Langmuir 1998, 14, 2129-2133
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Enhanced Barrier Properties of Alkanethiol-Coated Gold Electrodes by 1-Octanol in Solution Marla French and Stephen E. Creager* Department of Chemistry, Clemson University, Clemson, South Carolina 29634 Received November 10, 1997. In Final Form: January 14, 1998
The barrier properties of alkanethiol-coated gold electrodes are dramatically enhanced when the surfactant 1-octanol is present in the aqueous electrolyte solution contacting the electrode. Gold oxidation on the electrode surface and ferrocyanide oxidation in solution are suppressed to a positive potential limit of +1.6 V vs Ag/AgCl in an octanol-saturated pH 5 buffer at a dodecanethiol-coated gold electrode. Hydroxymethylferrocene oxidation in solution is also suppressed, though to a lesser extent. The enhanced barrier properties are thought to be caused by a thin layer of octanol atop the alkanethiol monolayer. The octanol fills in defects in the alkanethiol monolayer and increases the overall thickness of the barrier layer, thereby inhibiting reactions at defects and forcing electron transfer to occur over long distances across the barrier layer. The beneficial effect of octanol is obtained only when the monolayer surface is hydrophobic (e.g., at an alkanethiol monolayer) and is absent when the monolayer surface is hydrophilic and/or charged (e.g., at a mercaptoundecanoic acid monolayer in a basic buffer solution).
Introduction Barrier layers on electrodes can suppress background currents, control adsorption, reduce double-layer capacitance, inhibit corrosion, and generally enable control of electrode processes.1-3 These factors can be important in fundamental studies of electrochemical reactivity and in the design of electrochemical sensors, detectors, and other devices.4-12 Alkanethiolate self-assembled monolayers (SAMs) are particularly attractive for making barrier layers on gold electrodes because of their ease of formation, their flexibility regarding introduction of new chemical functionalities onto electrode surfaces, and their good physical, chemical, and electrochemical stability. Many prior studies of alkanethiol monolayers on gold have been made, and several recent reviews have been published.13-17 * Corresponding author. Telephone, 864-656-4995; fax, 864-6566613, e-mail,
[email protected]. (1) Finklea, H. O. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; Vol. 19, pp 109335. (2) Bard, A. J.; Abruna, H. D.; Chidsey, C. E.; Faulkner, L. R.; Feldberg, S. W.; Itaya, K.; Majda, M.; Melroy, O.; Murray, R. W.; Porter, M. D.; Soriaga, M. P.; White, H. S. J. Phys. Chem. 1993, 97, 7147-7173. (3) Murray, R. W. Molecular Design of Electrode Surfaces; Techniques of Chemistry Series; Murray, R. W., Ed.; John Wiley & Sons: New York, 1992; Vol. 22, p 427. (4) Murray, R. W.; Ewing, A. G.; Durst, R. A. Anal. Chem. 1987, 59, 379A. (5) Zhong, C. J.; Porter, M. D. Anal. Chem. 1995, 67, 7, A709. (6) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988, 332, 426-429. (7) Turyan, I.; Mandler, D. Anal. Chem. 1997, 69, 894-897. (8) Chailapakul, O.; Crooks, R. M. Langmuir 1993, 9, 884-888. (9) Wang, J.; Wu, H.; Angnes, L. Anal. Chem. 1993, 65, 1893-1896. (10) Quan, C.; Brajter-Toth, A. Anal. Chem. 1992, 64, 1998-2000. (11) Takehara, K.; Takemura, H.; Aihara, M.; Yoshimura, M. J. Electroanal. Chem. 1996, 404, 179-182. (12) Creager, S. E.; Olsen, K. G. Anal. Chim. Acta 1995, 307, 277289. (13) Ulman, A. Chem. Rev. 1996, 96, 1533. (14) Ulman, A. Characterization of Organic Thin Films; Ulman, A., Ed.; Butterworth-Heinemann: Boston, 1995. (15) Ulman, A. An Introduction to Ultrathin Organic Films. From Langmuir-Blodgett to Self-Assembly; Academic: San Diego, CA, 1991. (16) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437-463. (17) Bain, C. D.; Whitesides, G. M. Angew. Chem., Int. Ed. Eng. 1989, 28, 506-512.
