Anal. Chem. 1996, 68, 3143-3150
Reversibly Adsorbed Monolayers on Microelectrodes: Effect of Potential on the Adsorption Thermodynamics Robert J. Forster
School of Chemical Sciences, Dublin City University, Dublin 9, Ireland
Monolayers of 2-hydroxyanthraquinone (2OH-AQ) have been formed by reversible adsorption onto mercury microelectrodes. Cyclic voltammetry of these monolayers in contact with HClO4 as supporting electrolyte is nearly ideal and is consistent with reduction proceeding by a twoelectron, two-proton transfer mechanism. Cyclic voltammetry reveals that the dependence of the surface coverage Γ on the bulk concentration of 2OH-AQ is accurately described by the Langmuir isotherm over the concentration range from 2 × 10-8 to 2 × 10-6 M. A limiting surface coverage Γs of (1.0 ( 0.08) × 10-10 mol cm-2 and an adsorption coefficient β of (6.3 ( 0.6) × 106 M-1 are observed. Since adsorption is reversible in this system, the effect of changing the potential at which the monolayer is formed on the surface coverage, or the adsorption thermodynamics, cannot be investigated by ex situ measurement of Γ in a blank electrolyte solution. To address this issue, microsecond time scale chronoamperometry has been used to time-resolve double-layer charging and heterogeneous electron transfer to the adsorbed anthraquinone moieties. This approach allows the doublelayer capacitance Cdl to be measured even at potentials where the monolayer is redox active, thus allowing the effect of potential and bulk 2OH-AQ concentration on Cdl to be probed. Significantly, when Cdl is measured at the formal potential E°′ as the concentration of 2OH-AQ in solution is systematically varied, the values of Γs and β obtained are identical to those measured voltammetrically. This observation suggests that Cdl depends linearly on Γ, at least at E°′. Capacitance data have been used to determine the adsorption isotherms as the deposition potential was systematically varied from -0.400 to +0.300 V, where the monolayer is fully reduced and oxidized, respectively. These data suggest that the reduced form of the monolayer, trihydroxyanthracene, may be more compact than the oxidized anthraquinone films. Moreover, the magnitude of the free energy of adsorption ∆Gads changes significantly, with values of -41.4 ( 2.7 and -26.5 ( 0.3 kJ mol-1 being observed for fully reduced and oxidized monolayers, respectively. These differences in the free energies of adsorption may arise because of different extents of intermolecular hydrogen bonding in the oxidized and reduced films.
tion.1 These novel materials find diverse application, e.g., functionalizing inorganic solids provides a natural environment for protein immobilization,2 while depositing ultrathin, high-electricresistance layers onto conductors allows contemporary models of electron transfer to be tested.3 In particular, immobilizing redox-active species is an especially attractive approach to controlling the chemical and electronic properties of electrode surfaces. There has been a resurgence of interest in proton-coupled redox reactions because of their importance in catalysis, molecular electronics, and biological systems. Thin films of materials that undergo coupled electron and proton transfer reactions are attractive model systems for developing catalysts that function by hydrogen atom and hydride transfer mechanisms.4 In the field of molecular electronics, protonation provides the possibility of trapping electrons in a particular redox site, thus giving rise to molecular switches.5 In biological systems, the kinetics and thermodynamics of redox reactions are often controlled by enzyme-mediated acid-base reactions.6 Quinonoid monolayers, formed by spontaneous adsorption onto mercury, often exhibit nearly ideal electrochemical responses in low-pH electrolytes, making them attractive model systems for probing coupled proton and electron transfer reactions.7 Moreover, since one might expect the extent of intermolecular hydrogen bonding to be quite different for the oxidized and reduced forms, it may be possible to use these monolayers to probe hydrogen bonding in two dimensions. Furthermore, since both the oxidized and reduced forms are neutral, these supramo-
