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.     

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.     

Development and Optimization of an Ion‐selective Electrode for Serotonin Detection

Our studies are focused on the development of novel potentiometric sensors for the quantification of the neurotransmitter serotonin. Therefore, ion‐selective electrodes based on plasticized PVC membranes are applied. The electroactive part of the membrane consists of an ion pair complex formed between the protonated analyte and a carborane anion [Co(1,2‐C₂B₉H₁₁)₂]⁻. The analytical performance of the electrode was studied regarding sensitivity, concentration range, limit of detection and potential stability. The ion‐selective electrodes were optimized with respect to the material of the transducing element, as well as the membrane thickness and its composition. Stable, all solid state ISEs could be developed, using the non‐polar plasticizer NPOE and a graphite rod with high surface area as transducing element. We thus achieved a near Nernstian response over three decades of concentration (2.25⋅10^‐5‐1.00⋅10^‐2 M) and a limit of detection in the μ‐molar range for the optimized electrodes. The electrodes could successfully be miniaturized using carbon based screen printed electrodes.     

Highly Selective and Sensitive Triiodide PVC‐Membrane Electrode Based on a New Charge‐Transfer Complex of Bis(2,4‐Dimethoxybenzaldehyde)butane‐2,3‐Dihydrazone with Iodine

In this work, a highly selective membrane triiodide sensor based on a new charge‐transfer complex of bis(2,4‐dimethoxybenzaldehyde)butane‐2,3‐dihydrazone with iodine (Iodide Charge Transfer complex: ICT) as membrane carrier is introduced. The influences of five different solvent mediators on sensitivity and selectivity of the proposed sensor were considered. The best performance was obtained with the membrane composition containing 30% poly (vinyl chloride), 63% DBP, 5% ICT and 2% HTAB. The electrode shows a Nernstian behavior over a very wide triiodide ion concentration range (1.0 × 10^?7‐1.0 × 10^?2 M), and a detection limit value of 8.0 × 10^?8 M. The effect of pH on the potentiometric response of the sensor was also studied, and it was found that the response of the electrode is independent of the pH of the solution in the pH range of 4.0–10. The proposed sensor has a very fast response time (< 12 s), and good selectivities relative to a wide variety of common inorganic and organic anions, including iodide, acetate, bromide, chloride, fluoride, nitrite, nitrate, sulfite, sulfate, cyanide and thiocyanate. In fact the selectivity behavior of the proposed triiodide ion‐selective electrode shows great improvements compared to the previously reported electrodes for triiodide ion. The proposed membrane sensor can be used for at least 6 months without any divergence in the potentials. The electrode was successfully applied as an indicator electrode in the titration of triiodide with thiosulfate ion.     

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).     

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.     

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.     

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.     

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.     

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.     

On the Phase Shifts in the Dynamics of the H2O2–Na2S2O3–H2SO4–CuSO4 Oscillator Revealed by Comparison of Potentiometric Responses of Different Indicator Electrodes

The dynamics of the H₂O₂–Na₂S₂O₃–H₂SO₄–CuSO₄ homogeneous pH oscillator was studied in the flow reactor potentiometrically using different sensors: platinum electrode, Cu(II) ion‐selective electrode (Cu‐ISE), and pH‐electrode. It was found that for the flow rates close to two bifurcation values, between which the oscillations exist, there is a detectable phase shift between the response of the Cu‐ISE and other electrodes, while it practically vanishes for the intermediate flow rates. To explain both the oscillations of the Cu‐ISE potential and the relevant phase shift, the system's dynamics was studied both experimentally and numerically. The literature kinetic mechanism of the pH oscillator was extended for the dynamics of the copper(II) and copper(I) species in the form of thiosulfate complexes, and kinetic parameters of the redox equilibria, ensuring the oscillations, were estimated. It was found that the phase shift at the relatively low flow rates occurs due to limited efficiency of the supply of CuSO₄ catalyst, as the species of lowest concentration, to the reactor, and therefore it can be minimized either by increasing the flow rate of all reactants or, alternatively, by enhancing the model concentration of CuSO₄ in the feeding stream, for its fixed flow rate. This work is one more proof that it is useful to monitor the dynamics of the homogeneous oscillatory systems with more than one electrode, if the experimental potential–time courses are to be explained in terms of an appropriate kinetic mechanism.     