Defects are very important in controlling barrier properties at monolayer-coated electrodes.18,19 Redoxactive solutes can directly approach the electrode at defect sites larger than the size of the solutes and even at smaller defect sites; for examples, step edges and crystal grain boundaries in the underlying gold, and tilt domain boundaries and other minor packing defects within the monolayer. Solutes may approach the electrode more closely at such sites than they would at a region where the monolayer is highly ordered and free of defects. Also, defects can serve as nucleation sites for chemical and/or electrochemical degradation of the monolayer. In all cases, monolayer barrier properties are at least partially compromised by defects. Unfortunately, the behavior of defective monolayers is often erratic and irreproducible, which makes the behavior of monolayer-coated electrodes difficult to predict and control. In this paper, we present work on the barrier properties of alkanethiol monolayers on gold electrodes, with particular emphasis on the capacity of the monolayers to inhibit oxidation of the underlying gold electrode and that of two model redox-active solutes, ferrocyanide and hydroxymethylferrocene. Barrier properties were studied in the absence and presence of the neutral surface-active agent 1-octanol (C8H17OH). Octanol dramatically improves the barrier properties of the monolayers, enabling application of positive potentials up to +1.6 V vs Ag/AgCl at a dodecanethiol-coated gold electrode without damaging the monolayer. It is postulated that the octanol fills in defect sites in the alkanethiolate monolayer and forms an ultrathin liquid film on top of the monolayer and that it is the combination of the underlying alkanethiol monolayer and the octanol overlayer that is responsible for the unusually good barrier properties. Experimental Section Materials. Sodium ferrocyanide decahydrate (Fluka), hydroxymethylferrocene (STREM Chemicals), sodium acetate (18) Finklea, H. O.; Snider, D. A.; Fedyk, J.; Sabatani, E.; Gafni, Y.; Rubinstein, I. Langmuir 1993, 9, 3660-3667. (19) Finklea, H. O.; Avery, S.; Lynch, M.; Furtsch, T. Langmuir 1987, 3, 409-413.
S0743-7463(97)01226-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/19/1998
2130 Langmuir, Vol. 14, No. 8, 1998 anhydrous (J. T. Baker Inc.), sodium perchlorate anhydrous (Mallinckrodt), sodium phosphate monobasic (Mallinckrodt) and dibasic (Fisher), glacial acetic acid (Fisher), absolute ethanol (AAPER Alcohol and Chemical Company), 1-octanol (ACROS Organics), and all of the n-alkanethiols (Aldrich) were used as received from their respective manufacturers. 11-Mercaptoundecanoic acid was synthesized from 11-bromoundecanoic acid using thiourea as described previously.20 A Barnstead Nanopure system was used to purify water for all aqueous solutions. Working electrodes were constructed from gold wires (1 mm diameter, 99.9%, Alfa), sealed in EPON 825 epoxy resin (Shell) cured with 1,3-phenylenediamine (Aldrich) by a previously described procedure.21 Alumina (25, 5, 1, 0.3, and 0.05 mm) used in polishing the gold electrodes was purchased from Buehler. The concentrated acids used in preparing the dilute aqua regia (1:3:4 HNO3:HCl: H2O) for etching the electrodes were purchased from Fisher. Porous Vycor glass used in the preparation of reference electrodes was obtained from Bioanalytical Systems. Monolayer Preparation. After polishing, the gold working electrodes were etched in dilute aqua regia (1:3:4 HNO3:HCl: H2O) for 60 s to remove surface contamination and damaged surface layers left after polishing and thereby expose a fresh, clean, stable gold surface onto which the monolayer could be formed.22,23 Electrodes were then rinsed with deionized water followed by 2-propanol, and placed into a 1.0 mM solution of alkanethiol in ethanol. Monolayer formation was allowed to proceed for 20-24 h. Electrochemical Experiments. Electrochemical experiments were conducted in a three-electrode cell using a EG&G PAR model 362 scanning potentiostat with a double-junctioned Ag/AgCl/saturated KCl reference electrode and a Pt wire auxiliary electrode, with N2(g) sparging of the solution to remove dissolved oxygen. A 0.10 M sodium ferrocyanide solution was prepared by adding a measured quantity of solid sodium ferrocyanide to 10 mL of either a pH 5 buffered solution (0.10 M sodium perchlorate, 0.10 M acetic acid, and 0.18 M sodium acetate) or a pH 7 buffered solution (0.09 M sodium perchlorate, 0.01 M sodium phosphate monobasic, 0.02 M sodium phosphate dibasic) in a 25-mL threenecked pear-shaped flask. Similarly, 10 mL of 1.0 mM hydroxymethylferrocene was prepared by adding solid hydroxymethylferrocene to a buffered electrolyte solution. Octanolsaturated electrolytes were prepared by adding octanol in a dropwise manner to a measured volume of solution until no more would dissolve. This procedure usually required only a few drops for a 5-10-mL solution aliquot. (The saturation concentration of 1-octanol in water is 0.59 mg mL-1 at 25 °C24; presumably the electrolyte solutions used in this work are saturated to a concentration not greatly different from this.) Half-saturated solutions were prepared by dilution of saturated solutions (after removal of excess octanol) with octanol-free solutions. These solutions were used in some experiments to ensure that solutions were free from bulk-phase octanol, which we thought might form a “puddle” on the electrode and give spurious results.
