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.     

Potentiometric responses of ion-selective electrodes after galvanostatically controlled incorporation of primary ions

A new method of quantitative incorporation of primary cations into ion-selective membrane by means of galvanostatic cathodic polarization/conditioning, before measurement step, was proposed and tested on the example of potassium-selective electrode with ionophore - valinomycin in poly(vinyl chloride) based membrane and with polypyrrole solid contact. Open circuit potential values recorded after polarization can be quantitatively explained by changes of primary cations and ionophore concentration in the surface part of the membrane. The influence of potassium ions concentration in the membrane (in relation to ion exchange sites amount) on the shape of potentiometric calibration plots was also observed. Improved characteristics, with extended linear range, can be obtained for membrane of minor loading with primary cations (around 25%), the responses are relatively stable in course of following calibrations.     

Development of Miniature Solid Contact Ion Selective Electrodes for in situ Instrumentation

Using a standardized miniature format ion selective electrode, five different carbon based solid contact materials, including a novel commercially available graphene oxide assisted carbon nanotube dispersion (Flexiphene^TM), were compared. The electromotive force (EMF) response, stability, and behavior following storage was evaluated. Bulk resistance for the novel graphene oxide/carbon nanotube (GO‐CNT) based ISE was observed to be 0.09±0.03 MΩ, which is two orders of magnitude lower than reported for either component in isolation. The results for previously described solid contact materials are in general agreement with the literature, and the tradeoffs between solid contact layer material choices are discussed. Performance of GO‐CNT solid contact ion selective electrodes were then evaluated in a 3D‐printed fluidic array to determine their suitability for future in situ instruments.     

Comparison of Multi‐walled Carbon Nanotubes and Poly(3‐octylthiophene) as Ion‐to‐Electron Transducers in All‐Solid‐State Potassium Ion‐Selective Electrodes

Multi‐walled carbon nanotubes (MWCNTs) were compared with poly(3‐octylthiophene) (POT) as ion‐to‐electron transducer in all‐solid‐state potassium ion‐selective electrodes with valinomycin‐based ion‐selective membranes. MWCNTs and POT were mixed with the other components of the potassium ion‐selective membrane cocktail (valinomycin, KTpClPB, o‐NPOE, PVC, THF) which was then applied on a glassy carbon (GC) substrate to prepare single‐piece ion‐selective electrodes (SPISEs). Results from potentiometric and impedance measurements showed that the MWCNT‐based electrodes have a more reproducuible standard potential and a lower overall impedance than the electrodes based on POT. Both types of electrodes showed similar sensitivity to potassium ions and no redox sensitivity.     

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.     

An All‐solid‐state Polymeric Membrane Ca2+‐selective Electrode Based on Hydrophobic Alkyl‐chain‐functionalized Graphene Oxide

An all‐solid‐state polymeric membrane Ca²⁺‐selective electrode based on hydrophobic octadecylamine‐functionalized graphene oxide has been developed. The hydrophobic composite in the ion‐selective membrane not only acts as a transduction element to improve the potential stability for the all‐solid‐state Ca²⁺‐selective electrode, but also is used to immobilize Ca²⁺ ionophore with lipophilic side chains through hydrophobic interactions. The developed all‐solid‐state Ca²⁺‐selective electrode shows a stable potential response in the linear range of 3.0×10⁻⁷–1.0×10⁻³ M with a slope of 24.7±0.3 mV/dec, and the detection limit is (1.6±0.2 )×10⁻⁷ M (n =3). Additionally, due to the hydrophobicity and electrical conductivity of the composite, the proposed all‐solid‐state ion‐selective electrode exhibits an improved stability with the absence of water layer between the ion‐selective membrane and the underlying glassy carbon electrode. This work provides a simple, efficient and low‐cost methodology for developing stable and robust all‐solid‐state ion‐selective electrode with ionophore immobilization.     

Novel Solid‐Contact Ion‐Selective Microelectrodes for Localized Potentiometric Measurements

The design and properties of novel type of solid‐contact ionophore‐based ion‐selective microelectrodes are reported. The microelectrode is based on an insulated needle‐shaped metallic wire with an exposed apex. The ion‐to‐electron transducer is made of poly(3‐octylthiophene‐2,5‐diyl) and placed between an ion‐selective membrane and the metallic tip. The ion‐selective polyvinyl chloride‐based membrane is deposited atop the layer of conductive polymer. The length of the ion‐sensitive part of the electrode is less than 10 μm. pH and Mg²⁺‐selective microelectrodes were constructed and tested showing stable potential and fast response that are essential properties for the practical application of microelectrodes for localized scanning measurements.     

