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Generator-Collector Experiments at a Single Electrode: Exploring the General Applicability of This Approach by Comparing the Performance of Surface Immobilized versus Solution Phase Sensing Molecules Martin C. Henstridge, Gregory G. Wildgoose, and Richard G. Compton* Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QZ, United Kingdom Received July 6, 2009. Revised Manuscript Received July 27, 2009 We demonstrate proof-of-concept that generator-collector experiments can be performed at a single macroelectrode and used to determine mechanistic information. The practical advantages of such a system over conventional generator-collector techniques are also outlined. The single-electrode generator-collector technique is applied to study the known mechanism of oxygen reduction in aqueous conditions as a model system. We seek to demonstrate that the single-electrode generator-collector approach is capable of detecting local pH changes, immediately adjacent to the electrode surface during a redox reaction. Experiments are performed using a molecular pH probe attached to the electrode surface. Comparison of experimental data with numerical simulations verifies that the reduction of oxygen at pH 6.8 proceeds via a two-electron, two-proton mechanism. Experiments were also performed with a molecular pH probe dissolved in the electrolyte solution in order to explore the feasibility of this approach, which is potentially applicable to a much wider range of electrochemical systems.
1. Introduction Many chemical reactions are known to undergo homogeneous follow-up chemistry upon oxidation or reduction1-3 and as such are ideally suited to study using electrochemical techniques. Typically kinetic and/or mechanistic information concerning such reactions may be obtained using a generator-collector experimental arrangement.1-3 A reaction is initiated at a “generator” electrode, and redox-active intermediates are then detected at a second “collector” electrode in close proximity. Each generator-collector setup is characterized by a collection efficiency. This is the ratio of current detected at the collector relative to the generator electrode and is a function of the geometry of the electrochemical cell and the rate of mass transport of species between the collector and generator electrodes. Classical generator-collector systems include the use of double-channel electrodes, rotating ring-disk electrodes, and *Corresponding author: e-mail
[email protected]; Tel +44 (0)1865 275413; Fax +44 (0)1865 275410. (1) Brett, C. M. A.; Brett, A. M. O. Electrochemistry: Principles, Methods, and Applications; Oxford University Press: Oxford, 1993. (2) Compton, R. G.; Banks, C. E. Understanding Voltammetry; World Scientific: Singapore, 2007. (3) Organic Electrochemistry, 4th ed.; Lund, H., Hammerich, O., Eds.; CRC Press: London, 2000. (4) Albery, W. J.; Calvo, E. J. J. Chem. Soc., Faraday Trans. 1 1983, 79, 2583– 2596. (5) Albery, W. J.; Jones, C. C.; Mount, A. R. Compr. Chem. Kinet. 1989, 29, 129– 48. (6) Albery, W. J.; Mount, A. R. J. Chem. Soc., Faraday Trans. 1 1989, 85, 1181– 1188. (7) Alden, J. A.; Compton, R. G. Anal. Chem. 2000, 72, 198A–203A. (8) Amatore, C.; Belotti, M.; Chen, Y.; Roy, E.; Sella, C.; Thouin, L. J. Electroanal. Chem. 2004, 573, 333–343. (9) Amatore, C.; Da Mota, N.; Lemmer, C.; Pebay, C.; Sella, C.; Thouin, L. Anal. Chem. 2008, 80, 9483–9490. (10) Amatore, C.; Sella, C.; Thouin, L. J. Electroanal. Chem. 2006, 593, 194–202. (11) Brett, C. M. A.; Brett, A. M. C. F. O.; Compton, R. G.; Fisher, A. C.; Tyley, G. P. Electroanalysis 1991, 3, 631–6. (12) Compton, R. G.; Fisher, A. C.; Tyley, G. P. J. Appl. Electrochem. 1991, 21, 295–300. (13) Compton, R. G.; Laing, M. E.; Mason, D.; Northing, R. J.; Unwin, P. R. Proc. R. Soc. London A 1988, 418, 113–54.