French and Creager
Figure 1. Effect of barrier layers on the background voltammetry (0.2 V s-1) of monolayer-coated gold electrodes in pH 5 buffer solution. Top: Voltammetry at dodecanethiol-coated gold in buffer without octanol. Bottom: Same voltammetry at an identical monolayer-coated electrode in buffer that is halfsaturated with 1-octanol.
Gold Oxidation/Gold Oxide Reduction. Figure 1 illustrates the effect of the 1-octanol surfactant on the background cyclic voltammetry at a dodecanethiol-coated gold electrode. In the absence of octanol (Figure 1, top), the voltammetry at the coated electrode exhibits features associated with gold oxidation and oxide reduction that grow with increasing number of scans. The size of these features is ∼1% of what it would be at an uncoated electrode; however, the fact that they are there at all indicates that there are regions on the electrode (probably defect sites in the monolayer) where the underlying gold
is in direct contact with the electrolyte solution. On continued scanning, the features increase in size until eventually the bare electrode behavior is obtained. This behavior is similar to that described in many literature reports on the oxidative electrochemistry of alkanethiolate-coated gold electrodes.18,19,21,25-28 The bottom voltammogram in Figure 1 was obtained under identical conditions except that the electrolyte solution was 50% saturated with 1-octanol. The features associated with gold oxidation and oxide reduction are completely absent, even after multiple scans between 0 and +1.6 V versus the Ag/AgCl reference. The absence of features for gold oxidation and oxide reduction at potentials where they would be readily observed at a bare gold electrode indicates that there are now no regions where bare gold is directly exposed to the solution. The dodecanethiol monolayer and the 1-octanol surfactant have combined to generate a barrier layer on the electrode that is completely free of large, gross defects where gold is exposed to solution. Ferrocyanide Oxidation. The effectiveness of the barrier layers may also be assessed by examining the degree to which they can inhibit the electrode reactions of redox-active solutes. Figure 2 illustrates this behavior for oxidation of the model redox probe ferrocyanide. The top voltammogram was obtained at a bare gold electrode and exhibits all of the features expected for a chemically reversible redox reaction with E° ) +0.220 V. The small increase in current near +1.2 V on the forward scan corresponding to the onset of gold oxidation is barely visible on this current scale; however, the return peak for gold oxide reduction at +0.6 V is clearly visible. (Qualitatively similar behavior is seen for ferrocyanide oxidation at bare gold in octanol-containing solutions.) The next voltammogram was acquired in the same solution but at an electrode that was coated with a dodecanethiol monolayer.
(20) Creager, S. E.; Clarke, J. Langmuir 1994, 10, 3675-3683. (21) Groat, K. A.; Creager, S. E. Langmuir 1993, 9, 3668-3674. (22) Creager, S. E.; Hockett, L. A.; Rowe, G. K. Langmuir 1992, 8, 854-861. (23) Guo, L. H.; Facci, J. S.; Mclendon, G.; Mosher, R. Langmuir 1994, 10, 4588-4593. (24) Stephen, H.; Stephen, T. Solubilities of Inorganic and Organic Compounds; MacMillan: New York, 1963.
(25) Everett, W. R.; Welch, T. L.; Reed, L.; Fritschfaules, I. Anal. Chem. 1995, 67, 292-298. (26) Sondaghuethorst, J. A. M.; Fokkink, L. G. J. Langmuir 1995, 11, 2237. (27) Yang, D. F.; AlMaznai, H.; Morin, M. J. Phys. Chem. B 1997, 101, 1158-1166. (28) Sabatani, E.; Rubinstein, I. J. Phys. Chem. 1987, 91, 66636669.
