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Langmuir 2004, 20, 862-868
Modulating the Redox Properties of an Osmium-Containing Metallopolymer through the Supporting Electrolyte and Cross-Linking Robert J. Forster,*,† Darren A. Walsh,† Nicolas Mano,‡ Fei Mao,§ and Adam Heller‡ National Centre for Sensor Research, School of Chemical Sciences, Dublin City University, Dublin 9, Ireland; Department of Chemical Engineering, University of Texas at Austin, Austin, Texas, 78712-1062; and TheraSense Inc., 1360 South Loop Road, Alameda, California 94502 Received July 8, 2003. In Final Form: November 5, 2003 Thin films of the perchlorate salt of an [Os(N,N′-alkylated-2,2′-biimidazole)3]2+/3+-containing polymer have been formed on planar platinum microelectrodes. The electrochemical response associated with the Os2+/3+ couple occurs at -0.19 V. In aqueous perchlorate media at near-neutral pH the voltammetric response is close to that expected for an electrochemically reversible reaction involving a surface-confined reactant. Chronoamperometry conducted on a microsecond time scale indicates that the film and solution resistances are comparable for low concentrations of supporting electrolyte. However, for LiClO4 concentrations greater than 0.4 M, RFilm contributes less than 25% of the overall cell resistance. These results suggest that when the film is dehydrated and the density of redox centers is increased, electron or hole hopping dominates the rate of homogeneous charge transport through the film. The rate of homogeneous charge transport, characterized by DCT1/2Ceff, where DCT is the homogeneous charge transport diffusion coefficient and Ceff is the effective concentration of osmium centers within the film, depends weakly on the concentration of LiClO4 as supporting electrolyte decreasing from (8.1 ( 0.16) × 10-9 to (4.7 ( 0.4) × 10-9 mol cm-2 s-1/2 as the perchlorate concentration increases from 0.1 to 1.0 M. These values are about 2 orders of magnitude lower than those of the chemically cross-linked chloride salt of the polymer. The rate of heterogeneous electron transfer is unusually rapid in this system and increases from (5.2 ( 0.4) × 10-3 to (7.8 ( 0.4) × 10-3 cm s-1 on going from 0.1 to 0.4 M LiClO4 before becoming independent of the supporting electrolyte concentration at (9.2 ( 0.6) × 10-3 cms-1 for [LiClO4] g 0.6 M.
Introduction
Scheme 1
Electrodes coated with polymers containing dispersed redox centers are promising for chemical sensors, energy conversion devices, and electrosynthesis.1 This promise arises because of their synthetic flexibility and the ability to control the formal potential and hence their electrontransfer properties to target species in solution. In particular, developing devices such as sensors2,3 and biofuel cells4 requires the development of new electron transfer mediators with low formal potentials and rapid charge propagation through the electrocatalytic film. However, many redox centers that exhibit nearly ideal electrochemical responses in solution show highly complex and distorted behaviors when confined within ordered or disordered films. In this contribution, we report on the properties and charge transport dynamics of a metallopolymer that contains [Os(N,N′-alkylated-2,2′-biimidazole)3]2+/3+ centers in perchlorate-containing media (Scheme 1). This material has a negative formal potential (vs Ag/AgCl), making it attractive for application in enzyme-based biosensors and biofuel cells. Moreover, it exhibits a remarkably ideal electrochemical response associated with †
Dublin City University. University of Texas at Austin. § TheraSense Inc. * To whom correspondence should be addressed. ‡
(1) Forster, R. J.; Keyes, T. E.; Vos, J. G. Interfacial Supramolecular Assemblies; Wiley: Sussex, UK, 2003. (2) Rusling, J. F.; Forster, R. J. J. Colloid Interface Sci. 2003, 262, 1. (3) Forster, R. J.; Hogan, C. F. Anal. Chem. 2000, 72, 5576. (4) Katz, E.; Willner, I. J. Am. Chem. Soc. 2003, 125, 6803.
