)-26,28-dipropoxycalix[4]arene 3, its derivative 4 and 25,27-bis(6-diphenylphosphinomethylpyridylmethoxy)-26,28-dipropoxy-5,11,17,23-tetra-tert-butylcalix[4]arene 6 have been synthesized and characterized by 1H NMR spectroscopy and elemental analysis. The X-ray crystallographic structure was obtained for 4 and shows an infinite sheet aggregate through intermolecular hydrogen bonds and bridged methanol molecules in the solid state. Ion-selective electrodes (ISE) for Ag+ containing 3, 4 and 6 as neutral ionophores in a PVC membrane were prepared and their selectivity coefficients for Ag+ (KpotAg,M) were investigated against other alkali metal, alkaline-earth metal, lead, ammonium ions and some transition metal ions using the mixed-solution method. These ISEs showed excellent Ag+ selectivity towards most of the interfering cations examined, except for Hg2+ which has a relativly low interference (log KpotAg,M ≤ −1.5).

Novel Systems for Selective Cation Detection: Application of a Lipophilic Carborane Anion in Polymeric Ion-Selective Electrodes

Selectivity and sensitivity of PVC membrane electrode are dramatically influenced not only by the specific receptor, but also, mainly for neutral ionophores, by additional lipophilic co-receptor. For cation detection, tetraphenylborate derivatives have been commonly used. Here, we describe novel systems based on conjunction of the classic cation receptors and a highly lipophilic cobalticarborane anionic species. The superiority of this novel co-receptor is demonstrated by application of dibenzo-18-crown-6 and dicyclohexano-18-crown-6 as ionophores in conjunction with the classic lipophilic additive and the novel additive cobalticarborane. The presented data show much higher efficiency which makes this additive co-receptor ideal for cation detection and development of novel sensors.

Selectivity behavior and multianalyte detection capability of voltammetric ionophore-based plasticized polymeric membrane sensors.

The current response features ofvoltammetric ion-selective polymeric membranes doped with neutral ionophores in view of practical sensor development are elucidated. The membranes are designed to extract ions only under applied external potentials and interrogated by normal-pulse voltammetry and pulsed amperometry. They contain two polarizable interfaces to avoid loss of lipophilic ions at the sample side and to maximize the available potential window. A simple theoretical model is developed that describes the observed current at the end of an uptake pulse to the applied membrane potential, which is the sum of both boundary potentials (at the sample and inner electrolyte side) and the membrane internal iR drop. The results describe how the selectivity of the resulting sensor must be dependent on the applied potential. Evidently, the role of the applied potential is akin to incorporating lipophilic cationic and anionic sites with potentiometric ionophore-based membranes, which are well known to considerably affect membrane selectivity and to define the charge type of the assessed ions. This has important implications for sensor design, as the applied cell potential can be used to tune sensor selectivity. Theory also explains the role of the inner electrolyte on sensor behavior. A maximum measuring range is expected with ions in the inner electrolyte that are difficult to extract into the membrane. This corresponds to Kihara's experimental results and contrasts to common ion-selective electrode practice, where a salt of the analyte ion is normally present in the inner electrolyte. Separate and mixed solution experiments with membranes containing the sodium-selective ionophore tert-butyl calix[4]arene tetramethyl ester and the lithium ionophore ETH 1810 agree very well with theoretical expectations. Multianalyte detection capability with a single sensing membrane is demonstrated in a selectivity-modifying pulsed amperometric detection mode, where each applied voltage yields a different practical selectivity of the sensor. The sensor is altered from being sodium to potassium selective as the magnitude of the applied potential is repetitively varied within the pulse sequence. The sensors show high long-term stability under continuous measuring conditions over 15 h.

Micrometer-sized lithium ion-selective microoptodes based on a “tailed” neutral ionophore and a fluorescent anionic dye

The preparation and response features of a micrometer-sized lithium ion-selective optode based on a liquid membrane were examined. The optode membrane was a plasticized poly(vinyl chloride) (PVC)-based copolymer incorporating a lipophilic 14-crown-4 derivative as the neutral lithium ionophore and a dibromofluorescein derivative as the fluorescent anionic dye. The detection mode was a fluorescence change based on the ion-pair extraction/ion exchange principle caused by the fluorescent anionic dye and the lithium ionophore. The 5-μm-sized microoptode was prepared by the micropipette fabrication method and characterized by measuring the optical responses to Li + concentrations with the time-resolved photon counting method. The microoptode responded to lithium ion concentrations of ∼0.5 to ∼500 mM. The micrometer-sized lithium ion-selective microoptode was successfully obtained when a tailed ionophore was used. The anchor effect of the tailed ionophore was useful for the lithium ion-selective microoptode to resolve leaching of the ionophore, which is a significant problem in a microoptode based on a liquid membrane. This demonstration indicates that other ion-selective microoptodes can be obtained simply by replacing the tailed ionophore.

