Carrier-Based Ion-Selective Electrodes and Bulk Optodes. 1. General Characteristics

Mechanistic insights into the development of optical chloride sensors based on the [9]mercuracarborand-3 ionophore

Fluorescent sensing microspheres based on perhaps the most selective and practically useful chloride ionophore known, the recently reported [9]mercuracarborand-3 (MC-3), have been prepared and optimized for physiological measurements. In initial work, this ionophore was shown to yield functional optical sensing films in combination with an electrically neutral chromoionophore, ETH 5418. Unfortunately, however, these optodes suffered from unacceptably high levels of sodium interference under physiological conditions. To better understand the sensing mechanism, optical and potentiometric binding experiments were used to characterize the stoichiometry and the complex formation constants for this ionophore. It was found that the preferred stoichiometry is 1:2, rather than 1:1 as assumed earlier. The 1:2 complex is extremely stable (logbeta2 = 13.4), but a relatively strong 1:1 complex also exists (log K1 = 9.9). These characteristics were used to fabricate chloride optodes that make use of the stepwise ion-ionophore decomplexation equilibrium, by adding a calculated amount of lipophilic anion exchanger to the polymer film. Such optodes showed dramatically reduced sodium interference while maintaining the excellent selectivity of the traditional formulation. The optimized composition also shifted the measuring range to physiological conditions, making them useful for the assessment of chloride in undiluted and 10-fold-diluted blood at pH 7.4. After necessary alterations of the particle preparation procedure and sensor formulation, the new insights were used to fabricate mass-produced optical sensing microspheres with characteristics essentially identical to those of the optode sensing films.

Synthesis of an ammonium ionophore and its application in a planar ion-selective electrode

A modular technique was used to synthesize an ammonium-selective ionophore based on a cyclic depsipeptide structure. The ionophore was incorporated into a planar ion-selective electrode sensor format and the selectivity tested versus a range of metal cations in a commercial clinical diagnostic "point-of-care" instrument. Four sensor membrane formulations were tested, all of which consisted of plasticized PVC. Formulations differed as to the type of plasticizer used and whether an ionic additive was present. It was found that the membrane containing the polar plasticizer nitrophenyl octyl ether in the absence of ionic additive exhibited near-Nernstian behavior (slope, 60.1 mV/decade at 37 degrees C) and possessed high selectivity for ammonium ion over lithium and the divalent cations, calcium and magnesium (log K(POT)NH4+(j) = -7.3, -4.4, and -7.1 for lithium, calcium, and magnesium ions, respectively). The same membrane also exhibited sodium and potassium selectivity that was comparable to that reported for nonactin (log K(POT)NH4+(j) = -2.1 and -0.6 for sodium and potassium, respectively, compared to -2.4 and -0.9 in the case of nonactin). Membranes containing the less polar plasticizer, dioctyl phthalate, showed sub-Nernstian behavior (slope, <50 mV/decade at 37 degrees C). In all cases, the presence of the ionic additive potassium tetrakis(4-chlorophenyl)borate substantially reduced the selectivity observed. The flexible modular synthetic technique developed and reported here will allow the cyclic depsipeptide structure to be tuned for optimum selectivity.

Mars Surveyor Program '01 Mars Environmental Compatibility Assessment wet chemistry lab: A sensor array for chemical analysis of the Martian soil