the Os2+/3+ redox reaction when in contact with aqueous perchlorate supporting electrolyte. In general, the propagation of charge through redox active films of this type involves self-exchange reactions that are driven by a concentration gradient of immobile oxidized and reduced redox sites and does not require the presence of an
10.1021/la035229h CCC: $27.50 © 2004 American Chemical Society Published on Web 12/30/2003
Osmium-Containing Metallopolymer
electrical potential gradient. However, the overall response is frequently limited by the movement of chargecompensating counterions required to maintain electroneutrality within the layer. An apparent electron diffusion coefficient of 5.8 × 10-6 cm2 s-1 has been reported for the cross-linked chloride salt of the polymer.5 This electron diffusion coefficient is very large for films of redox polymers and approaches the diffusion of chloride in aqueous solution. Here, we consider the effect of using perchloratecontaining supporting electrolyte on the film structure and consequentially on the charge transport properties. To address the issue of ion content within the film, we have probed the film resistance of the perchlorate salt of the polymer as the concentration of LiClO4 is systematically varied from 0.1 to 1.0 M. The ideality of the voltammetry has allowed us to obtain a detailed understanding of mass and charge transport through the material. In this study, we have conducted short time scale (tens of milliseconds) measurements using coated microelectrodes6-9 to determine the rate of charge transport through the metallopolymer film. The nature of the rate-limiting step in switching the oxidation state has been probed by analyzing the dependence of the homogeneous charge transport diffusion coefficient, DCT, on the concentration of supporting electrolyte. Beyond determining the homogeneous charge transport diffusion coefficient, we have also determined the standard heterogeneous electron-transfer rate, k°. These data are revealing of the differences between an electrode/solution and an electrode/ metallopolymer film interface, especially those factors that control double-layer effects. Experimental Section Materials. The synthesis of the PVP-[Os(N,N′-dialkylated2,2′-biimidazole)3]2+/3+ was previously described.10 Glucose oxidase (GOx) from Aspergillus niger (EC 1.1.3.4, 191 U mg-1) was purchased from Fluka (Milwaukee, WI) and poly(ethylene glycol) (400) diglycidyl ether (PEGDGE) from Polysciences, Inc. (Warrington, PA). All solutions were made with deionized water passed through a purification train (Sybron Chemicals Inc, Pittsburgh, PA). Apparatus and Procedures. Microelectrodes were prepared using platinum microwires of radii between 1 and 25 µm sealed in a glass shroud that were mechanically polished as described previously.11 Electrochemical cleaning of the electrodes was carried out by cycling in 0.1 M H2SO4 between potential limits chosen to initially oxidize and then reduce the surface of the platinum electrode. Excessive cycling was avoided in order to minimize the extent of surface roughening. The real surface areas were determined by calculating the charge under the platinum oxide reduction peak. Typical surface roughness values were between 1.6 and 2.0, and all surface coverages quoted are calculated on the basis of this microscopic electrode area. Adherent layers were obtained by evaporation of the required volume of a 1% solution of the metallopolymer in PBS buffer on the electrode or by mechanically transferring a solid sample of the polymer directly onto the microelectrode surface. Mechanical abrasion12,13 was used primarily to produce relatively thick films (Γ > 1 × 10-7 mol cm-2) and involved abrading the microelectrode off a solid sample of the metallopolymer on a weighing paper. SEM imaging reveals that these films consist of a collection of (5) Mano, N.; Mao, F.; Heller, A. J. Am. Chem. Soc. 2003, 125, 6588. (6) Forster, R. J. Chem. Soc. Rev. 1994, 289. (7) Forster, R. J. In Encyclopedia of Analytical Chemistry; Meyers, R., Ed.; Wiley: New York, 2000. (8) Forster, R. J.; Keyes, T. E. J. Phys. Chem. B 1998, 102, 10004. (9) Forster, R. J. Phys. Chem. Chem. Phys. 1999, 1, 1543. (10) Mao, F.; Mano, N.; Heller, A. J. Am. Chem. Soc. 2002, 125, 4951. (11) Forster, R. J.; Faulkner, L. R. J. Am. Chem. Soc. 1994, 116, 5444. (12) Bond, A. M.; Scholtz, F. J. Phys. Chem. 1991, 95, 7640. (13) Forster, R. J.; Keyes, T. E.; Bond, A. M. J. Phys. Chem. B 2000, 104, 6389.
Langmuir, Vol. 20, No. 3, 2004 863 random array of particles with dimensions between approximately 0.5 and 5 µm. Beyond minor differences in the initial cycles, the voltammetric responses from films prepared using both approaches were indistinguishable. In the case of films formed by droplet evaporation onto macroscopic electrodes, the resulting films were visibly smooth and shiny and were free from any obvious aggregates when viewed under an optical microscope at magnifications up to 40×. Carbon Fiber Electrodes. Prior to their coating, the 7 µm diameter fibers (0.0044 mm2) were made hydrophilic by exposure to 1 Torr of O2 plasma for 3 min.14 The anodic catalyst solution was made as follows. 100 µL of 40 mg/mL GOx in 0.1 M NaHCO3 was oxidized by 50 µL of 7 mg/mL of NaIO4 in dark for 1 h, and then 2 µL of the periodate-oxidized GOx was mixed with 8 µL of 10 mg/mL of the polymer and 0.5 µL droplet of 2.5 mg/mL of PEGDGE. 5 µL of this solution was applied to the fiber. The resulting electrocatalyst consisted of the cross-linked adduct of 39.6 wt % GOx, 59.5 wt % redox polymer, and 0.9 wt % PEDGE. In studies where the electrolyte concentration was varied, all experiments were performed using freshly prepared films. Thus, the error bars presented, e.g., on homogeneous charge transport rates, include the interfilm variability. Typically, the interfilm variability is approximately twice the intrafilm value; i.e., repeated measurements on a single film are typically reproducible to within approximately 2%. Cyclic voltammetry was performed using a CH Instruments model 660 or a CH Instruments model 832 electrochemical workstation and a conventional three-electrode cell. All solutions were deoxygenated thoroughly using nitrogen, and a blanket of nitrogen was maintained over the solution during all experiments. Potentials are quoted with respect to a CH Instruments Ag/AgCl reference electrode in which the electrolyte concentration is 3.0 M NaCl. All experiments were performed at room temperature (22 ( 3 °C). For short time scale experiments (
