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

Direct potentiometric bicarbonate/carbon dioxide sensing based on polymeric membranes doped with selective meso-bisubstituted calix[4]pyrrole ionophores

We evaluated in this work the properties of a promising class of HCO3- ionophores, which have not been recognized previously. Three types of neutral or charged calix[4]pyrroles with meso-bisubstituted groups were evaluated as ionophores for polymeric membrane HCO3- selective electrodes. Optimizing membrane components, such as ionophores, lipophilic additives and plasticizers, yielded ISEs exhibiting Nernstian response to HCO3- with improved linear range and detection limits, while the selectivity sequence differs significantly from the Hofmeister series. Interference from important biological and environmental species was reduced significantly, especially that from SCN-, NO3-, Br- and Cl-, which are always at high concentrations in related samples. In order to provide more insights into the properties of the ionophores and performance of the proposed ISEs, the stability constants of anion-ionophore complexes in the membrane phase were determined. Studies on the influence of the sample solution pH demonstrated that the proposed ISEs can be employed in a wide pH range of 3.0-9.0 with fast response (<30 s), good reversibility and long shelf life. Moreover, the proposed ISEs were used to quantify the concentration of HCO3- and dissolved CO2 in mineral and beverage samples with good recoveries.

A tutorial on potentiometric data processing. Analysis of software for optimization of protonation constants

Defining the distribution of the chemical species in a multicomponent system is a task of great importance with applications in many fields. To clarify the identity and the abundance of the species that can be formed by the interaction of the components of a solution, it is fundamental to know the formation constants of those species. The determination of equilibrium constants is mainly performed through the analysis of experimental data obtained by different instrumental techniques. Among them, potentiometry is the elective technique for this purpose. As such, a survey was run within the NECTAR COST Action - Network for Equilibria and Chemical Thermodynamics Advanced Research, to identify the most used software for the analysis of potentiometric data and to highlight their strengths and weaknesses. The features and the calculation processes of each software were analyzed and rationalized, and a simulated titration dataset of a hypothetic hexaprotic acid was processed by each software to compare and discuss the optimized protonation constants. Moreover, further data analysis was also carried out on the original dataset including some systematic errors from different sources, as some calibration parameters, the total analytical concentration of reagents and ionic strength variations during titrations, to evaluate their impact on the refined parameters. Results showed that differences on the protonation constants estimated by the tested software are not significant, while some of the considered systematic errors affect results. Overall, it emerged that software commonly used suffer from many limitations, highlighting the urgency of new dedicated and modern tools. In this context, some guidelines for data generation and treatment are also given.

Design and application of potentiometric sensors for the determination of mefenamic and phenylanthranilic acids

Characteristics, performance and applications of potentiometric membrane sensors are described for the determination of mefenamic and phenylanthranilic ions. Ion associates of mefenamic, ClO₄^-, and phenylanthranilic ions with crystal violet (counter-cation) as ion exchange sites have been used as ionophores in the plasticized one- and two-layer membrane ion-selective electrodes. The LOD is reported to be 8.4 × 10^-5 M for mefenamic acid, and 5.1 × 10^-5 M for phenylanthranilic acid. The cations of basic dyes (crystal violet) are characterized by significant delocalization of the positive charge and polarizability. This may explain the better selectivity of the developed sensors. These sensors were used for the direct assay of mefenamic and N-phenylanthranilic acids in model solutions and applications studied in commercial pharmaceutical preparations.

A feasible, fast and reliable method for estimating ion-site association constants in plasticized PVC ion-selective electrode membranes

A feasible, fast and reliable method for estimating ion association constants in PVC plasticized membranes of ion-selective electrodes from potentiometric data has been theoretically and experimentally substantiated. The method is based on the established fact of complete dissociation of salts of quaternary ammonium cations R₄N ⁺ An^‒ (except for those containing methyl substituents at the nitrogen atom) in a membrane plasticized with o-nitrophenyl octyl ether (o-NPOE). Therefore, the boundary potential at the interface of the membrane with an aqueous solution of R₄N⁺ depends only upon the concentrations of the corresponding solution and the ion exchanger in the membrane and is independent of the presence of a lipophilic ionic additive (LIA), which makes it possible to use such ions as reference ones in the internal filling solution. If the ions studied (i⁺) are capable of forming ion associates with the ion exchanger, then the introduction of LIA into the membrane will lead to a decrease in the concentration of free i⁺ ions and to a corresponding increase in the boundary potential, from which the ion association constant can be directly calculated. The results obtained agree with the known literature data and the results of quantum chemical calculations. The prospective of applying the proposed method to the study of other membrane compositions is discussed.