The Mars Environmental Compatibility Assessment (MECA) instrument was designed, built, and flight qualified for the now canceled MSP (Mars Surveyor Program) '01 Lander. The MECA package consisted of a microscope, electrometer, material patch plates, and a wet chemistry laboratory (WCL). The primary goal of MECA was to analyze the Martian soil (regolith) for possible hazards to future astronauts and to provide a better understanding of Martian regolith geochemistry. The purpose of the WCL was to analyze for a range of soluble ionic chemical species and electrochemical parameters. The heart of the WCL was a sensor array of electrochemically based ion-selective electrodes (ISE). After 20 months storage at -23 degrees C and subsequent extended freeze/thawing cycles, WCL sensors were evaluated to determine both their physical durability and analytical responses. A fractional factorial calibration of the sensors was used to obtain slope, intercept, and all necessary selectivity coefficients simultaneously for selected ISEs. This calibration was used to model five cation and three anion sensors. These data were subsequently used to determine concentrations of several ions in two soil leachate simulants (based on terrestrial seawater and hypothesized Mars brine) and four actual soil samples. The WCL results were compared to simulant and soil samples using ion chromatography and inductively coupled plasma optical emission spectroscopy. The results showed that flight qualification and prolonged low-temperature storage conditions had minimal effects on the sensors. In addition, the analytical optimization method provided quantitative and qualitative data that could be used to accurately identify the chemical composition of the simulants and soils. The WCL has the ability to provide data that can be used to "read" the chemical, geological, and climatic history of Mars, as well as the potential habitability of its regolith.

An absorption-based surface plasmon resonance sensor applied to sodium ion sensing based on an ion-selective optode membrane

A surface plasmon resonance (SPR) sodium ion sensor using an ion optode membrane film was experimentally and theoretically described based on an absorption-based SPR principle proposed in our previous article (Kurihara, K; Suzuki, K. AnaL Chem. 2002, 74, 696-701). The sodium ion concentrations from 10(-6) to 10(-1) have been successfully determined not only by the resonance angle diagnosis of the SPR curve but also by the minimum reflectance one. The ion optode film was plasticized poly(vinyl chloride) including a neutral sodium ionophore, a pH-sensitive cationic dye, and an anionic additive. Its optical absorption intensity changed with the sodium ion concentrations. The SPR ion sensor physically measured the complex refractive index caused by the absorption in the ion optode film. We have exhaustively investigated the experimental response behavior of the SPR curve relative to the sodium ion concentrations by comparison with numerically simulated SPR curves using a three-layer Fresnel equation including experimental values for the sodium ion optode membrane film. As predicted by the absorption-based SPR principle, the SPR curve behavior of the SPR ion sensors depended on two factors: one was the relation between the excitation frequency of the light source and the absorption maximum frequency in the ion optode film while the other was the gold metallic thickness in the Kretchmann configuration. The concept and practical theory of an absorption-based SPR sensor not only have been proved by the experimental results of the SPR sodium ion sensor but also have successfully allowed flexible ion sensing in an SPR sensor, which would be very difficult without the absorption mechanism in the ion optode film.

Application of an absorption-based surface plasmon resonance principle to the development of SPR ammonium ion and enzyme sensors

Two new types of surface plasmon resonance (SPR) sensors that can determine the concentration of ammonium cations and urea were realized based on the previously reported theory of the absorption-based SPR measurement method. The change of the dielectric constant caused by the change of the light absorption characteristics of dyes incorporated in a sensing membrane phase is utilized in these SPR sensors. The determination of ions using the SPR sensor was realized by detecting the SPR signals of the minimum reflectance related to the change of absorption spectra of the dye in the ion optode membrane consisting of an ammonium-selective ionophore (TD19C6) and a lipophilic cationic dye (KD-M11) that shows absorption spectral changes due to protonation and deprotonation. A SPR enzyme sensor that can determine the concentration of urea was prepared by the combination of this ion optode membrane and an enzyme membrane based on urease. With the newly developed SPR sensors, the intensity changes of the reflectance at the fixed SPR resonance angle are monitored, which is different from conventional SPR sensors where usually the change of the SPR resonance angles is detected. In a continuous-flow experiment using the SPR ion sensor for NH4+ ion determination, a dynamic measurement range from 10(-5) to 10(-2) M was achieved. In the case of the enzyme-based SPR urea sensor, a dynamic range from 10(-4) to 10(-1) M was observed in a stopped-flow batch arrangement. It is expected that this sensing technique can be applied for the SPR-based detection of a wide range of low molecular weight analytes.

