Ion-selective Polymeric Membrane Research Articles

Enzyme assays using sensor arrays based on ion-selective carbon nanotube field-effect transistors

In the fields of clinical diagnostics and point-of-care diagnosis as well as food and environmental monitoring there is a high demand for reliable high-throughput, rapid and highly sensitive assays for a simultaneous detection of several analytes in complex and low-volume samples. Sensor platforms based on solution-processable electrolyte-gated carbon nanotube field-effect transistors (CNT-FETs) are a simple and cost-effective alternative for conventional assays. In this work we demonstrate a selective as well as direct detection of the products of an enzyme-substrate interaction, here the for metabolic processes important urea-urease system, with sensors based on spray-coated CNT-FETs. The selective and direct detection is achieved by immobilizing the enzyme urease via certain surface functionalization techniques on the sensor surface and further modifying the active interfaces with polymeric ion-selective membranes as well as pH-sensitive layers. Thereby, we can avoid the generally applied approach for a field-effect based detection of enzyme reactions via detecting changes in the pH value due to an on-going enzymatic reaction and directly detect selectively the products of the enzymatic conversion. Thus, we can realize a buffering-capacity independent monitoring of changes in the substrate concentration.

Flexible Electrolyte-Gated Ion-Selective Sensors Based on Carbon Nanotube Networks

We demonstrate the ion-selective response of an electrolyte-gated carbon nanotube network-based field-effect transistor fabricated on a flexible polyimide substrate. Selective response toward the two prominent second messengers for cell–cell communication, namely K+ and Ca2+, is demonstrated by modifying the carbon nanotube network with different polymeric ion-selective membranes. The sensing mechanism relies on the transduction of the ionic signal in an electrical one due to an ion-activity dependent change of the membrane potential at the membrane/electrolyte interface, which leads to a change in the effective gate-potential affecting the charge transport in the semiconducting channel. These sensors can be successfully used to selectively detect concentrations of primary ions down to a concentration in the micrometer range even in solutions with a highly concentrated background of interfering ions. Our approach allows the realization of low-cost, flexible, portable and multipurpose biosensing devices.

Understanding Side Reactions in K–O2 Batteries for Improved Cycle Life

Superoxide based metal-air (or metal-oxygen) batteries, including potassium and sodium-oxygen batteries, have emerged as promising alternative chemistries in the metal-air battery family because of much improved round-trip efficiencies (>90%). In order to improve the cycle life of these batteries, it is crucial to understand and control the side reactions between the electrodes and the electrolyte. For potassium-oxygen batteries using ether-based electrolytes, the side reactions on the potassium anode have been identified as the main cause of battery failure. The composition of the side products formed on the anode, including some reaction intermediates, have been identified and quantified. Combined experimental studies and density functional theory (DFT) calculations show the side reactions are likely driven by the interaction of potassium with ether molecules and the crossover of oxygen from the cathode. To inhibit these side reactions, the incorporation of a polymeric potassium ion selective membrane (Nafion-K(+)) as a battery separator is demonstrated that significantly improves the battery cycle life. The K-O2 battery with the Nafion-K(+) separator can be discharged and charged for more than 40 cycles without increases in charging overpotential.

Concanavalin A electrochemical sensor based on the surface blocking principle at an ion-selective polymeric membrane

We report on a new electrochemical sensor for Concanavalin A. It is based on blocking the surface of plasticized PVC membranes that were covalently modified with D-mannose using click chemistry. The interaction of D-mannose with Concanavalin A on the surface perturbs the flux of a marker ion for which the ion-selective membrane is responsive, and this results in a change in the electrochemical signal. The sensor was characterized using a variety of electrochemical protocols, and results were confirmed by quartz crystal microbalance experiments. The lowest limit of detection (10 μg mL−1) was obtained using a membrane containing a cation exchanger and tetrabutylammonium ion as the marker ion.