There is currently intense interest in the construction, characterization, and properties of highly ordered monolayers formed on solid supports using self-assembly and spontaneous adsorp-
(1) (a) Ulman, A. An Introduction to Ultrathin Organic Films: From LangmuirBlodgett to Self-Assembly; Academic Press: London, UK, 1991. (b) Murray, R. W. In Molecular Design of Electrode Surfaces; Murray, R. W., Ed.; Wiley: New York, 1992; Chapter 1. (c) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (d) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (e) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682. (f) Finklea, H. O.; Snider, D. A.; Fedik, J. Langmuir 1990, 6, 371. (g) Chidsey, C. E. D. Science 1991, 251, 919. (h) Hickman, J. J.; Ofer, D.; Zhou, C.; Wrighton, M. S.; Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1991, 113, 1128. (i) Duevel, R. V.; Corn, R. M. Anal. Chem. 1992, 64, 337. (2) Sackmann, E. Science 1996, 271, 43. (3) (a) Forster, R. J.; Faulkner, L. R. J. Am. Chem. Soc. 1994, 116, 5444. (b) Forster, R. J.; Faulkner, L. R. J. Am. Chem. Soc. 1994, 116, 5453. (c) Forster, R. J.; O’Kelly, J. P. J. Phys. Chem. 1996, 100, 3695. (d) Cheng, J.; Sa`ghiSzabo´, G.; Tossell, J. A.; Miller, C. J. J. Am. Chem. Soc. 1996, 118, 680. (e) Li, T. T.-T.; Weaver, M. J. J. Am. Chem. Soc. 1984, 106, 6107. (f) Rowe, G. K.; Creager, S. E. Langmuir 1991, 2307. (g) Finklea, H. O.; Hanshew, D. D. J. Am. Chem. Soc. 1992, 114, 3173. (4) Meyer, T. J. J. Electrochem. Soc. 1984, 131, 221C. (5) Blaho, J. K.; Goldsby, K. A. J. Am. Chem. Soc. 1990, 112, 6132. (6) (a) Function of Quinones in Energy Conserving Systems; Trumpower, B. L., Ed.; Academic Press, New York, 1982. (b) Dryhurst, G.; Kadish, K. M.; Scheller, F.; Renneberg, R. Biological Electrochemistry; Academic Press, London, 1982; Vol. 1, Chapter 1.
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© 1996 American Chemical Society
lecular assemblies may provide an opportunity to probe the effect of potential on the strength of adsorption without the added complication of changing electrostatic effects.8 Voltammetry provides a powerful insight into the effect of the applied potential on the surface coverage, the free energy of adsorption, and the associated kinetics for electroactive films that form on electrode surfaces by irreversible adsorption. However, when adsorption is reversible, one cannot use the traditional approach to measuring the effect of the deposition potential on the coverage, i.e., immersing a clean electrode in a deposition solution under potential control, followed by ex situ measurement of the surface coverage in a blank electrolyte solution. Here, we consider how time-resolved electroanalysis7a,9 can be used to investigate the electron transfer dynamics and the potential dependence of the free energy of adsorption for 2-hydroxyanthraquinone (2OH-AQ) monolayers that adsorb reversibly onto mercury microelectrodes. The advent of microelectrodes and high-speed instrumentation makes it possible to separate electrochemical processes, such as double-layer charging and heterogeneous electron transfer, that occur on a microsecond time scale.10 For example, the short time scale approach employed here allows the interfacial capacitance to be measured even at potentials where the monolayers are redox active. Capacitance data have been used to probe the effect of the applied potential on the adsorption thermodynamics by measuring Cdl as the concentration of 2OH-AQ in solution is systematically varied. When the surface coverage is measured at the formal potential E°′ of the quinone/hydroquinone reaction, the values of the saturation surface coverage Γs and the adsorption parameter β obtained from capacitance data are identical to those measured voltammetrically. This observation suggests that the modified interface is described by a parallel capacitor model, at least at E°′. Capacitance measurements have been used to determine the adsorption isotherms as the deposition potential is systematically varied from -0.400 to +0.300 V. In this way, the potential dependence of the free energy of adsorption ∆Gads has been probed, thus providing an insight into the differences in hydrogen bonding that exist between 2OH-AQ and trihydroxyanthracene monolayers. EXPERIMENTAL SECTION Apparatus and Procedures. The preparation of hemispherical mercury microelectrodes, as well as the electrochemical instrumentation and procedures employed, was described previously.7a Materials. 