Lonophore-based lithium ion film optode realizing multiple color variations utilizing digital color analysis

Digital color analysis (DCA), utilizing colors themselves or digital information of colors, can not only be applied to various quantitative analysis using chromaticity coordinates but can also be used to develop suitable sensors for visual colorimetry based on the characteristics of human visual perception by virtual simulations based on digital color information. To achieve a clear visual color variation for lithium ion determination, we designed and prepared a color-changeable film sensor (film optode) by the use of two kinds of lipophilic dyes, KD-C4 and KD-M11, whose colors and pKa values are different. This film sensor is a plasticized PVC membrane containing the mixture of two kinds of dyes with the lithium ionophore TTD14C4 and the lipophilic anionic additive tetrakis-[3,5-bis(trisfluoromethyl)phenyl]borate sodium salt dihydrate. The simulation of the color variation using the mixed dyes was evaluated by plotting the values on a uniform chromaticity scale diagram in a*b* coordinates, after converting the tristimulus values of each dye into its L*a*b* values. When the lithium ion concentration was actually determined by the PVC film optode containing the mixed dyes whose molecular ratio of KD-C4/KD-Ml 1 was 3:1, the hues of red --> orange --> yellow --> green --> blue could be realized in the range of 10(-6)-1 M. This observed color variation was similar to the result of the virtual simulation based on DCA.

Lanthanum(IlI) PVC membrane electrodes based on 1,3,5-trithiacyclohexane

Novel plasticized polymeric membrane (PPME) and membrane-coated graphite (MCGE) electrodes based on 1,3,5-trithiacyclohexane for highly selective determination of La3+ ion have been developed. The electrodes exhibit Nernstian responses over very wide concentration ranges (8.0 x 10(-6)-5.0 x 10(-2) M for PPME and 4.0 x 10(-8)-1.0 x 10(-2) M for MCGE). The limit of detections were 5.0 x 10(-6) and 2.0 x 10(-8) M for PPME and MCGE, respectively. The electrodes possess a fast response time of approximately 10 s and can be used for at least 6 months without observing any deviation. The proposed electrodes revealed excellent selectivities for La3+ over a wide variety of alkali, alkaline earth, transition, and heavy metal ions and could be used in a pH range of 5.0-8.0. The practical utility of the electrodes has been demonstrated by their use as indicator electrodes in the potentiometric titration of La3+ ions with EDTA and in determination of F- in some mouthwash preparations.

Flow cytometric ion detection with plasticized poly(vinyl chloride) microspheres containing selective lonophores

The use of flow cytometry as a rapid, high-throughput diagnostic tool for the analysis of ions is described. Monodisperse, uniform microspheres, which obey bulk optode theory and are governed by bulk extraction processes rather than surface phenomena, were prepared under mild, nonreactive conditions using a sonic stream particle casting apparatus. As an initial example demonstrating the utility of this approach, microspheres that contained a H+-selective fluorescent chromoionophore (ETH 5294), a cation-exchanger (NaTFPB), and either a highly sodium-selective (sodium ionophore X) or a potassium-selective ionophore (BME-44) were prepared. Separate solution analysis of sodium- and potassium-selective microspheres resulted in the generation of functional response curves using peak channel fluorescence intensities. The selectivity observed for both types of particles is sufficient for the clinical determination of Na+ and K+. Furthermore, sodium- and potassium-selective microspheres were analyzed in parallel using sodium sample solutions, resulting in the successful determination of sodium ion concentrations and providing important information about the selectivity of the potassium-selective sensors over sodium. This work demonstrates the potential applicability of flow cytometry as a means for developing multiplexed, rapid, high-throughput analyses for clinically relevant ions.

Potentiometric determination of fentanyl in pharmaceutical formulations

A poly (vinyl chloride) membrane electrode with dibutyl phthalate as plasticizer based on the fentanyl-phosphotungstate ion-association complex as ion-exchange site for the determination of fentanyl citrate in injections was developed. A linear response for 1 x 10(-5)-1 x 10(-2) mol dm(-3) drug with a slope of 55.9 +/- 0.4 mV per decade was established. The optimum pH range was 1-7. The lower detection limit was 5.43 x 10(-6) mol dm(-3) fentanyl citrate (1.827 microg cm(-3), 2.33 x 10(-6) mol dm(-3) fentanyl). There were negligible interference from a number of inorganic cations, structural analogues, and some common drug additives in injections. The electrode proposed had been successfully applied to determine fentanyl citrate in injections. The results correlated well with those obtained by the United States Pharmacopoeia standard procedure.

