The oxidation of ascorbate at copolymeric sulfonated poly(aniline) coated on glassy carbon electrodes C. Sanchis a , M.A. Ghanem b,1 , H.J. Salavagione a,2 , E. Morallón a, ⁎, P.N. Bartlett b a Departamento de Química Física e Instituto de Materiales (IUMA), Universidad de Alicante, Ap. 99, E-03080 Alicante, Spain b School of Chemistry, University of Southampton, Southampton SO17 1BJ, UK abstract article info Article history: Received 11 January 2010 Accepted 17 June 2010 Available online 27 June 2010 Keywords: Sulfonated poly(aniline) Ascorbate Electrocatalysis Kinetics Self-doped poly(aniline)s as electrode coatings to catalyze ascorbate oxidation are revisited in this article. Sulfonated poly(aniline) (SPAN) was deposited on glassy carbon electrodes as a copolymer of aniline and its sulfonated derivative, 2-aminobenzenesulfonic acid (2-ABSA). The resulting deposits are reproducible and show good stability and electroactivity at pH N 7, enabling studies at typical physiological pH values. Calibration curves were obtained using a rotating disc electrode at a sampling potential of 0.2 V, displaying linear dependence in the region 0–20 mM ascorbate. A kinetic model based on the Michaelis–Menten reaction mechanism, previously validated for poly(aniline) composites, was used to analyse the form of the calibration curve leading to values of the effective reaction constants K ME and k′ ME . New calibration curves constructed for different sampling potentials were used to elucidate the rate limiting step at saturated kinetics. Rotating disc voltammetry performed at increasing pH (from pH 2 to 9) showed a dramatic decrease in the limiting current, without any evidence for a change in the reaction mechanism. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Ascorbate is an important analyte in food and beverages, plant physiology and in medicine. For example elevated levels of ascorbate in plant tissues are an indicator of stress [1–6], it is known that ascorbate is a vital antioxidant in the brain [7], and low levels of ascorbate in humans have been found in patients with stroke [8]. Direct oxidation of ascorbate at carbon or platinum electrodes is kinetically slow and is usually accompanied by fouling of the electrode surface or interference from other species in the matrix. Consequently many groups have investigated the possibility of using activated or modified electrode surfaces for ascorbate measurement. For example in previous works electrode surfaces modified with immobilized quinone groups [9], adsorbed TCNQ [10], and conducting polymers [11–17] have all been used. Conducting polymers are attractive electrode materials for ascorbate oxidation because of the ease with which they can be deposited on the electrode surface and the ability to vary their properties by the choice of counterion and film thickness [18]. Lyons et al. [12] showed that poly(pyrrole) doped with chloride or dodecylbenzene sulfonate could be used to catalyse the electro- oxidation of ascorbate and presented results for a detailed kinetic analysis. In earlier work one of us reported the use of poly(aniline), PANI, as an electrode for ascorbate oxidation [13] and described its use for ascorbate detection in beverages [19], in vivo and in plant leaves [20]. PANI has the advantage that it has excellent stability and can be deposited reproducibly if grown under carefully controlled conditions. However for application in neutral solutions the polymer suffers from the problem of deprotonation leading to loss of con- ductivity [21,22]. To overcome this, in earlier work the PANI was deposited as a composite with poly(vinylsulfonate) [23,24], poly (styrenesulfonate) [25,26], or other polyanionic species [27,28]. In these composites the compensating anions for the conducting emeraldine state of the polymer are entrapped within the film so that loss of protonation can only occur if balancing counter cations enter the film in exchange for protons, a process which is less favourable thus extending the conductivity of the PANI to higher pH. An alternative and attractive approach is to use self-doped forms of PANI, that is PANIs in which the balancing negative charge for the conducting emeraldine form of the polymer is covalently bound to the polymer chain itself. This approach has been developed by Barbero's group [29–31] and applied by others [32] using post functionalisation of the PANI. A disadvantage of this approach is that it can be difficult to control the functionalisation reaction and to fully chemically characterise the resulting polymer or to be sure that its composition is uniform throughout. An alternative approach is to copolymerise aniline with a suitable substituted aniline derivative such as aminobenzoic acid or amino- benzenesulfonic acid (ABSA) [33]. When aminobenzenesulfonic acid is used the resulting polymer is basically the same as the sulfonated Bioelectrochemistry 80 (2011) 105–113 ⁎ Corresponding author. Tel.: + 34 965909590; fax: + 34 965903537. E-mail address: morallon@ua.es (E. Morallón). 1 Permanent address: Science & Math. Department, Faculty of Petroleum & Mining Engineering, Suez Canal University, Suez, Egypt. 2 Instituto de Ciencia y Tecnología de Polímeros (ICTP-CSIC) Dpto. de Física e Ingeniería c/ Juan de la Cierva, 3. 28006. Spain. 1567-5394/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.bioelechem.2010.06.006 Contents lists available at ScienceDirect Bioelectrochemistry journal homepage: www.elsevier.com/locate/bioelechem