Krzysztof Maksymiuk

All-solid-state reference electrodes based on conducting polymers

A novel construction of solution free (pseudo)reference electrodes, compatible with all-solid-state potentiometric indicator electrodes, has been proposed. These electrodes use conducting polymers (CP): polypyrrole (PPy) or poly(3,4-ethylenedioxythiophene) (PEDOT). Two different arrangements have been tested: solely based on CP and those where the CP phase is covered with a poly(vinyl chloride) based outer membrane of tailored composition. The former arrangement was designed to suppress or compensate cation- and anion-exchange, using mobile perchlorate ions and poly(4-styrenesulfonate) or dodecylbenzenesulfonate anions as immobilized dopants. The following systems were used: (i) polypyrrole layers doped simultaneously by two kinds of anions, both mobile and immobilized in the polymer layer; (ii) bilayers of polypyrrole with anion exchanging inner layer and cation-exchanging outer layer; (iii) polypyrrole doped by surfactant dodecylbenzenesulfonate ions, which inhibit ion exchange on the polymer/solution interface. For the above systems, recorded potentials have been found to be practically independent of electrolyte concentration. The best results, profound stability of potentials, have been obtained for poly(3,4-ethylenedioxythiophene) or polypyrrole doped by poly(4-styrenesulfonate) anions covered by a poly(vinyl chloride) based membrane, containing both anion- and cation-exchangers as well as solid potassium chloride and silver chloride with metallic silver. Differently to the cases (i)-(iii) these electrodes are much less sensitive to the influence of redox and pH interferences. This arrangement has been also characterized using electrochemical impedance spectroscopy and chronopotentiometry.

Lowering the resistivity of polyacrylate ion-selective membranes by platinum nanoparticles addition.

The effect of platinum nanoparticles introduction into polyacrylate membranes was examined. Platinum nanoparticles were added to the membrane cocktail before photopolymerization of the poly(n-butyl acrylate) based ion-selective membranes. Thus obtained sensors were characterized with significantly lowered electrical resistance and increased stability of potential readings compared to classical poly(n-butyl acrylate) membranes. The analytical parameters of platinum nanoparticle containing membranes were well comparable with those of classical membranes.

Kinetics and mechanism of charge-transfer reactions between conducting polymers and redox ions in electrolytes

Charge-transfer reactions of selected ionic redox couples were studied on rotating glassy carbon electrodes covered with conducting polymers of different membrane properties: poly-N-methylpyrrole as the anion exchanger or poly-N-methylpyrrole with immobilized poly(4-styrenesulfonate) ions as the cation exchanging matrix. The electrochemical and spectroscopic (EDAX) results obtained with the redox systems: Fe(CN)3−4−6, Ru(NH3)3+2+, Eu3+/Eu2+, Co(en)3+2+3 and Fe(C2O4)3−4−3 pointed to reactions proceeding at the polymer-solution interface. The data were analysed on the basis of a model in which the charge transfer was regarded as a redox reaction between polymeric sites in the film and the redox species in the solution. The rate of electron transfer was found to be: (i) proportional to the concentration of the oxidized or reduced polymeric sites; (ii) dependent to some extent on the Donnan potential prevailing at the interface; and (iii) correlated with the thermodynamic driving force of the reaction between the polymer and redox species.

Studies on Spontaneous Charging/Discharging Processes of Polypyrrole in Aqueous Electrolyte Solutions

Spontaneous processes of polypyrrole (PPy) films in electrolyte solutions resulting from polymer oxidation by dissolved oxygen or polymer redox capacitance discharging and their influence on open circuit potentials were investigated. Chronopotentiometric and chronoamperometric methods were adapted to study these processes. Based on the data obtained and open circuit potential vs. time measurements, Tafel plots, rest (zero-current) potentials and dependences of discharging/charging current on time were determined. Currents resulting from discharge of oxidized polypyrrole are slightly higher for thicker films, while oxidation currents of neutral PPy increase additionally with rising H+ ions and oxygen concentrations. The influence of concentration and kind of electrolyte solution on recorded currents and rest potentials was also taken into consideration. Results of these experiments can be helpful in evaluation of the polymer charge stability. They are also of significance in the explanation of the dependence of open circuit potential of PPy on concentration of solution components, described in the literature, important for analytical applications of conducting polymers.

Photopolymerized polypyrrole microvessels.

