Poly(3,4-ethylenedioxythiophene) (PEDOT) doped with carbon nanotubes as ion-to-electron transducer in polymer membrane-based potassium ion-selective electrodes Zekra Mousavi, Johan Bobacka, Andrzej Lewenstam, Ari Ivaska * Åbo Akademi University, Process Chemistry Centre, c/o Laboratory of Analytical Chemistry, Biskopsgatan 8, FI-20500 Åbo-Turku, Finland article info Article history: Received 7 April 2009 Received in revised form 8 June 2009 Accepted 11 June 2009 Available online 13 June 2009 Keywords: All-solid-state ion-selective electrode Potassium ion sensor Conducting polymers PEDOT Multi-walled carbon nanotubes abstract Negatively charged multi-walled carbon nanotubes (MWCNTs) were used as dopants in the electrochem- ical synthesis of the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT). The resulting electroactive film, PEDOT(CNT), was used as ion-to-electron transducer (solid contact) in potassium ion-selective electrodes (K + -ISEs) based on plasticized PVC membrane containing valinomycin as neutral ionophore. Potentiometric measurements were carried out to study the analytical performance of solid-contact K + -ISEs, the influence of dissolved O 2 and CO 2 on the potential of the electrodes, and the formation of the interfacial aqueous film. The prepared electrodes were also characterized by cyclic vol- tammetry (CV), electrochemical impedance spectroscopy (EIS), and chronopotentiometry. The experi- mental results showed that PEDOT(CNT) has the capability to function as solid contact in fabrication of K + -ISEs. These electrodes, based on PEDOT(CNT) as ion-to-electron transducer, showed high sensitivity and selectivity to K + ion which can be related to the plasticized PVC-based ion-selective membrane con- taining valinomycin. The stability of the electrode potential, however, was found to depend on the con- ducting substrate used for deposition of the PEDOT(CNT) film. Results from the CV and EIS revealed that the PEDOT(CNT) contact exhibits high redox capacitance that is favorable for a solid contact. Ó 2009 Published by Elsevier B.V. 1. Introduction Potentiometric ion sensors, or ion-selective electrodes (ISEs), are the most frequently used chemical sensors in analytical appli- cations in medical, environmental, and industrial analysis [1–4]. As an analytical device, ISE offers many advantages such as selectivity, fast response, simple analytical procedure, and relatively low cost. The necessity of robust, maintenance-free, and reliable ISEs led to the invention of the so called coated-wire electrode (CWE) which is a polymeric membrane-based all-solid-state ISE [5,6]. However, the configuration of CWE involves some drawbacks such as drift in the electrode potential due to the high charge-transfer resis- tance and low double-layer capacitance at the interface between the electrically conducting substrate and the ionically conducting ion-selective membrane (ISM) [7,8]. To solve this problem, solid- contact ISEs were fabricated by using a layer with mixed electronic and ionic conductivity to act as ion-to-electron transducer be- tween the ISM and the solid substrate [9]. The excellent and unique properties of the electrically conducting polymers (ECPs) make them attractive materials that can function as ion-to-electron transducers in solid-contact ISEs [10–12]. Poly(3,4-ethylenedioxy- thiophene) (PEDOT), is one of the most interesting conducting polymers that has been used for this purpose [8,13,14]. On the other hand, carbon nanotubes (CNTs) with remarkable structural, electrical, mechanical, and thermal properties have gen- erated a considerable interest since being discovered by Iijima in 1991 [15]. It has been shown that incorporating CNTs into a poly- mer matrix enhances the mechanical and electrical properties of the original polymer [16–18]. Another important application of CNTs was incorporating them in electrochemical transducers in the fabrication of biosensing devices [19,20]. The ability of CNTs to function as gas, pressure, temperature, and mass sensors has been demonstrated [21]. Carbon nanotubes have also been used as the ion-to-electron transducing layer in ISEs [22,23]. The proce- dure used to prepare these electrodes, however, was tedious and time consuming. Also other carbon-based materials with high sur- face area have successfully been used for the same purpose [24,25]. The aim of this study was to combine the properties of conduct- ing polymers and carbon nanotubes to prepare a composite that can be used as ion-to-electron transducer in poly (vinyl chloride) (PVC)-based ion-selective electrodes. The conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) doped with negatively charged multi-walled carbon nanotubes (MWCNT) was electro- chemically synthesized to prepare the composite material PED- OT(CNT) that was coated with a PVC-based K + -selective 0022-0728/$ - see front matter Ó 2009 Published by Elsevier B.V. doi:10.1016/j.jelechem.2009.06.005 * Corresponding author. Tel.: +358 2 215 4420; fax: +358 2 215 4479. E-mail address: ari.ivaska@abo.fi (A. Ivaska). Journal of Electroanalytical Chemistry 633 (2009) 246–252 Contents lists available at ScienceDirect Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem