Available online at www.sciencedirect.com Electrochimica Acta 53 (2008) 3955–3959 Current rectification by mediating electroactive polymers Gabriel Ybarra a , Carlos Moina a , M. In´ es Florit b , Dionisio Posadas b,∗ a Centro de Investigaci´ on sobre Electrodeposici´ on y Procesos Superficiales, Instituto Nacional de Tecnolog´ ıa Industrial, CC 157, (1650) San Mart´ ın, Argentina b INIFTA, Facultad de Ciencias Exactas, UNLP, Suc. 4, CC 16, (1900) La Plata, Argentina Received 29 May 2007; received in revised form 23 August 2007; accepted 29 August 2007 Available online 1 September 2007 Abstract In this work we briefly review the theoretical basis for the electrochemical rectification in mediated redox reactions at redox polymer modified electrodes. Electrochemical rectification may have two distinct origins. It is either caused by a slow kinetics of the reaction between the external redox couple and the mediator or it is originated by a slow electronic transport within the film under an unfavorable thermodynamic condition. We show experimental results for the redox mediation reaction of poly(o-aminophenol) (POAP) on the Fe 2+/3+ and on the Fe(CN) 6 3-/4- redox couples in solution that prove the proposed mechanisms of electrochemical rectification. © 2007 Elsevier Ltd. All rights reserved. Keywords: Mediation kinetics; Redox electroactive polymers; Current rectification; Current potential response 1. Introduction Redox mediators are substances that, forming part of a film that covers a conducting electrode, mediate the electronic trans- fer between a redox species in an external solution and the electrode. They are substances containing redox centers that can be oxidized and reduced in a reversible way. An exam- ple of redox mediators is electroactive polymers [1,2]. They are classified into redox and conjugated polymers [3]. The first ones possess, either as a part of the monomer unit or as a coordination compound, chemical groups that participate in the redox reaction. Examples of this kind of polymers are poly(o- aminophenol) (POAP) and Os(II) bipyridil-polyvynilpyridile. In the first one, the redox centers are the amino groups that oxidize to imino ones. In the second, it is the coordinated Os(II), which is oxidized to Os(III). On the other hand, conjugated polymers are characterized by becoming electronic conductors when oxidized (positively doped) to a certain degree. Examples of this class of polymers are polyaniline, polypyrrole and polythiophene. Of course, there are a wide variety of electroactive polymers that can be obtained by changing the coordination metal, ligands, substituents and copolimerization, etc. ∗ Corresponding author. Tel.: +54 221 4257291/430; fax: +54 221 4254642. E-mail address: dposadas@inifta.unlp.edu.ar (D. Posadas). Redox mediation has attracted the attention of many researchers [1,2], mainly because the possibility of tuning the electrochemical reactivity of the coated electrode to a specific end, both by choosing appropriate redox mediators and by con- trolling the films characteristics. This is because the redox agent in the film, which mediates the electron transfer, determines the kinetics of the electron transfer process between the mediator and the external redox couple [4]. Also, because it is possible to design mediators with certain specific properties, such as ion selectivity [5,6], conductivity [7] and formal potential [8], etc. One of the most interesting aspects of these systems and processes is the possibility of designing organic electronic devices, such as diodes and transistors. Chidsey and Murray have described bilayer electrodes, based on electroactive or con- jugated polymers, which may act as diodes [9]. Alleman et al. [10] showed that a monolayer modified electrode may act as an electrochemical rectifier. Electrochemical systems, based on electroactive polymers that show current rectification have been reported [11–15]. However, this phenomenon was not clearly recognized as current rectification by a single layer of elec- troactive polymer. Recently, an analysis of the kinetics of redox mediation under steady state conditions was published in which the reasons for current rectification by mediating redox polymers were singled out [16]. We will consider the requirements that a redox mediator must fulfill to be able to act as an electrochemi- cal rectifier and we will also show experimental results obtained 0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2007.08.061