Electrochimica Acta 104 (2013) 536–541 Contents lists available at SciVerse ScienceDirect Electrochimica Acta j our nal homep age : www.elsevier.com/locate/electacta pH sensors based on polypyrrole nanowire arrays Grzegorz D. Sulka a,1 , Katarzyna Hnida a,∗ , Agnieszka Brzózka b,1 a Department of Physical Chemistry & Electrochemistry, Jagiellonian University, Ingardena 3, 30060 Krakow, Poland b Faculty of Non-Ferrous Metals, AGH University of Science and Technology, Al. Mickiewicza 30, 30059 Krakow, Poland a r t i c l e i n f o Article history: Received 30 August 2012 Received in revised form 21 November 2012 Accepted 17 December 2012 Available online 24 December 2012 Keywords: Anodization Anodic aluminum oxide (AAO) Polypyrrole (PPy) Nanowire arrays pH a b s t r a c t The hydroquinone monosulfonate-doped polypyrrole (PPy-HQS) nanowires were successfully fabricated by potentiostatic electropolymerization of pyrrole (Py) inside the pores of home-made through-hole anodic aluminum oxide (AAO) membranes. The AAO templates with a nominal pore diameter of 80 nm were prepared by a two-step anodization process. The potentiostatic electropolymerization of HQS- doped polymer nanowires was carried out in 0.1 M NaClO 4 , or 0.1 M LiClO 4 or 0.1 M citric acid containing 0.05 M pyrrole and 0.05 M potassium hydroquinone monosulfonate. The synthesized PPy-HQS nanowire arrays were tested as potential potentiometric pH sensors. It was found that pH sensors based on PPy- HQS nanowires exhibited better electrochemical performance toward pH sensing than those based on PPy-HQS thin films. The best potentiometric response to pH changes and a very good stability in time showed the sensor based on the PPy-HQS nanowires polymerized in a 0.1 M LiClO 4 solution. © 2012 Elsevier Ltd. All rights reserved. 1. Introduction From material science point of view, the advancement in nano- technology provides a way to produce various nanoscale objects with higher precision and enhanced performance. Recently, there has been widespread interest in nanomaterials science largely due to optical, electrical, and mechanical properties of nanostructured materials that are usually different from those of the bulk materials. Moreover, nanomaterials such as: nanotubes, nanowires, nanorods, nanodots and nanoporous materials are attractive for a range of applications because of their intrinsic large surface area. Compared with bulk conducting polymers, one dimensional (1-D) polymer nanowires are expected to display improved performance in tech- nological applications due to their metal-like high conductivity, low energy optical transitions, low ionization potential, large surface area, and light weight. Among -conjugated polymers, polypyrrole (PPy) obtained by electropolymerization or chemical oxidation, is one of the most extensively studied intrinsically conducting polymer. The PPy main chain consists of alternating single and double bonds in order to allow the formation of delocalized electronic states and conse- quently a large energy gap [1,2]. However, PPy in the undoped state is a p-type semiconductor and its conductivity is rather low [3]. In order to maintain neutrality of the polymer backbone structure, the ∗ Corresponding author. Tel.: +48 12 663 22 69; fax: +48 12 634 05 15. E-mail address: hnida@chemia.uj.edu.pl (K. Hnida). 1 ISE member. positive charges (polarons and bipolarons) are counter balanced by the incorporation of anions from the electrolyte solution. A primary doping of PPy with anions, occurring during the chemical or elec- trochemical polymerization, increases the electrical conductivity of polymer many orders of magnitude [3]. On the other hand, a secondary doping can be accomplished by introduction of an elec- trically neutral gas into a conducting polymer. Owing to its relatively easy preparation, high levels of conduc- tivity, tunable physicochemical properties and morphologies with good environmental stability [4], PPy has been found particularly interesting for a potential scientific and commercial applications like electrodes and additives for solid-state batteries [5], energy storage devices [6], field-emission devices [7], nanoscale actua- tors for nanoelectromechanical systems (NEMS) and nanorobots [8], artificial muscles [9], controlled drug release [10], electrocatal- ysis [11] micro and nanoelectronics [12]. Obviously, polypyrrole and its nano-sized composites (e.g., with metal nanoparticles, car- bon nanotubes) [13] offer tremendous technological applications as chemical and electrochemical sensors based on a reversible change of PPy conductivity from insulator to metal caused by a doping/de-doping process [14]. The chemical modification and/or biochemical functionalization of polypyrrole electrodes toward molecular recognition give almost unlimited possibilities for recog- nition of specific molecules, ions and gases [15–18]. For example, polypyrrole-based chemical sensors for detection of pH changes [19], NO 3 - anions [20], Cu 2+ ions [21], gases [22], and biological sensors for glucose [23], cholesterol [24], DNA [25], viruses [26], drugs [27], antibiotic [28] sand anybodies [29] have been recently reported in the literature. Although various PPy-based sensors are 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.12.064