Results and Discussion
Surfactant-Enhanced Barrier Properties of Au Electrodes
Figure 2. Effect of barrier layers on ferrocyanide oxidation ([Fe(CN)64-] ) 0.10 M) at monolayer-coated gold electrodes in pH 5 buffer. Top: Voltammetry at an uncoated gold electrode in octanol-free buffer. Next: A single voltammetric scan at a dodecanethiol-coated electrode in octanol-free buffer. Next: Voltammetry at an identical dodecanethiol-coated gold electrode in buffer that is half-saturated in octanol. Bottom: Same voltammetry as just mentioned, with the positive limit extended from +1.6 V to +1.8 V. All voltammograms were recorded at 0.2 V s-1.
The peak corresponding to ferrocyanide oxidation has shifted from approximately +0.3 V at the bare electrode to approximately +1.1 V at the monolayer-coated electrode, indicating that the monolayer acts as a barrier layer restricting close approach of ferrocyanide ions to the electrode and forcing oxidation to occur by a long-range electron-transfer process across the monolayer and/or via electron transfer at structural defect sites (“pinholes”) in the monolayer.19,29,30 The return scan in this voltammogram exhibits a very unusual “crossing” phenomenon whereby the oxidative current remains high on the return scan even at potentials where oxidation was suppressed on the forward scan. This behavior is expected only in those special cases where something happens on the forward scan that enables an electrode reaction to occur that was initially suppressed. We postulate that in this case, the scan to positive potentials has either generated defect sites in the dodecanethiol monolayer or has caused existing defect sites to grow, thereby generating an array of ultramicroelectrodes and compromising the monolayer barrier properties.18 (29) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568. (30) Becka, A. M.; Miller, C. J. J. Phys. Chem. 1993, 97, 6233-6239.
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Indeed, if a second forward scan was initiated in Figure 2B following the scan shown, it would look quite similar to the bare electrode response, indicating that the monolayer barrier properties have been irreversibly compromised. The next voltammogram in Figure 2 (third from top) was acquired under identical conditions to those just mentioned except that the electrolyte solution was 50% saturated with 1-octanol. The octanol causes a remarkable suppression of the ferrocyanide oxidation current all the way out to +1.6 V (an overpotential of approximately +1.3 V). This suppression is not the result of a permanent passivation of the electrode surface because when the potential is scanned to +1.8 V (bottom voltammogram in Figure 2), the current sharply increases at the positive limit, and on continued scanning, the reversible response for the ferrocyanide/ferricyanide redox couple is slowly recovered. Thus, the barrier layer properties can be compromised if a very positive potential is applied. Even so, the suppression of gold oxidation, ferrocyanide oxidation, and oxidative defect formation in the barrier layer out to +1.6 V versus Ag/AgCl in an aqueous electrolyte is remarkable and to our knowledge unprecedented for a monolayer-coated electrode. The properties of the combined alkanethiol/1-octanol barrier layers depend critically on the chain length of the alkanethiol. Figure 3 illustrates this dependence for electrodes coated with dodecanethiol, decanethiol, and octanethiol monolayers, all of which are in contact with ferrocyanide-containing electrolyte solutions that are saturated with 1-octanol. The general trend is that ferrocyanide oxidation and irreversible monolayer disruption both occur at progressively less positive potentials as alkanethiol chain length decreases. The dodecanethiol, decanethiol, and octanethiol monolayers retained good barrier properties out to +1.6, +1.3, and +1.0 V, respectively; however, in each case, increasing the positive potential limit by 0.2 V resulted in irreversible monolayer disruption and loss of barrier properties. The fact that the barrier properties depend on monolayer chain length indicates that the octanol alone is not responsible for controlling the overall barrier properties. We believe that the enhancement of monolayer barrier properties by octanol is caused by the presence of an ultrathin octanol layer on top of the alkanethiol monolayer. Scheme 1 illustrates the structure that we envision for such a surface layer. A similar phenomenon was previously studied by Creager and Rowe31 who used voltammetry and interfacial capacitance measurements to study ferrocene-containing alkanethiol monolayers on gold in contact with electrolytes containing aliphatic alcohols, and also by Bain and co-workers32,33 who used sum-frequency spectroscopy to study decanol and dodecyl sulfate aggregation onto hexadecanethiol monolayers on gold. Longchain alcohols are known to be surface-active in water,34 so it seems plausible that they would also tend to aggregate at a hydrophobic solid surface in water. Similar aggregation phenomena of alcohols on low-energy surfaces have been reported by Miller and co-workers35 and Goss and co-workers36 in their work on the effect of alcohols on the (31) Creager, S. E.; Rowe, G. K. Langmuir 1993, 9, 2330-2336. (32) Ward, R. N.; Davies, P. B.; Bain, C. D. J. Phys. Chem. B 1997, 101, 1594-1601. (33) Bain, C. D.; Davies, P. B.; Ward, R. N. Langmuir 1994, 10, 20602063. (34) Hommelen, J. R. J. Colloid Sci. 1959, 14, 385. (35) Miller, C. J.; Widrig, C. A.; Charych, D. H.; Majda, M. J. Phys. Chem. 1988, 92, 1928-36. (36) Goss, C. A.; Miller, C. J.; Majda, M. J. Phys. Chem. 1988, 92, 1937-1942.
