Coulometric Response of H+-Selective Solid-Contact Ion-Selective Electrodes and Its Application in Flexible Sensors†

The analytical performance of H⁺-selective solid-contact ion-selective electrodes (SCISEs) based on solid contact polyaniline doped with chloride (PANI(Cl)) and poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT(PSS)) was characterized by a developed coulometric signal transduction method. PEDOT(PSS) solid contact is covered by PVC based H⁺-selective membrane. The obtained coulometric signal demonstrates that the cumulated charge can be amplified by increasing the capacitance of solid contact. SCISEs covered with spin-coated membrane behave faster amperometric response than electrodes with drop-cast membrane. In contrast to earlier works, the amperometric response and impedance spectrum demonstrates H⁺ transfer through SCISEs is independent from the thickness of membrane. The exceptional behavior of PANI(Cl) H⁺-SCISEs shows that the capacitance estimated from impedance spectrum at low frequency 10 mHz and coulometric signal of PANI(Cl) based SCISEs is influenced by the applied potentials, whereas PEDOT(PSS) solid contact is independent from the chosen applied potentials. Furthermore, preliminary investigations of coulometric signal transduction on flexible pH sensor implies its potential applications in wearable sensors for sweat ion concentration detection.      

Electrochemical properties of photocurable membranes for all-solid-state chemical sensors

The development of ion-selective electrodes with inner solid contact is described using photocurable acrylated polyurethane matrices and electron-ion exchanger (EI), which provides a reversible transition from electrical conductivity in the metal to ionic conductivity in the membrane phase. The application of a photocurable polymer matrix gives the possibility to use modern photolithographic techniques for the formation of all-solid-state chemical sensors. The influence of the polymer matrix and of the preparation of the membrane on the electro-chemical properties of calcium-selective membrane sensors is shown. For carbonate-selective membranes the possibility of improvement of electrochemical characteristics by incorporation of the anionic additive tetrakis (4-chlorophenyl) borate was studied.     

Evaluation,Pitfalls and Recommendations for the “Water Layer Test” for Solid Contact Ion‐selective Electrodes

The “water layer test” is a crucial validation step of solid‐contact ion‐selective electrodes. It can confirm or contest the claim that the tested electrode is indeed a genuine solid contact electrode without an aqueous film between the ion‐selective membrane and its solid contact. Information about the presence of a water layer is essential for the interpretation of drifts in the electrode potentials commonly experienced with solid contact electrodes. Since its publication, the water layer test has been ubiquitously used, but without a standardized protocol the interpretation (or misinterpretation) of the test results led to uncertainties in the conclusions. Through both experiments and simulations based on theoretical models we have investigated the experimental parameters that can influence the results of the water layer test. We propose guidelines to minimize the possibility of misinterpretation of the results of the water layer test by considering the key factors that affect the shape of transients recorded during the water layer test. Most importantly, we emphasize the importance of allowing sufficient time for conditioning the tested electrode before the water layer test and providing adequate time for equilibration during the experiment. Using a thin ion‐selective membrane and thin solid‐contact layer for the tests is also recommended.     

Electrochemical properties of photocurable membranes for all-solid-state chemical sensors

The development of ion-selective electrodes with inner solid contact is described using photocurable acrylated polyurethane matrices and electron-ion exchanger (EI), which provides a reversible transition from electrical conductivity in the metal to ionic conductivity in the membrane phase. The application of a photocurable polymer matrix gives the possibility to use modern photolithographic techniques for the formation of all-solid-state chemical sensors. The influence of the polymer matrix and of the preparation of the membrane on the electro-chemical properties of calcium-selective membrane sensors is shown. For carbonate-selective membranes the possibility of improvement of electrochemical characteristics by incorporation of the anionic additive tetrakis (4-chlorophenyl) borate was studied. Received: 14 June 1997 / Revised: 23 September 1997 / Accepted: 9 October 1997     

A solid-state redox buffer as interface of solid-contact ISEs

Graphite with a surface-confined redox buffer system was used as solid contact in solid-contact ion-selective electrodes (SC-ISE). Potentiostatically preconditioning of the redox buffer ensures that the ratio of oxidized and reduced groups is unity, i.e., a maximum buffer capacity. These SC-ISEs exhibited a very high reproducibility and stability of potentials. Graphite modification was achieved by generating carboxylic surface groups via oxidation with nitrosulfuric acid and subsequent modification of the acidic groups with n-(2,5-dimethoxyphenyl)ethyl-1-amine (DMPEA). Composite electrodes of modified graphite and polymethylmetharcrylate (PMMA) as binder were used as solid-contact of polymer membrane K⁺-and F^?-sensitive SC-ISEs. The solid-state redox buffer decreased potential variation from several hundred mV to just a few mV.     