Ion‐selective Electrodes with 3D Nanostructured Conducting Polymer Solid Contact

Until now both ion‐to‐electron transducers as well as large surface area nanostructured conducting materials were successfully used as solid contacts for polymer‐based ion‐selective electrodes. We were interested to explore the combination of these two approaches by fabricating ordered electrically conducting polymer (ECP) nanostructures using 3D nanosphere lithography and electrosynthesis to provide a high surface area and capacitive interface for solid contact ion‐selective electrodes (SC‐ISEs). For these studies we used poly(3,4‐ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT(PSS)) films with 750 nm diameter interconnected pores as the intermediate layer between a glassy carbon electrode and a Ag⁺ ‐selective polymeric membrane. We also investigated the feasibility of loading the voids created in the polymer film with a lipophilic redox mediator (1,1’‐dimethylferrocene) to provide the respective ISEs with well‐defined/controllable E⁰ values. These expectations were fulfilled as the standard deviation of E⁰ values were reduced with almost an order of magnitude for 3D nanostructured SC‐ISEs filled with the redox mediator as compared to their redox mediator‐free analogs. The detrimental effect of the redox mediator extraction into the plasticized PVC‐based ion‐selective membrane (ISM) was efficiently suppressed by replacing the PVC‐based ISMs with a low diffusivity silicone rubber matrix.     

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.     

Intercalation Compounds as Inner Reference Electrodes for Reproducible and Robust Solid‐Contact Ion‐Selective Electrodes

With billions of assays performed every year, ion‐selective electrodes (ISEs) provide a simple and fast technique for clinical analysis of blood electrolytes. The development of cheap, miniaturized solid‐contact (SC‐)ISEs for integrated systems, however, remains a difficult balancing act between size, robustness, and reproducibility, because the defined interface potentials between the ion‐selective membrane and the inner reference electrode (iRE) are often compromised. We demonstrate that target cation‐sensitive intercalation compounds, such as partially charged lithium iron phosphate (LFP), can be applied as iREs of the quasi‐first kind for ISEs. The symmetrical response of the interface potentials towards target cations ultimately results in ISEs with high robustness towards the inner filling (ca. 5 mV dec^?1 conc.) as well as robust and miniaturized SC‐ISEs. They have a predictable and stable potential derived from the LiFePO₄/FePO₄ redox couple (97.0±1.5 mV after 42 days).     

Implementation of a Chloride‐selective Electrode Into a Closed Bipolar Electrode System with Fluorimetric Readout

Applicability of a bipolar electrode system was tested for arrangements containing a typical ion‐selective electrode (ISE) connected with an electrode coated by a conducting polymer characterized by electroluminescence. In this case a selective response of the ISE membrane at one pole of the bipolar electrode is transduced to a fluorimetric signal obtained by reduction of the conducting polymer at the second pole. This signal transformation mode was studied on example of a simple closed bipolar electrode system composed of all‐solid‐state chloride‐selective electrode with polypyrrole transducer as the sensing pole and the reporting pole represented by electrode coated by poly(3‐octylthiophene) (POT) layer characterized by fluorescence in the neutral state. In this system selective and linear dependences of fluorimetric signal on logarithm of chloride ions concentration in turn‐on mode were recorded for optimized external voltage applied. Alternatively, a concept of cascade bipolar electrode system with incorporation of additional bipolar electrode being a polarization source for the sensing bipolar electrode with ISE and POT layer was also tested. A significant advantage of the cascade system is its possibility to work spontaneously without external polarization. For this case also linear calibration plots of fluorimetric signal vs. logarithm of analyte concentration were recorded.     

A Solid Potassium Ion Selective Electrode

Abstract

A solid membrane potassium selective electrode has been developed. The following parameters of the potassium ion electrode were investigated; Nernstian response to potassium ion activity, response time to step changes in potassium ion activity, the effect of pH on potassium response, and the electrode response to other cations.     

Description of the Effects of Non‐ion‐exchange Extraction and Intra‐membrane Interactions on the Ion‐selective Electrodes Response within the Interface Equilibria‐triggered Model

The recently proposed interface equilibria‐triggered dynamic diffusion model of the boundary potential has proven its high predictive efficiency for quantification of the ion exchange and co‐extraction effects at the interface, as well as of the trans‐membrane transfer effect, on the electrode response. It is applicable for both ion exchanger‐based and neutral carrier‐based electrodes. In this communication, the adaptability of this model to more complex cases, when non‐ion‐exchange extraction processes at the interface (partition of organic acids’ and bases’ molecular forms and extraction of ionic associates) are coupled with protolytic equilibria in the aqueous phase and with self‐solvation process in the membrane phase, is demonstrated. By the example of electrodes reversible to ions of highly lipophilic physiologically active bases and acids (amiodarone, verapamil, vinpocetine, salicylic acid), it is shown that the peculiarities of their functioning, such as a very strong pH effect on the potential of cation‐selective electrodes, non‐monotonic pH dependence of the potential and super‐Nernstian response slope in certain pH region for a salicylate‐selective electrode, are well described within the model.     