Ion chromatography detector based on solid-state ion-selective electrode array

A variety of neutral carrier type ionophores for monovalent cations were employed to prepare solid-state cation-selective electrodes (SSEs) for use as a detector in single-column ion chromatography (IC). The polyurethane-based pseudoreference electrode made it possible to assemble an array type SSE detector for IC. An SSE-based detector provides not only the overall chromatogram for the separated ion species (monensin methyl ester–nonactin-based membrane), but also the enhanced chromatogram for specified ions of interest (valinomycin as K + and nonactin for NH 4 +). This feature makes it possible to perform highly quantitative analysis with low detection limits even if the separation efficiency of the ion-exchange is not sufficient. Since SSE-based IC detectors are easily miniaturized and replaceable at low cost, they are an ideal component of a portable IC system.

Determination of complex formation constants of 18 neutral alkali and alkaline earth metal ionophores in poly(vinyl chloride) sensing membranes plasticized with bis(2-ethylhexyl)sebacate and o-nitrophenyloctylether

A segmented sandwich membrane method is used to determine complex formation constants of 18 electrically neutral ionophores in situ in solvent polymeric sensing membranes. These ionophores are commonly used in potentiometric and optical sensors, and knowledge of such binding information is important for ionophore and sensor design. In this method, two membrane segments are fused together, with only one containing the ionophore, to give a concentration-polarized sandwich membrane. Unlike other approaches, this method does not require the use of a reference ion in the sample and/or a second ionophore in the membrane, and is typically pH insensitive. The following ionophores responsive for the common cations lithium, sodium, potassium, magnesium and calcium are characterized and discussed: valinomycin, BME-44, bis[(benzo-15-crown-5)-4′-ylmethyl]pimelate, ETH 157, ETH 2120, bis[(12-crown-4)methyl]dodecylmethylmalonate], 4- tert-butylcalix [4] arene tetraacetic acid tetraethyl ester, ETH 149, ETH 1644, ETH 1810, 6,6-dibenzyl-14-crown-4, N, N, N′, N′, N″, N″-hexacyclohexyl-4,4′,4″-propylidyne tris(3-oxabutyramide), ETH 1117, ETH 4030, ETH 1001, ETH 129, ETH 5234, and A23187. The logarithmic complex formation constants range from 4.4 to 29 and compare well to published data for ionophores that were characterized earlier. From the observed complex formation constants, maximum possible selectivities are calculated that would be expected if interfering ions show no binding affinity to the ionophore, and the values are compared with experimental findings. Each ionophore is characterized in poly(vinyl chloride) membranes plasticized either with a polar (NPOE) or a nonpolar plasticizer (DOS). Membranes based on NPOE always show larger complex formation constants of the embedded ionophore.

Lifetime of ion-selective electrodes based on charged ionophores

The lifetime of solvent polymeric ion-selective electrodes (ISEs) is limited by leaching of the membrane components into the sample solutions. In this article, leaching of charged ionophores is discussed. Because of the electroneutrality principle, the loss of the charged ionophore into the sample must be accompanied by parallel transport of an ion of the opposite charge sign into the sample or by ion exchange with a sample ion of the same charge sign. Because ionic sites of high lipophilicity are available, the loss of ionic sites is, in general, not a concern. Therefore, it is assumed here that the cotransported or ion-exchanging ions are primary or interfering ions forming complexes with the ionophore. A general theory that allows quantification of ionophore lipophilicities and a discussion of changes in the membrane composition and selectivity with time is presented. A high complex stability and high analyte concentrations diminish the rate of ionophore loss into the sample if a charged ionophore is coextracted from the membrane into the sample together with an analyte ion of opposite charge. On the other hand, if the charged ionophore has the same charge sign as the ion that it binds, a large binding constant and high analyte concentrations enhance ionophore leaching into the sample. The model is applied to interpret results for an electrically charged ionophore, for which selectivity changes as a function of the leaching time were observed and the lipophilicity was determined with potentiometric measurements. Using the lipophilicities of neutral ionophores, as described previously, and the lipophilicities of charged ionophores, as described here, a direct comparison of the expected leaching rates of charged and neutral ionophores has become possible.