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wall-jet electrode systems,2,4-17 occasionally performed in conjunction with spectroelectrochemical techniques.5,18-20 All of these classical techniques are hydrodynamic systems. This is because the dimensions of the generator and collector electrodes and/or the separation between them are usually constructed on the millimeter scale so that stagnant, diffusion-only conditions would lead to relatively large transit times between the two electrodes, resulting in poor collection efficiencies. Recent advances by the work of del Campo et al.21 and Marken et al.22 have successfully fabricated generator/collector electrode arrangements where the electrode separation is of the order of micrometers or even a few hundreds of nanometers. Both of these latter approaches remove the need for hydrodynamic conditions but require complex fabrication processes and computationally expensive, nontrivial 2-D or even 3-D numerical simulations in order to infer useful mechanistic or kinetic data. Similarly, the introduction of scanning electrochemical microscopy techniques can also be used to reduce the electrode separation to the submicrometer scale.23-26 However, the problem still remains that the widespread use of any of the techniques mentioned above is limited by the specialist equipment (bipotentiostats, rotating electrode controllers, hydrodynamic flow cells, spectrometers, (14) Cooper, J. A.; Compton, R. G. Electroanalysis 1998, 10, 1182–1187. (15) Gooding, J. J.; Hall, E. A. H. Anal. Chem. 1998, 70, 3131–3136. (16) Klymenko, O. V.; Oleinick, A. I.; Amatore, C.; Svir, I. Electrochim. Acta 2007, 53, 1100–1106. (17) Wang, J. Analytical Electrochemistry, 3rd ed.; Wiley-VCH: Weinheim, 2006. (18) Compton, R. G.; Hillman, A. R. Chem. Br. 1986, 22, 1088–92. (19) Prieto, F.; Webster, R. D.; Alden, J. A.; Aixill, W. J.; Waller, G. A.; Compton, R. G.; Rueda, M. J. Electroanal. Chem. 1997, 437, 183–189. (20) Waller, A. M.; Compton, R. G. Compr. Chem. Kinet. 1989, 29, 297–352. (21) Menshykau, D.; del Campo, F. J.; Mu~noz, F. X.; Compton, R. G. Sens. Actuators, B 2009, DOI: 10.1016/j.snb.2008.12.064. (22) French, R. W.; Collins, A. M.; Marken, F. Electroanalysis 2008, 20, 2403– 2409. (23) El-Giar, E. E.-D. M.; Wipf, D. O. J. Electroanal. Chem. 2007, 609, 147–154. (24) Hess, C.; Borgwarth, K.; Heinze, J. Electrochim. Acta 2000, 45, 3725–3736. (25) McKay, L.; LeSuer, R. J. Electrochim. Acta 2008, 53, 8305–8309. (26) Sugihara, T.; Kinoshita, T.; Aoyagi, S.; Tsujino, Y.; Osakai, T. J. Electroanal. Chem. 2008, 612, 241–246.
Published on Web 09/11/2009
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X,Y,Z piezo-electric controllers, etc.) and the elaborate fabrication of the very small electrodes that are required. We are interested in developing generator-collector systems that can be used with a single macroelectrode, under stagnant diffusion-only conditions, with a widely available standard singlechannel potentiostat. Furthermore, mechanistic and/or kinetic information can be extracted from such a simple system using commercially available 1-D electrochemical simulation packages such as Digisim. In order to demonstrate proof-of-concept of this approach, we have chosen to study the well-known two-electron, two-proton reduction mechanism of oxygen at a single edge-plane pyrolytic graphite electrode (EPPG) which forms hydrogen peroxide. This is the generation step. The collection step is achieved at the same electrode and in the same scan by examining the cyclic or linear sweep voltammetric response of a “molecular pH probe” to changes in the local proton concentration immediately adjacent to the electrode surface as the reduction of oxygen proceeds. By comparing the experimentally observed shift in local pH with that calculated from numerical simulation, the number of protons involved in the reduction of oxygen can be determined. As it is established that the number of protons involved in the reduction of oxygen is equal to two, we can demonstrate that the single electrode generator-collector system is capable of easily distinguishing between the two-electron and hypothetical one-, two-, or four-proton mechanisms, so offering proof-of-concept for this type of generator-collector electrochemical system. We envisage that this system could readily be applied for example in areas such as fuel cell research or to the study of peroxidase or oxidase enzymes where there it is key to establish if a given catalytic system is reducing oxygen in a two-electron, two-proton mechanism to form hydrogen peroxide or in a preferred fourelectron, four-proton reduction forming water as the product. We also seek to examine the general applicability of this approach by comparing two embodiments of the single-electrode generator-collector system. The first is where the pH probe molecule, anthraquinone, is covalently attached to the electrode surface (AQ-EPPG), and the second is where the pH probe, anthraquinone-2-sulfonate (AQS, chosen because this anthraquinone derivative is more soluble in water than anthraquinone itself), is simply present in the electrolyte solution. While the modification of the electrode surface with anthraquinone groups is relatively rapid and facile, having the molecular sensing probe in the solution phase is envisaged to be more widely applicable. The advantages and disadvantages of the two embodiments are explored and discussed.
