Neutral Ionophores Research Articles

Peculiarities of the potentiometric response of ISEs with membranes containing two neutral ionophores and an excess of ion-exchanger: Experiment and modeling

The potentiometric response of ion-selective electrodes (ISEs) with membranes simultaneously containing two ionophores: valinomycin and lithium ionophore Li VIII, as well as the cation exchanger potassium tetrakis(p-Cl-phenyl) borate in different concentrations is studied in KCl, LiCl and KCl + LiCl solutions. It is shown that ISEs with a membrane containing an excess of ionophores over the ion-exchanger provide Nernstian response to K+ and to Li+ ions. However, membranes with an excess of the ion-exchanger over valinomycin show Nernstian response to K+ only in mixed KCl + LiCl solutions with constant ratios of the concentrations of these electrolytes. On the contrary, the response to Li+ is obtained both in mixed and pure LiCl solutions. The results are semi-quantitatively explained using a simple mathematical model and respective computer simulation.

Transfer of Sodium Ion across Interface between Na+-Selective Electrode Membrane and Aqueous Electrolyte Solution: Can We Use Nernst Equation If Current Flows through Electrode?

Electrochemical impedance and chronopotentiometric measurements with Na+-selective solvent polymeric (PVC) membranes containing a neutral ionophore and a cation exchanger revealed low-frequency resistance, which is ascribed to Na+ ion transfer across the interface between the membrane and aqueous solution. The attribution is based on the observed regular dependence of this resistance on the concentration of Na+ in solutions. The respective values of the exchange current densities were found to be significantly larger than the currents flowing through ion-selective electrodes (ISEs) during an analysis in non-zero-current mode. This fact suggests that the interfacial electrochemical equilibrium is not violated by the current flow and implies that the Nernst equation can be applied to interpret the data obtained in non-zero-current mode, e.g., constant potential coulometry.

Finding a perfect match of ion-exchanger and plasticizer for ion-selective sensors

Application of neutral ionophore based ion-selective sensors requires presence of ion-exchanger in the receptor phase, silently assuming that it is not only soluble but also dissociates to ions in the applied plasticizer. Although for typically applied ion-selective membrane constituents (plasticizers - ion-exchanger pairs) dissociation of ion-exchangers to ions is proven by theoretical (or close to) performance of resulting sensors, search for alternative plasticizers or ion-exchangers requires a method allowing estimation of the match of properties of involved compounds. In this work we propose a simple optical approach allowing estimation of ion-exchanger interactions with plasticizer. The results were confirmed by conductivity studies of model plasticizers solutions. The estimated dissociation constants of model ion-exchangers in plasticizers used are in excellent agreement with the results of optical studies. It was shown that solubility coupled with poor dissociation to ions of ion-exchanger affects performance of the resulting ion-selective membrane. Rational choice of properties of ion-exchanger and plasticizer allows finding a perfect match of the two, that results in improvements in performance of sensors (e.g. detection limits). As model sensors potassium and sodium ion-selective electrodes with poly(vinyl chloride) (PVC) based membranes, plasticized with classical plasticizer bis(2-ethylhexyl sebacate) or biodegradable alternative acetyl tributyl citrate, were prepared and studied using selected ion-exchangers.

Looking into the ion-selective nanoptode bulk – Alternative mechanism of optical signal transduction for ratiometric ion-selective probes applicable in broad pH range

Ion-selective sensors, including optodes, explore ion-exchange at the interface to observe analyte ion binding to neutral ionophore in the probe bulk. Typically the optical transducer used is a charged dye - incorporation of analyte cation requires expulsion of positively charged ion from the probe, the process is giving rise to observable optical signal. We propose an alternative approach focused on the probe bulk and allowing application of uncharged dye as the optical transducer. The neutral solvatochromic dye is not involved in maintaining electroneutrality, but its optical spectrum reflects local changes occurring in the probe bulk in response to analyte incorporation, giving rise to analytical signal dependent on the analyte concentration changes in the sample. The proposed optical transduction mechanism offers increase of emission for increase of analyte contents in the sample in a broad concentration range, a linear dependence of optical signal on logarithm of concentration was observed. Moreover, this approach results in significant benefits: it mitigates the cross sensitivity to hydrogen ions concentration changes observed for classical optodes, allowing application of probes in unbuffered samples as well as in wide pH range, including acidic solution. The system requires the presence of adjunctive, cationic species in the probe, that are exchanged for analyte cations preferred in the system due to interactions with ionophore. Depending on the choice of adjunctive cation ratiometric sensing can be achieved. It is also shown that such system does not require the presence of a surfactant, yet allows formation of stable emulsion of nanoprobes.