Response Mechanism of Hyperpolarization-Based Polyion Nanosensors

The last decade has witnessed a rapid development of nano- and microparticle-based optical ion sensors, including ion-selective optodes (ISOs). While the application of nano-ISOs has shown promising performance for sensing inorganic ions, polyion sensing using nanoscale ISOs has encountered significant interference in complex samples such as blood plasma. Recently, we have reported on a new polyion sensing principle that operates through a novel mechanism to overcome this challenge. The new sensing mechanism showed improved characteristics not observed with conventional ion-exchange type sensors, but the precise mechanism of operation remained thus far unclear. This paper aims to clarify how protamine, the arginine-rich target polycation, behaves during optical signal transduction to give dramatically improved selectivity. Based on thermodynamic data, sensor performance and ζ-potential analysis, two discrete phases of protamine extraction are identified. Initially, protamine extracts into the bulk nanosensor phase, a process that is concurrent with the optical signal change. This is then followed by protamine accumulation onto the nanosensor surface, which starts only upon saturation of the optical signal change. The data indicate that the improved selectivity is due to the inability of small ions to form a sufficiently strong interaction with an active sensing ingredient, DNNS^-. Any exchange of one inorganic cation for another therefore remains optically silent, suppressing matrix effects. Moreover, the recognition of protamine is shown to be an exhaustive extraction process, making the response independent of the nature and concentration of the initial small cation in the nanosensor phase.

Determination of Single-Ion Partition Coefficients between Water and Plasticized PVC Membrane Using Equilibrium-Based Techniques

An experimentally simple method for the direct determination of single-ion partition coefficients between water and a PVC membrane plasticized with o-NPOE is suggested. The method uses the traditional assumption of equal single-ion partition coefficients for some reference cation and anion, in this case tetraphenylphosphonium (TPP⁺) and tetraphenylborate (TPB^-). The method is based on an integrated approach, including direct study of some salts' distribution between water and membrane phases, estimation of ion association constants, and measurements of unbiased selectivity coefficients for ions of interest, including the reference ones. The knowledge of distribution coefficients together with ion association constants allows for direct calculation of the multiple of the single-ion partition coefficients for the corresponding cation and anion, while the knowledge of unbiased selectivity coefficients together with ion association constants allows for immediate estimation of the single-ion partition coefficients for any ion under study, if the corresponding value for the reference ion is known. Both potentiometric and extraction studies are inherently equilibrium-based techniques, while traditionally accepted methods such as voltammetry and diffusion are kinetical. The inner coherent scale of single-ion partition coefficients between water and membrane phases was constructed.

Highly Selective Potentiometric Sensing of Biologically Relevant Pyrophosphate and Lysophosphatidic Acid Using N-Alkyl/Aryl Ammonium Resorcinarenes/Extended-Resorcinarenes as Ionophores

The ionophore properties of four kinds of N-alkyl/aryl ammonium resorcinarenes and extended-resorcinarenes were inspected for the first time to fabricate polymeric membrane electrodes for determination of biologically relevant pyrophosphate (PPi) and lysophosphatidic acid (LPA). The proposed ion selective electrodes (ISEs) showed significant preference for PPi and LPA with significant selectivity pattern differences from the Hofmeister series. To gain further insight into the performances of the developed ISEs, the binding constants of ionophore-anion complexes in the plasticized membrane phase were investigated, along with the optimized geometries and calculated electrostatic potential. Nernstian potential responses with good reversibility to target anions can be observed when shifting the optimized membranes in aqueous solutions in the concentration range from 10^-6.5 to 10^-2.3/10^-2.2 M. Moreover, potentiometric sensings of PPi and LPA in mineral water and artificial serum were achieved in low μM concentration range, demonstrating their promising real-world applications. These results provide a promising avenue for the development of polymeric membrane electrodes for biological relevant anions and will broaden the scope of potentiometric sensing.

Anchoring H-Bond Donating/Accepting Pyrrolic Derivatives on Preorganized Scaffolds: Conformationally Switchable Bipedal/Tripodal and Locked Molecular Cage Ionophores for Potentiometric Sensing of Phosphate and Fluoride

The ionophore properties of a myriad of conformationally switchable bipedal/tripodal receptors and locked molecular cages were evaluated here for the first time to fabricate potentiometric sensors for the determination of environmentally important phosphate and fluoride. Owing to the competent ionophore properties such as high binding selectivity and affinity, the developed ion-selective electrodes displayed response preference for phosphate and fluoride with a selectivity pattern that differs distinctly from traditional Hofmeister series. Binding constants of the ionophore-anion complexes are determined to underscore how modifications in the preorganization and H-bond donating/accepting ability of a given series of ionophores can be exploited to improve the performance for potentiometric sensing. While conformationally switchable bipedal/tripodal ionophores prefer tetrahedral oxyanions, locked molecular cages shift their preference to spherical halides gradually. Nernstian potential responses with good reversibility to target anions can be observed when shifting the optimized membrane electrodes in aqueous solutions within the concentration range of 10^-6.5-10^-2.0 M. Moreover, potentiometric determination of phosphate and fluoride in mineral water, soil, and tap water samples was achieved in a low μM concentration range with high accuracy, confirming their promising utility in real world applications.

