Johan Bobacka

Potentiometric Ion Sensors

Åbo Akademi University, Process Chemistry Centre, c/o Laboratory of Analytical Chemistry, Biskopsgatan 8, FI-20500 Turku-Åbo, Finland; Faculty of Material Science and Ceramics, AGH-University of Science and Technology, Al. Mickiewicza 30, PL-30059 Cracow, Poland; and Åbo Akademi University, Process Chemistry Centre, c/o Center for Process Analytical Chemistry and Sensor Technology (ProSens), Biskopsgatan 8, FI-20500 Turku-Åbo, Finland

Potential Stability of All-Solid-State Ion-Selective Electrodes Using Conducting Polymers as Ion-to-Electron Transducers.

Demanding analytical applications such as on-line process analysis and clinical analysis require robust, reliable, and maintenance-free ion sensors of high potential stability. In this work the stability of the electrode potential of all-solid-state ion-selective electrodes using conducting polymers as ion-to-electron transducers is critically evaluated by using chronopotentiometry and electrochemical impedance spectroscopy. This study is focused on the relationship between the potential stability of the electrode and the capacitance of the solid contact where ion-to-electron transduction takes place. The influence of this capacitance on the potential stability of all-solid-state ion-selective electrodes is studied experimentally by using conducting polymer layers of different thickness as solid contacts in potassium ion-selective electrodes based on a solvent polymeric membrane. Because of its excellent environmental stability, the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) is used as a model compound for the solid contact material. Chronopotentiometry is found to be a convenient and fast experimental method to critically evaluate the potential stability of different types of ion-selective electrodes.

Electrochemical impedance spectroscopy of oxidized poly(3,4-ethylenedioxythiophene) film electrodes in aqueous solutions

The electrochemical properties of oxidized (p-doped) poly(3,4-ethylenedioxythiophene) (PEDOT) film electrodes in aqueous solutions were investigated by electrochemical impedance spectroscopy (EIS). PEDOT was electrochemically deposited on platinum from aqueous solutions containing 0.01 M 3,4-ethylenedioxythiophene (EDOT) and 0.1 M supporting electrolyte: KCl, NaCl or poly(sodium 4-styrenesulfonate) (NaPSS). Impedance spectra were obtained for Pt/PEDOT electrodes at dc potentials where PEDOT is in the oxidized (p-doped) state. Electrodes with PEDOT films of different thickness, containing different doping ions, were investigated in contact with different aqueous supporting electrolyte solutions. The EIS data were fitted to an equivalent electrical circuit in order to characterize the electrochemical properties of the Pt/PEDOT film electrodes. Best fits to the experimental impedance data were obtained for an equivalent circuit where the total bulk (redox) capacitance of the polymer film is composed of the diffusional pseudocapacitance in series with a second bulk capacitance. The results imply that the PEDOT film contains an excess of supporting electrolyte, which facilitates ion diffusion and gives rise to a large diffusional pseudocapacitance.

Transduction mechanism of carbon nanotubes in solid-contact ion-selective electrodes.

Porous carbon materials and carbon nanotubes were recently used as solid contacts in ion-selective electrodes (ISE), and the signal transduction mechanism of these carbon-based materials is therefore of great interest. In this work the ion-to-electron transduction mechanism of carbon nanotubes is studied by using electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). Single-walled carbon nanotubes (SWCNT) are deposited on glassy carbon (GC) disk electrodes by repetitive spraying, resulting in SWCNT layers with thicknesses of 10, 35, and 50 mum. The impedance spectra of these GC/SWCNT electrodes in contact with aqueous electrolyte solution show a very small resistance and a large bulk capacitance that is related to a large effective double layer at the SWCNT/electrolyte interface. Interestingly, the impedance response of GC/SWCNT is very similar to that of poly(3,4-ethylenedioxythiophene) (PEDOT) film electrodes studied earlier under the same experimental conditions. The same equivalent circuit is valid for both types of materials. The reason is that both materials can be described schematically as an asymmetric capacitor where one side is formed by electronic charge (electrons/holes) in the SWCNT wall or along the conjugated polymer chain of PEDOT and the other side is formed by ions (anions/cations) in the solution (or in the ion-selective membrane when used as a solid contact in ISE).

