Barrier Properties of Organic Monolayers on Glassy Carbon Electrodes

Alison J. Downard* and Maureen J. Prince†. Department of Chemistry, University of Canterbury, Private Bag 4800,. Christchurch, New Zealand. Received...
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Langmuir 2001, 17, 5581-5586

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Barrier Properties of Organic Monolayers on Glassy Carbon Electrodes Alison J. Downard* and Maureen J. Prince† Department of Chemistry, University of Canterbury, Private Bag 4800, Christchurch, New Zealand Received April 3, 2001. In Final Form: May 29, 2001 The barrier properties of phenyl layers covalently attached to glassy carbon electrodes by the aryldiazonium reduction method have been interpreted using a model of electron transfer at defect sites and closely spaced microscopic pinholes. The surface coverage of phenyl groups determines the effective average thickness of the modifying layer which is most likely less than that of a closely packed monolayer of phenyl groups. The voltammetric responses of eight redox probes in aqueous media at polished and modified glassy carbon electrodes were examined in order to evaluate the barrier properties. With the exception of the MV+/0 couple, the films are much more blocking toward electron-transfer reactions of solution species than is predicted on the basis of the average film thickness. Comparisons of pairs of redox couples show that the electron-transfer kinetics of hydrophobic probes are slowed less than those of hydrophilic probes at the modified electrodes. This finding supports the notion that hydrophobic/hydrophilic interactions between solution species and the monolayer restricts the approach of redox probes to the monolayer surface, forcing electron transfer at the modified electrodes to occur over a distance significantly greater than that defined by the monolayer. Under experimental conditions where adsorption of MV+ and MV0 is not important, electrode modification has no apparent effect on the kinetics of the MV+/0 couple, indicating that these redox species interact closely with the phenyl layer.

Introduction Assemblies of thin organic films on electrode surfaces are widely used for fundamental studies of interfacial electron transfer and are also of interest for electroanalytical sensing systems. For both these applications, the barrier properties of the films are critically important. Studies of electron transfer require the film to function as an insulating spacer, maintaining a constant distance between the redox center and the electrode. Analytical applications require the film to provide selective barrier properties by accumulating the target analyte or repelling interferents or by allowing the target analyte to undergo electron transfer at a greater rate or at a potential different from that of interfering species. A large amount of work has focused on the barrier properties of alkanethiol self-assembled monolayers (SAMs) on gold electrodes,1 but much less is known about the barrier properties of organic monolayers on carbon electrodes. Recently, new methods have been developed which offer the opportunity to assemble a wide variety of monolayers on carbon surfaces.2 These methods involve formation of a covalent bond between the modifier and a surface carbon. In the most widely utilized procedure, reduction of an aryl diazonium salt at a carbon electrode leads to the generation of an aryl radical which couples with a surface carbon.3,4 Scheme 1 shows the proposed mechanism for this reaction. * To whom correspondence should be addressed. Telephone: 64-3-3642501. Fax: 64-3-3642110. E-mail: a.downard@ chem.canterbury.ac.nz. † Present address: Department of Chemistry, University of WisconsinsMilwaukee, Milwaukee, WI 53201-0413. (1) Finklea, H. O. In Electroanalytical Chemistry; Bard, A. J., Rubenstein, I., Eds.; Dekker: New York, 1996; Vol. 19, pp 109-335. (2) Downard, A. J. Electroanalysis 2000, 12, 1085-1096. (3) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Saveant, J.-M. J. Am. Chem. Soc. 1997, 119, 201-207. (4) Delamar, M.; Hitmi, R.; Pinson, J.; Saveant, J. M. J. Am. Chem. Soc. 1992, 114, 5883-5884.