We report on the preparation of water-filled polymer microvessels through the photopolymerization of pyrrole in a water/chloroform emulsion. The resulting structures were characterized by complementary spectroscopic and microscopic techniques, including Raman spectroscopy, XPS, SEM, and TEM. The encapsulation of fluorescent, magnetic, and ionic species within the microvessels has been demonstrated. Confocal microscopy and fluorescence anisotropy measurements revealed that the encapsulated chromophore (Rhodamine 6G) resides within voids in the capsules; however, strong interaction of the dye with polypyrrole results in a measurable decrease in its rotational dynamics. Microvessels loaded with ferrofluid exhibit magnetic properties, and their structures can be directed with an external magnetic field. TEM measurements allowed imaging of individual nanoparticles entrapped within the vessels. The application of Cu(2+)-loaded microvessels as a transducer layer in all-solid-state ion-selective electrodes was also demonstrated.

Metallic and non-metallic redox response of conducting polymers

Abstract 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(NH 3 ) 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.

Electrode processes of the Pb(II)/Pb(Hg) and Zn(II)/Zn(Hg) systems in water + hexamethylphosphortriamide mixtures

Abstract The kinetic parameters of the Pb(II)/Pb(Hg) and Zn(II)/Zn(Hg) electrode processes have been determined in water + hexamethylphosphortriamide (HMPT) mixtures. Charge-transfer rate constants of both reactants drastically decrease for about three orders of magnitude in a narrow HMPT concentration range from 0 to 1 vol. %. At higher concentrations of the organic solvent the changes in the rate constant are small. Gibbs energies of transfer of Pb(II) and Zn(II) ions from water to water+ HMPT solutions have been calculated on the basis of potentiometric measurements using the ferrocene electrode. The Zn(II)/Zn(Hg) process has been analysed in detail and the results obtained suggest that the existing models of electrode reactions in mixed solvents do not apply to this system. An experimental equation describing the rate constant in terms of the dependence on solvent composition has been proposed.

Method of achieving desired potentiometric responses of polyacrylate-based ion-selective membranes.

We introduce a simple procedure allowing preparation of cation-selective electrodes with poly( n-butyl acrylate)-based membranes containing different proportions of primary and interfering ions introduced already at the membrane preparation step, by using two different liphophilic salts of the same anion. With this approach the time required to achieve saturation of polyacrylate membranes with primary ions can be significantly shortened. Moreover, depending on the ratio of the primary and interfering ions introduced to the membrane cocktail, different potentiometric responses are obtained ranging from typical (with micromolar detection limit), through lower detection limits to super-Nernstian ones.

Conducting polymer membranes for low activity potentiometric ion sensing

Conducting polymer (CP) films, used as ion-sensing membranes under open circuit potentiometric conditions, are usually characterised with rather high detection limit, in the range of 10(-4)-10(-5)moldm(-3). This effect is unfavourable, not only from the point of view of CP applications in potentiometry as ion sensitive membranes, but also when these materials are used as ion-to-electron transducers (solid contacts) for ion-selective electrodes. The theoretical considerations presented underline the crucial role of spontaneous processes of polymer charging/discharging-the source of observed high detection limit of sensors comprising CP layer under zero current conditions. Although the mechanism of occurring process is different from that observed for plastic, solvent polymeric based ion-selective electrodes, the ultimate result-alteration of activity of electrolyte at the membrane/solution interface leading to elevation of the detection limit-is the same. The method of estimation of parameters characterising spontaneous charge transfer processes is presented. The values obtained can be used to calculate the resulting polymer/solution interface activity of electrolyte ions, thus the detection limit of CP membrane can be theoretically predicted. A method of lowering of the detection limit of conducting polymer membranes, applying galvanostatic polarisation to compensate the spontaneous process of polymer charging/discharging, is presented. The experimental results obtained for poly(pyrrole), poly(N-methylpyrrole) and poly(3,4-ethylenedioxythiophene) are in good accordance with predictions of the presented model.

On the nature of the potentiometric response of polypyrrole in acidic solutions

Abstract The potentiometric response of electrochemically coated polypyrrole layers was studied both in hydrochloric acid and in alkali metal chloride solutions. The H + ions were found to exert an influence, different from that of alkali metal cations, resulting in a positive slope of the open-circuit potential vs. log[H + ] dependence. This was found also for concentrated KCl solutions, where the Donnan potential is close to zero. Voltammetric and coulometric experiments demonstrated that this behaviour cannot be explained only by the acid-base properties of polypyrrole. Additionally, redox reactions between the polymer film and solution components, whose equilibrium constants (and thus final open-circuit potential) may be pH dependent, should be taken into consideration. On the basis of model calculations, the influence of the concentration of redox species on the potential of a polymer electrode is discussed.