Silver-ligand complex as an additive in polymeric membranes of screen-printed fluoride-selective electrodes

Fluoride-selective electrodes with membranes containing zirconium porphyrin and dithizone-silver complex were developed. Silver disc electrodes and devices fabricated by screen-printing with silver ink were used as transducers. Dithizone-silver complex ensures potential stability of prepared electrodes, forming a redox couple with silver metal transducers. Moreover, it serves as lipophilic cationic site in the membrane, inducing zirconium porphyrin to act as a neutral ionophore. Electrodes with these membranes showed the following selectivity sequence: ClO4− > F− > NO3– > Cl−. Linear response towards fluoride was observed from 10−4 M to 10–1 M. Charged carrier mechanism is also possible, requiring polar plasticizer and addition of anionic lipophilic sites to the membrane. While membranes with ionophore working as charged carrier show better discrimination of lipophilic anions (selectivity sequence: F− > ClO4− > NO3– > Cl−), dynamic range of fluoride response is narrower in this case (10−4 M to 10−2 M), as compared to membranes with ionophore working as neutral carrier. Screen-printed electrodes with membranes containing dithizone-silver complex were integrated with reference electrodes with membranes containing ionic liquid, prepared on the same transducers. These devices were successfully employed for determination of fluoride ion in pharmaceutical samples.

Ion–ionophore interactions in polymeric membranes studied by thin layer voltammetry

In ion sensing applications, selective ion-receptor complexation is the molecular basis for endowing the sensing material with selectivity. In this work, thin polymeric membrane-based ion transfer voltammetry is used to investigate ion-receptor complexation, using a range of electrically neutral ionophores and surfactants as examples. Previous studies lacked a convincing approach to eliminate the influence from transducing layer, resulting in deviations of the observed binding constants compared to potentiometric methods. A recently developed method allows for subtracting the potential changes of the transducing layer, thereby overcoming this challenge. Using this approach, a range of ionophores are assessed. Valinomycin for the detection of potassium gave a logarithmic complex formation constant in the membrane of 9.69 ± 0.25 with a 1:1 stoichiometry. Lithium ionophore VI for lithium gave a logarithmic stability constant of 5.97 ± 0.06 with 1:2 complexes; while sodium ionophore X for sodium (7.57 ± 0.03, 1:1) and calcium ionophore IV for calcium (21.57 ± 0.25, 1:3) were also characterized, in addition to their complexes with potential interfering ions. The complex formation of three surfactants with potassium are also explored in membranes containing valinomycin, with Brij-35 (4.88 ± 0.08, 1:1), Triton X-100 (5.63 ± 0.10, 1:1), F-127 (4.63 ± 0.49, 1:1). Limitations of the approach are discussed, which includes the need for electrochemical reversibility and a sufficiently high lipophilicity to adequately retain the components in the membrane

Registration of small (below 1%) changes of calcium ion concentration in aqueous solutions and in serum by the constant potential coulometric method

Small changes (down to 0.1%) of Ca2+ ion activity in pure solutions of CaCl2, model solutions mimicking blood serum, and blood serum sample are registered by constant potential coulometric measurements, using solid contact ion-selective electrodes (ISEs) with poly(vinyl chloride) (PVC) membrane containing ETH 1001 as neutral ionophore. A new measurement protocol is proposed aimed at a significant improvement of the signal-to-noise ratio in the current measurements. A procedure is developed for the PEDOT-PSS deposition on glassy carbon from mixed water-acetonitrile solution. Data are obtained in support of Kragh-Hansen and Vorum’s approach for the quantification of the interaction between Ca2+ ions and bovine serum albumin (BSA).