2-Hydroxyanthraquinone (2OH-AQ) was obtained from Aldrich Chemical Co. It was recrystallized from purified water three times using decolorizing charcoal. The recrystalli(7) (a) Forster, R. J. Langmuir 1995, 11, 2247. (b) Chambers, J. Q In The Chemistry of Quinonoid Compounds Vol. II; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1988; Chapter 12. (c) Inoue, H.; Hida, M. Bull. Chem. Soc. Jpn. 1982, 55, 1880. (d) Keita, B.; Nadjo, L. J. Electroanal. Chem. 1984, 163, 171. (e) He, P.; Crooks, R. M.; Faulkner, L. R. J. Phys. Chem. 1990, 94, 1135. (f) Zhang, J.; Anson, F. C. J. Electroanal. Chem. 1992, 331, 945. (g) Soriaga, M. P.; Hubbard, A. T. J. Am. Chem. Soc. 1982, 104, 3937. (h) Brown, A. P.; Anson, F. C. J. Electroanal. Chem. 1978, 92, 133. (i) Gamage, R. S. K. A.; McQuillan, A. J.; Peake, B. M. J. Chem. Soc., Faraday Trans. 1991, 87, 3653. (8) Bretz, R. L.; Abrun ˜a, H. D. J. Electroanal. Chem. 1995, 388, 123. (9) (a) Forster, R. J.; Faulkner, L. R. Anal. Chem. 1995, 67, 1232. (b) Forster, R. J. The Analyst 1996, 121, 733. (10) Forster, R. Chem. Soc. Rev. 1994, 23, 289.
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Figure 1. Cyclic voltammograms (solid lines) for a 20 µm radius mercury microelectrode immersed in a 10 µM solution of 2OH-AQ in 1.0 M HClO4. Scan rates, from top to bottom, are 50, 20, 10, and 5 V/s. The current scale is on the right-hand side. The dashed line is the cyclic voltammogram observed for the same electrode immersed in a 10 mM solution of 2OH-AQ. The scan rate is 50 V/s. The current scale is on the left-hand side. In all cases, cathodic currents are up and anodic currents are down. The initial potential is +0.250 V.
zation and subsequent storage were carried out in the dark to avoid photochemical decomposition. RESULTS AND DISCUSSION General Electrochemical Properties. Figure 1 shows representative cyclic voltammograms for a 20 µm radius mercury microelectrode immersed in a 10 µM solution of 2OH-AQ in 1.0 M HClO4 as the scan rate is systematically varied from 5 to 50 V/s. For scan rates below 10 V/s, the voltammetric response obtained is consistent in all respects with that expected for an electrochemically reversible reaction involving a surface-confined species.11 For example, the peak shapes are independent of scan rate, and the peak height scales linearly with the scan rate ν, unlike the ν1/2 dependence expected for a freely diffusing species.12 Therefore, it appears that the anthraquinone adsorbs onto the surface of the mercury microelectrode to give an electroactive film. Figure 1 also shows the cyclic voltammogram observed at 50 V/s when the 2OH-AQ concentration is increased to 10 mM. With this high bulk concentration of anthraquinone, the currents observed for reduction/oxidation of the solution phase species are much larger than those found for the surface-confined species. Therefore, information about the formal potential of the freely diffusing anthraquinone can be obtained. Figure 1 shows that the formal potentials for the monolayer and solution phase species are approximately +0.05 and -0.10 V, respectively, where the supporting electrolyte is 1.0 M HClO4. That E°′ shifts in a positive potential direction by ∼0.15 V upon surface confinement indicates that the reduced form of 2OH-AQ is more strongly adsorbed than the oxidized form by ∼15 kJ mol-1. In a later section, we consider how capacitance measurements can be used to probe the potential dependence of the free energy of adsorption at potentials where the monolayer is not redox active. (11) (a) Laviron, E. J. Electroanal. Chem. 1974, 52, 395. (b) Brown, A. P.; Anson, F. C. Anal. Chem. 1977, 49, 1589. (12) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; Wiley: New York, 1980.