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French and Creager
Figure 4. Effect of monolayer hydrophobicity on barrier properties. Top: electrode is coated with a 11-mercaptoundecanoic acid monolayer and studied in a pH 7 electrolyte that contains 0.1 M Fe(CN)64- and is saturated in 1-octanol. Bottom: electrode is coated with a decanethiol monolayer and is studied in a pH 5 electrolyte that also contains 0.10 M Fe(CN)64- and is saturated in 1-octanol.
Figure 3. Alkanethiol chain length effects on the barrier properties of monolayer-coated electrodes. The electrolyte contains 0.10 M Fe(CN)64- and is saturated in 1-octanol. The top pair of voltammograms is for an electrode coated with a 1-octanethiol monolayer at positive potential limits of +1.0 V and +1.2 V. The center pair is for an electrode coated with 1-decanethiol monolayer scanned to +1.3 V and +1.5 V. The bottom pair is for an electrode coated with a 1-dodecanethiol monolayer scanned to +1.6 V and +1.8 V. Scheme 1
lateral mobility of surfactants on silanized metal oxide surfaces, and also by Barton and co-workers37 in their work on alcohol aggregation in surfactant adlayers on liquid chromatography stationary phases. We believe that the alcohol overlayer should fill in defect sites in the underlying monolayer and provide added overall thickness to the surface layer. Both of these effects should contribute to the observed improvement in monolayer barrier properties in the presence of octanol. Further support for this physical picture of the barrier layer comes from experiments with monolayers in which (37) Barton, J. W.; Fitzgerald, T. P.; Lee, C.; O’Rear, E. A.; Harwell, J. H. Sep. Sci. Technol. 1988, 23, 637-660.
charged, hydrophilic surfaces are exposed to the aqueous electrolytes containing 1-octanol and ferrocyanide. The top voltammogram in Figure 4 is for a monolayer of 11mercaptoundecanoic acid in an electrolyte buffered at pH 7, and the bottom voltammogram is for a monolayer of decanethiol in an electrolyte buffered at pH 5. Both electrolytes contain 0.1 M sodium ferrocyanide and are saturated in octanol. The carboxylic-acid-containing monolayer is hydrophilic to begin with and, at pH 7, it should also be at least partially deprotonated and therefore negatively charged.38 One might expect that this negative charge, when combined with the effect of octanol in filling in defect sites, would improve suppression of ferrocyanide oxidation because ferrocyanide is a tetra-anion and should be electrostatically repelled from the negatively charged electrode surface. Such improvement was not observed; rather, the decanethiol monolayer is much more effective than the mercaptoundecanoic acid monolayer in suppressing ferrocyanide oxidation in the presence octanol. We believe that this result is obtained because octanol has only a very weak affinity for the negatively charged monolayer surface and therefore the thin octanol layer that forms on the hydrophobic monolayer is not formed on the charged monolayer. Apparently, a thin octanol layer on top of an alkanethiol monolayer is more effective than a high negative surface charge density in suppressing ferrocyanide oxidation. Hydroxymethylferrocene Oxidation. The properties of molecular barrier layers on electrodes can depend on the nature of the redox molecule that is used to probe the barrier. We therefore sought to broaden the scope of this study be examining a second redox-active solute, hydroxymethylferrocene, that is very different in character from ferrocyanide. Hydroxymethylferrocene is nearly the same size as ferrocyanide; however, it is a neutral molecule that is only sparingly soluble in water but is soluble in nonpolar organic solvents such as 1-octanol and alkanes. If the high charge, good solubility in water, and poor solubility in alkanes of ferrocyanide are important to achieving efficient suppression of ferrocyanide oxidation, then we might expect hydroxymethylferrocene to behave differently. Figure 5 presents a series of voltammograms for hydroxymethylferrocene oxidation at a bare gold electrode, a dodecanethiol-coated gold electrode, and a dodecanethiolcoated gold electrode in contact with an octanol-saturated (38) Bain, C. D.; Whitesides, G. M. Langmuir 1989, 5, 1370-1378.
