An advanced potentiometric pH transducer based on processible polyaniline (PCPAn) is reported on. Both glassy carbon and screen-printed carbon electrodes modified with PCPAn by dip-coating exhibited a fully reversible potentiometric response of approximately 90 mV/pH unit over the range from pH 3 to 9. Such a significantly higher potentiometric response of PCPAn-modified electrodes as compared with those of existing devices is explained on the basis of the thermodynamics of polyaniline redox reactions. The PCPAn-based pH transducers exhibit both good operational stability and prolonged shelf life and display a negligible response toward singly charged cations. The new thick-film pH transducer was employed for designing a potentiometric biosensor for urea. In the model solution which mimics blood serum, the urea-sensitive electrode has a detection limit of 10(-5) M urea and a maximum response of approximately 120 mV. The attractive performance characteristics are advantageous over those of existing pH sensors and offer great promise for sensing and biosensing applications.

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Thermodynamics of ion transfer across the liquid|liquid interface at a solid electrode shielded with a thin layer of water-immiscible organic solvent is investigated. The shielded electrode has been made by using a commercial hydrophobic carbon filament material (LSG) as an electrode support and a specially synthesized water-insoluble electroactive polymer (polyphenotiazine, PPTA) as a mediator. Oxidation of PPTA dissolved in the organic layer is accomplished with anion transfer from the aqueous phase to compensate for the positive charge of the mediator. The formal potential of the electrode shielded with PPTA solution in nitrobenzene (NB) is proportional to the Gibbs free energy of anion transfer across the liquid|liquid interface. An enhanced potential window as compared to a “classic” four-electrode setup allowed direct determination of Gibbs free energy for fluoride ion transfer from water to NB. For less polar organic solvents the formal potential of a shielded electrode is mainly determined by the enthalpy of anion transfer across the liquid|liquid interface, whereas the entropy term of Gibbs free energy causes only a minor effect.