New solid-state glass electrodes by using zinc oxide thin films as interface layer

Novel glass electrodes for the determination of cations with reversible internal solid contact are introduced. They are based on a semiconducting zinc oxide layer with a maximum thickness of 1 μm in contact with ion selective glasses on one side and with a metal layer on the other side. The metal oxide layer is thereby generated either by ultrasonic spray pyrolysis from zinc acetate solution, by electrochemical deposition from zinc nitrate solution or by spin coating from a dispersion of ZnO in an organic binder. A following activation in a palladium chloride solution allows the chemical reductive deposition of NiP as electronic conductor. Dipping-type and flow through electrodes as well as planar glass electrodes in thick film technology fabricated in the above-mentioned method are described. In this case gold electrodes are applied by screen printing on isolated steel substrates. The zinc oxide layers, created in different manners, are covered afterwards with cation selective glasses in thick film technology. They cause a stabilisation of the half-cell potentials of the all solid state indicator electrodes proved by suitable measurements.     

Conductivity detection for conventional and miniaturised capillary electrophoresis systems

Since the introduction of capillary electrophoresis (CE), conductivity detection has been an attractive means of detection. No additional chemical properties are required for detection, and no loss in sensitivity is expected when miniaturising the detector to scale with narrow-bore capillaries or even to the microchip format. Integration of conductivity and CE, however, involves a challenging combination of engineering issues. In conductivity detection the resistance of the solution is most frequently measured in an alternating current (AC) circuit. The influence of capacitors both in series and in parallel with the solution resistance should be minimised during conductivity measurements. For contact conductivity measurements, the positioning and alignment of the detection electrodes is crucial. A contact conductivity detector for CE has been commercially available, but was withdrawn from the market. Microfabrication technology enables integration and precise alignment of electrodes, resulting in the popularity of conductivity detection in microfluidic devices. In contactless conductivity detection, the alignment of the electrodes with respect to the capillary is less crucial. Contactless conductivity detection (CCD) was introduced in capillary CE, and similar electronics have been applied for CCD using planar electrodes in microfluidic devices. A contactless conductivity detector for capillaries has been commercialised recently. In this review, different approaches towards conductivity detection in capillaries and chip-based CE are discussed. In contrast to previous reviews, the focus of the present review is on the technological developments and challenges in conductivity detection in CE.     

All-solid-state planar miniature ion-selective chloride electrode

All-solid-state ion-selective electrodes with plastic membrane (poly(vinyl chloride) (PVC), bis(2-ethylhexyl) sebacate (DOS), methyltri-n-tetradecylammonium chloride (MTTACl)), a conducting poly(pyrrole) (PPy) film doped either with chloride ions (PPyCl) or hexacyanoferrate(II) ions (PPyFeCN), and glassy carbon (GC) or screen-printed graphite layer (S-PG) as an inner electric contact were investigated. All the electrodes show close to Nernstian response, but their lifetimes vary. The at least 2-months lifetime of screen-printed electrodes is only achieved for the electrodes containing PPyFeCN (cation-exchanging film). Shorter lifetime of other screen-printed electrodes, i.e. without PPy, or with PPyCl (anion-exchanging film), was attributed to the diffusion of anionic products of the hydrolysis of organic components of the graphite paste used to prepare the electric contact. The properties of miniature, screen-printed electrodes comprising PPyFeCN solid contact, were comparable to those ion-selective electrodes with PPy solid contact (regardless the ion-exchanging characteristic of the polymer) deposited on GC electric contact.     