Hydrogen ion-selective electrolyte-gated organic field-effect transistor for pH sensing

A H+ ion-selective electrolyte-gated organic field-effect transistor (IS-EGOFET) with a broad detection range between pH 3 and pH 12, is presented. This pH sensor relies on an integrated EGOFET used as a transducer in combination with an ionophore-doped polymeric ion-selective membrane serving as a sensing element. The broad detection range was possible through a dynamic measurement protocol comprising a readjustment of the gate voltage, which ensures a stable device operation at a constant working point. The effectiveness of this dynamic approach is confirmed by stability investigations. On the basis of this pH sensor concept, the importance of an appropriate gating electrolyte is highlighted, giving insights into the working mechanism of EGOFETs.

A microfluidic chip for ammonium sensing incorporating ion-selective membranes formed by surface tension forces

A microfluidic chip incorporating a series of polymeric ion-selective membranes (ISMs) is proposed for the detection of ammonium ions. In the proposed approach, the ISMs are formed by means of a simple flow-through forming technique, in which a polymer liquid doped with ammonium ionophores is injected into the microchip and attached to a series of diamond-like microstructures patterned along the center of the microchannel under the effects of surface tension forces and two neighboring sheath flows. The experimental results show that the proposed microsensor exhibits a linear response when detecting ammonium ions with a concentration of 10−4 to 103 ppm (R2 ∼ 0.9689). Furthermore, the detection limit is shown to be as low as 0.1 ppb. Finally, the time response when measuring 0.01 ppm of ammonium ions is just 4 s (95% standard), which is far faster than that of typical commercial ISE products (∼minutes) Overall, the results presented in this study show that the proposed microchip provides a low-cost yet highly-sensitive means of performing the continuous detection of ammonium ions.

Polymeric ion-selective membrane functionalized gate-electrodes: Ion-selective response of electrolyte-gated poly (3-hexylthiophene) field-effect transistors

We demonstrate the ion-selective response of an electrolyte-gated organic field-effect transistor (IS-OFET) towards the two prominent second messengers for cell–cell communication: K+ and Ca2+. The ion-selective response is achieved by modifying the gate-electrode with different polymeric ion-selective membranes. The sensing mechanism relies on the transduction of the ionic signal in an electrical one due to a concentration-dependent change in the membrane potential at the gate/electrolyte interface which leads to a change in the effective gate potential and thus alters charge transport in the semiconducting channel. These sensors can be successfully used to selectively detect concentrations of primary ions down to a concentration in the μM range even in solutions with an ionic background of 150mM. Our approach represents a valuable strategy for the realization of portable, multi-purpose and low-cost biosensing devices.

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

Novel Fe3+ 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 8.3×10−8–1.0×10−1molL−1, a lower detection limit of 2.3×10−8molL−1, and a near Nernstian slope of 19.5±0.4mVdecade−1 of activity with a response time of 10s. The CGE shows a shelf life of 6weeks and in view of high selectivity, it can be used to quantify Fe3+ ion in water, soil, vegetable and medicinal plants. It can also be used as an indicator electrode in potentiometric titration of EDTA with Fe3+ ion.

Electrochemical control of the standard potential of solid-contact ion-selective electrodes having a conducting polymer as ion-to-electron transducer

This work addresses the well-known problem of variations in the standard potential (E°) of solid-contact ion-selective electrodes (SC-ISEs) that have a conducting polymer (CP) as ion-to-electron transducer covered by a polymeric ion-selective membrane. Poly(3,4-ethylene dioxythiophene) doped with poly(sodium 4-styrenesulfonate), i.e. PEDOT(PSS), was electrodeposited on glassy carbon (GC) disk electrodes and used as the solid contact for three different types of PVC-based membranes in order to elucidate the possibility to electrochemically control E° for this type of SC-ISE. The GC/PEDOT(PSS) electrode was thus coated with potassium-selective membranes with and without the lipophilic salt tetradocedylammonium tetrakis(4-clorophenyl)borate (ETH-500) and by a cation-sensitive membrane without ionophore. The results show that the standard potential of the studied types of SC-ISEs can be shifted by applying a potential that deviates from the open-circuit potential of the electrode in the chosen electrolyte solution or by applying current pulses in the nA range.

