Available online at www.sciencedirect.com Electrochimica Acta 53 (2008) 4219–4227 Electrochemical, spectroscopic and electrogravimetric detection of oligomers occluded in electrochemically synthesized poly(3,4-ethylenedioxythiophene) films Edgar Ventosa, Alvaro Colina, Ar´ anzazu Heras, Alberto Mart´ ınez, Oscar Orcajo, Virginia Ruiz, Jes´ us L ´ opez-Palacios ∗ Departamento de Qu´ ımica, Universidad de Burgos, Pza. Misael Ba˜ nuelos s/n, E-09001 Burgos, Spain Received 12 September 2007; received in revised form 23 November 2007; accepted 23 December 2007 Available online 6 January 2008 Abstract This paper shows that oligomers are retained inside the polymer film during the electrosynthesis of poly(3,4-ethylenedioxythiophene), PEDOT, in aqueous media. The behavior of the electrochemically generated PEDOT film is highly dependent on the presence of these oligomers. A detailed study of the release of oligomers trapped in the PEDOT film has been carried out using bidimensional spectroelectrochemistry (BSEC), spectroelectrochemical quartz crystal microbalance (SEQCM) and scanning electrochemical microscopy (SECM). These multiresponse techniques have allowed us to determine when these EDOT oligomers are released into solution and to investigate their electrochromic properties. Mass spectroscopy measurements revealed that most of these oligomers consist of four or six monomer units, which seem to be the most stable species in aqueous solution. © 2008 Elsevier Ltd. All rights reserved. Keywords: PEDOT; Conducting polymers; Oligomers; Spectroelectrochemistry; Electrochemistry 1. Introduction Conducting polymer (CPs) films exhibit very interesting redox properties such as controlled transport of ions through them. Electrical switching of CPs involves changes in their elec- tronic charge that are accompanied by equal changes in the ionic charge; these processes need exchange of counterions between the polymer film and the electrolyte to balance the charge. These properties of CPs have attracted great interest in the development of electrochemical release actuators and separation devices of charged species. In this respect, the suitability of conducting polymers for drug delivery [1–3], release of bioactive molecules [3–5], electrochemically controlled solid-phase microextraction [6,7] and solid-state ion-selective electrodes [8–10] has been demonstrated. The synthesis of conducting polymers can be performed either chemically or electrochemically, and in both cases reten- ∗ Corresponding author. Tel.: +34 947 258817; fax: +34 947 258831. E-mail address: jlopal@ubu.es (J. L ´ opez-Palacios). tion of oligomers inside the polymer structure takes place [11]. In general, by electrochemical switching of the polymer, the film undergoes a transformation with the number of electro- chemical cycles, as revealed by differences between successive cyclic voltammograms. Therefore, the as-synthesized polymer film needs to be treated until its behavior is stable prior to be used in any application [12,13]. Changes during the first poten- tial cycles have been ascribed to the memory effect [14–16], the break-in effect [17,18] and/or the release of oligomers from the polymer matrix [19,20]. Among conducting polymers, poly(3,4-ethylendioxythio- phene), PEDOT, is a very interesting one due to its low band- gap, high environmental stability, and high conductivity and transparency in the doped state [21–23]. This relatively new conducting polymer has been proposed as an alternative to tra- ditional polymers as the electroactive component in biosensors [5,24]. PEDOT films are usually synthesized in organic media [25–29] or in aqueous solutions with surfactants [30–32] due to its low solubility in water (2.1 g L -1 at 20 ◦ C). However, poly- merization of the monomer in aqueous media without any kind of surfactant has proven possible [33–35], leading to a poly- 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2007.12.064