Surfactant-Enhanced Barrier Properties of Au Electrodes
Figure 5. Effect of barrier layers on hydroxymethylferrocene (1 mM) oxidation at monolayer-coated electrodes in pH 5 buffer. Top: Voltammetry at an uncoated gold in octanol-free buffer. Middle: Voltammetry at a dodecanethiol-coated electrode in octanol-free buffer. Bottom: Voltammetry at an identical dodecanethiol-coated gold electrode in buffer that is halfsaturated in octanol.
electrolyte. This sequence is nearly identical to the top three voltammograms in Figure 2, except that the redoxactive solute is now hydroxymethylferrocene instead of ferrocyanide. The top voltammogram shows the expected signature for reversible oxidation and rereduction of hydroxymethylferrocene at E° ) +0.235 V, as well as peaks for gold oxidation beginning near +1.0 V and oxide reduction near +0.6 V. (These peaks are more prominent here than in Figure 2 because of the higher current sensitivity, which in turn was used because of the much lower concentration of hydroxymethylferrocene compared to ferrocyanide.) At the dodecanethiol-coated electrode, the peak for hydroxymethylferrocene oxidation has shifted slightly from approximately +0.26 V to +0.33 V. This shift is consistent with suppression of hydroxymethylferrocene oxidation by a barrier layer that forces electron transfer to occur over a long distance, however the effect is much smaller than that shown in Figure 2 for suppression of ferrocyanide oxidation by the same monolayer. Furthermore, addition of 1-octanol to the electrolyte solution does have the effect of further suppressing hydroxymethylferrocene oxidation; however, the effect is again much smaller than for suppression of ferrocyanide oxidation at the same dodecanethiol/1-octanol-coated electrode. We believe that there are two reasons for the very different behavior of ferrocyanide and hydroxymethyl-
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ferrocene at these monolayer-coated electrodes. First, as already suggested, the large negative charge and poor solubility of ferrocyanide in organic solvents probably make it easier for the alkanethiol/octanol layers to prevent close approach of ferrocyanide to the electrode. Hydroxymethylferrocene is neutral and highly soluble in organic solvents; therefore, it should have a greater affinity than ferrocyanide for the alkanethiol/octanol surface layers. Indeed, it may be possible for hydroxymethylferrocene to partition into the thin octanol layer atop the alkanethiol monolayer, thereby promoting electrooxidation. Second, the homogeneous electron self-exchange rate constant for ferrocene/ferricenium is 102 to 104 times higher than that for ferrocyanide/ferricyanide.39-45 This difference probably reflects a number of factors, most significantly the activation energies and the energies associated with forming the encounter complexes (work terms) in the two systems. All other factors being the same, hydroxymethylferrocene oxidation will be more difficult to suppress if the intrinsic redox kinetics for ferrocene oxidation are faster than for ferrocyanide oxidation. The marked difference in the behavior of these two simple, reversible, one-electron redox molecules at an electrode coated with a relatively simple molecular monolayer is suggestive of the degree of selectivity that could be achieved in the electrochemical response if the analytes and the surface layer are both carefully chosen. The present combination of a suitable pair of redox-active solutes with a highly selective modifying layer suggests some novel analytical applications, for example using one redox molecule to mediate the electrode reaction of another redox molecule in a controlled manner. These applications are being explored in ongoing work that will be reported separately. Acknowledgment. This work was partially supported with funds from the National Science Foundation, Grant # CHE 90616370, for which the authors are most grateful. LA971226R (39) McManis, G. E.; Nielson, R. M.; Gochev, A.; Weaver, M. J. J. Am. Chem. Soc. 1989, 111, 5533-5541. (40) Nielson, R. M.; McManis, G. E.; Safford, L. K.; Weaver, M. J. J. Phys. Chem. 1989, 93, 2152-2157. (41) Weaver, M. J.; Phelps, D. K.; Nielson, R. M.; Golovin, M. N.; McManis, G. E. J. Phys. Chem. 1990, 94, 2949-2954. (42) Yang, E. S.; Chan, M.-S.; Wahl, A. C. J. Phys. Chem. 1980, 84, 3094-3099. (43) Komarynsky, M. A.; Wahl, A. C. J. Phys. Chem. 1975, 79, 695699. (44) Wahl, A. C. Z. Elektrochem. 1960, 64, 90-93. (45) Shporer, M.; Ron, G.; Lowenstein, A.; Navon, G. Inorg. Chem. 1965, 4, 361-364.