Scheme 1

Several studies have investigated the influence of diazonium-derived monolayers on interfacial electrontransfer rates of solution species.5-13 Monolayers of 4-phenylacetate and 4-carboxyphenyl exhibit barrier properties that can be rationalized by considering the charge of the monolayer and redox probe.5,6,10 On the other hand, the barrier properties of 4-nitrophenyl-modified electrodes are difficult to understand, particularly when comparing results from different studies.5-9 Recent work indicates that different conditions used for film preparation probably accounts for the apparent variability in behavior.12 The kinetics of chlorpromazine oxidation have been investigated at GC electrodes modified with seven different neutral monolayers.13 A linear relationship between the natural log of the observed rate constant and monolayer thickness was obtained, where the monolayer thickness was calculated by assuming the phenyl rings are oriented perpendicular to the electrode surface. The linearity of the ln ko vs d plot was tentatively suggested to indicate compact monolayers providing a constant spacing between the GC surface and solution-based chlorpromazine. (5) Saby, C.; Ortiz, B.; Champagne, G. Y.; Belanger, D. Langmuir 1997, 13, 6805-6813. (6) Ortiz, B.; Saby, C.; Champagne, G. Y.; Belanger, D. J. Electroanal. Chem. 1998, 455, 75-81. (7) Chen, P.; McCreery, R. L. Anal. Chem. 1996, 68, 3958-3965. (8) DuVall, S. H.; McCreery, R. L. Anal. Chem. 1999, 71, 4594-4602. (9) DuVall, S. H.; McCreery, R. L. J. Am. Chem. Soc. 2000, 122, 6759-6764. (10) Downard, A. J.; Roddick, A. D.; Bond, A. M. Anal. Chim. Acta 1995, 317, 303-310. (11) Downard, A. J.; Roddick, A. D. Electroanalysis 1997, 9, 693698. (12) Downard, A. J. Langmuir 2000, 16, 9680-9682. (13) Yang, H.-H.; McCreery, R. L. Anal. Chem. 1999, 71, 4081-4087.

10.1021/la010499q CCC: $20.00 © 2001 American Chemical Society Published on Web 08/11/2001

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This paper reports a study of the barrier properties of a phenyl layer assembled on glassy carbon (GC) electrodes via reduction of benzenediazonium tetrafluoroborate. The degree of order of the phenyl monolayer and the monolayer thickness are examined, and eight redox probes are used to explore the barrier properties. A phenyl layer was chosen for this study because it should produce a very hydrophobic surface, thus amplifying any probe-monolayer hydrophobic interactions. A phenyl layer is also the thinnest barrier possible using the aryldiazonium modification strategy. Thin layers are particularly interesting because well-ordered monolayers cannot be assembled on gold electrodes using short-chain alkanethiols. Experimental Section Reagents. Acetonitrile (BDH, HiperSolv) was dried by passing through a column of activated neutral alumina (Merck). All other reagents were used as received. Chlorpromazine hydrochloride (CPZ) was supplied by Sigma; hydroxymethylferrocene (FcOH) and hexammineruthenium trichloride ([Ru(NH3)6]Cl3) were supplied by Strem Chemicals. Potassium hexachloroiridate (K2[IrCl6]) and 4-methylcatechol (4-MC) were from Aldrich and potassium ferrocyanide (K4[Fe(CN)6]) was from The British Drug Houses Ltd. Iron(II) trisbipyridal sulfate ([Fe(bpy)3]SO4) was prepared by published procedures.11 Methyl viologen diiodie ([MV]I2) was synthesized by stirring 4,4′-dipyridyl with a 5 M excess of methyl iodide in dry acetonitrile for 4 days at 22 °C. The orange precipitate was collected by filtration. Tetrabutylammonium tetrafluoroborate ([Bu4N]BF4) was prepared as described earlier.12 Aqueous solutions were prepared with triply distilled water and (with the exception of FcOH) ∼ 1 mM electroactive species. FcOH solutions were 0.5 mM. Electrolytes were analytical reagent grade with the exception of [Et4N]Cl (laboratory reagent grade). Synthesis of Benzenediazonium Tetrafluoroborate. Aniline (0.49 mL, 5 mmol) was added to 2 mL of 40% fluoroboric acid prediluted with 2 mL of water, and the solution was then cooled in an ice bath. A solution of 0.346 g (5 mmol) of sodium nitrite in 0.7 mL of water was added slowly to the stirred solution maintaining the temperature near 10 °C. The mixture was then cooled to below 0 °C in a salt/ice bath. The cream precipitate was collected on a sintered glass filter which had been cooled with ice water and washed with 1 mL volumes of cold 5% fluoroboric acid followed by ice-cold methanol and ether. The product was air-dried and stored at room temperature in the dark in a vacuum desiccator. Electrodes, Electrode Modification, and Electrochemical Procedures. Working electrodes were glassy carbon (GC) disks (area ) 0.07 cm2) fabricated from rods of Atomergic Chemetals VC-10 glassy carbon embedded in Teflon. Before use, electrodes were polished with 1 µM diamond paste and ultrasonicated in analytical reagent grade acetone for 3 min. The auxiliary electrode was a platinum wire, an SCE was used as reference in aqueous solutions, and a Ag/Ag+ (10-2 M AgNO3 in CH3CN-0.1 M [Bu4N]BF4) reference electrode was employed in acetonitrile solutions. All potentials in acetonitrile solution are reported vs the ferrocenium/ferrocene couple (Fc+/0) after comparison with in situ ferrocene. GC electrodes were modified in an oxygen-free dry acetonitrile solution of 5 mM phenyl diazonium tetrafluoroborate and 0.1 M [Bu4N]BF4. Unless stated otherwise, a potential of -1.1 V vs Fc+/0 was applied for 10 min. After modification the electrodes were ultrasonicated in analytical reagent grade acetone for 5 min, and the integrity of the coating was checked by recording a cyclic voltammogram of 1 mM [Ru(NH3)6]3+ in 0.2 M phosphate buffer, pH 7. Between measurements in different solutions, modified electrodes were rinsed with TDW and ultrasonicated in analytical reagent grade acetone for 3 min. Cyclic voltammograms were obtained with a PAR 173 potentiostat with a Model 273 interface, a Par Model 175 universal programmer, and a Graphtec WX1200 recorder. Solutions were purged with nitrogen gas prior to obtaining voltammograms. Cyclic voltammograms were recorded at a scan rate of 100 mV s-1.

