X-ray Photoelectron Spectroscopy Study of Counterion Incorporation in Poly(3,4-ethylenedioxythiophene) Sarah A. Spanninga, † David C. Martin, †,‡,§ and Zhan Chen †,|, * Macromolecular Science and Engineering Center and Departments of Materials Science and Engineering, Biomedical Engineering, and Chemistry, UniVersity of Michigan, Ann Arbor, Michigan 48109 ReceiVed: August 8, 2008; ReVised Manuscript ReceiVed: February 6, 2009 Poly(3,4-ethylenedioxythiophene) (PEDOT) is widely used in organic electronics and biomaterial coatings because of its outstanding electrical properties and chemical stability. In this research, X-ray photoelectron spectroscopy (XPS) was used to investigate incorporation of different counterions during electrochemical polymerization into PEDOT. The counterions probed included both a polyanion, poly(sodium 4-styrene- sulfonate) (PSSNa), and small anions including lithium perchlorate, sodium chloride, and sodium phosphate monobasic monohydrate. In addition to these counterions, an ion mixture, phosphate-buffered saline solution (PBS), was also examined. For such mixtures, the chlorine anion from sodium chloride was found to act as the counterion during PEDOT electrochemical polymerization in PBS solution. This is important because PEDOT is being considered for biomedical applications, which may be prepared in the presence of PBS or other mixtures of ions. Various mixtures of PSSNa, lithium perchlorate, sodium p-toluenesulfonate (TosNa) and PBS counterions were investigated. We detected that the polyanion, PSS - , preferentially incorporated into PEDOT in comparison to ClO 4 - and Cl - anions when ion mixtures were used. Results were supplemented by those obtained via other analytical techniques including electrochemical impedance spectroscopy, cyclic voltammetry, and scanning electron microscopy. Introduction Poly(3,4-ethylenedioxythiophene) (PEDOT) (Figure 1) is a highly conductive, π-conjugated polymer that can be polym- erized either by oxidative chemical polymerization or by electrochemical polymerization. Given the high conductivity of PEDOT, ∼300 S/cm, some of its first applications were antistatic coatings and organic light-emitting devices. 1 Recent studies have taken advantage of both the electronic and ionic conductivity of PEDOT for use as a biological tissue interfacing agent. 2,3 Previously, conductive polymer research into biological ap- plications has centered upon polypyrrole. 4-7 The superior thermal 8 and chemical stability 9,10 of PEDOT, in comparison to polypyrrole, have been utilized as motivation for its use as an interfacing agent. The incorporation of counterions into PEDOT affects both the polymer surface morphology and electrical properties of the PEDOT polymer film. Therefore, studying the process of counterion incorporation can greatly improve basic understand- ing of these film properties. Of particular interest is the electrochemical polymerization of 3,4-ethylenedioxythiophene (EDOT) in phosphate-buffered saline solution (PBS). PBS is a common biological buffer and is required for the incorporation of biological components for applications such as biosensors 11-15 and implant coatings. 2 Understanding PEDOT counterion incorporation in PBS is therefore a prerequisite for the incor- poration of biological components into PEDOT. PEDOT is a highly conductive polymer. PEDOT conductivity varies with the type of polymerization and dopants used, for example conductivities of ∼300 S/cm 1 and ∼100 S/cm 16 have been found for the chemically polymerized and organic chemical vapor deposited versions, respectively. For electrochemical polymerization, PEDOT can also be doped with other counterions. 1,17 The counterions used in this research include the polyanion poly(sodium 4-styrenesulfonate) (PSSNa), lithium perchlorate (LiClO 4 ), sodium chloride (NaCl), sodium phosphate monobasic monohydrate (NaH 2 PO 4 · H 2 O), and ions in phosphate-buffered saline solution (PBS) (including KH 2 PO 4 , NaCl, and Na 2 HPO 4 ). X-ray photoelectron spectroscopy (XPS) is an ultra-high- vacuum, surface-sensitive technique 18-21 that was used to investigate the differences in binding energy as a result of different counterion incorporation. Electrochemical impedance spectroscopy, cyclic voltammetry, and scanning electron mi- croscopy were used to supplement the XPS study. Impedance spectroscopy and cyclic voltammetry were used to characterize the film’s electrical properties, while scanning electron micros- copy was used to investigate the resulting morphology. Previous XPS studies have been used to deduce how chemical bonding in PEDOT was affected by the presence of various * To whom correspondence should be addressed: phone 734-615-4189; fax 734-647-4865; e-mail zhanc@umich.edu. † Macromolecular Science and Engineering Center. ‡ Department of Materials Science and Engineering. § Department of Biomedical Engineering. | Department of Chemistry. Figure 1. (Left) Poly(3,4-ethylenedioxythiophene) (PEDOT). (Right) Poly(sodium 4-styrenesulfonate) (PSSNa). J. Phys. Chem. C 2009, 113, 5585–5592 5585 10.1021/jp811282f CCC: $40.75  2009 American Chemical Society Published on Web 03/17/2009