2. Experimental Section 2.1. Reagents and Equipment. All chemical reagents were purchased from Aldrich (Gillingham, UK), were of analytical grade, and used as received without further purification with the exception of anthraquinone-2-diazonium tetrafluoroborate which was synthesized as described in section 2.2. All solutions were prepared with deionized water of resistivity not less than 18.2 MΩ cm at 298 K (Millipore water systems, UK). Solutions of known pH were prepared as follows: pH 1.0, 0.1 M HCl; pH 2.0, 0.01 M HCl; pH 4.6, 0.1 M acetic acid + 0.1 M sodium acetate; pH 6.8, 0.025 M potassium dihydrogen phosphate + 0.025 M disodium hydrogen phosphate; pH 9.2, 0.05 M disodium tetraborate; pH 12.0, 0.01 M NaOH; pH 13.0, 0.1 M NaOH; pH 14.0, 1.0 M NaOH. In addition all solutions contained 0.1 M KCl as supporting electrolyte. pH measurements were preformed using a pH213 Hannah Instruments meter. Voltammetric measurements were recorded using a μ-Autolab (EcoChemie, Utrecht, Netherlands) computer-controlled potentiostat using GPES (Version 4.7). All experiments were conducted Langmuir 2010, 26(2), 1340–1346
using a standard three-electrode configuration using an edgeplane pyrolytic graphite working electrode (EPPG, 4.5 mm diameter, Le Carbone, Sussex, UK), a graphite rod counter electrode (Alfa Aesar), and a saturated calomel reference electrode (SCE, Radiometer, Copenhagen, Denmark). The EPPG working electrode surface was renewed as necessary by successive polishing using 1.0 and 0.3 μm alumina slurries (Buehler Micropolish II) on soft lapping pads (Buehler). The electrode was thoroughly rinsed with pure water and briefly sonicated after each polishing step to remove any adhered microparticles. Deaerated and aerated electrolyte solutions were prepared by bubbling either nitrogen or oxygen gases (BOC gases, Guildford, UK), respectively, for 20 min prior to performing any voltammetric experiments. All experiments were performed at 20 ( 2 °C. Numerical simulations were performed using the commercially available Digisim software package (BASi Technicol) run on a PC equipped with 512 Mb of RAM and a 2.5 GHz Intel Pentium processor.
2.2. Synthesis of Anthraquinone-2-diazonium Tetrafluoroborate and Electrode Modification. Anthraquinone-2diazonium tetrafluoroborate, used in the surface modification of the EPPG electrode, was synthesized according to the method of Milner.27 500 mg of 2-aminoanthraquinone was added to a slurry of 50% molar excess of nitrosonium tetrafluoroborate in 50 mL of dichloromethane. The slurry was gently stirred for 1 h in an ice-water bath before the solvent was removed under vacuum. The resulting anthraquinone-2-diazonium tetrafluoroborate product was then stored under nitrogen at -5 °C prior to use. The chemical modification of the EPPG electrode with 2-anthraquinone groups was achieved as follows: a freshly polished EPPG electrode was immersed in a stirred solution of 5 mM anthraquinone-2-diazonium tetrafluoroborate prepared using deionized water cooled to ca. 5 °C. The solution was allowed to warm to room temperature and stirred for a further 30 min, after which the EPPG electrode modified with 2-anthraquinone groups (AQ-EPPG) was removed and rinsed with copious quantities of pure water and acetonitrile to remove any unreacted diazonium salt.
3. Results and Discussion 3.1. Comparison of the Voltammetric Response to Varying Solution pH of Anthraquinone-Based “Reporter” Molecules Covalently Attached to the Electrode Surface and in the Solution Phase. In order to compare the performance of the anthraquinone-based sensing molecules either attached to the EPPG electrode surface or diffusion in solution, it is first necessary to characterize the anthraquinone-modified EPPG electrode (AQ-EPPG) to confirm that it is indeed chemically modified. To this end a cyclic voltammetric protocol was performed in pH 6.8 phosphate buffer which is detailed in the Supporting Information.28-30 Having characterized the AQ-EPPG electrode, we next compare the voltammetric response of the AQ-EPPG electrode and a 1.0 mM AQS solution (recorded in the case of AQS at an unmodified EPPG electrode) over a range of pH from 1.0 to 13.0. For both the AQ-EPPG and the AQS systems the peak potential of the oxidative and reductive peaks was found to shift to more negative potentials with increasing solution pH. A plot of peak potential vs pH revealed that below pH 10 both the AQEPPG and the AQS system exhibited a linear variation in peak (27) Milner, D. J. Synth. Commun. 1992, 22, 73–82. (28) Heald, C. G. R.; Wildgoose, G. G.; Jiang, L.; Jones, T. G. J.; Compton, R. G. ChemPhysChem 2004, 5, 1794–1799. (29) Leventis, H. C.; Streeter, I.; Wildgoose, G. G.; Lawrence, N. S.; Jiang, L.; Jones, T. G. J.; Compton, R. G. Talanta 2004, 63, 1039–1051. (30) Wildgoose, G. G.; Pandurangappa, M.; Lawrence, N. S.; Jiang, L.; Jones, T. G. J.; Compton, R. G. Talanta 2003, 60, 887–893.
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potential with pH, as shown in parts a and b of Figure 1, respectively. The gradients of the plot in the region below pH 10 are in good agreement with that predicted by the appropriate form of the Nernst equation for a reversible redox system undergoing an n-electron, m-proton reduction or oxidation where m = n (in this case m = n = 2 for the anthraquinone/anthrahydroquinone redox couple28,30). E ¼E-
2:303mRT pH nF
However, Figure 1b reveals that the AQS system exhibits a deviation from linear behavior above ca. pH 10.4, while the linear range for the AQ-EPPG electrode extends to beyond pH 12.0. The break in the plot of peak potential vs pH for the AQS system can be related to the solution phase pKa values for the removal of protons from the anthrahydroquinone sulfonate which are ca. pKa1 = 10 and pKa2 = 12.31 In the case of the AQ-EPPG system, where the anthraquinone molecules are covalently attached to the graphite surface, the pKa values are shifted to beyond 12.0. Such large pKa shifts have been documented recently for several species covalently attached to carbon surfaces, including anthraquinone groups.31-33 It has been suggested that the difference in the surface pKa values and the solution phase pKa values is related to changes in the solvation entropy and the structure of the solvation shell surrounding these molecules on the relatively hydrophobic carbon surfaces.31-33 It is worth noting that the use of a solution phase redox active molecular probe in a single electrode generator-collector experiment may provide a wider range of candidates to choose from, thus leading to wider general applicability of this method. However, the use of surface immobilized species may allow one to extend the useful sensing range of the molecular probe, in this case with respect to the accessible pH range that can be measured, which, while less widely applicable, may be advantageous in certain systems. Having characterized the voltammetric behavior of the AQ-EPPG and AQS systems as molecular pH probes, we can now apply them to demonstrate proof-of-concept of performing generator-collector experiments at a single electrode and compare the use of surface bound vs solution phase sensing molecules using the reduction of molecular oxygen as a model system. 3.2. Studying the Reduction of Dioxygen Using the Single-Electrode Generator-Collector System. The electroreduction of molecular oxygen on graphite electrodes is well-known to follow an irreversible two-electron, two-proton reduction mechanism in aqueous solutions.34 It is therefore an ideal model system to use to demonstrate proof-of-concept of our ability to perform generator-collector experiments at a single electrode. Cyclic voltammetry was performed in 1.0 mM phosphate buffer (containing 0.1 M KCl as supporting electrolyte) adjusted to pH 6.8, using both the AQ-EPPG and the AQS system in the presence and absence of dissolved (saturated) molecular oxygen over a range of scan rates from 50 to 500 mV s-1. Parts a and b of Figure 2 show the overlaid voltammetric response of the AQEPPG electrode and the AQS system (recorded at an unmodified EPPG electrode), respectively, under identical conditions in both (31) Masheter, A. T.; Abiman, P.; Wildgoose, G. G.; Wong, E.; Xiao, L.; Rees, N. V.; Taylor, R.; Attard, G. A.; Baron, R.; Crossley, A.; Jones, J. H.; Compton, R. G. J. Mater. Chem. 2007, 17, 2616–2626. (32) Abiman, P.; Crossley, A.; Wildgoose, G. G.; Jones, J. H.; Compton, R. G. Langmuir 2007, 23, 7847–7852. (33) Abiman, P.; Wildgoose, G. G.; Crossley, A.; Jones, J. H.; Compton, R. G. Chem.;Eur. J. 2007, 13, 9663–9667. (34) Sljukic, B.; Banks, C. E.; Compton, R. G. J. Iran. Chem. Soc. 2005, 2, 1–25.