Mass-produced lonophore-based fluorescent microspheres for trace level determination of lead ions

The development and characterization of small, uniform, and mass-produced plasticized PVC-based sensing microspheres in view of rapid trace level analysis of lead ions is reported. Micrometer-sized particles obtained via an automated casting process were rendered selective for lead ions by doping them with highly selective components in a manner analogous to traditional optode sensing films. Single particles that contained the lipophilic ionophore N,N,N',N'-tetradodecyl-3-6-dioxaoctane-1-thio-8-oxodiamide (ETH 5493), the chromoionophore ETH 5418 together with a lipophilized indocarbocyanine derivative as internal reference dye (DiIC18), and lipophilic ion-exchanger sites sodium tetrakis[3,5-bistrifluoromethylphenyl]borate, yielded measurable lead responses at the low nanomolar level in pH buffered solutions. The detection limit for single particles was 3 x 10(-9) M at pH 5.7. The microspheres were fabricated via a reproducible formation of polymer droplets within a flowing aqueous phase followed by collection of spherical particles of approximately 13 microm in size. The particles were immobilized and assayed individually in a microflow cell via fluorescence microscopy. Selectivity patterns found were in agreement with those reported earlier for the lead-selective ligand ETH 5493, and all response functions were fully described by theory. In contrast to optode films that necessitated very long equilibration times and large sample volumes in diluted samples of analyte, particles exhibited extremely enhanced equilibrium response times. Thus, for lead sample concentrations at and above 5 x 10(-8) M, response times were approximately 3 min, whereas at the detection limit, complete equilibrium was recorded after just 15 min, with required sample volumes on the order of 1 mL This new class of microspheres appears to be suitable for rapid and sensitive ion detection at trace levels in environmental and biological applications.

A fiber-optic fluorescence sensor for lithium ion in acetonitrile

A novel fiber-optic fluorescence sensor based on a controlled-release reagent for the determination of lithium ion in organic solvents is proposed. The fluorogenic indicator 2-(2-hydroxyphenyl)benzoxazole is contained in a mini-polyethylene tube as the reagent reservoir and is brought into contact with the analyte solution by diffusion across a poly(vinyl chloride) (PVC) membrane to form a strongly fluorescent complex at the membrane/solution interface. The fluorescence signals produced are measured via two joined optical fibers positioned closely to the backside of the PVC membrane for light illumination and collection. The sensor is useful for measuring Li+ at concentrations in acetonitrile ranging from 1.0 x 10(-6) to 1.0 x 10(-2) M with a detection limit of 3.0 x 10(-7) M. The steady-state response can be reached within seconds, and the signal changes are fully reversible. The sensor shows minimal interference effects from other alkali metal and alkaline earth metal cations and has good stability and durability when stored in acetonitrile solutions.

Rational design of potentiometric trace level ion sensors. A Ag+-selective electrode with a 100 ppt detection limit

Submicromolar to picomolar lower detection limits have recently been obtained with various polymer membrane ion-selective electrodes by minimizing biases due to ion fluxes through the membrane. For the best performance, the compositions of the membrane and inner solution should be optimized for each application. Given the number of parameters to be adjusted, it has been difficult to find the best parameters for a target sample. In this paper, a much simplified and more practical steady-state model of zero-current ion fluxes is derived, which is based on measurable parameters. The model allows one to predict achievable lower detection limits for a membrane with given selectivities. It can also be used to predict the optimal composition of the inner filling solution for the measurement of samples with a known, typical ionic background. Selectivity coefficients of monovalent and divalent analyte ions required for desired detection limits in drinking water are calculated. As an application of the proposed general recipe, a silver-selective electrode is developed on the basis of the ionophore O,O''-bis[2-(methylthio)ethyl]-tert-butylcalix[4]arene. With the predicted optimal composition of the inner electrolyte, its lower detection limit is found to be 10(-9) M or 100 ppt Ag+ with an ionic background of 10(-5) M LiNO3, which is very close to the expected value.