Effects of neutral ionophores on membrane electrical characteristics of NG108-15 cells

The effects of several K +-selective neutral ionophores on membrane electrical characteristics of differentiated NG108-15 (neuroblastoma X glioma hybrid) cells were examined. Specifically, alterations in membrane resting potential ( V m), input resistance ( R in) and electrically-induced action potential generation were determined upon bath application of enniatin (0.1–10 μg/ml), nonactin (0.1–10 μM) and valinomycin (0.1–10 μM). Although some cells exhibited a slight hyperpolarization and/or reduced R in, i.e. membrane electrical correlates of enhanced K + loss, neither V m nor R in were significantly altered by any of the ionophores. However, valinomycin and especially nonactin affected action potentials induced by electrical stimulation. This was apparent in the ablation of action potentials in some cells and in the occurrence of degenerative changes in action potential shape in others. The simultaneous administration of the neutral ionophores and the protonophore CCCP or the superfusion of enniatin, nonactin or valinomycin in high (50 mM) glucose-containing physiological solution did not yield more extensive alterations in V m or R in. These data suggest that the neutral ionophores are unable to materially enhance K + flux above the relatively high resting level in NG108-15 cells. Thus, alterations in action potentials appear to be unrelated to K + transport activity.

Cationic or anionic sites? Selectivity optimization of ion-selective electrodes based on charged ionophores

The influence of ionic sites on the selectivities of ionophore-based ion-selective electrodes (ISEs) is described on the basis of a phase boundary potential model. The discussion presented here is significantly more general than previous ones. It is formulated for primary and interfering ions of any charges and it is valid for ISEs based on electrically charged or neutral ionophores. Furthermore, it also applies to membranes that contain more than one type of complex of the primary or interfering ion. It has been believed thus far that only ionic sites of the same charge sign as the primary ion improve the selectivities of ISEs based on charged ionophores. However, it is shown here that the charge sign of the ionic sites that give the highest potentiometric selectivities depends on the charge number of the primary and interfering ions and on the stoichiometry of their complexes with the ionophore. The validity of our model was confirmed experimentally with three ISEs based on different charged ionophores. ISEs based on lasalocid or 1-(N,N-dicyclohexylcarbamoyl)-2- (N,N-dioctadecylcarbamoyl)ethylphosphonic acid monomethyl ester (ETH 5639) as the ionophore responded selectively to Sr2+ or Mg2+, respectively, and discriminated well against other alkaline earth cations when their membranes contained anionic sites. These two electrodes are the first examples of ISEs based on charged ionophores for which maximum selectivities are obtained with membranes containing ionic sites with a charge sign opposite to that of the primary ion. On the other hand, the experimental F- selectivities of membranes based on oxo(5,10,15,20-tetraphenylporphyrinato)molybdenum-(V) improved gradually when the concentration of anionic sites was increased from 0 to 75 mol%. The selectivity-modifying influence of ionic sites for these three types of ISEs can be explained by considering the different stabilities of the 1:2 ion-ionophore complexes of the primary and of the interfering ions.

Facilitated ion transfer across oil∣water interfaces. Part III. Algebraic development and calculation of cyclic voltammetry experiments for the formation of a neutral complex

Abstract Computer simulations of cyclic voltammetry experiments at liquid ∣ liquid interfaces are presented for the transfer of ions (M z + ) assisted by z charged ligands (L − ). The main difference, when compared with neutral ligands, is that in the case presented here, all the complexes formed have a different charge and the flux of the ligands has to be taken into account in the definition of the current, in addition to that of the metal ion and those of the ( z −1) charged complexes (the complex with the highest stoichiometry being neutral). The particular cases of M + /L − and M 2+ /L − systems are presented. The effect, on the cyclic voltammetry response, of various parameters such as association constants, metal and ligand concentration, is analysed and results are compared to the case of neutral ionophores.

Ion-selective field effect transistor (ISFET)-based calcium ion sensor with photocured polyurethane membrane suitable for ionised calcium determination in milk

Ca 2+-ion-sensitive ISFETs with photocured polyurethane-based polymer membranes with alkylphosphate exchanger (DOPPCa) and three neutral carrier ionophores (ETH 1001, ETH 129 and ETH 5234) and dioctylsebacate as plasticizer have been studied in pure CaCl 2 solutions and in background solutions containing Na +, K +, Mg 2+ ions in concentrations normally found in milk. In case of DOPPCa and ETH 1001 the influence of Mg 2+ ions on the slope of the response curve is reported. ISFETs with membranes containing ETH 129 are shown to have high selectivity, more than 6 weeks life-time in constant contact with a solution and high precision of ionised calcium determination. Experimental results on determination of calcium ions in a whole milk are presented.