Oxazinoindolines as Fluorescent H+ Turn-On Chromoionophores For Optical and Electrochemical Ion Sensors

We propose here a new class of fluorescent H(+) turn-on oxazinoindoline (Ox) dyes as chromoionophores suitable for the development of polymeric ion-selective membranes. Through rational design, oxazinoindolines with varying basicities and tunable absorption bands that extend to the near-infrared region were prepared. The basicities of the compounds were evaluated by potentiometry and fluorescence spectroscopy. By comparing the potentiometric lower detection limit with chromoionophore I (pKa = 14.80 ± 0.03 in PVC-NPOE), pKa values were determined for Ox Y, Ox R, and Ox B to be 9.80 ± 0.03, 12.85 ± 0.03, and 12.95 ± 0.03, respectively. Unlike conventional pH indicators that involve only a protonated and deprotonated form, a third species, arising from the thermal isomerization of the Ox compounds in polar solvents, may influence the overall protonation equilibrium. Interestingly, this results in an apparent pKa increase in the fluorescence mode when more polar PVC-NPOE membranes were utilized. The isomerization is suppressed in nonpolar PVC-DOS membranes. Na(+)-selective optical sensors were successfully prepared using the Ox dyes as an early application of the compounds in ion-selective optical sensors. The compounds were also successfully evaluated in a dynamic electrochemistry (chronopotentiometry) sensing mode for the detection of total alkalinity.

Determination of Water Uptake of Polymeric Ion-Selective Membranes with the Coulometric Karl Fischer and FT-IR-Attenuated Total Reflection Techniques

The water uptake of plasticized poly(vinyl chloride) (PVC) and silicone rubber (SR) based calcium-selective membranes which are commonly used in solid-contact and coated-wire ion-selective electrodes (SC-ISEs and CWEs) was quantified with the oven based coulometric Karl Fischer (KF) technique. Two different membrane types were studied: (1) the plasticized PVC or SR (RTV 3140) membrane matrix without other added membrane components and (2) the full Ca(2+)-selective membrane formulation consisting of the membrane matrixes, potassium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate and calcium ionophore IV (ETH 5234) or calcium ionophore I (ETH 1001). The membranes were contacted for 24 h either asymmetrically from one side or symmetrically from both sides with deionized water (DIW) or 0.1 M solutions of CaCl(2), KCl, or NaCl. It was found that the water uptake was higher for symmetrically contacted membranes. The highest water uptake (0.15-0.17 wt %) was obtained for the plasticized PVC based Ca(2+)-selective membranes in DIW, whereas the water uptake was lower in 0.1 M electrolyte solutions. Symmetrically contacted Ca(2+)-selective SR membranes had much lower water uptake in 0.1 M CaCl(2) (0.03 wt %) than their plasticized PVC counterparts (0.1 wt %). However, the (noncontacted) SR membranes contained initially much more water (0.09-0.15 wt %) than the PVC membranes (0.04-0.07 wt %). Furthermore, in good accordance with the KF measurements, it was verified with FT-IR-attenuated total reflection (ATR) spectroscopy that the water content at the substrate/membrane interface and consequently in the whole membrane was influenced by the electrolyte solution.

The influence of an intermediate layer on the composition stability of a polymeric ion-selective membrane

This paper focuses on the influence of the different materials used as an intermediate layer between ionophore doped polymeric membranes and metallic conductors on ion-selective membranes composition stability. The quantitative determination of the membranes components loss from of solid contact ion-selective electrodes (SC-ISEs) during contact with aqueous solutions was performed by stripping voltammetry. A correlation between the leaching of the lipophilic salt rate and the presence of carbon black and a conducting polymer with different dopants layers is observed. The sodium selective SC-ISEs with Printex U carbon black presented in this paper for the first time showed the theoretical potentiometric response (59.11mV/pNa) and a very good long-term potential stability (16.4μVh−1).