Embedding of Functionalized Coordination Cages and a Molecular Knot in a Polymeric Membrane for Potentiometric Sensing of Environmentally Important Oxyanions and Halides

Three kinds of coordination cages and a molecular knot with inductively activated ⁺P-H, N-H, or C-H hydrogen bond donors anchoring in the functionalized cavities were inspected as ionophores to develop polymeric membrane ISEs for potentiometric sensing of environmentally important oxyanions and halides. The proposed ISEs displayed significant preference for perrhenate, phosphate, or chloride with a selectivity pattern distinctively different from the sequence depending on the Gibbs free energy of hydration owing to the high degree of shape, charge, and size selectivity originating from the rigidity and complementarity of the binding cavities. To gain further insight into the response characters of the proposed ISEs, the binding constants of ionophore-anion complexes in the membrane phase were investigated, and the binding affinity, together with the Hofmeister series, correlates well with the determined selectivity pattern of the proposed ISEs. Optimizing the composition of the membrane such as lipophilic additives and plasticizers produced ISEs displaying Nernstian/near-Nernstian potentiometric responses to primary anions with a wide linear range, improved detection limits, good reversibility, and satisfying lifetime. Potentiometric determination of perrhenate, phosphate, and chloride in river water, mineral water, and artificial serum samples was achieved with good recovery and accuracy using the proposed ISEs, demonstrating their potential for real-life applications. These results will shed new light on how novel ionophores could be designed for potentiometric sensing and broaden the scope of host-guest chemistry of coordination cages and molecular knots.

Recent improvements to the selectivity of extraction-based optical ion sensors

Optical sensors continue to demonstrate tremendous potential across a wide range of applications due to their high versatility and low cost. This feature article will focus on a number of recent advances made in improving the performance of extraction-based optical ion sensors within our group. This includes the progress of anchored solvatochromic transduction to provide pH and sample volume independent optical responses in nanoemulsion-based sensors. A recent breakthough is in polyion sensing in biological fluids that uses a novel indirect transduction mechanism that significantly improves the selectivity of dinonylnaphthalenesulfonate-based protamine sensors and its potential applications beyond polyion sensing. The role of particle stabilizers in relation to the response of emulsified sensors is shown to be important. Current challenges in the field and possible opportunities are also discussed.

Selectivity remains a constant challenge in the development of optical extraction-based sensors. Fortunately, there are several mechanistic and compositional changes with the potential to improve selectivity without developing new ionophores.

Ionophore-Based Ion-Selective Electrodes in Non-Zero Current Modes: Mechanistic Studies and the Possibilities of the Analytical Application

Abstract

This mini review briefly describes (i) literature data on the non-zero current measurements with ionophore-based ion-selective electrodes (ISEs) aimed at fundamental studies of the mechanism of their potentiometric response, and (ii) the data on the possibilities of analytical applications of ISEs in voltametric and constant potential chronoamperometric/coulometric modes, in particular the K+ ion assay in blood serum with the sensitivity of 0.1%. A special attention is paid to the basics of voltammetry and chronoamperometry/coulometry with the ionophore-based ISEs, and to how and why these methods differ from the classical voltammetry and coulometry.

Surfactants for Optode Emulsion Stabilization without Sacrificing Selectivity or Binding Constants

We compare here the effect of surfactants on ion-selective membranes measured via voltammetry and optode emulsions measured optically. Cyclic voltammetry on a thin-film ion-selective membrane is shown to be a useful screening technique for the estimation of effective complex formation constants and selectivity coefficients for different surfactants with various cations. This technique is particularly useful for its ability to identify separate ion-transfer events (free, surfactant complexed, ionophore complexed) for a specific membrane. However, we also caution against the over-reliance on this technique as changes in membrane characteristics are observed following surfactant partitioning. Of the surfactants explored here, a zwitterionic sulfobetaine-based surfactant was found to stabilize sensors without reducing effective binding constants and selectivity, with greatly superior characteristics to other commonly utilized surfactants. Those include Brij-35, F-127, and Triton X-100, all of which showed significant binding to so-called free ions in the membrane, resulting in peak potential shifts of 199 ± 10, 180 ± 24, 278 ± 11 mV, respectively, for potassium following the subtraction of transducing layer effects. This peak shift translated to a much larger undesired free ion response in optode emulsions. The selectivity in emulsion-based systems was also shown to decrease in the presence of nonionic surfactants compared to that containing the zwitterion.

Hydrogen Bond-Based Macrocyclic and Tripodal Neutral Ionophores for Highly Selective Polymeric Membrane Sulfate-Selective Electrodes

Four hydrogen bond-based macrocyclic and tripodal neutral receptors with increasing conformational complementarity with sulfate were used for the first time as ionophores to develop polymeric membrane sulfate-selective electrodes. Optimizing the membrane composition such as ionophores, lipophilic additives, and plasticizers yielded ISEs which showed Nernstian response to sulfate with the best selectivity so far and improved detection limits (a slope of -29.8 mV/dec in the linear range of 1 × 10^-6-1 × 10^-1 M with a detection limit of 5 × 10^-7 M), which led to the success of the determination of sulfate in drinking water samples and neomycine tablets. The anion-ionophore complex constants in the membrane phase were determined and correlated with the selectivity sequence of the ISEs. Studies on the influence of pH of the sample solution demonstrated that the developed ISEs can be operated in a wide pH range of 3-8 with fast response and rapid (in 1 min) and long lifetime. The success of these ionophores represents a feasible strategy for overcoming the "Hofmeister series" by employing a combination of complementarity and hydrogen bonds.