PVC‐Based on Thiopyrilium Derivatives Membrane Electrodes for Determination of Histamine

This study describes novel histamine‐selective electrodes, having their basis on thiopyrilium (TP) derivatives as suitable ionophores. The electrodes were prepared by incorporating the TP derivatives into plasticized poly(vinyl chloride) (PVC) membranes. These electrodes demonstrate high selectivity as far as the histamine response is considered, as compared with many common inorganic anions and other kinds of amino acids. The influence of membrane composition, pH and the effect of lipophilic cationic and anionic additives on the response characteristics of the electrode were investigated. The resulting sensor based on 2,6‐bis(4‐dimethyl amino phenyl)‐4‐phenyl thiopyrilium perchlorate (TP4) responds to histamine in a wide concentration range from 5.0 × 10^?6 to 1.0 × 10^?1 M with a slope of 54.8 ± 0.6 mV decade^?1 and detection limit of 3.0 μ mol L^?1 (～0.3 ppm). The electrode illustrates fast response time and good long‐term stability (more than 2 months). The ability to design histamine‐selective electrodes based on new thiopyrilium derivatives and both to alter selectivity and improve the response characteristics through structural changes to the charged ionophore, has been investigated. The prepared electrode was used for the determination of histamine in a synthetic human serum sample. Consequently, satisfactory recovery results were obtained over a wide concentration range of histamine.     

Design of an Imipramine‐Selective Electrode Based on an Ion‐Pair and its Application to Pharmaceutical Analysis

Ion‐selective electrodes play an important role in pharmaceutical analysis due to their simplicity, rapidity and accuracy over some other analytical methods. This research introduces the design of an ion‐pair of imipramine‐tetraphenyl borate (IMP‐TPB) based PVC membrane sensor for IMP hydrochloride determination. Effect of the membrane composition, the pH influence and the effect of lipophilic anionic additives on the response characteristics of the electrode were investigated. After a series of experiments, the best electrode performance was accomplished with a membrane composition of 30% PVC, 63% DBP, 5% (IMP‐TPB) and 2% (NaTPB). This electrode illustrated a fast (15 s), stable and Nernstian response across a relatively wide IMP hydrochloride concentration range (10^?5 to 10^?5 M), in the pH range of (3.0–8.0), and it can be used for at least two months without any measurable change in sensitivity. Additionally, the interference between IMP and several drugs and common inorganic anions was negligible as shown by the potentiometric selectivity coefficient data. The membrane sensor was successfully applied to the determination of IMP in tablets and human urine samples, and very good recovery results were obtained.     

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.     

Potentiometric Determination of Non‐Ionic Surfactants by Liquid Membrane Electrodes

It is well known that non‐ionic surfactants (NIS) influence remarkably the potentiometric measurements with liquid membrane ion selective electrodes (ISEs), interfering particularly on performance of ISEs for earth‐alkali metals, for which the loss of selectivity with regard to alkali metals has been documented. These studies indicate that such interferences are due to the extraction of surfactants within the membrane, where a competition takes place between the originally present ionophore and the surfactant which also acts as a ligand for alkali metals. The interpretation of such phenomena enabled one to exploit this interference for analytical purposes by membrane/solution extraction experiment monitored by UV measurements and by impedance FRA analysis on coated wire electrodes. Using Ca/Mg ISEs based on the neutral ionophore ETH 4030, it has been established that the logarithm of the Ca/Mg over Na potentiometric selectivity constant is linearly correlated with the concentration of NIS like Tegopren 5863 and Triton X‐100. The proposed method has been applied for the development of a new potentiometric analytical procedure for the determination of Tegopren 5863 in synthetic seawater (SSW), ranging from 0.25 to 5 ppm. Our procedure consists in the exposure of the electrode to stirred SSW containing the surfactant; the progressive extraction of Tegopren 5863 causes a growth in electrode's sensitivity to Na⁺ and K⁺, losing selectivity for Ca²⁺ and Mg²⁺. In turn this induces an increase of EMF, as all these ions are present in the studied matrix. The potential drift was monitored for 15 hours, showing that the process reaches thermodynamic equilibrium after about 12 hours of exposure. This method presents a value of 210 ppb of Tegopren 5863 as detection limit.     

Direct Observation of a Li‐Ionic Space‐Charge Layer Formed at an Electrode/Solid‐Electrolyte Interface

When two different materials come into contact, mobile carriers redistribute at the interface according to their potential difference. Such a charge redistribution is also expected at the interface between electrodes and solid electrolytes. The redistributed ions significantly affect the ion conduction through the interface. Thus, it is essential to determine the actual distribution of the ionic carriers and their potential to improve ion conduction. We succeeded in visualizing the ionic and potential profiles in the charge redistribution layer, or space‐charge layer (SCL), formed at the interface between a Cu electrode and Li‐conductive solid electrolyte using phase‐shifting electron holography and spatially resolved electron energy‐loss spectroscopy. These electron microscopy techniques clearly showed the Li‐ionic SCL, which dropped by 1.3 V within a distance of 10 nm from the interface. These techniques could contribute to the development of next‐generation electrochemical devices.     