Lipophilic Multi‐walled Carbon Nanotube‐based Solid Contact Potassium Ion‐selective Electrodes with Reproducible Standard Potentials. A Comparative Study

The two most promising approaches for preparing solid contacts (SCs) for polymeric membrane based ion‐selective electrodes (ISEs) are based on the use of large surface areas conducting materials with high capacitance (e. g., various carbon nanotubes) and redox active materials (e. g. conducting polymers). While many of the essential requirements for the potential stability of SCISEs were addressed, the E⁰ reproducibility and its predictability, that would enable single use of such electrodes without calibration is still a challenge, i. e., the fabrication of electrodes with sufficiently close E⁰ and slope values to enable the characterization of large fabrication batches through the calibration of only a small number of electrodes. The most generic solution seems to be the adjustment of the E⁰ potential by polarization prior to the application of the ion‐selective membrane. This approach proved to be successful in case of conducting polymer‐based solid contacts, but has to be still explored for capacitive solid contact based ISEs, which is the purpose of this paper. We have chosen a well‐established highly lipophilic multi‐walled carbon nanotube (MWCNT), i. e. octadecane modified MWCNT (OD‐MWCNT), that is investigated in the comparative context of a similarly lipophilic conducting polymer solid contact (a perfluorinated alkanoate side chain functionalized poly(3,4‐ethylenedioxythiophene)). While, the OD‐MWCNT based SCISEs had inherently small standard deviation of their E⁰ values (less than 5 mV) this could be further improved by external polarization and short circuiting the SCISEs.     

Thin Layer Membrane Systems as Rapid Development Tool for Potentiometric Solid Contact Ion‐selective Electrodes

The use of thin membrane layer ion‐selective electrodes (of ～200 nm thickness) as rapid diagnosis tool is proposed. While conventional solid contact systems (with a membrane of ～250 μm thickness) may exhibit a satisfactory stability for regular laboratory use, a signal degradation can still be distinguished over a longer period of time but this requires tedious and time consuming tests. By diminishing the thickness of the membrane by a factor of 10³ approximately, diffusion processes happen faster, and the lifetime is significantly reduced. This would ordinarily be a strong drawback but not if the aim is to detect a membrane deterioration in a shorter time frame. This characteristic makes thin membrane systems an ideal tool for rapid complications identification in the development process of conventional solid contact electrodes. The approach is demonstrated here in the development of an all new solid contact probe for anions. PEDOT?C₁₄, a conducting polymer, was used for the first time in a solid contact electrode with an anion exchange membrane for the detection of nitrate. The thin layer configuration was used to optimise the polymerisation parameters as well as the membrane composition without having to run week‐long trials. A stable conventional solid contact electrode was in the end successfully developed and exhibited a lower detection limit of 10^?5.5 M for nitrate with a stable Nernstian response for several days.     

Poly(3-octylthiophene) as solid contact for ion-selective electrodes: contradictions and possibilities

The hydrophobic conductive polymer, poly(3-octylthiophene) (POT), is considered as uniquely suited to be used as an ion-to-electron transducer in solid contact (SC) ion-selective electrodes (ISEs). However, the reports on the performance characteristics of POT-based SC ISEs are quite conflicting. In this study, the potential sources of the contradicting results on the ambiguous drift and poor potential reproducibility of POT-based ISEs are compiled, and different approaches to minimize the drift and the differences in the standard potentials of POT-based SC ISEs are shown. To set the potential of the POT film, it has been loaded with a 7,7,8,8-tetracyanoquinodimethane (TCNQ/TCNQ^·?) redox couple. An approximately 1:1 TCNQ/TCNQ^·?ratio in the POT film has been achieved through potentiostatic control of the potential of the redox couple-loaded conductive polymer. It is hypothesized that once the POT film has a stable, highly reproducible redox potential, it will provide similarly stable and reproducible interfacial potentials between the POT film and the electron-conducting substrate and result in SC ISEs with excellent reproducibility and potential stability. Towards this goal, the potentials of Au, GC, and Pt electrodes with drop-cast POT film coatings were recorded in KCl solutions as a function of time. Some of the POT films were loaded with TCNQ and coated with a K⁺-selective membrane. The improvement in the potential stabilities and sensor-to-sensor reproducibility as a consequence of the incorporation of TCNQ in the POT film and the potentiostatic control of the TCNQ/TCNQ^·?ratio is reported.     