Influence of Electrolyte Coextraction on the Response of Indicator-Based Cation-Selective Optodes

Here, we report on systematic investigation of the impact of coextraction of the aqueous electrolyte and anion interference on the response of cation-selective bulk optodes. It is evident that to deliberately manage the properties of chemical sensors and to apply them in routine analysis, one should have exhaustive insight into their operation mechanism. Despite the extensive research in the field of ionophore-based optodes and numerous attempts for their practical application, the understanding of how coextraction of an aqueous electrolyte influences its response characteristics has not been developed thus far. Meanwhile, the electrolyte coextraction determines the detection limits of analogous ion-selective electrodes. A theoretical model based on phase distribution equilibrium is proposed to quantitatively describe the effect of Donnan exclusion failure on the response of polymeric plasticized optodes. The theoretical conclusions are confirmed by the results obtained with Na⁺/pH-selective optodes based on a neutral chromoionophore as a model system in solutions containing anions of various lipophilicities (Cl^-, NO₃^-, I^-, SCN^-, and ClO₄^-). For the first time, it is shown that coextraction leads to a significant shift of the response range of the optodes as well as to nonmonotonic response curves due to the transition from cationic to anionic response. An approach to estimate the coextraction constants of electrolytes from the optode response curves is proposed. The limitations in the applicability of optodes due to co-ion interference are explored. It is found that neglecting anion interference can cause dramatic errors in the results of analyses with optical sensors.

Polymeric Membrane Fluoride-Selective Electrodes Using Lewis Acidic Organo-Antimony(V) Compounds as Ionophores

Four Lewis acidic organo-antimony(V) compounds with strong binding affinity to fluoride were used for the first time as ionophores to fabricate polymeric membrane fluoride-selective electrodes. Improved detection limits and significant anti-Hofmeister selectivity could be achieved by optimizing ionophores, lipophilic additives, and plasticizers. Membrane electrodes fabricated with tetrakis-(pentafluorophenyl)stibonium (ionophore 2) performed best in detection limit, sensitivity, and selectivity. Optimal performance was obtained by fluoride with a slope of -59.5 mV/decade in the linear range of 1 × 10^-5 to 4 × 10^-2 M and a detection limit of 5 × 10^-6 M. Studies on the influence of sample solution pH demonstrate that the best pH for fluoride determination is pH 3.0. All of the electrodes studied respond rapidly (in 1 min) in different concentrations of fluoride solutions. The anion-ionophore complex constants in the membrane phase determined using the segmented sandwich membrane method correlate well with the solution-phase binding data and determined selectivity sequence of the ion-selective electrodes. The possibility of real life application of the optimized electrodes was assessed by determination of fluoride concentrations in tap water.

Emulsion Doping of Ionophores and Ion-Exchangers into Ion-Selective Electrode Membranes

Ion-selective electrodes (ISEs) are widely used analytical devices to selectively measure ionic species. Despite significant advances in recent years, ion-selective membranes are still mostly prepared in the same manner, by preloading the selective components into a solvent that is subsequently cast into a membrane or film. This paper describes an alternative method to prepare ISE membranes by mass transfer of the sensing components from an emulsion phase. Specifically, blank (undoped) plasticized poly(vinyl chloride) (PVC) membranes mounted into an electrode body are immersed into an aqueous solution containing analyte ions and an appropriate emulsion of the desired sensing components to allow their transfer into the membrane. The concept is demonstrated with conventional membrane electrodes containing an inner solution as well as all-solid-state electrodes. It is shown to be universally useful for the realization of ISEs for K⁺, Na⁺, Ca²⁺, and NO₃^-.

Polymeric Membrane Electrodes Using Calix[4]pyrrole Bis/Tetra-Phosphonate Cavitands as Ionophores for Potentiometric Acetylcholine Sensing with High Selectivity