PVC Membrane Electrode Modified by Lawson as Synthetic Derivative Ionophore for Determination of Cadmium in Alloy and Wastewater

In this study, a PVC membrane electrode was modified by a derivative of lawason that is a neutral ionophore for Cd (II) determination in alloy and wastewater as real samples. The modified electrode showed a linear electrochemical response to Cd (II) concentration between 5.0×10−8 to 1.5×10−1 molL-1 (LOD = 2.51×10−8 molL-1). In selecting and determining the level of an effective composite of the membrane, the highest results were achieved in the membrane composition 33% PVC, 64.0% TEHP, 1.5% NaTPB, and 1.5% ionophore. The time it takes for the response to reach its maximum and remain stable at this electrode is very favorable (response time < 12 s). The responses of the electrode in consecutive uses for two months did not deviate significantly. In investigating the disturbance of other ions and compounds in the measurement of cadmium, the designed electrode had a very good selectivity for cadmium. The best conditions for measuring cadmium with this electrode are in an acidic environment (pH range of 2.8–6.5). In this research, potentiometric titration with the use of chromate as titrant was used in water and alloy samples.

Ni(II)-selective PVC membrane sensor based on 1,2,4-triazole bis Schiff base ionophore: Synthesis, characterization and application for potentiometric titration of Ni2+ ions against EDTA

This study involves the preparation and investigation of a novel and highly selective poly(vinyl chloride)-based membrane of 2-((5-(2-hydroxy-3-methoxybenzylideneamino)-2H-1,2,4-triazol-3-ylimino)methyl)-6-methoxyphenol Schiff base ligand (HMBT), which is a neutral ionophore with sodium tetraphenyl borate (STB) in the form of an excluder and o -nitrophenyloctyl ether ( o -NPOE) in the form of solvent mediators (plasticizing) as a Ni(II)-selective electrode. The observation of optimal performance was done wherein the membrane was shown to have the HMBT–PVC–NPOE-STB composition of 4:32:63:1.It worked effectively across a broad range of concentration (1.0 × 10 −8 to 1.0 × 10 −2 mol L −1 ). Meanwhile, the Nernstian slope was recorded as 29.3 mV per decade of activity between pH 3.0 and 8.0. The response time of this electrode was fast at 11 s which was used for a span of 100 days with sound reproducibility. According to the selectivity coefficients for trivalent, divalent, and monovalent cations, excellent selectivity was indicated for Ni(II) ions across a large number of citations, whereas no interference was caused by anions like PO 4 3− , SO 4 2− and Cl − . The proposed method in this study was applied successfully to determine Ni(II) content in different samples of water, obtaining suitable recoveries. Additionally, the probed sensor is utilized as indicator electrode when considering Ni 2+ ion potentiometric titration against EDTA. In addition, the chelate’s geometry and structure of the complex formed between Ni 2+ ions and HMBT, abbreviated as HMBT-Ni 2 , was evaluated by separating the solid product. Complex structure was confirmed based on alternative analytical and spectral methods to be structured in the bimetallic form with the formula [Ni 2 (HMBT)(H 2 O) 2 Cl 2 ]. The diamagnetic nature of the complex, which was concluded from the room temperature magnetic moment measurement combined with the UV–Vis measurement, suggested the square planar geometry around the Ni centers.

A Novel Potentiometric Ion Selective Electrode Based on Piperine for the Determination of Fe(III) Ions

A new potentiometric sensor highly selective to Fe3+ was prepared using piperine as a neutral ionophore. The electrode exhibits a linear response for Fe3+ ions in the citrate buffer medium over a concentration range from 1 × 10–4 to 1 M (detection limit of 6 × 10–5 M) with a Nernstian slope of 20.5 mV per decade. The electrode can be successfully used in the pH range of 3.15–7.85. It has a fairly good discriminating ability toward Fe3+ ions among some other metal ions such as K+, Ca2+, Mg2+, Cu2+, Ba2+, Co2+, Al3+ and Cr3+ with selectivity coefficients of –2.11, –9.02, –10.38, –8.34, –8.12, –8.79, –8.41 and –3.69, respectively. The electrode can be used for 10 weeks without any considerable divergence in the observed slope. It has been successfully validated using atomic absorption spectrometry and used in the determination of Fe3+ in chewable iron capsules.

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.

In situ ATR-FTIR spectroscopic imaging of PVC, plasticizer and water in solvent-polymeric ion-selective membrane containing Cd2+-selective neutral ionophore

Concentration profiles of water, polymer and plasticizer in the vicinity of the membrane/water interface were studied by in situ ATR-FTIR spectroscopic imaging with Cd2+-selective membranes as a model system. Two membrane compositions have been studied, both contained polyvinylchloride (PVC) as matrix, 2-nitrophenyl octyl ether (oNPOE) as plasticizer, Cd2+-selective neutral ionophore ETH 1062, and cadmium tetrakis (p-Cl-phenyl)borate as ion-exchanger. One of the membranes contained also a lipophilic electrolyte ETH 500. Prior to measurements, the membranes with overall thickness of 700 μm were equilibrated with water for 1 week. It is shown that in the membranes the surface layers with a thickness of 150 μm are enriched with water, whereas in the bulk of the membranes practically no water was detected. Overall water uptake and also kinetics of evaporation of absorbed water have been studied gravimetrically. Much slower process of leak of the membrane components to water was studied by UV–Vis absorption spectroscopy and gravimetrically, and ion-exchanger and plasticizer are identified as leaking components.