Dithizone Modified Gold Nanoparticles Films for Potentiometric Sensing

For the first time, application of a membrane composed of gold nanoparticles decorated with complexing ligand for potentiometric sensing is shown. Gold nanoparticles drop cast from a solution form a porous structure on a substrate electrode surface. Sample cations can penetrate the gold nanoparticles layer and interact with ligand acting as a charged ionophore, resulting in Nernstian potentiometric responses. Anchoring of complexing ligand on the gold surface abolishes the necessity of ionophore application. Moreover, it opens the possibility of preparation of potentiometric sensors using chelators of significantly different selectivity patterns further enhanced by the absence of polymeric membrane matrix. This was clearly seen, for example, for gold nanoparticles stabilizing the applied ligand-dithizone-thiol conformation leading to a high potentiometric selectivity toward copper ions, much higher than that of ionophores typically used to induce selectivity for polymeric ion-selective membranes.

Novel monohydrogenphosphate ion-selective polymeric membrane sensor based on phenyl urea substituted calix[4]arene

A highly selective and sensitive PVC membrane, containing phenylurea substituted calix[4]arene was found to be a suitable ionophore for monohydrogen phosphate (HPO42−) ions that exhibited a Nernstian response (with a slope of −29.4±0.3mVdecade−1). The working concentration range of the electrode was 6.0×10−8–1.0×10−1molL−1 with a detection limit of 2.0×10−8molL−1. The response time of the sensor in the whole concentration range is very short (<8s), with a considerable lifetime of at least 15 weeks. The sensor possesses the advantages of short response time, low detection limit and a high selectivity towards a large number of inorganic anions. The developed sensor was used as an indicator electrode in potentiometric titration of monohydrogen phosphate ion with barium chloride. The proposed sensor was successfully applied for the direct determination of monohydrogen phosphate in real life samples.

Biparametric Potentiometric Analytical Microsystem Based on the Green Tape Technology

AbstractThe construction and evaluation of a Low Temperature Co‐fired Ceramics (LTCC)‐based microsystem prototype as a possible on‐site microanalyzer to monitor the presence of two ions in water simultaneously is presented. The approach has been assayed to detect nitrate and chloride ions as model analytes by means of integrating in the same single substrate an ion selective polymeric membrane to nitrate and two screen‐printed Ag/AgCl electrodes. One supplies the chloride ion concentration and the other is to complete the potentiometric detection system as the reference electrode. The results obtained by the full characterization of the microanalyzer prove the relevance of the proposal and the possibility to be transferred to real‐world samples and environmental monitoring.

ChemInform Abstract: Macrocyclic Amides as Ionophores for Lead-Selective Membrane Electrodes.

A series of ionophores based on diaza 18-crown-6 units possessing amide and sulfonamide functions were prepared. Their complexing properties were investigated in solvent polymeric membrane electrodes. The influence of the structure of ligands, prepared on the selectivity of complexation, was examined with special respect to lead ions. Substantial changes were observed, when methyl substituent in benzene rings was replaced with hydrogen, bromomethyl or nitro group. All results were rationalized on basis of steric and electronic effects. The synthesis and complexation studies of supramolecular compounds is of continuous interest [1]. Several types of ionophores were designed for medicine, biochemistry and environmental pollution monitoring [2]. For pollution monitoring some of ionophores, selective towards heavy metal cations, are of general interest. Previously, we have shown that macrocyclic polyether diamides can be readily prepared [3] and successfully applied in ion-selective electrodes [4], showing high selectivity towards Pb 2+ over other divalent cations. This result prompted us to the conclusion that the introduction of external amide function into the macrocyclic ring can increase the stability of electrodes and improve the complexing properties of ionophores towards divalent cations. This paper deals with the preparation of a series of external macrocyclic diamides derived from diaza-18-crown-6. Complexing properties of prepared compounds were investigated in ion-selective polymeric membranes system with a special respect to lead cation.

Determination of the leaching of polymeric ion-selective membrane components by stripping voltammetry

This paper focuses on the quantitative determination of the loss of the components from plastic membranes of ion-selective electrodes (ISEs) during contact with aqueous bathing solutions. The leaching processes, which affect the ISE responses, are rarely characterized by independent methods. For this purpose, differential pulse cathodic stripping voltammetry (DP CSV) is used. This method, owing to its high sensitivity, acceptable recovery and accuracy, is a good tool to characterize the kinetics of leakage of the lipophilic salts. Sodium tetraphenylborate (NaTPB) leakage from the PVC-based sodium-selective membrane containing two different plasticizers, o-nitrophenyl octyl ether (o-NPOE) or di(2-ethylhexyl) sebacate (DOS) is presented. Correlation between the rate of leaching of the lipophilic salt and dielectric constants of the plasticizers is observed. The data obtained by DP CSV correlate well with potentiometric and electrochemical impedance responses. The observed outflow of TPB(-) is associated with decreasing potentiometric sensitivity to sodium and increasing bulk membrane resistance.