Aroylthioureas: new organic ionophores for heavy metal ion selective electrodes. A nuclear magnetic resonance study

The 1H and 13C NMR spectra of four series of 1-aroylthiourea derivatives in DMSO-d6 are reported. The NH signals for 3-alkyl substituted aroylthioureas are identified by their multiplicity and by homonuclear irradiation experiments. Correlation analyzes are made for NH, CO and CS signals in order to determine the best way to modulate the nucleophilic character of the CS group, as thioureas are well-known ionophore groups. Almost all 1,3-substituted thioureas (Series 2-4) show the reported chelated structure with the exception of those with CF3, CN and NO2 groups. Pyridine group promote a different equilibrium in solution. The fragment -CO-NH- transmits poorly the electronic effects of substituents in the aroylgroup.

Chromium(III) porphyrin as a selective ionophore in a salicylate-selective membrane electrode

The construction, potentiometric response properties, and applications of a novel ion-selective electrode with high selectivity toward salicylate are described. Chromium(III) tetraphenylporphyrin chloride was used as ion carrier into plasticized PVC membrane. This ionophore is capable of serving as both a positively charged and neutral carrier, depending on the pH of the sample solution. The influence of several variables was investigated to optimize the potentiometric response and selectivity of the electrode. The resulting electrode demonstrates a near-Nernstian response over a wide range of salicylate concentration (10(-6)-10(-1) M). This electrode has a fast response time and micromolar detection limit and could be used over a wide pH range (3-9). The proposed electrode showed very high selectivity for salicylate over a number of common inorganic and organic anions. The specific interaction of salicylate with the central metal of porphyrin is described based on UV-visible absorption spectra. The electrode was successfully applied to the determination of salicylate in pharmaceutical preparations and clinical samples.

Evaluation of the separate equilibrium processes that dictate the upper detection limit of neutral ionophore-based potentiometric sensors

The upper detection limit of polar ionophore-based ion-selective electrode membranes is predicted by utilizing the coextraction constant of dissociated electrolyte, the stability constant of the ionophore, and the membrane composition. The coextraction constant of dissociated electrolytes into the polar poly(vinyl chloride) membrane plasticized with o-nitrophenyl octyl ether (PVC-NPOE) is here measured by a novel approach. The sandwich membrane technique is utilized, with one membrane segment containing a lipophilic cation exchanger and the other containing an anion exchanger. This yields information about the coextraction constant and the free ion concentrations of the electrolyte in the two segments. Predictions correlate quantitatively with the upper detection limit observed for ion-selective electrodes based on the ionophores valinomycin, tert-butylcalix[4]arene tetraethyl ester, and calcimycin. The difficulties of the prediction of the upper detection limit for nonpolar poly(vinyl chloride) membranes plasticized with bis(2-ethylhexyl sebacate) (PVC-DOS) due to ion association are discussed in detail. A thermodynamic cycle experiment with a series of sandwich membranes shows that the principal processes governing the upper detection limit of PVC-DOS membranes are identical to those for the PVC-NPOE membranes. However, the stability of the ion pairs between the ionophore-metal ion complexes and the extracted anion are different from that of ion pairs formed between the same anion and the lipophilic anion exchanger. This makes it difficult to quantitatively predict the upper detection limit on the basis of simple apparent coextraction and complexation data alone. The approach reported herein is useful not only for mechanistic purposes but also to shed light onto the many cases where coextraction effects need to be understood but are not directly experimentally accessible.