A handful of bis/tetra-phosphonate calix[4]pyrroles with recognition sites embedding in hydrophobic cavitands were evaluated for the first time as ionophores for polymeric membrane Ach⁺-selective electrodes. Highly selective potentiometric Ach⁺ could be achieved over its analogues, especially for Ch⁺, which differs only by an acetate tail from Ach⁺. The superior performance of the proposed ISEs might be ascribed to a dual-site binding mode, in which the trimethylammonium head and acetate tail were accommodated by the phosphonate group-bridged aryl walls and the bowl-shaped aromatic cavity, through cation-π/charge-dipole interaction and the convergent four N-H···O hydrogen bonds, respectively. To gain more insight into the performance of the proposed ISEs, the cation-ionophore complex constants in the membrane phase were determined, and the binding affinity trend correlates well with the selectivity pattern. These results suggest that conformational preorganization of the ionophores and complementary weak interactions do change the selectivity of the ionophores. Studies on the influence of the sample solution pH demonstrated that the developed ISEs can be employed in a wide pH range of 4.0-9.6 with a fast response (<60 s), good reversibility, and long lifetime. Optimizing the membrane components, such as ionophores, lipophilic additives, and plasticizers, yielded ISEs, showing Nernstian responses to Ach⁺ with improved linear ranges and detection limits (a slope of -59.5 mV/dec in the linear range of 1 × 10^-6-1 × 10^-2 M with a detection limit of 3 × 10^-7 M), which led to the success of the determination of Ach⁺ in spiked urine and milk samples.

Spectroelectrochemical Evidence of Interconnected Charge and Ion Transfer in Ultrathin Membranes Modulated by a Redox Conducting Polymer

Previous publications have demonstrated the tuning of ion-transfer (IT) processes across ion-selective membranes (ISMs) with thicknesses in the nanometer order by modulating the oxidation state of a film of a conducting polymer, such as poly(3-octylthiophene) [POT], that is in back-side contact. Attempts on the theoretical description of this charge transfer (CT)-IT system have considered the Nernst equation for the CT, while there is no empirical evidence confirming this behavior. We present herein the first experimental characterization of the CT in POT films involved in different CT-IT systems. We take advantage of the absorbance change in the POT film while being oxidized, to monitor the CT linked to nonassisted and assisted ITs at the sample-ISM interface, from one to three ionophores, therefore promoting a change in the nature and number of the ITs. The CT is visualized as an independent sigmoid in different potential ranges according to the assigned IT. Herein, we have proposed a simple calculation of the empirical CT utilizing the mathematical Sigmoidal-Boltzmann model. The identification of the physical meaning of the mathematical definition of CT opens up new possibilities for the design of sensors with superior analytical features (mainly in terms of selectivity) and the calculation of apparent binding constants in the ISM.

Fluorous-Phase Ion-Selective pH Electrodes: Electrode Body and Ionophore Optimization for Measurements in the Physiological pH Range

Because of their low polarity and polarizability, fluorous sensing membranes are both hydrophobic and lipophobic and exhibit very high ion selectivities. Here, we report on a new fluorous-membrane ion-selective electrode (ISE) with a wide sensing range centered around physiologically relevant pH values. The fluorophilic tris[perfluoro(octyl)butyl]amine (N[(CH₂)₄R_f8]₃) was synthesized and tested as a new H⁺ ionophore using a redesigned electrode body that provides excellent mechanical sealing and much improved measurement reliability. In a challenging 1 M KCl background, these fluorous-phase ISEs exhibit a sensing range from pH 2.2 to 11.2, which is one of the widest working ranges reported to date for ionophore-based H⁺ ISEs. High selectivities against common interfering ions such as K⁺, Na⁺, and Ca²⁺ were determined (selectivity coefficients: logK _H, K ^pot = - 11.6; logK _H, Na ^pot = - 12.4; logK _H, Ca ^pot < - 10.2). The use of the N[(CH₂)₄R_f8]₃ ionophore with its -(CH₂)₄- spacers separating the amino group from the strongly electron-withdrawing perfluorooctyl groups improved the potentiometric selectivity as compared to the less basic tris[perfluoro(octyl)propyl]amine ionophore. The use of N[(CH₂)₄R_f8]₃ also made the ISE less prone to counter anion failure (i.e., Donnan failure) at low pH than the use of tris[perfluoro(octyl)pentyl]amine with its longer -(CH₂)₅- spacers, which more effectively shield the amino center from the perfluorooctyl groups. In addition, we exposed both conventional plasticized PVC-phase pH ISEs and fluorous-phase pH ISEs to 10% serum for 5 days. Results show that the PVC-phase ISEs lost selectivity while their fluorous-phase counterparts did not.

t-Butyl calixarene/Fe2O3@MWCNTs composite-based potentiometric sensor for determination of ivabradine hydrochloride in pharmaceutical formulations

Ivabradine hydrochloride (IVB) has shown high medical importance as it is a medication for lowering the heart rate for the symptomatic chronic heart failure and symptomatic management of stable angina pectoralis. The high dose of IVB may cause severe and prolonged bradycardia, uncontrolled blood pressure, headache, and blurred vision. In this study, a highly sensitive carbon-paste electrode (CPEs) was constructed for the potentiometric determination of IVB in pharmaceutical formulations. t-Butyl calixarene (t-BCX) was used as an ionophore due to its ability to mask IVB in the cavity via multiple H-bonding at the lower rim, as estimated quantitatively by the sandwich membrane method (Log β_ILn = 8.62). Besides, the use of multi-walled carbon nanotubes decorated with Fe₂O₃ nanoparticles (Fe₂O₃@MWCNTs) as an additive for the paste electrode significantly improved the detection limit of the sensor up to 36 nM, with Nernstian response of 58.9 mV decade^-1 in the IVB linear dynamic range of 10^-3-10^-7 M in aqueous solutions. The constructed sensors showed high selectivity against interfering species that may exist in physiological fluids or pharmaceutical formulations (e.g. Na⁺, K⁺, NH₄⁺, Ca²⁺, Mg²⁺, Ba²⁺, Fe³⁺, Co²⁺, Cr³⁺, Sr²⁺, glucose, lactose, maltose, glycine, dopamine, and ascorbic acid). The sensors were successfully employed for IVB determination in the pharmaceutical formulations (Savapran®).