Figure 1. Cyclic voltammograms (ν ) 100 mV s-1) recorded in aqueous solutions of (A) 1 mM [Ru(NH3)6]3+, 0.2 M KCl and (B) 0.5 mM FcOH, 0.2 M KCl at (a) polished GC; (b-d) GC modified in 5 mM benzenediazonium, 0.1 M [Bu4N]BF4-CH3CN for 10 min at Emod ) (b) -0.96 V, (c) -1.16 V, and (d) -1.36 V vs Fc+/0.

Results and Discussion Preparation and Verification of Modified Electrodes. To examine the selectivities of a barrier layer, it is necessary to modify the electrode with a layer that strongly blocks the electron-transfer reactions of some solution species but not others. Previously we have shown that the surface coverage and blocking properties of diazonium-derived surface layers depend on the potential used for film formation.12 Figure 1 shows cyclic voltammograms of Ru(NH3)63+ and hydroxymethylferrocene (FcOH) in aqueous solution (0.2 M KCl) at polished and phenyl-modified GC electrodes. The modified electrodes were prepared by 10 min electrolysis in a 5 mM solution of benzenediazonium tetrafluoroborate (CH3CN-0.1 M [Bu4N]BF4) at three different potentials. As the electrolysis potential is decreased from -0.96 to -1.36 V vs Fc+/0, the phenyl layer becomes more blocking toward both probe species. However, at each modified surface the response of Ru(NH3)63+ is attenuated significantly more than that of FcOH. For the present study where the selectivity properties of the layer are of interest, 10 min electrolysis at -1.1 V vs Fc+/0 in a 5 mM solution of the diazonium salt was used. Under these conditions, the barrier properties of the resultant monolayers have easily observed selectivity differences for these solution species. To ensure reproducibility of results, after each electrode modification the integrity of the monolayer was assessed by cyclic voltammograms (scan rate ) 100 mV s-1) of 1 mM Ru(NH3)63+ in 0.2 M phosphate buffer, pH 7. Voltammogram a of Figure 2 was obtained at a polished GC electrode, and voltammograms b and c at electrodes modified in benzenediazonium solution at Emod ) -1.1 V (two separate modifications). Voltammogram b shows very low currents even at quite negative potentials and is featureless over the region where oxidation of Ru(NH3)63+ occurs at the polished electrode. In contrast, voltammogram c shows a small sigmoidal response (E1/2 ∼ -0.3 V) at the potential of the [Ru(NH3)6]3+/2+ couple. The origin of the sigmoidal response was not investigated further, but only modified electrodes that gave a response similar to voltammogram b, were used for further measurements. Modification of GC electrodes to produce monolayers of the type shown in Figure 2b required very clean conditions (cell, electrodes, and solvent). Recrystallization of the diazonium salt was found not to influence the modification outcome, but the most reliable results were obtained with samples of the diazonium salt which were less than 3