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Figure 1. Plots of oxidative and reductive peak potential vs pH for (a) the AQ-EPPG system and (b) the 1.0 mM AQS system recorded at an unmodified EPPG electrode. All data obtained from cyclic voltammetry performed at 100 mV s-1.
the presence and absence of dissolved oxygen. There are three things to note. First, despite anthrahydroquinone being known to electrocatalyze the reduction of oxygen at low pH,34,35 the peak current and the peak potential of the oxygen reduction are almost identical at the AQ-EPPG electrode and an unmodified EPPG electrode, while in the presence of AQS in solution the oxygen reduction has shifted to a slightly more negative potential, which may possibly be due to some small degree of adsorption of the AQS onto the electrode surface actually retarding the electron transfer kinetics slightly. This suggests that at pH 6.8 very little, if any, electrocatalysis by the anthraquinone moieties is occurring. To explain this lack of electrocatalysis, one has to consider that for the anthrahydroquinone to catalyze the reduction of oxygen there must be some match between the AQ and O2 reduction peak potentials. This does indeed occur at low pH vlaues,34,35 and electrocatlaysis is observed. However, the oxygen reduction peak potential and the AQ reduction peak potential do not vary with pH by the same amount due to the irreversibility of the oxygen reduction (hence the Nernst equation cannot be applied). Hence, at pH close to 7 the AQ reduction potential no longer matches that of oxygen, and electrocatlaysis is not observed. Second, the fact that the voltammetric characteristics of the reduction of oxygen are almost identical at a bare EPPG electrode and at the AQ-EPPG electrode indicates that the chemical modification of the electrode surface with the molecular sensing probes has not altered the electrode kinetics to any great extent. This is important if meaningful data are to be collected using generator-collector (35) Banks, C. E.; Wildgoose, G. G.; Heald, C. G. R.; Compton, R. G. J. Iran. Chem. Soc. 2005, 2, 60–64.
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experiments at a single electrode. Third, both the oxidative and reductive peaks corresponding to the anthraquinone/anthrahydroquinone redox couple in both the AQS and AQ-EPPG systems are present but shifted to more negative potentials in the presence of oxygen when compared to their peak potentials in the absence of oxygen. Furthermore, the ratio of oxidative and reductive peak currents in either system approaches unity, again indicating a lack of any electrocatalytic reduction of oxygen by the anthraquinone moieties (in the case of an EC0 electrocatalytic mechanism the ratio of the reductive peak to the oxidative peak would be much greater than one, with the corresponding oxidative peak being very much smaller than the reductive peak or not observable34,35). The observed shift of the anthraquinone redox couple to more negative potentials in the presence of oxygen suggests that these molecular pH probes are sensing a reduction in the local pH at the electrode surface resulting from the consumption of protons as oxygen is reduced to form hydrogen peroxide. Thus, we have a generation step, the reduction of oxygen and concomitant change in local proton concentration, which is sensed by the “collector”;the anthraquinone molecular pH probes;at a single electrode. In order to validate this claim we shall now examine if the single electrode generator-collector electrode can be used to quantitatively determine the number of protons consumed during the reduction of oxygen by comparing the experimental results with numerical simulations. 3.3. Comparison of Experiment vs Simulation. In order to verify that the observed shift in potential for both the AQ-EPPG and the AQS reduction peaks are due to the increase in local pH caused by the uptake of protons by the reduction of oxygen, the experimental peak potentials were compared to those calculated using Digisim simulations. Diffusion coeffcients in the simulations were set to literature values for each species,36 with DO2 = 2.2 10-5 cm2 s-1. The diffusion coeffcient of the proton varies between 5.0 10-5 and 9.3 10-5 cm2 s-1 depending on the ionic strength of the solution;37 however, using the values at either extreme of this range was found to have a negligibly small effect on the simulated local proton concentration, and so the typical literature value of 9.0 10-5 cm2 s-1 was used. The buffer components were simulated using the following literature values for the pKa and rate constants:38 kf