A highly selective polymeric membrane barium(II) electrode based on a macrocyclic tetrabasic ligand as neutral carrier

A novel barium(II) ion-selective polymeric membrane electrode based on a macrocyclic tetrabasic ligand that derived from the reaction of 4,6-diacetylresorcinol and ethylenediamine in the molar ratio 1:1 as neutral carrier was described. The electrode, with optimum membrane composition, exhibited an excellent near-Nernstian response for Ba(2+) ion ranging from 3.6 x 10(-6) to 1.0 x 10(-1) mol/L with a detection limit of 1.9 x 10(-6) mol/L and a slope of 29.7 +/- 0.2 mV/dec in pH 4.0 nitrate buffer solution at 25 degrees C. It had a relatively fast response time (10 s), good selectivity towards Ba(2+) ion, and could be used over a wide pH range of 2.5-7.5. Then, the response mechanism was discussed in view of UV-visible spectroscopy and the A.C. impedance technique. Finally, the proposed electrode was successfully applied to the detection of Ba(2+) ions in real samples as well as in sulfate(II) ions in water.

Metalloporphyrin-based acetate-selective electrodes as detectors for enzymatic acetylcholine determination in flow-injection analysis system

In this work two Zr(IV)-porphyrins were tested as potential acetate-selective ionophores. It is shown that these compounds show increased selectivity towards acetate ion, with selectivity sequence: Cl − < NO 3 − < SCN − < ClO 4 − < acetate − < F −, when incorporated into polymeric ion-selective membranes. Among tested ionophores, Zr(IV)-tetra(tert-butylphenyl)porphyrin was found to be the best in terms of response time (20 s) and lower detection limit (2 × 10 −4 M acetate −). Designed electrodes were used as detectors in flow-injection analysis system for acetylcholine determination. Enzymatic hydrolysis was used to generate acetate ions from acetylcholine, employing acetylcholinesterase immobilized on Amberlite. The optimization of enzymatic reactions and flow-injection analysis system configuration were performed and enzyme immobilization procedures have been evaluated. It is shown that resulting flow-injection system exhibits good working parameters, such as reproducibility (standard deviation ≤3.5%), sampling rate (60 samples per hour) and lifetime (over a week).

Limitations of Current Polarization for Lowering the Detection Limit of Potentiometric Polymeric Membrane Sensors

Ion fluxes across polymeric ion-selective membranes are a decisive parameter dictating the lower detection limit of potentiometric ion sensors. An applied current was earlier proposed to counteract such fluxes and reduce the detection limit to ultratrace levels. So far, however, the method has not been used in practical situations since the correct current amplitude requires prior knowledge of the sample composition. This paper explores the use of the stir effect to evaluate the optimal current by theory and experiments. It is shown that the traditionally used steady-state model assuming a uniform distribution of ion exchanger in the membrane, fixed with time, violates the electroneutrality condition. A modified steady-state model is introduced that allows for a concentration tilt of the ion exchanger and predicts that a stir effect can indeed be utilized to find the optimal current. Ideally, by choosing the optimal current and very long measurement times, the thermodynamic detection limit might be obtained. However, in practice the stir effect declines at low concentrations and the conditions are far from steady state. Therefore, the improvement of the lower detection limit achievable by galvanostatic control is only about 1 order of magnitude. A numerical finite-difference approximation is shown to trace the experimental potential responses of silver-selective electrodes well and to reproduce the stir effect adequately, even for different conditioning protocols. The stir effect is successfully used to improve the detection limit of electrodes with ill-optimized inner solutions; however, significant improvements beyond what is commonly feasible by chemical optimization does not seem to be easily achievable. The results indicate that with conventional membranes the possibility of improving the detection limit by current polarization is much more limited than assumed so far.