Flow-through calcium-selective electrode: application in flow-injection analysis and ion chromatography

The use of solid contact flow-through calcium-selective electrodes as potentiometric detectors in flow-injection analysis and non-suppressed ion chromatography is discussed. Owing to the high selectivity of the membrane electrode based on tetratolyl-m-xylylenediphosphine dioxide, it can be used to monitor trace amounts of calcium ions in the presence of a 100-fold excess of alkali metal, ammonium and magnesium ions. The detection limit is about 1 × 10^?6 M. The composition and thickncss of the calcium selective membrane influence both the detection limit and selectivity of the electrode. The sensitivity of this potentiometric detector in ion chromatography relative to alkaline earth and heavy metals is significantly higher than that of a commercial conductivity detector.     

Gold nanoparticles solid contact for ion-selective electrodes of highly stable potential readings

Internal solution free ion-selective electrodes were prepared applying for the first time gold nanoparticles as a solid contact layer. The presence of a layer of gold nanoparticles stabilized with aliphatic thiols at the back side of the membrane resulted in highly stable potentiometric responses of the sensors, good selectivities and close to Nernstian slopes. Electrochemical studies have confirmed that the applied material is effectively working as capacitive solid contact, yielding high stability sensors.     

Advantages of Amperometric Readout Mode of Ion‐selective Electrodes under Potentiostatic Conditions

Amperometric responses of all‐solid‐state ion‐selective electrodes, recorded under potentiostatic conditions, were studied on example of potassium‐selective sensors with polypyrrole solid contact, at potential corresponding to reduction of the solid contact material and accompanying transfer of potassium ions across the membrane. Selective and stable in time linear dependences of current vs. logarithm of analyte concentration were recorded, resulting from high membrane resistance and changing membrane potential. The influence of experimental parameters as applied potential or thickness of the membrane was discussed. Advantages of the amperometric mode compared to potentiometric one relate to possibility of tailoring analytical parameters (sensitivity, magnitude of the signal) as well as over one order of magnitude decrease of the detection limit. The latter effect is achieved due to externally forced incorporation of potassium ions from the solution to the membrane, compensating their spontaneous release to the sample solution. A method of experimental setup simplification was proposed, with application of two‐electrode system, which can be used in the absence of external polarization source. The required driving force for the current flow was assured by spontaneous oxidation process occurring at the second electrode, coupled with reduction of the solid contact material of the ion‐selective electrode. In this case also stable in time calibration plots can be recorded.     

Calcium-selective electrodes with pvc membranes and solid internal contacts

The response of calcium-selective electrodes based on PVC membranes has been investigated. The membrane, containing the calcium salt of di-n-octylphenylphosphoric acid, is in direct contact with the internal electrode made of silver, silver/silver chloride, or teflonized graphite. Evidence is given to support the hypothesis that the oxygen half-cell is responsible for the potential at the solid contact—membrane interface. Silver or teflonized graphite contacts provide better potential stability and analytical sensitivity than Ag/AgCl contacts. The electrodes can be used for determination of calcium in waters.     

Potentiometric sensor for hydrogen ion based on N,N'-dialkylbenzylethylenediamine neutral carrier in a poly(vinyl chloride) membrane with polyaniline solid contact

Hydrogen ion selective solid contact electrodes based on N,N'-dialkylbenzylethylenediamine (alkyl=butyl, hexyl, octy, decyl) are prepared. Solid contact electrodes and coated wire electrodes have been fabricated from polymer cocktail solutions based on N,N'-dialkylbenzylethylenediamine (alkyl=butyl, hexyl, octy, decyl). We showed that the response range and slopes are influenced by the alkyl chain length. Response ranges and Nernstian slops for solid contact electrodes were slightly better than for coated wire electrodes. Solid contact electrodes were shown linear selective to hydrogen ion in the pH ranges pH 4.5-13.0, 4.2-13.1, 3.4-13.0 and 3.0-13.2. and their Nernstian slopes were 49.7, 50.8, 51.5 and 53.7 mV pH(-1) at 20+/-0.2 degrees C, respectively. Stability is also improved especially when compared with coated wire electrodes. The 90% response time was <2 s and their electrical resistance varied in the range 2.37-2.76 MOmega. Especially solid contact electrodes with N,N'-didecylbenzylethylenediamine have shown the best selectivity and reproducibility of e.m.f..     