H2 PO4 - F s s H þ þ HPO42 R pKa ¼ 7:20;
kf ¼ 1:77 103 s -1
The initial proton concentration was set to correspond to the measured solution pH. The values of the electrochemical parameters for each electron transfer step were confirmed as optimized by varying each parameter separately and individually until values that produced a satisfactory fit to experimental data were obtained. For both the surface-bound and solution-phase pHprobe cases, simulations were performed for three cases corresponding to the known two-electron, two-proton reduction mechanism of oxygen at a carbon electrode and also for the hypothetical two-electron, one-proton and two-electron, fourproton mechanisms for comparison. 3.3.1. Simulation of the AQ-EPPG System. Digisim is designed to simulate the voltammetry of species diffusing in (36) Ferrel, R. T.; Himmelblau, D. M. J. J. Chem. Eng. Data 1967, 12, 111–115. (37) Choi, P.; Jalani, N. H.; Datta, R. J. J. Electrochem. Soc. 2005, 152, 1548– 1554. (38) Bewick, A.; Fleishmann, M.; Hiddleston, J. N.; Wynne-Jones, L. Discuss. Faraday Soc. 1965, 39, 149–158.
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Figure 2. Comparison of the voltammetry recorded in 1.0 mM pH 6.8 phosphate buffer (scan rate 500 mV s-1) of (a) the AQ-EPPG system and (b) the 1.0 mM AQS system in the absence (dashed lines) and presence (solid lines) of saturated dissolved oxygen.
solution and is not capable of accurately modeling the voltammetry of surface-bound species. Therefore, in order to attempt to simulate the observed shift in reduction peak potential of the in the AQ-EPPG system, it was necessary to first simulate the reduction of oxygen at the AQ-EPPG electrode (Figure 3) as a two-electron, two-proton reduction mechanism using the parameters given in Table 1 (note that no attempt to simulate the anthraquinone reduction peak was made). This is then used to extract the simulated value of the local proton concentration (see section 1.2 and Figure 3a,b in the Supporting Information), and hence the local pH, at the surface of the electrode at the potential that the AQ-EPPG reduction peak is observed experimentally. Using the calibration plot for the AQ-EPPG system shown in Figure 1a, this simulated value of the local pH is then converted to a peak potential for the AQ-EPPG system and compared to the experimentally observed value. For comparison, simulations were additionally performed for the hypothetical cases where the reduction of oxygen involves either a two-electron, one-proton reduction or a two-electron, four-proton reduction mechanism, and these are shown in Figure 4 along with the simulated peak potential for the twoelectron, two-proton mechanism. Reassuringly, it is evident from Figure 4 that best fit to the experimental data is obtained when the reduction of oxygen is simulated as a two-electron, twoproton mechanism, as is expected. In the cases where the mechanism proposed to involve one or four protons, the former predicts too few protons being consumed by the reduction of DOI: 10.1021/la902418v
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Figure 3. Comparison of the experimental (dashed line) and simulated (solid line) voltammetry of oxygen reduction at the AQEPPG electrode (scan rate 100 mV s-1). Table 1. Parameters Used in the Digisim Simulations of Oxygen Reduction E E C C buffer
O2 + e- h O2E = -0.08 V O2- + e- h O22E = -0.15 V O22- + H+ h HO2K = 1010 HO2- + H+ h H2O2 K = 1010 Buf + H+ h HBuf K = 1.58 107
k° = 10-7 cm s-1 -4
k° = 10
-1
cm s
R = 0.61 R = 0.5
kf = 1010 s-1 kf = 1010 s-1 kf = 2.8 1010 s-1
Figure 4. Comparison of experimental (crosses) and simulated (squares) AQ peak potential. + denotes the experimental AQ peak potential in the absence of oxygen, and denotes experimental AQ peak potential in the presence of dissolved oxygen. The one-proton mechanism is shown by white squares, the two-proton mechanism is shown by gray squares, and the four-proton mechanism is shown by black squares.