Calixarene-facilitated transfer of alkali metal ions across the polarised liquid–liquid interface

The transfer of sodium and potassium ions across the water–1,2-dichloroethane interface by the neutral ionophore tetraethyl-p-tert-butyl calix[4]arene tetraacetate has been investigated using voltammetric methods. The electrochemical data has been used to determine the stoichiometry of the metal: ligand complex and the appropriate association constants between the metal ions and the ligand in the organic phase. This data is correlated with structural studies, including the crystal structure of the Na+-calixarene complex, which is reported for the first time.

Optical Urea Biosensor Based On Ammonium Ion Selective Membrane

ABSTRACT An optical urea biosensor was developed by immobilizing an urease enzyme layer on a thin ammonium-selective polymer membrane. The ammonium optical membrane utilized dichlorofluorescein octadecyl ester (DCFOE) as anionic chromophore and nonactin as neutral ionophore. The urease layer was coated on the top of the ammonium layer by gelatin entrapment combined with glutaradehyde cross-linking. Hydrolysis of urea catalyzed by urease produced ammonium ion, which was extracted into the-polymer film to form complexes with nonactin. A proton was released which resulted in a color change of the optical membrane due to charge neutrality principle. The biosensor

The use of a phenylpyrazine liquid crystalline material with a liquid crystalline solvent mediator as an ion-selective electrode

Liquid crystalline 2-n-hexyloxy-5-(4-bromophenyl)pyrazine was investigated as a novel neutral ionophore for use in ion selective electrodes. Two membrane compositions were studied. The first membrane included o-nitrophenyl octyl ether as a plasticising solvent mediator, and in the second membrane the plasticising mediator was replaced with a commercial non-ionophoric liquid crystal material (E7 Merck). The inclusion of E7, which exhibits solvent mediator properties, ensured the ionophore was present in a room temperature liquid crystalline phase. For membranes containing the E7, improved Nernstian response was observed for ammonium ions over the same concentration range, together with improved selectivity when used as ammonium and caesium selective electrodes. This is believed to be the first calamitic liquid crystalline ionophore polymer system described and may have potential in direct manipulation of ionophores in polymer membrane electrodes, using established liquid crystal switching technology.

Determination of complex formation constants of lipophilic neutral ionophores in solvent polymeric membranes with segmented sandwich membranes.

A potentiometric method to determine ionophore complex formation constants in solvent polymeric membrane phases, proposed originally by Russian researchers, is critically evaluated and compared to other established methods. It requires membrane potential measurements on two-layer sandwich membranes, where only one side contains the ionophore. The resulting initial membrane potential reflects the ion activity ratio at both aqueous phase--membrane interfaces and can be conveniently used to calculate complex formation constants in situ. This method is potentially useful, since it does not require the use of a reference ion or second ionophore in the measurement. In this paper, the five ionophores valinomycin, BME-44, ETH 2120, tert-butylcalix[4]arene tetraethyl ester, and S,S'-methylenebis(diisobutyldithiocarbamate) are characterized in poly(vinyl chloride) (PVC) plasticized with dioctyl sebacate (DOS) and compared with other established methods. The resulting formation constants correspond well to literature values. The influence of varying membrane concentrations and different anionic site additives is studied and found to be relatively small. Experiments are also performed with and without lipophilic inert electrolytes and with ionophore-free sandwich membranes to illustrate the effect of ion pairing and the membrane internal diffusion potential on the response of such sandwich membranes. These experiments suggest that ions are completely associated in PVC-DOS membranes, but that such ion pairs are rather nonspecific. Diffusion potentials seem to play a minor role with these systems. The results are explained with theory. This work indicates that the characterization of electrically charged ionophores, anion-selective ionophores, and ionophores in membrane matrixes other than PVC plasticized with DOS may now be experimentally accessible.

Overcoming the pH dependency of optical sensors: a pH-independent chloride sensor based on co-extraction

A chloride-selective optode is presented which is based on a polymer-stabilized emulsion system which consists of a hydrogel into which plasticizer droplets are incorporated containing a neutral chloride-selective ionophore. A solvatochromic cationic fluorescent dye is located in the hydrogel. When the ionophore extracts chloride out of the aqueous solution into the plasticizer droplets, the positively charged dye is co-extracted for reasons of electroneutrality. This concerted reaction causes a decrease of micro-polarity near the dye, resulting in an increase of its fluorescence intensity along with a bathochromic shift of the emission maximum. This system can be used for pH-independent reversible sensing of chloride in the 10–100 mM concentration range with response times of the order of 3 min.