Conducting Polymer‐Based Solid‐State Ion‐Selective Electrodes

Conducting polymers, i.e., electroactive conjugated polymers, are useful both as ion‐to‐electron transducers and as sensing membranes in solid‐state ion‐selective electrodes. Recent achievements over the last few years have resulted in significant improvements of the analytical performance of solid‐contact ion‐selective electrodes (solid‐contact ISEs) based on conducting polymers as ion‐to‐electron transducer combined with polymeric ion‐selective membranes. A significant amount of research has also been devoted to solid‐state ISEs based on conducting polymers as the sensing membrane. This review gives a brief summary of the progress in the area in recent years.     

Voltammetric Properties of All‐solid State Ion‐selective Electrodes with Multiwalled Carbon Nanotubes‐poly(3‐octylthiophene‐2,5‐diyl) Nanocomposite Transducer

Voltammetric response of an all‐solid‐state ion‐selective electrode was studied on example of potassium‐selective sensor with poly(vinyl chloride) based membrane and nanocomposite transducer containing poly(3‐octylthiophene‐2,5‐diyl) and multiwalled carbon nanotubes. Factors limiting the rate of the electrochemical process and the response were discussed. The challenge in voltammetric applications of ion‐selective electrodes is thickness of the plastic membrane. It was found that although a relatively thick ion‐selective membrane was applied, as typically used in potentiometric studies, the position of the reduction peak, corresponding to potassium ions incorporation, was dependent on ions concentration in a Nernstian manner. This opens possibility of deviation from the paradigm of ultrathin membranes in voltammetric applications, thus potentially extending the sensors lifetime. The high resistance of the membrane did not affect the voltammetric characteristics, because the resistance was independent of ions concentration in solution. On the other hand, high resistance results in charge trapping effect in the solid contact material, leading to advantageous retention of the oxidized‐conducting state of the solid contact, independently of the applied electrode potential.     

Methanol electrooxidation on platinum mesh electrodes bonded to solid polymer electrolytes

Methanol electrooxidation on smooth platinum electrodes bonded to solid polymer electrolytes was studied in water and acid solution by voltammetric measurements with different scanning rates. An enhancement of the oxidation rates was observed in these systems as compared to identical platinum electrodes in contact with liquid electrolytes. This electrocatalytic effect strongly depends on the measuring conditions and on the electrode potential. The reasons for the catalytic effects at different potentials are discussed. Received: 8 January 1997 / Accepted: 1 December 1997     

Mixed Aza‐Thioether Crowns Containing a 1,10‐Phenanthroline Sub‐Unit as Neutral Ionophores for Silver Ion

Three different recently synthesized aza‐thioether crowns containing a 1,10‐phenanthroline sub‐unit (L1–L3) and a corresponding acyclic ligand (L4) were studied to characterize their abilities as silver ion ionophores in PVC‐membrane electrodes. Novel conventional silver‐selective electrodes with internal reference solution (CONISE) and coated graphite‐solid contact electrodes (SCISE) were prepared based on one of the 15‐membered crowns containing two donating S atoms and two phenanthroline‐N atoms (L1). The electrodes reveal a Nernstian behavior over wide Ag⁺ ion concentration ranges (1.0×10^?5?1.0×10^?1 M for CONISE and 5.0×10^?8?4.0×10^?2 M for SCISE) and very low limits of detection (8.0×10^?6 M for CONISE and 3.0×10^?8 M for SCISE). The potentiometric response is independent from pH of the solution in the pH range 3.0–8.0. The electrodes manifest advantages of low resistance, very fast response and, most importantly, good selectivities relative to a wide variety of other cations. The electrodes can be used for at least 2 months (for CONISE) and 4 months for (SCISE) without any appreciable divergence in potentials. The electrodes were used as an indicator electrode in the potentiometric titration of Ag⁺ ion and in the determination of silver in photographic emulsions and in radiographic and photographic films.