Amr AEG, A. Al-Omar M, H. Kamel A, Khalifa NM. Improved Solid-Contact Nitrate Ion Selective Electrodes Based on Multi-Walled Carbon Nanotubes (MWCNTs) as an Ion-to-Electron Transducer

Possible improvement of the performance characteristics, reliability and selectivity of solid-contact nitrate ion-selective electrodes (ISE) (SC/NO3--ISE) is attained by the application of a nitron-nitrate (Nit+/NO3-) ion association complex and inserting multi-walled carbon nanotubes (MWCNTs) as an ion-to-electron transducer between the ion sensing membrane (ISM) and the electronic conductor glassy carbon (GC) substrate. The potentiometric performance of the proposed electrode revealed a Nernstian slope -55.1 ± 2.1 (r² = 0.997) mV/decade in the range from 8.0 × 10-8-1 × 10-2 M with a detection limit of 2.8 × 10-8 (1.7 ng/mL). Selectivity, repeatability and reproducibility of the proposed sensors were considerably improved as compared to the coated disc electrode (GC/NO3--ISE) without insertion of a MWCNT layer. Short-term potential stability and capacitance of the proposed sensors were tested using a current-reversal chronopotentiometric technique. The potential drift in presence of a MWCNT layer decreased from 167 μVs-1 (i.e., in absence of MWCNTs) to 16.6 μVs-1. In addition, the capacitance was enhanced from 5.99 μF (in absence of MWCNTs) to 60.3 μF (in the presence of MWCNTs). The presented electrodes were successfully applied for nitrate determination in real samples with good accuracy.

Gold Plate Electrodes Functionalized by Multiwall Carbon Nanotube Film for Potentiometric Thallium(I) Detection

Solid-contact potentiometric ion-selective electrodes (SC-ISEs) for thallium determination have been designed using multiwall carbon nanotubes (MWCNTs) as the ion-to-electron transducer. Dispersed MWCNTs were drop-casted over a gold plate electrode. Two different crown ethers were used in the sensing membrane for the recognition of thallium (I). Sensorsbased on dibenzo-18-crown-6 (DB18C6) as a neutral carrier and NaTPB as an anionic additive exhibited a near Nernstian response of 57.3 mV/decade towards Tl+ ions over the activity range 4.5 × 10-6-7.0 × 10-4 M, with a limit of detection of 3.2 × 10-7 M. The time required to achieve 95% of the steadyequilibrium potential was <10 s. The complex formation constant (log βML) between dibenzo-18-crown-6 and thallium (I) (i.e., 5.99) was measured using the sandwich membrane technique. The potential response was pH independent over the range 3.0-9.5. The introduction of MWCNTs as an electron-ion-transducer layer between gold plate and the sensing membrane lead to a smaller membrane resistance and a large double layer capacitance, which was proven using impedance spectra and chronopotentiometry (i.e., 114.9 ± 12 kΩ, 52.1 ± 3.3 pF, 200 ± 13.2 kΩ, and 50 ± 4.2 µF). Additionally, reduction ofthe water layer between the sensing membrane and the underlying conductor wastested. Thus, it is clear that MWCNTs can be used as a transducing layer in SC-ISEs. The proposed sensor was introduced as an indicator electrode for potentiometric titration of single and ternary mixtures of I-, Br-, and S2- anions.

Potentiometric Selectivities of Ionophore-Doped Ion-Selective Membranes: Concurrent Presence of Primary Ion or Interfering Ion Complexes of Multiple Stoichiometries