Evaluating the Effect of four Hosting Ionophores on the Performance of Anion Selective Potentiometric Sensor

A systematically designed practical approach was carried out for the optimization of an anion selective electrode for the determination of an anionic laxative, docusate sodium (DS). The PVC membrane composition and the sensor assembly were systematically optimized. Different sensors were fabricated using tetradodecylammonium bromide (TDAB), tridodecylmethylammonium bromide, tetraheptylammonium bromide as ion exchangers. The effect of ionophore was evaluated using four different host‐guest ionophores, namely; calix[8]arene, β‐cylodextrin, hydroxypropyl‐β‐cylodextrin and carboxymethyl‐β‐cylodextrin to reach the optimum membrane composition. Sensors were constructed in both liquid membrane and solid contact sensor‐assemblies. The slope, linear range, LOQ and response time for each sensor was calculated to assess their performance characteristics. Best Nernstian slope of ?61.38 mV/decade and lowest quantification limit of 7.62×10^?7 M was achieved by the sensor containing TDAB as ion exchanger and Calix[8]arene as ionophore in the PVC matrix using the gold wire solid contact sensor assembly. Electrode selectivity was assured in the presence of DS potential degradation product, common interfering ions and industrial excipients of tablet and gel formulations. Validation was carried out regarding the ICH validation parameters.     

The Influence of the Conditioning Procedure on Potentiometric Characteristics of Solid Contact Calcium‐Selective Electrodes in Nanomolar Concentration Solutions

A solid contact calcium‐selective electrode with a poly(pyrrole) intermediate layer doped with the calcium ion‐complexing ligand Tiron was successfully constructed and examined towards the possibility of lowering the detection limit. Two calcium ionophores, ETH 1001 and ETH 129, were compared. A variety of different shapes of potentiometric responses can be obtained in a controlled manner by applying an appropriate conditioning procedure. The lifetime of the investigated electrodes was longer than 1 year. The modified poly(pyrrole) solid contact electrode was compared with the coated wire type. An inductively coupled plasma mass spectrometry with laser ablation analysis (LA ICP MS) was used to confirm the transmembrane ion fluxes.     

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.     

Highly Selective All‐Plastic,Disposable, Cu2+‐Selective Electrodes

Conducting polymers (CP) remain a promising material to construct stable potential all‐solid‐state ion‐selective potentiometric electrodes. The unique properties of poly(3,4‐ethylenedioxythiophene) doped with poly(4‐styrenesulfonate) ions, PEDOT‐PSS: high CP stability and affinity of doping anions towards Cu²⁺ ions, make it highly attractive for construction of all‐solid‐state copper(II)‐selective electrodes with outstanding selectivity. The additional benefits can arise from solution processability of commercially available PEDOT‐PSS system. This material was highly promising for a new sensor arrangement, i.e. to obtain disposable, planar and flexible all‐plastic Cu²⁺‐selective electrodes. These sensors can be obtained by casting a commercially available dispersion of PEDOT‐PSS (Baytron P) on a plastic, non‐conducting support material. The CP being both electrical lead and ion‐to‐electron transducer, was covered with plastic, solvent polymeric Cu²⁺ selective membrane. This extremely simple arrangement, after conditioning in dilute Cu²⁺ solution, was characterized with linear Nernstian responses within the activities range from: 0.1 to 10^?4 M, followed by super‐Nernstian responses for lower activities. The latter result points to effective elimination of primary ions leakage from the plastic membrane / transducer phase and has resulted in significantly improved selectivities. Obtained log K values were equal to ?7.6 for Co²⁺, ?7.4 for Zn²⁺, ?7.2 for Ca²⁺ and ?6.8 for Na⁺, respectively.     

A Solid‐State Reference Electrode Based on a Self‐Referencing Pulstrode

The design of solid‐state reference electrodes without a liquid junction is important to allow miniature and cost‐effective electrochemical sensors. To address this, a pulse control is proposed using an Ag/AgI element as reliable solid‐state reference electrode. It involves the local release of iodide by a cathodic current that is immediately followed by an electromotive force (EMF) measurement that serves as the reference potential. The recapture of iodide ions is achieved by potentiostatic control. This results in intermittent potential values that are reproducible to less than one millivolt (SD=0.27 mV, n=50). The ionic strength is shown to influence the activity coefficient of released iodide in accordance with the extended Debye–Hückel equation, resulting in a predictable change of the potential reading. The principle is applied to potentiometric potassium detection with a valinomycin‐based ion‐selective electrode (ISE), demonstrating a completely solid‐state sensor configuration.     

Tailoring Solution Cast Poly(3,4‐dioctyloxythiophene) Transducers for Potentiometric All‐Solid‐State Ion‐Selective Electrodes

A novel concept of tailoring potentiometric responses of all‐solid‐state ion‐selective electrodes was introduced. The effect of composition and resulting properties of the conjugated polymer transducer, placed between the electrode support and ion‐selective membrane, on analytical characteristic of obtained sensors was studied.     

Lead Ion‐Selective Polypyrrole Solid‐Contact Electrode Based on Crown Ether

Abstract

The lead(II) ion selective solid‐contact electrode based on polypyrrole film, covered with a polyvinyl chloride membrane has been prepared. Polypyrrole film was used as a mediating layer of the solid‐contact electrode due to the conductivity. Crown ether has been used as ionophores in polyvinyl chloride cocktail solutions. This solid‐contact electrode based on benzo‐15‐crown‐5 exhibited Nernstian‐response within 30 s response time over concentration range, 1×10^?1～5×10^?7 M. The selectivity of this electrode to other metal cations was comparatively good. This electrode showed much better results, such as detection range, slope, response time and reproducibility than conventional ion selective electrode and coated wire electrode.