Determination of Norepinephrine Alone and in the Presence of Ascorbic and Uric Acids Using a Gold Electrode Modified with Gold Nanoparticles and Self‐Assembled Layers of meso‐2,3‐Dimercaptosuccinic Acid

This paper reports on the modification of gold electrodes with self‐assembled layers (SAMs) composed of meso‐2,3‐dimercaptosuccinic acid, cysteamine and gold nanoparticles, respectively and their application to quantitative determination of norepinephrine alone and in the presence of ascorbic and uric acids in solution at pH 7. The modification was carried out on two kinds of templates: a bare gold electrode (2D electrode) and a gold electrode coated in the first step with gold nanoparticles (3D electrode). Cyclic voltammograms reveal an enhancement of the norepinephrine electrooxidation in comparison to a bare, (non‐modified) gold electrode. The oxidation peaks for norepinephrine, ascorbic acid and uric acid have a peak‐to‐peak separation that enables their selective determination even in a complex mixture.     

Highly Selective and Sensitive Triiodide PVC‐Based Membrane Electrode Based on a New Charge Transfer Complex of 2‐(((2‐(((E)‐1‐(2‐Hydroxyphenyl) Methylidine) Amino) Phenyl) Imino) Methyl) Phenol for Nano‐Level Monitoring of Triiodide

Abstract

A highly selective and sensitive triiodide sensor based on a 2‐(((2‐(((E)‐1‐(2‐hydroxy phenyl) methylidine) amino) phenyl) imino) methyl) phenol with iodine (CTC) as membrane carrier was developed. The electrode revealed a Nernstian behavior over a very wide triiodide‐ion concentration range (5.0×10^?8–1.0×10^?2 M), and relatively low detection limit (3.0×10^?8 M). The potentiometric response is independent of the pH of solution in the pH range of 3.0–10.0. The electrodes manifest advantages of low resistance, very fast response (<12 s), and most importantly, good selectivities relative to a wide variety of inorganic and organic anions, including iodide, bromide, chloride, fluoride, sulfite, sulfate, cyanide, thiocyanate, and acetate. In fact, the selectivity behavior of the proposed triiodide ion‐selective electrode shows great improvements compared to the previously reported electrodes for the triiodide ion. The proposed membrane sensor can be used for at least 6 months without any significant divergences in the potential. The electrode was successfully applied as an indicator electrode in the titration of triiodide with thiosulfate ion.     

Potentiometric Quantification of the Benzoate Ion in Nonalcoholic Beverages Using a Solid Electrode Modified with a Polypyrrole‐Based Selective Membrane

This work presents the results obtained on the construction of a potentiometric ion‐selective electrode based on a polypyrrole (PPy) selective membrane to quantify the benzoate ion in nonalcoholic beverages. The electrode modification with benzoate (Benz^?1) ion doped‐Ppy was carried out under an imposed potential, while the electrosynthesis optimization was undertaken using the modified Simplex method, such that the film’s sensitivity was maximized toward the benzoate ion. The maximum sensitivity recorded was ?52.02±1.55 mV/decade [Benz^?1] using a graphite powder‐araldite resin composite electrode. During the modified electrode’s characterization it was found out that the response and drift were relatively short, namely 2 min and 0.4 mV min^?1 respectively, within the 7 to 9 pH range, exhibiting a detection limit of 7×10^?4 mol L^?1 and a quantification range of 3×10^?3 at 10^?1 mol L^?1. Selectivity coefficients were evaluated with the Matched Potential Method obtaining in all cases values much less than 1. The benzoate concentration in commercially available juices and sweetened beverages was evaluated comparing the results obtained with the proposed ion‐selective electrode and those of HPLC, the usually accepted technique. A statistical analysis of the results led to the conclusion that there exists no meaningful difference between the results obtained with both techniques, which shows the usefulness of the ion‐selective electrode to proceed with the quantification in real samples.     

Triiodide PVC Membrane Electrode Based on a Charge‐Transfer Complex of Iodine with Ditertbutyl‐Dicyclohexyl‐18‐Crown‐6

A new triiodide ion‐selective electrode based on a charge‐transfer complex of iodine with ditertbutyl‐dicyclohexyl‐18‐crown‐6 (t‐Bu)₂DC18C6 as membrane carrier was prepared. The electrode has a linear dynamic range from 6.3 × 10^?3‐5 × 10^?6 with a Nernstian response of 58.6 ± 1 mV decade^?1 and a detection limit of 1.3 × 10^?6 M. The response time of the sensor was 25 s. The membrane could be used for two months without any divergence in potentials. The electrode exhibits an anti‐Hofmeistetr selectivity sequence with a preference for triiodide at pH 2.0‐10.0. The response mechanism of the electrode was investigated by Uv‐Vis spectroscopic technique. The electrode can be used for the determination of ascorbic acid in orange juice.