The selectivities of ionophore-doped ion-selective electrode (ISE) membranes are controlled by the stability and stoichiometry of the complexes between the ionophore, L, and the target and interfering ions (I ^zi and J ^zj, respectively). Well-accepted models predict how these selectivities can be optimized by selection of ideal ionophore-to-ionic site ratios, considering complex stoichiometries and ion charges. These models were developed for systems in which the target and interfering ions each form complexes of only one stoichiometry. However, for a few ISEs, the concurrent presence of two primary ion complexes of different stoichiometries, such as IL ^zi and IL₂ ^zi, was reported. Indeed, similar systems were probably often overlooked and are, in fact, more common than the exclusive formation of complexes of higher stoichiometry unless the ionophore is used in excess. Importantly, misinterpreted stoichiometries misguide the design of new ionophores and are likely to result in the formulation of ISE membranes with inferior selectivities. We show here that the presence of two or more complexes of different stoichiometries for a given ion may be inferred experimentally from careful interpretation of the potentiometric selectivities as a function of the ionophore-to-ionic site ratio or from calculations of complex concentrations using experimentally determined complex stabilities. Concurrent formation of JL ^zj and JL₂ ^zj complexes of an interfering ion is shown here to shift the ionophore-to-ionic site ratio that provides the highest selectivities. Formation of IL _n-1 ^zi and IL _n ^zi complexes of a primary ion is less of a concern because an optimized membrane typically contains an excess of ionophore, but lower than expected selectivities may be observed if the stepwise complex formation constant, K_ILn, is not sufficiently large and the ionophore-to-ionic site ratio does not markedly exceed n.

Ion Aggregation and R3N+-C(R)-H···NR3 Hydrogen Bonding in a Fluorous Phase

Potentiometric selectivities show that in fluorous ion-selective electrode membranes the tetrabutylammonium ion binds to fluorophilic proton ionophores. For the ionophore bis[3-(perfluorooctyl)propyl](2,2,2-trifluoroethyl)amine, this type of interaction is confirmed by the effect of the ionophore on the ionic conductivity of perfluoro(perhydrophenanthrene) solutions of a fluorophilic NBu₄⁺ salt. In this system, ion pairs, triple ions, and higher ionic aggregates dominate over single ions, and the ionophore increases the conductivity by favoring the formation of ion aggregates with a net charge. These observations are consistent with the formation of R₃N⁺-C(R)-H···NR₃ type hydrogen bonds between the nitrogen atom of the ionophore and the hydrogen atoms in the α position to the positively charged quaternary ammonium center of NBu₄⁺. Similar interactions were observed in a number of crystalline phases. To date, observations of C-H···N type hydrogen bonds in liquid phases have been very few, and solution-phase N⁺-C-H···N type hydrogen bonds have not been reported previously. Interestingly, no interactions between NBu₄⁺ and the more basic ionophore tridodecylamine were observed in conventional plasticized poly(vinyl chloride) membranes doped with the ionophore tridodecylamine, emphasizing the uniquely low polarity of fluorous phases.

Nano-level monitoring of Er(III) by fabrication of coated graphite electrode based on newly synthesized Schiff base as neutral carrier

Plasticized membranes using N-(-3-((thiazol-2-ylimino)methyl)benzylidene)thiazol-2-amine (S1) and 5-((-3-((5-mercapto-1,3,4-thiadiazol-2-ylimino)methyl)benzylidene)amino)-1,3,4-thiadiazole-2-thiol (S2) have been prepared and explored as Er (III) selective electrodes. Effect of various plasticizers viz. dibutylphthalate, tri-n-butylphosphate, dioctylphthalate, acetophenone, 1-chloronapthalene, o-nitrophenyloctylether, and anion excluders viz. sodium tetraphenylborate and potassium tetrakis-p-(chlorophenyl)borate was studied in detail and improved performance was observed. Optimum performance was observed for the membrane electrode having a composition of S2: PVC: o-NPOE: KTpClPB in the ratio of 4: 38: 55: 3 (w/w, mg). The performance of the PME based on S2 was compared with CGE. The electrodes exhibit Nernstian slope for Er (III) ion with detection limit 5.4 × 10(-8)mol L(-1) for PME and 6.1 × 10(-9)mol L(-1) for CGE. The response time for PME and CGE was found to be 12s and 9s respectively. The practical utility of the CGE has been demonstrated by its usage as an indicator electrode in potentiometric titration of EDTA with Er (III) solution and determination of fluoride ions in mouthwash solution. The proposed electrode was also applied to the determination of added Er(3+) ion in water and binary mixtures. It is found that the electrode could be able to recover the Er(3+) ion in 96.2-99.5%.

Potassium ion-selective fluorescent and pH independent nanosensors based on functionalized polyether macrocycles

We present here a new family of pH insensitive ion-selective optical sensors based on emulsified nanospheres containing densely functionalized 15-, 16-, 18- and 20-membered pyreneamide derivatives. These compounds were successfully synthesized by the reaction of α-diazo-β-ketoesters with cyclic ethers of the desired size in the presence of dirhodium complexes followed by a stereo-selective tandem amidation-transposition process and characterized by 1H-NMR, 13C-NMR, IR, HR-ESI-MS, UV-VIS and fluorescence spectroscopy and potentiometry. Their unique structure consisting of a crown ether ring linked to pyrene moieties through amide groups exhibits on-off switchable behavior upon binding of specific cations and allows one to incorporate these chemosensors as fluorescent ionophores into ion-exchange nanospheres. The nanosphere matrix is composed of bis(2-ethylhexyl)sebacate (DOS), poly(ethylene glycol) (PEG), sodium tetrakis 3,5-bis(trifluoromethyl)phenyl borate and pyreneamide functionalized 18-crown-6 ether (18C6). These optode nanoparticles exhibit a strong affinity to the potassium cation over other metal ions up to the millimolar concentration range in an exhaustive detection mode. The logarithmic complex formation constant was determined using the segmented sandwich membrane method and was found to be 6.5 ± 0.3 (SD) in PVC membrane plasticized with NPOE and 5.3 ± 0.3 (SD) in DOS with a 1 : 1 complex stoichiometry. The nanosensors were characterized in broad range of pH from 4 to 10 and the same linear calibration curves were obtained in the concentration range from 10-7 M to 10-5 M and thus the pH dependent response was largely overcome. These nanosensors are sufficiently stable, simple to prepare, exhibit a rapid response and their nanoscale size makes them suitable for sensing purposes in samples of limited dimensions.