oxygen and the latter too many, across the whole range of scan rates studied. 3.3.2. Simulation of the AQS System. Since the AQS is a solution-phase species, both the AQS reduction and oxygen reduction can be modeled simultaneously by Digisim. Thus, direct comparison between theory and experiment is possible. However, in order for the peak potential of the two-electron, 1344 DOI: 10.1021/la902418v
Figure 5. Comparison of experimental (squares) and simulated (crosses) pH dependence of AQS peak potential.
two-proton AQS couple to exhibit the pH dependence observed experimentally, the simulation parameters must obey a series of rules developed by Smith et al.39 and described in more detail in section 1.3 of the Supporting Information. The AQS system only exhibits a pH-dependent peak potential up to ca. pH 10.4; as such the simulated Ka for the deprotonation of the anthrahydroquinone species was set to give a pKa value of 11. The exact parameters used for the simulation of the AQS system are shown in Scheme 1 of the Supporting Information. Note that, first, the homogeneous rate constants were set to be so fast as to ensure that the “C” steps are equilibrated and, second, fast electrochemical kinetics (k° . 1 cm s-1) are assumed. Using the parameters given in Scheme 1 (Supporting Information), which conform to the above rules, simulations were carried out for the same pH values used in the experimental plot of peak potential vs pH for the AQS system recorded at an unmodified EPPG electrode (Figure 1b). The simulated peak potentials are plotted against pH in Figure 5 along with the experimental values for comparison. Very good agreement between experiment and simulation is observed, as shown. Note that because the “C” steps are assumed equilibrated and the electrode kinetics are reversible, then any precise sequence of “E” and “C” steps is not implied by the modeling. Rather Digisim is modeling the process AQ + 2H+ + 2e- h AQH2 with fast electron transfer and homogeneous chemical steps over the whole pH range studied. Having satisfied ourselves that we have a good fit between experiment and simulation for the AQS reduction peak, we then combined this with our simulation for the reduction of oxygen as shown in Figure 6a, while the comparison of simulated and experimentally observed values of the AQS reduction peak potential over the range of scan rates studied is given in Figure 6b. Again, simulated values obtained when the reduction of oxygen was modeled so as to involve concomitant uptake of only one or four protons are also shown for comparison. It is clear from Figure 6b that, although each of the three simulated mechanisms (one, two, or four protons involved in the oxygen reduction) does produce small differences in the simulated values of the AQS reduction peak potential, these differences are not sufficiently large as to enable the AQS system to reliably differentiate between any of the proposed mechanisms. This is likely due to the fact that the local pH adjacent to the EPPG electrode (39) Smith, E. T.; Davis, C. A.; Barber, M. J. J. Anal. Biochem. 2003, 323, 114– 121.
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where a single-electrode generator-collector system has been used for mechanistic studies. We have recently demonstrated that a single-electrode generator-collector approach can be used to determine the number of protons and electrons involved in the initial oxidation mechanism of the biologically relevant molecule, serotonin.40 Using a different redox active molecule as the pH-sensitive reporting molecule for the “collection” step to the anthraquinone groups used in this report, namely the arylhydroxylamine/arylnitroso couple, the mechanism was found to initially involve the transfer of two electrons but only one proton (for further details see ref 40). Although, in this second example, only the case where the reporter molecular redox system was covalently attached to the electrode surface was explored, the fact that different reporter molecular systems can be employed (which can be chosen for the system under investigation) and that the single-electrode generatorcollector approach can be used to differentiate which, of several possible mechanisms, is likely correct further demonstrates that this approach may have more widespread and general applicability to the elucidation of redox mechanisms in complex systems.