Mediatorless sugar/oxygen enzymatic fuel cells based on gold nanoparticle-modified electrodes.

We report on the fabrication and characterisation of a gold-nanoparticle (AuNP)-based mediatorless sugar/oxygen biofuel cell (BFC) operating in neutral sugar-containing buffers and human physiological fluids, such as blood and plasma. First, Corynascus thermophilus cellobiose dehydrogenase (CtCDH) and Myrothecium verrucaria bilirubin oxidase (MvBOx), used as anodic and cathodic bioelements, respectively, were immobilised on gold electrodes modified with 20 nm AuNPs. Detailed characterisation and optimisation of a new CDH/AuNP-based bioanode were performed and the following fundamental parameters were obtained: (i) the redox potential of the haem-containing centre of the enzyme was measured to be 75 mV vs. NHE, (ii) the surface coverage of CtCDH was found to be 0.65 pmol cm(-2) corresponding to a sub-monolayer coverage of the thiol-modified AuNPs by the enzyme, (iii) a turnover number for CtCDH immobilised on thiol-modified AuNPs was calculated to be ca. 0.5 s(-1), and (iv) the maximal current densities as high as 40 μA cm(-2) were registered in sugar-containing neutral buffers. Second, both biomodified electrodes, namely the CtCDH/AuNP-based bioanode and the MvBOx/AuNP-based biocathode, were combined into a functional BFC and the designed biodevices were carefully investigated. The following characteristics of the mediator-, separator- and membrane-less, miniature BFC were obtained: in phosphate buffer; an open-circuit voltage of 0.68 V, a maximum power density of 15 μW cm(-2) at a cell voltage of 0.52 V and in human blood; an open-circuit voltage of 0.65 V, a maximum power density of 3 μW cm(-2) at a cell voltage of 0.45 V, respectively. The estimated half-lives of the biodevices were found to be >12, <8, and <2 h in a sugar-containing buffer, human plasma, and blood, respectively. The basic characteristics of mediatorless sugar/oxygen BFCs were significantly improved compared with previously designed biodevices, because of the usage of three-dimensional AuNP-modified electrodes.

Influence of oxygen and carbon dioxide on the electrochemical stability of poly(3,4-ethylenedioxythiophene) used as ion-to-electron transducer in all-solid-state ion-selective electrodes

Abstract The electrochemical stability of poly(3,4-ethylenedioxythiophene) (PEDOT) is studied in view of its use as ion-to-electron transducer (solid contact) in all-solid-state ion-selective electrodes (ISEs). PEDOT is electrochemically deposited on glassy carbon (GC) and the resulting GC/PEDOT electrodes are studied by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and potentiometry. Valinomycin-based all-solid-state K + -ISEs are constructed by placing a K + -selective poly(vinyl chloride) (PVC)-based membrane on the GC/PEDOT electrode (solid contact). The influence of dissolved O 2 and CO 2 on the potential of the GC/PEDOT electrodes and of all-solid-state K + -ISEs is studied. PEDOT is compared with polypyrrole (PPy) as the solid contact material. A significant difference between the two conducting polymers (CPs) is that PEDOT is less sensitive to O 2 and CO 2 (pH) than PPy. Therefore, PEDOT is a promising new candidate as ion-to-electron transducer (solid contact) in all-solid-state ISEs based on solvent polymeric membranes that are permeable to O 2 and CO 2 .

Mechanism of ionic and redox sensitivity of p-type conducting polymers: Part 1. Theory

Equations have been derived to describe the open-circuit potentiometric response of a p-type conducting polymer membrane. The final model is derived step by step from the model of a single-metal electrode by using the concept of solvated electron and from the model of a solid-state ion-selective and/or electron- selective electrode containing a sparingly soluble metal-rich salt in the membrane. All electrodes considered, i.e. the metal, ion-selective and polymer electrodes, are presented as systems based on an ionic salt of the M+e− type. The ionic and redox sensitivities of a p-type conducting polymer are described by assuming two possible equilibration pathways. In the first approach the ion-electron exchange process at the polymer surface without a change of the oxidation state of the polymer is discussed and in the second the metathesis of the polymer surface with a concurrent change in the oxidation state of the polymer is considered. The equations derived allow us to interpret the anionic, cationic and redox sensitivities and to discuss the observed changes in the standard potentials.

