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.     

Electrochemical Behaviour of Poly(benzopyrene) Films Doped with Eriochrome Black T as a Pb2+‐Sensitive Sensors

Poly(benzopyrene) films were electrosynthesized on glassy carbon disk electrodes from benzo(a)pyrene by cyclic voltammetry in the presence of eriochrome black T, which forms strong complexes with lead ions. In consequence, by conditioning the films in high concentration of lead (0.1 mol dm^?3 Pb(NO₃)₂) potentiometric sensitivity to lead ions down to 10^?5 mol dm^?3 Pb²⁺ was induced, and a novel type of lead‐sensitive electrode was obtained. The electrode is characterized by high stability of the potential readouts, good reproducibility of the calibration curves as well as a minor hysteresis effects. The performance of our lead‐sensitive electrode was favourable compared to PEDOT and PPy‐based electrodes doped with eriochrome black T. We conclude that poly(benzopyrene) doped with eriochrome black T is a new electroactive material that may be applied in sensor technology.     

Solid-contact ion-selective electrodes for aromatic cations based on pi-coordinating soft carriers

Ion-selective electrodes (ISEs) based on pi-coordinating carriers were prepared and investigated as potentiometric sensors for aromatic cations, using N-methylpyridinium as a model aromatic cation. Derivatives of tetraphenylborate were studied as charged carriers in plasticized poly(vinyl chloride) membranes. Furthermore, neutral compounds containing pi-coordinating anthryl groups were studied as neutral carriers. Bis(2-ethylhexyl)sebacate (DOS) and 2-nitrophenyl octyl ether (o-NPOE) were used as non-polar and polar plasticizer, respectively. ISEs were constructed by using poly(3,4-ethylenedioxythiophene) (PEDOT) as solid-contact material. Conventional ISEs with internal filling solution were used for comparison. The potentiometric responses of the ISEs were investigated using N-methylpyridinium as primary ion. The results show that the selectivity of the ISEs is influenced significantly by both the plasticizer and the charged carriers, while the neutral carriers studied have only a minor influence on the selectivity. The role of cation-pi interactions between aromatic cations and the membrane components is discussed.     

Influence of anionic additive on Hg(2+) interference on Ag(+)-ISEs based on [2.2.2]p,p,p-cyclophane as neutral carrier

The influence of the anionic additive on the Hg²⁺ interference on Ag⁺-ion-selective electrodes (Ag⁺-ISEs) based on [2.2.2]p,p,p-cyclophane as π-coordinating neutral carrier was studied by potentiometric measurements. The weakly coordinating carborane anion (7,8,9,10,11,12-hexabromocarborane, CB₁₁H₆Br₆⁻) was compared with the commonly used tetrakis(4-chlorophenyl borate) (TpClPB⁻) as the anionic additive in plasticized PVC membrane-based Ag⁺-ISEs. The Hg²⁺ interference was reduced by six orders of magnitude when replacing TpClPB⁻ with CB₁₁H₆Br₆⁻ as the anionic additive, resulting in determined by the matched potential method (MPM). This indicates that weakly coordinating carboranes may be particularly useful as anionic additives in ISEs for soft cations.     

Metallic and non-metallic redox response of conducting polymers

The potentiometric response of electrodes coated with polypyrrole or poly(N-methylpyrrole) films with different doping anions was studied in solutions containing the redox couples: Fe(CN)₆^3−/4−, Ru(NH₃)₆^3+/2+ and Fe(Ill)/Fe(II). The stable potential measured with the electrodes was the potential of the redox couple. The response time was instant for polypyrrole doped with dodecylsulphate ions, PPy(DS) and slow for the polymers doped with mobile anions. On the basis of electrochemical measurements and chemical analysis by EDAX spectroscopy it was found that with the PPy(DS) electrode the potentiometric response was of the ‘metallic’ type, with no change in the oxidation state of the bulk polymer. With the other polymer systems studied reduction or oxidation of the polymer bulk took place when it was in contact with a redox couple in the solution.     

Solid‐contact Acetate‐selective Electrode Based on a 1,3‐bis(carbazolyl)urea‐ionophore

The construction and study of solid‐contact acetate‐selective electrodes is described using a 1,3‐bis(carbazolyl)urea derivative as a neutral hydrogen‐bonding ionophore and poly(3,4‐ethylenedioxythiophene) as the solid contact. It was shown recently that this ionophore binds acetate (logK_ass=4.98) that is used as primary ion in this study. The electrodes show linearity over the activity range of 10^?4.50–10^?1.10 with a sub‐Nernstian slope of ?51.3 mV per decade and a detection limit of 10^?5.00. The anion‐selectivity pattern of these electrodes deviates markedly from the Hofmeister pattern. When adding ionophore to the membrane the logarithm of the selectivity coefficient (logK) for SCN^? decreased from 6.5 to 1.2, logK for I^? decreased from 5.7 to 0.9, logK for NO₃^? decreased from 4.3 to 0.6 and logK for Br^? decreased from 3.3 to 0.1. The selectivity coefficients of hydrophilic anions such as Cl^?, F^?, HPO₄^2?, and SO₄^2? are significantly lower than in case of the ionophore‐free membrane. It was discovered that the constructed electrodes are also relatively selective to bicarbonate. This work is an important step towards the further development of solid‐contact anion‐selective electrodes.     

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.