Tweezer-type neutral carrier-based calcium-selective membrane electrode with drastically reduced anionic interference

A new calcium-selective ionophore N,N-dioctyl-3alpha,12alpha-bis(N-heptyl-N-methylcarbamoyl-methoxyacetamidoacetoxy)-5beta-cholan-24-amide (denoted as BACA), was synthesized, and its potentiometric performance has been evaluated in comparison with that of the best known calcium-selective neutral carriers, ETH 129 and ETH 1001. The 1H NMR spectra of BACA titrated with Ca(SCN)2 suggest that BACA forms a 1:1 complex, tweezing a calcium ion between the two parallel diamide groups substituted on a rigid bile acid frame. The calcium-selective membrane based on BACA was less selective to calcium (log K(Ca2+ j)POT = -4.2, -4.2, -4.6, and -4.8, respectively, for j = Mg2+, Li+, Na+, and K+) than those based on ETH 129 (log K(Ca2+ j)POT = -4.4, -4.3, -5.4, and -5.4, respectively, in the same order) and ETH 1001 (log K(Ca2+ j)POT = -4.4, -4.4, -5.4, and -5.4), implying that BACA forms a weaker calcium complex than the other two ETH compounds. In our experimental conditions, potentiometrically determined complex formation constants of calcium-selective neutral carriers (log beta(Ca2+ L)) were 15.2, 14.0, and 8.6 for ETH 129, ETH 1001, and BACA, respectively. A slightly reduced calcium selectivity of BACA, however, affects the anionic interference-free calcium-selective membrane; the BACA-based membrane exhibits a Nernstian response up to 10(-1) M Ca2+ in the presence of lipophilic anions (e.g., SCN-, ClO4-, salicylate, and I-) and anionic surfactant, whereas the ETH 129- and ETH 1001-based ones suffer from serious anionic interference showing a curvature or leveled off response over about 10(-4) M. It was demonstrated that such a trade off does not affect the analytical performance of BACA-based electrode in most applications, including clinical analysis.

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.

Perbrominated closo-dodecacarborane anion, 1-HCB11Br11-, as an ion exchanger in cation-selective chemical sensors

The 2,3,4,5,6,7,8,9,10,1 1,12-undecabromocarborane anion, 1-HCB11Br11- (UBC-) has been evaluated for its suitability as an ion exchanger in solvent polymeric membrane electrodes and bulk optodes. Experiments comparing the chemical stability of the perhalogenated carborane anion to that of the best lipophilic tetraphenylborate, 3,5-[bis(trifluoromethyl)phenyl]borate (TFPB-), demonstrated that in the presence of 0.2 M acetic acid TFPB- was completely lost within 6 h, while the concentration of UBC- decreased by less than 10% in the same time period. Thin-film bulk optodes containing BME-44 as potassium-selective ionophore, ETH 5294 as chromo-ionophore, and UBC- as ionic sites exhibited a K+ response similar to analogous optodes containing TFPB-, with comparable selectivities over Na+ and Ca2+. Potentiometric measurements evaluating the selectivity behavior of UBC- in both ionophore-free and ionophore-containing electrodes were performed. Ionophore-free PVC membranes containing UBC- as ion exchanger and either DOS or NPOE as plasticizer also demonstrated selectivity similar to TFPB--containing membranes. Sodium-selective membranes containing the ionophore 4-tert-butylcalix[4]arenetetraacetic acid tetraethyl ester (sodium ionophore X) and UBC- as ionic sites showed a Nernstian response for sodium and selectivity comparable to that found in analogous electrodes containing TFPB-.

Highly selective iodide membrane electrode based on a cerium salen

A highly selective PVC membrane electrode based on a cerium-salen complex was prepared. The sensor displays an anti-Hofmeister selectivity sequence with a preference for iodide ion over many common organic and inorganic anions. The proposed electrode exhibits a near-Nernstian behavior over a wide concentration range (5.0 x 10(-2) - 8.0 x 10(-6) M) with a slope of 57.5 mV per decade, and a detection limit of 6.0 x 10(-6) M. The electrode has a very fast response time and can be used in the pH range of 3.0 - 1 1.0. It was applied, as an indicator electrode, in potentiometric titration of Ag+ ions.