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Figure 3. Schematic representation of disordered monolayers. Monolayer a has lower surface coverage and a smaller ATWM than monolayer b. Figure 2. Cyclic voltammograms (ν ) 100 mV s-1) recorded in 1 mM [Ru(NH3)6]3+, 0.2 M phosphate buffer, pH 7, at (a) polished GC; (b,c) GC modified in 5 mM benzenediazonium, 0.1 M [Bu4N]BF4-CH3CN for 10 min at Emod ) -1.1 V vs Fc+/0.

weeks old. These observations suggest that trace amounts of hydrophobic contaminants may adsorb on the polished electrode and prevent coupling of the phenyl radical to the carbon surface. When appropriate procedures were used, the success rate for modification was very high; however, the integrity of the monolayer was always verified by recording cyclic voltammograms of Ru(NH3)63+ as described above, prior to making other measurements. Mechanism of Electron Transfer at Phenyl-Modified Electrodes. Voltammograms b-d in Figure 1 and voltammogram b in Figure 2 show apparent decreases in standard heterogeneous rate constants at the modified electrodes compared with voltammograms recorded at polished electrodes. We now examine the origin of the voltammetric changes after electrode modification by considering the nature of the modifying layer. At one extreme the layer could be perfectly ordered and compact, forming a spacer of uniform thickness across which electron transfer occurs. A second possibility is that the layer is well-ordered but has microscopic pinholes with an average spacing which is small compared with the diffusion layer. The pinholes allow access of solution species directly to the electrode surface. A third possibility is that the monolayer is substantially disordered with microscopic pinholes and defects where the layer is thinner than the ideal thickness. At defect sites, solution species can approach more closely to the surface than at other areas but do not have direct access to the electrode surface. The voltammograms in Figure 1 strongly suggest that the phenyl monolayers are not compact and highly ordered. The standard heterogeneous rate constants for electron transfer for solution probes appear to decrease as the electrolysis potential for monolayer formation becomes more negative. In earlier work we demonstrated that at constant electrolysis time, surface coverages increase as the modification potential becomes more negative and at each potential film growth stops at submonolayer coverage.12 Hence, the voltammograms in Figure 1 show that standard heterogeneous rate constants appear to decrease as the surface coverage of phenyl groups increases. Considering that (with the possible exception of films formed at the most negative potentials) the surface coverages are submonolayer, the films cannot be compact and of uniform maximum thickness. In fact highly ordered monolayers would not be expected to form on GC which has a microscopically rough surface.