Thin polymeric membrane ion-selective electrodes for trace-level potentiometric detection

In this work, we describe a novel method to improve the detection limits of the non-classical polymeric membrane ion-selective electrodes (ISEs) which are conditioned with highly discriminated ions instead of primary ions. It is based on a thin-layer ISE membrane with a thickness of 5 μm, which is coated on ordered mesoporous carbon used as solid contact. The diffusion of the primary ion from the surface of the sensing membrane to the bulk of the membrane could be avoided by the proposed thin membrane configuration. Since the detection sensitivity of the non-classical ISEs depends on the accumulation of the primary ion in the interfacial layer of the sensing membrane, a lower detection limit can be obtained. By using the copper ion as a model, the present potentiometric sensor shows a significantly improved detection sensitivity compared to the conventional ISE with a membrane thickness of ca. 200 μm. Low detection limits of 0.29 and 0.53 nM can be obtained in 0.01 and 0.5 M NaCl, respectively. In addition, the proposed sensor exhibits an excellent reversibility by using a neutral proton-selective ionophore incorporated in the thin membrane.

Development of Chromium(III)-selective Potentiometric Sensor by Using Synthesized Pyrazole Derivative as an Ionophore in PVC Matrix and its Applications

A novel poly(vinyl chloride) membrane potentiometric sensor for chromium(III) ions based on the use of 5,5′-(1,4-phenylene)bis(3-(naphthalen-1-yl)-4,5-dihydro-1H-pyrazole-1-carbothioamide) as a neutral ionophore was developed. The optimum composition of the best performing membrane contained ionophore, potassium tetrakis (p-chlorophenyl) borate (KTpClPB), dibutyl phthalate (DBP), and poly(vinyl chloride) (PVC) in the ratio of 5.5:1.5:55:38 (mg). The sensor exhibits a working concentration range of 1.0 × 10–5–1.0 × 10–1 mol L−1 and a detection limit of 1.7 × 10–6 mol L−1. The sensor shows good selectivity for chromium(III) ions over a number of cations including alkali, alkaline earth, heavy and transition metals. The response time of the sensor is 8 s. In addition, the developed sensor shows good reusability and stability. The sensor operates in the wide pH range of 5.0–11.0. The sensor could be used as an indicator electrode in the quantification of Cr3+ ions by potentiometric titration against ethylenediaminetetraacetic acid (EDTA). Finally, this sensor was successfully used for the determination of chromium(III) in commercial water, purification water and wastewater.

Characterization of a Ag+-Selective Electrode Based on Naphthalimide Derivative as Ionophore

A naphthalimide derivative has been explored as neutral ionophore for Ag+-selective electrode. Potentiometric response revealed that electrode based on the proposed ionophore with 2-nitrophenyl octyl ether as solvent in a poly (vinyl chloride) membrane matrix shows a measuring range of 1.0×10-6-1.0×10-2 M with a slope of 50.4±0.3 mV/decade. This electrode has high selectivity to Ag+ with respect to alkaline, alkaline earth and other heavy metal ions.

Tripodal Squaramide Derivative as a Neutral Chloride Ionophore for Whole Blood and Sweat Chloride Measurement

AbstractA tripodal squaramide derivative was synthesized and characterized as a new neural ionophore for chloride ion. Squaramide based compounds are known to be better hydrogen bonding receptors than urea derivatives because they donate four hydrogen bonds to anions, while the tripodal structure provides a 3D geometry optimum for spherical anion complexation. The ionophore demonstrated high selectivity over salicylate and heparin that are major interferences for chloride measurement in biological samples with direct potentiometric method. The new ionophore based solid‐contact electrodes were fabricated and showed the measuring range of 10−5 to 1 M chloride ion in the presence of 1 mM of salicylate and heparin with near Nernstian slope of 55 mV/decade. The sensor was successfully applied for the chloride measurement in undiluted heparinized whole blood and artificial sweat samples with our clinical analyzer.