4. Conclusions
Figure 6. (a) Comparison of experimental (dotted line) and simulated (dashed and solid lines) voltammetry of oxygen reduction in 1.0 mM AQS solution. The dashed line is simulated for the absence of AQS, i.e., O2 reduction only; the solid line is simulated for the presence of 1.0 mM AQS. All voltammetry was recorded/simulated at 100 mV s-1. (b) Comparison of experimental (crosses) and simulated (squares) AQS peak potential. + denotes the experimental AQS peak potential in the absence of oxygen, and denotes experimental AQS peak potential in the presence of dissolved oxygen. The one-proton mechanism is shown by white squares, the two-proton mechanism is shown by gray squares, and the four-proton mechanism is shown by black squares.
surface is >10.4 (extracted from the simulated proton concentration profiles) for all of the scan rates studied. Hence, although using a solution phase molecular probes (which may be chosen so as to be sensitive to species other than the proton depending on the type of mechanism under investigation) may have wider general applicability than attaching the probe molecules to the electrode surface, the latter may in certain circumstances such as the example shown here offer distinct advantages. Furthermore, such surface modifications do not necessarily have to be particularly lengthy or complex, and yet they enable single-electrode generator-collector experiments to be performed without any specialist equipment, expensive and time-consuming electrode fabrication methodologies, or the need for complex multidimensional numerical simulations programs. One caveat in using this approach is that the molecular “reporter” systems chosen for the “collection” step must not undergo any significant redox reaction with any species involved/produced in the target system under study. As yet another example of the wider applicability of this approach, we draw the reader’s attention to a second example Langmuir 2010, 26(2), 1340–1346
Proof-of-concept has been demonstrated that generator-collector experiments can be performed at a single macroelectrode and that this system can be used to obtain mechanistic data, exemplified by the determination of the number of protons involved in the aqueous electroreduction of oxygen as a model system. Our method holds distinct advantages over classical approaches, such as the use of dual-channel electrodes or rotating ring-disk electrode setups, in that the need for specialist equipment (such as bipotentiostats, double electrode fabrication, etc.) and the requirement for hydrodynamic conditions and complex multidimensional numerical simulation are avoided. Furthermore, we have examined the advantages and disadvantages of using molecular probes in solution or chemically bound to the electrode surface to enable the collection step to be performed at the same electrode as the generation step. The use of solution-phase molecular probes is envisaged to be more generally applicable and removes the need to consider any possible influence that attaching the probe molecule to the electrode may have. However, in certain circumstances (such as the example provided herein where the molecular probe is pHsensitive and the pKa values of the molecular probe are increased from their solution-phase values by attachment to the graphite surface)30-33 using a surface-bound molecular pH probe may be preferable. We also note that the method of surface modification using diazonium salts can be applied to several other commonly used electrode substrates other than carbon, such as silicon, iron, cobalt, nickel, copper, zinc platinum, gold, and boron-doped diamond.41-45 We envisage that, by careful choice of molecular probe (i.e., probes that have redox potentials sufficiently close to the system of interest or are selectively reactive toward intermediates generated in a mechanism under consideration), the (40) Henstridge, M. C.; Wildgoose, G. G.; Compton, R. G. J. Phys. Chem. C 2009, 113, 14285-14289. (41) Bernard, M.; Chauss, A.; Cabet-Deliry, E.; Chehimi, M. M.; Pinson, J.; Podvorica, F.; Vautrin-UL, C. Chem. Mater. 2003, 15, 3450–3462. (42) Henry de Villeneuve, C.; Pinson, J.; Bernard, M. C.; Allongue, P. J. Phys. Chem. B 1997, 101, 2415–2420. (43) Adenier, A.; Bernard, M.; Chehimi, M. M.; Cabet-Deliry, E.; Desbat, B.; Fagebaum, O.; Pinson, J.; Podvorica, F. J. Am. Chem. Soc. 2001, 123, 4541–4549. (44) Kullapere, M.; Seinberg, J.; M€aeorg, U. G. M.; Schiffrin, D. J.; Tammeveski, K. Electrochim. Acta 2009, 54, 1961–1969. (45) Foord, J. S.; Hao, W.; Hurst, S. Diamond Relat. Mater. 2007, 16, 877–880.
DOI: 10.1021/la902418v
1345
Article
approach we have outlined in this report can readily be extended to study the kinetics and mechanisms of a wide range of interesting chemical or biochemical reactions. Acknowledgment. G.G.W. thanks St. John’s College, Oxford, for support via a Junior Research Fellowship.
1346 DOI: 10.1021/la902418v
Henstridge et al.
Supporting Information Available: Voltammetric characterization of the AQ-EPPG electrode system, Digisim modeling parameters and concentration profiles, and the schemeof-squares Digisim parameters used to model the AQS system. This material is available free of charge via the Internet at http://pubs.acs.org.
Langmuir 2010, 26(2), 1340–1346