A calixarene-based ion-selective electrode for thallium(I) detection

Three new calixarene Tl(+) ionophores have been utilized in Tl(+) ion-selective electrodes (ISEs) yielding Nernstian response in the concentration range of 10(-2)-10(-6)M TlNO3 with a non-optimized filling solution in a conventional liquid contact ISE configuration. The complex formation constants (logβIL) for two of the calixarene derivatives with thallium(I) (i.e. 6.44 and 5.85) were measured using the sandwich membrane technique, with the other ionophore immeasurable due to eventual precipitation of the ionophore during these long-term experiments. Furthermore, the unbiased selectivity coefficients for these ionophores displayed excellent selectivity against Zn(2+), Ca(2+), Ba(2+), Cu(2+), Cd(2+) and Al(3+) with moderate selectivity against Pb(2+), Li(+), Na(+), H(+), K(+), NH4(+) and Cs(+), noting that silver was the only significant interferent with these calixarene-based ionophores. When optimizing the filling solution in a liquid contact ISE, it was possible to achieve a lower limit of detection of approximately 8nM according to the IUPAC definition. Last, the new ionophores were also evaluated in four solid-contact (SC) designs leading to Nernstian response, with the best response noted with a SC electrode utilizing a gold substrate, a poly(3-octylthiophene) (POT) ion-to-electron transducer and a poly(methyl methacrylate)-poly(decyl methacrylate) (PMMA-PDMA) co-polymer membrane. This electrode exhibited a slope of 58.4mVdecade(-1) and a lower detection limit of 30.2nM. Due to the presence of an undesirable water layer and/or leaching of redox mediator from the graphite redox buffered SC, a coated wire electrode on gold and graphite redox buffered SC yielded grossly inferior detection limits against the polypyrrole/PVC SC and POT/PMMA-PDMA SC ISEs that did not display signs of a water layer or leaching of SC ingredients into the membrane.

Construction and performance characteristics of polymeric membrane electrode and coated graphite electrode for the selective determination of Fe³⁺ ion

Novel Fe(3+) ion-selective polymeric membrane electrodes (PMEs) were prepared using three different ionophores N-(4-(dimethylamino)benzylidene)thiazol-2-amine [L1], 5-((3-methylthiophene-2yl) methyleneamino)-1,3,4-thiadiazole-2-thiol [L2] and N-((3-methylthiophene-2yl)methylene)thiazol-2-amine [L3] and their potentiometric characteristics were discussed. Effect of various plasticizers and anion excluders was also studied in detail and improved performance was observed. The best performance was obtained for the membrane electrode having a composition of L2:PVC:o-NPOE:NaTPB as 3:38.5:56:2.5 (w/w; mg). A coated graphite electrode (CGE) was also prepared with the same composition and compared. CGE is found to perform better as it shows a wider working concentration range of 8.3×10(-8)-1.0×10(-1)molL(-1), a lower detection limit of 2.3×10(-8)molL(-1), and a near Nernstian slope of 19.5 ± 0.4 mVdecade(-1) of activity with a response time of 10s. The CGE shows a shelf life of 6 weeks and in view of high selectivity, it can be used to quantify Fe(3+) ion in water, soil, vegetable and medicinal plants. It can also be used as an indicator electrode in potentiometric titration of EDTA with Fe(3+) ion.

Study of Cobalt(III) Corrole as the Neutral Ionophore for Nitrite and Nitrate Detection via Polymeric Membrane Electrodes

Cobalt(III) 5, 10, 15-tris(4-tert-butylphenyl) corrole was synthesized and incorporated into plasticized poly(vinyl chloride) membranes and studied as a neutral carrier ionophore via potentiometry. This cobalt(III) complex has binding affinity to nitrite, and the resulting membrane electrode yields reversible and Nernstian response toward nitrite. Enhanced nitrite selectivity is observed over other anions, including lipophilic anions such as thiocyanate and perchlorate when an appropriate amount of lipophilic cationic sites are added to the membrane phase. Detection limit to nitrite is ca. 5 µM. Using tributylphosphate as the plasticizer with the cobalt(III) corrole species yields electrodes with enhanced nitrate selectivity.