Single-Piece Solid Contact Cu2+-Selective Electrodes Based on a Synthesized Macrocyclic Calix[4]arene Derivative as a Neutral Carrier Ionophore.

Herein, a facile route leading to good single-walled carbon nanotubes (SWCNT) dispersion or poly (3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) based single-piece nanocomposite membrane is proposed for trace determination of Cu2+ ions. The single-piece solid contact Cu2+-selective electrodes were prepared after drop casting the membrane mixture on the glassy-carbon substrates. The prepared potentiometric sensors revealed a Nernstian response slope of 27.8 ± 0.3 and 28.1 ± 0.4 mV/decade over the linearity range 1.0 × 10−3 to 2.0 × 10−9 and 1.0 × 10−3 to 1.0 × 10−9 M with detection limits of 5.4 × 10−10 and 5.0 × 10−10 M for sensors based on SWCNTs and PEDOT/PSS, respectively. Excellent long-term potential stability and high hydrophobicity of the nanocomposite membrane are recorded for the prepared sensors due to the inherent high capacitance of SWCNT used as a solid contact material. The sensors exhibited high selectivity for Cu2+ ions at pH 4.5 over other common ions. The sensors were applied for Cu2+ assessment in tap water and different tea samples. The proposed sensors were robust, reliable and considered as appealing sensors for copper (II) detection in different complex matrices.

Fast Potentiometric Analysis of Lead in Aqueous Medium under Competitive Conditions Using an Acridono-Crown Ether Neutral Ionophore.

Lead is a particularly toxic heavy metal that is present above acceptable levels in the water of many countries. This article describes a quick detection method of lead(II) ions using a polyvinyl chloride (PVC)-based ion-selective membrane electrode containing an acridono-crown ether ionophore by potentiometry. The electrochemical cell exhibits a Nernstian response for lead(II) ions between the concentration range of 10−4 to 10−2 M, and can be used in the pH range of 4–7. The applicability of this sensor was verified by measuring a multicomponent aqueous sample. Under the given conditions, this electrode is suitable for the selective quantitative analysis of lead(II) ions in the presence of many additional metal ions.

Solid-Contact Potentiometric Sensors and Multisensors Based on Polyaniline and Thiacalixarene Receptors for the Analysis of Some Beverages and Alcoholic Drinks.

Electronic tongue is a sensor array that aims to discriminate and analyze complex media like food and beverages on the base of chemometrics approaches for data mining and pattern recognition. In this review, the concept of electronic tongue comprising of solid-contact potentiometric sensors with polyaniline and thacalix[4]arene derivatives is described. The electrochemical reactions of polyaniline as a background of solid-contact sensors and the characteristics of thiacalixarenes and pillararenes as neutral ionophores are briefly considered. The electronic tongue systems described were successfully applied for assessment of fruit juices, green tea, beer, and alcoholic drinks They were classified in accordance with the origination, brands and styles. Variation of the sensor response resulted from the reactions between Fe(III) ions added and sample components, i.e., antioxidants and complexing agents. The use of principal component analysis and discriminant analysis is shown for multisensor signal treatment and visualization. The discrimination conditions can be optimized by variation of the ionophores, Fe(III) concentration, and sample dilution. The results obtained were compared with other electronic tongue systems reported for the same subjects.

Water Hardness Electrodes with Ionophores Containing Oxy- and Ester-Groups

Lipophilic compounds combining oxy- and ester-groups are synthesized and studied as neutral ionophores in plasticized PVC membranes for the development of novel water hardness ion-selective electrodes. Electrodes based on the ionophores studied showed a higher selectivity to calcium over magnesium ions. However, for electrodes based on hexadecyl-4-hydroxybutanoate or decyloxybutanol this preference turned to be rather low: logKCaMg=–(0.5–0.7). Electrodes with membranes containing hexadecyl-4- hydroxybutanoate, 0.3 M as a neutral ionophore and bis[4-(1,1,3,3-tetramethylbutyl)phenyl]phosphate, 0.01 M as a charged ionophore, in combination with Ca2+-selective electrode based on ETH 1001 as ionophore, proved to be suitable for measurements of water hardness, also for the measurement of Mg2+ ion content in artificial aquarium fish-breeding water samples and in samples modeling electrolyte composition of blood serum.