Langmuir 1996, 12, 5681-5688
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Tailored Polymers To Probe the Nature of the Bioelectrochemical Interface K. S. Ryder,†,‡,§ D. G. Morris,‡ and J. M. Cooper*,† Bioelectronics Research Centre, Department of Electronics & Electrical Engineering, University of Glasgow, Glasgow G12 8QQ, U.K., and Department of Chemistry, University of Glasgow, Glasgow G12 8QQ, U.K. Received May 6, 1996. In Final Form: July 26, 1996X A range of pyrrole monomers with carboxyl derivatives both at the N-, and β-ring positions were synthesized and, subsequently, were polymerized electrochemically at carbon, gold, and platinum electrodes. The resulting polymers, which were characterized with both electrochemical and spectroscopic methods, were then used to investigate the importance of polymer oxidation potential, polymer functionality, and backbone conductivity on the redox interaction with the small redox protein, cytochrome c. By choosing monomer substituents with varying side-chain length and steric bulk, it was possible to probe the nature of the protein-polymer interaction and to show how the heterogeneous rate constants, ks, as an estimate for electron exchange between the electrode functionalized poly(pyrroles) and the protein, varied as a consequence of the structure of the matrix. The method provides a novel route for the modification of a wide range of conducting surfaces for the study of biological interfacial reactions, particularly those involving biomolecular recognition.
Introduction Study of the interaction of biological molecules with electronic materials is of fundamental interest in the field of molecular electronics and bioelectronics, with applications in the development of submicrometer-sized electronic components as well as in novel biosensor configurations. Previously, one approach toward producing functionalized electrode surfaces has been in the construction of selfassembled monolayers (SAMs) of thiolates on gold resulting in highly defined surfaces.1,2 Studies using these systems have shown that control of the interfacial molecular structure plays a central role in promoting direct electronic communication between biological molecules and electronic substrates.2 As one prerequisite to such interfacial design, materials that have been used have needed to exhibit good biological compatibility in order to avoid protein denaturation (as is often observed at conventional unmodified metallic electrodes). One particular example of biocompatibility that may be required of a modified electrode involves the ability of the surface to recognize a protein, in order to confer a degree of specificity and/or speed of function to a device by effecting electron transfer.2 As a consequence, over the last decade, there have been extensive efforts focused on the creation of modified surfaces to study heterogeneous electron transfer reactions involving redox proteins, such as cytochrome c.2,3 Techniques to date have been based upon producing chargematching across the bioelectrochemical interface, with, for example, positively charged amino acids (especially lysines) surrounding the biological redox center of cytochrome c, aligning with electronegative functionalities on * To whom correspondence may be addressed. † Department of Electronics and Electrical Engineering. ‡ Department of Chemistry. § Present address: Department of Chemistry, University of Aberdeen, Old Aberdeen AB9 2UE, U.K. X Abstract published in Advance ACS Abstracts, October 1, 1996. (1) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (2) Eddowes, M. J.; Hill, H. A. O. J. Chem. Soc., Chem. Commun. 1977, 771. Barker, D.; DeGleria, K.; Hill, H. A. O.; Lowe, V. J. Eur. J. Biochem. 1990, 190, 171. (3) Cooper, J. M.; Morris, D. G.; Ryder, K. S. J. Chem. Soc., Chem. Commun. 1995, 697.
S0743-7463(96)00440-4 CCC: $12.00
the surface, and so bringing the protein into a close and oriented proximity. Using this biophysical model, welldefined reversible electron transfer has now been rationalized for a variety of heme proteins at gold electrodes, modified with heterobifunctional SAMs of biocompatible materials, e.g., N-acetylcysteine or 4,4′-bipyridine. The SAMs serve not only to create the bioelectrochemical interface but also to shield the protein from the highenergy metal surface and prevent structural unfolding and nonspecific adsorption of the protein.2 We have adopted a novel alternative approach, analogous to that involving SAMs,2 which is based upon using electrochemically polymerized functionalized pyrroles to produce thin films ( 30 mV s-1) in Figure 5. Conclusion Use of conducting polymers as surface modifiers already has proved to be a rich vein of pure and applied research, due to the flexibility of the well-defined physical and organic chemistries involved. In this paper, we have now clearly demonstrated a novel application of these polymers for the functionalization of surfaces in order to study biological interfacial redox reactions. Although a considerable body of work has been carried out using SAMs (23) The value for ks for cyt c at N-acetylcysteine modified gold was measured using the mathematical treatment described in this paper. The Au electrode was cleaned by polishing with 0.03 µm alumina slurry and incubation in aqua regia. The electrode was then washed in distilled H2O and was modified by placing in a 2 mM solution of N-acetylcysteine, in water. All other experimental details are as for the polymer-modified electrochemical experiments.
Figure 8. Plot of heterogeneous rate constant, ks, as a function of the number of carbon atom spacers between the pyrrole ring and the carboxylic acid group (inclusive). Numerical values are given in Table 3.
to modify Au surfaces for bioelectrochemistry, e.g., refs 2, 3, and 18, our results, described here, provide a generic method for tailoring a wider variety of electrode materials to facilitate biomolecular recognition. Indeed, in future, the use of these polymers, substituted with peptide-like motifs as a method for creating surfaces capable of biomolecular recognition, may present us with the opportunity to customize interfaces for creation of a more stable class of biosensor, in which the surface modifier replaces the biological component. These novel conducting materials may also offer the possibility of producing biocompatible conducting surfaces that are not prone to protein fouling in the same manner as many metals.3 Acknowledgment. The authors are grateful to the EPSRC and ESPRIT Contract No. TOPFIT 7282 for funding this research, and to Professor Francis Garnier, CNRS, Thiais, France, for advice in the preparation of 4. LA960440X