A perchlorate-selective membrane electrode based on a Cu(II) complex of the ligand 1,4,8,11-tetra(n-octyl)-1,4,8,11-tetraazacyclotetradecane

The new cyclic polyazacycloalkane 1.4,8,11-tetra(n-octyl)-1,4,8,11-tetraazacyclotetradecane (L1) was synthesised and the copper(II) complex [Cu(L1)]2+ characterised. Different electrodes were prepared using the [Cu(L1)]2+ complex as ionophore, PVC as plastic matrix and o-nitrophenyl octyl ether (NPOE), bis(2-ethylhexyl) sebacate (BEHS) or dibutyl phthalate (DBP) as plasticizers. The electrode containing DBP showed a Nernstian response over a wide pH range and a fast response time (ca. 3 s) whereas NPOE and BEHS gave near-Nernstian slopes. Selectivity coefficients for the different anions with respect to perchlorate were calculated. The response of the electrodes basically followed the Hofmeister sequence, suggesting that interaction of the ionophore with the anions is via electrostatic forces rather than due to anion coordination to the axial sites of the square-planar [Cu(L1)]2+ complex.

Selectivity of lithium electrodes: correlation with ion-lonophore complex stability constants and with interfacial exchange current densities

Lithium-selective electrodes with solvent polymeric membranes based on two different dicyclohexylamide neutral ionophores are studied systematically. The selectivity of lithium response is studied by means of the ordinary potentiometric experiments. Stability constants of lithium, sodium, and potassium ions with the neutral ionophores are measured by means of the segmented sandwich membrane method. Charge transfer through the membrane bulk and across the membrane/solution interface is studied by means of electrochemical impedance spectroscopy. Well-resolved Faradaic impedance semicircles are obtained, allowing calculation of exchange current densities for lithium, sodium, and potassium. It is clearly demonstrated that the potentiometric selectivity coefficients correlate well with thermodynamic equilibrium parameters. The correlation with exchange current densities also exists, although it is low, and seems rather qualitative than quantitative. The results are treated in favor of equilibrium at the membrane/solution interface. It is also concluded (tentatively) that the kinetic description is equivalent to the equilibrium one, giving evidence that ion-ionophore complexes form directly at the interface.

Ketocyanine dyes: H+ selective lonophores for use in integrated waveguides absorbance optodes

The optical and analytical characteristics of a series of neutral H+-selective chromoionophores in PVC membranes are described. Such indicators have been synthesized so that they can operate in bulk optode membranes as the chemically active region of integrated waveguide absorbance optodes (IWAOs), lambdamax near 780 nm. Their spectral characteristics, acid-base properties, chemical stability, and solubility in the membrane phase are given and discussed. The response characteristics are first tested in a conventional absorbance/transmittance flow cell. They offer a wide range of pKa's in PVC membranes, good sensitivity as a result of their high molar absortivities, excellent solubility in the plasticizer, and quick response times. They present good chemical stability in common laboratory conditions when stored in the dark, and the absence of leaching guarantees a long lifetime. Membranes have been finally applied as the sensing region of an integrated waveguide optode, demonstrating the extraordinary sensitivity improvement while preserving the remaining analytical features. Calibration curve slopes are multiplied by 3-28, and response times lower than 2 min are obtained.

Potentiometric estimation of the stability constants of ion-lonophore complexes in ion-selective membranes by the sandwich membrane method: theory, advantages, and limitations

Segmented sandwich membrane method of studying stoichiometry and stability constants of ion-ionophore complexes in ion-selective membranes is considered in detail. The experimental data (reported earlier in Russian) concerning complexes of various ions with valinomycin, with H+-selective neutral ionophore hexabutyltriamidophosphate, and with anion-binding neutral ionophore p-hexyl trifluoroacetylbenzoate is presented in a compact form. Advantages of titration technique in the sandwich membrane method (the presence of an internal criterion of reliability, and the possibility of direct determination of complex stoichiometry coefficients) are specially addressed. Biases of the estimates of the constants caused by ion-pair formation in real membranes and by diffusion potential are analyzed by means of computer simulations. The possibility of revealing two coexisting complexes with different compositions is also discussed.