All solid-state poly(vinyl chloride) membrane ion-selective electrodes with poly(3-octylthiophene) solid internal contact

All-solid-state ion-selective electrodes were prepared by using poly(3-octylthiophene)(POT) as solid contact material. A film of POT (thickness approximately 0.25 µm) was deposited on a solid substrate (platinum, gold or glassy carbon) by electrochemical polymerization of 3-octylthiophene. The POT layer was subsequently coated with an ion-selective membrane (ISM) to produce a solid-contact ion-selective electrode (SCISE), SCISEs for several ions (Li+, Ca2+ and Cl–) were prepared and investigated. The compositions of the ion-selective membranes were the same as normally used for the same ions in poly(vinyl chloride)(PVC)-based ion-selective electrodes (ISEs) with internal filling solution. The potentiometric response of the SCISEs was studied and compared with that of coated-wire electrodes (CWEs) prepared by coating the bare substrate with the same ion-selective membrane. The potentiometric slopes, limits of detection and response times of the SCISEs were similar to those of the corresponding CWEs, but the long-term stability of the potential was different for the two type of electrodes. The SCISEs exhibited a more stable electrode potential than the corresponding CWEs. However, the stability of the SCISEs was found to be influenced by the substrate material and this was studied in detail for the Ca-SCISE and Ca-CWE. For comparison, a Ca-ISE with internal filling solution was also used. By using glassy carbon as the substrate it was possible to obtain a Ca-SCISE exhibiting a standard potential that was almost as stable (ESCISE= 259.3 ± 1.3 mV, drift = 0.23 mV d–1) as for the conventional Ca-ISE (EISE= 61.4 ± 0.5 mV, drift = 0.16 mV d–) and significantly more stable than for the Ca-CWE, during a time period of 8 d. The most stable Ca-CWE, prepared by using glassy carbon as substrate, showed a potential drift of –3.8 mV d–1(ECWE= 269.6 ± 10.2 mV) during testing for 8 d. Electrochemical impedance spectrometry was used to understand the charge-transfer mechanisms of the different types of ion-selective electrodes studied. The impedance response of the electrodes was modelled by equivalent electrical circuits.

Kinetics of electron transfer between Fe(CN)63−/4− and poly(3,4-ethylenedioxythiophene) studied by electrochemical impedance spectroscopy

The electron transfer between the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) and the Fe(CN)63−/4− redox couple in aqueous solution was investigated by electrochemical impedance spectroscopy (EIS). PEDOT was electrochemically deposited on platinum from aqueous solutions containing 0.01 M 3,4-ethylenedioxythiophene (EDOT) and 0.1 M poly(sodium 4-styrenesulfonate) (NaPSS) as supporting electrolyte. Pt/PEDOT(PSS) electrodes with polymer films of different thickness were investigated at different concentrations of the redox couple in 0.1 M KCl background electrolyte solution. Impedance spectra were obtained at the dc-potential corresponding to the formal redox potential of Fe(CN)63−/4− (E°′≈220 mV) where the polymer is in the oxidized and electrically conducting state. The EIS data for the electrodes were fitted to an equivalent electrical circuit. The standard rate constant (ko) for electron-transfer between Pt/PEDOT(PSS) and Fe(CN)63−/4− was determined by calculations based on the Butler–Volmer equation. The diffusion coefficient (D) of the redox couple was also calculated from the EIS data.

Plasticizer-free all-solid-state potassium-selective electrode based on poly(3-octylthiophene) and valinomycin

Abstract All-solid-state potassium-selective electrodes with plasticizer-free membranes were prepared by incorporation of valinomycin as the ionophore and potassium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate as the lipophilic additive in a semiconducting conjugated polymer matrix of poly(3-octylthiophene). The membrane components were dissolved in chloroform and deposited on glassy carbon by solution casting resulting in all-solid-state potassium sensors. The analytical performance of the potassium sensors were studied by potentiometric measurements. Electrochemical impedance spectroscopy was used to obtain information about the charge transfer and double layer charging processes in the electrodes. The results show that a plasticizer-free all-solid-state potassium-selective electrode can be prepared and that the electron transfer at the glassy carbon|membrane interface plays a significant role in the signal transduction.