Scheme 1
To probe the number of electrons transferred in the quinone/ hydroquinone reaction, controlled potential bulk electrolysis of a 1 mM solution of 2OH-AQ was performed at -0.500 V. The electrolysis was terminated when the current flow was reduced to 0.995). Modeling the experimental data using a Frumkin isotherm,12,17 which considers adsorbate-adsorbate interactions, gave interaction parameters that were close to zero. This observation suggests that concentration-dependent attractions or repulsions between adsorbates exert little influence over the thermodynamics of adsorption. The value of Γs is (1.0 ( 0.08) × 10-10 mol cm-2, which corresponds to an average area of occupation per molecule of ∼185 Å2. This value is consistent with the range found by Faulkner and co-workers7e for monolayer coverages of 2,6-anthraquinone disulfonic acid (190-210 Å2) but is distinctly larger than the 126-138 Å2 indicated by the work of Soriaga and Hubbard for a flat orientation of 1,5and 2,6-anthraquinone disulfonic acid on platinum.14b This difference in the area of occupation has been interpreted previously in terms of differences in the surface bonding at platinum and mercury.14a The adsorption coefficient for this system is (6.3 ( 0.6) × 106 M-1. This value is similar to that reported by us and others for the adsorption of anthraquinonedisulfonic acids7a,e (7.1 × 106 M-1), and its magnitude confirms that 2OH-AQ is strongly adsorbed onto mercury. Chronoamperometry. For an ideal electrochemical reaction involving a surface-bound species, the Faradaic current following a potential step that changes the redox composition of the 3146
Analytical Chemistry, Vol. 68, No. 18, September 15, 1996
(2)
where k is the apparent rate constant for the overall reaction and Q is the total charge passed during exhaustive electrolysis of the monolayer, equal to nFAΓs, where n is the number of electrons transferred, F is Faraday’s constant, and A is the electrode area. Figure 4 shows the current response for a 20 µm radius mercury microelectrode modified with a dense 2OH-AQ monolayer following a potential step from -0.400 V to +0.150 V. This potential step corresponds to an overpotential (η ≡ E - E°′) of 100 mV. Figure 4 shows that two current decays, corresponding to double-layer charging and Faradaic current flow, respectively, are separated on a 100 µs time scale. These two processes are time-resolved because the time constant for double-layer charging is much smaller than that of the Faradaic reaction.12 In this study, the electron transfer dynamics are probed only under those circumstances where the time constant of double-layer charging is at most one-fifth that of the Faradaic reaction. This condition has been satisfied by selecting the radius of the mercury microelectrode. While fast charging of the electrochemical double-layer is undoubtedly important, the effects of ohmic losses must also be considered.12 When Faradaic and charging currents flow through a solution, they generate a potential that acts to reduce the applied potential by an amount iR, where i is the total current that flows through the solution. This ohmic drop can lead to severe distortions of experimental responses, resulting in inaccurate measurements of the heterogeneous electron transfer rate. As illustrated in Figure 4, the Faradaic currents that flow in these high-speed chronoamperometric experiments are typically in the low microampere range. Given that the cell resistance in this experiment is ∼3000 Ω, the average iR drop is less than 30 mV for this system. However, the situation regarding ohmic effects is much more demanding when large overpotentials are employed because larger Faradaic currents flow at short times. For example, the Faradaic currents observed at a 20 µm radius microelectrode at short times in a potential step experiment employing a 400 mV overpotential would of the order of 100 µA,
resulting in massive ohmic losses. We have used three strategies to minimize the effects of uncompensated resistance in the largeamplitude potential step experiments. First, we use a relatively high supporting electrolyte concentration (g0.2 M). Second, we use smaller electrodes to measure k at large overpotentials. This approach is useful, since the resistance increases with decreasing electrode radius, but the current decreases as the square of the radius, leading to reduced ohmic effects for smaller electrodes.10 Third, and perhaps most importantly, we extract rate constants only from data obtained relatively late in the lifetime of the current transient, i.e., when the anticipated iR drop is