Considering the roughness of the GC surface, it is reasonable to assume that the phenyl monolayer is structurally disordered with defect sites of various “depths”. There are also likely to be areas of microscopic pinholes; however, for simplicity we assume only defects. Recently, Tong and co-workers have developed the concept of “average thickness of the whole monolayer” (ATWM) to describe the blocking behavior of structurally disordered SAMs on gold electrodes.14,15 They show that when there are defects of different depths with different fractional coverages, the apparent effect on the standard heterogeneous rate constant of a solution redox probe can be modeled by assuming an ATWM. Clearly the ATWM will be less than that of an ideal, defect-free monolayer. This concept is useful for the present work where the decrease in apparent standard heterogeneous rate constant as the surface coverage increases can be attributed to an increase in the ATWM. Figure 3 shows schematically how the ATWM could increase as the number of defect sites or their depth decreases. Using appropriate bond lengths, a thickness of ∼5.9 Å is calculated for a perfect monolayer of phenyl groups. Hence, we assume that, for the phenyl monolayers used for this study, the ATWM is less than 5.9 Å. The concept of ATWM can be usefully applied if the phenyl layers contain microscopic, closely spaced pinholes in addition to defects. However, simple quantitative treatment of electrode kinetics at the modified electrodes is not possible in this case. Voltammetry at Phenyl Monolayers. Redox probes for this study were chosen to give a range of hydrophobicity and molecular size.16,17 CPZ, FcOH, 4-MC, Fe(bpy)32+, and MV2+ are relatively hydrophobic compared with Ru(NH3)63+, IrCl62-, and Fe(CN)63-. The average diameters of Ru(NH3)63+, IrCl62-, and Fe(CN)63- are similar (∼6.0 ( 0.5 Å), and the hydrophobic species range in size from the relatively large Fe(bpy)32+ (diameter ∼ 14 Å) to the much smaller FcOH (diameter ∼ 4.5 Å) and 4-MC. Cyclic voltammograms were recorded for all redox probes in 0.2 M phosphate buffer, pH 7, at polished and phenyl-modified GC electrodes. Figure 4 shows the voltammograms obtained at 100 mV s-1 for Ru(NH3)63+, IrCl62-, Fe(CN)63-, 4-MC, CPZ, FcOH, and Fe(bpy)32+, and Figure 5A shows voltammograms for MV2+ . Inspection (14) Cui, X.; Jiang, D.; Diao, P.; Li, J.; Tong, R.; Wang, X. J. Electroanal. Chem. 1999, 470, 9-13. (15) Diao, P.; Guo, M.; Jiang, D.; Jia, Z.; Cui, X.; Gu, D.; Tong, R.; Zhong, B. J. Electroanal. Chem. 2000, 480, 59-63. (16) Slowinski, K.; Slowinska, K. U.; Majda, M. J. Phys. Chem. B 1999, 103, 8544-8551. (17) Belanger, S.; Stevenson, K. J.; Mudakha, S. A.; Hupp, J. T. Langmuir 1999, 15, 837-843.

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Downard and Prince Table 1. Cyclic Voltammetric Peak Potentials at Polished and Phenyl-Modified GC Electrodesa Ep/V vs SCEb redox probe

polished GC

modified GC |Ep(mod) - Ep(pol)|/V

Ru(NH3)63+ IrCl62Fe(CN)634-MC CPZ FcOH Fe(bpy)32+ MV2+

-0.31 0.63 0.09 0.23 0.62 0.19 0.83 -0.75, -1.09

c c c 1.2 1.2 0.6 1.2 -1.0, -1.09

>1.0 >1.3 >0.9 1.0 0.6 0.4 0.4 0.25, 0

a Cyclic voltammograms recorded in 0.2 M phosphate buffer, pH 7, at υ ) 100 mV s-1. b Peak potentials on the forward scan. cNo peak observed.

Figure 4. Cyclic voltammograms (ν ) 100 mV s-1) recorded in 0.2 M phosphate buffer, pH 7, of (A) Ru(NH3)63+, (B) IrCl62-, (C) Fe(CN)63-, (D) 4-MC, (E) CPZ, (F) FcOH, and (G) Fe(bpy)32+: (s) polished GC and (--) phenyl-modified GC (Emod ) -1.1 V vs Fc+/0).

are observed at modest overpotentials and there is a peak in each voltammogram. Table 1 lists the shifts in peak potentials on the forward scan for each probe species at the modified electrodes. Figure 5A shows results for voltammetry of MV2+ in phosphate buffer solution. At the polished electrode (curve a), there are the expected two stepwise reduction processes with some adsorption of the doubly reduced, neutral species on the electrode evidenced by oxidative stripping on the reverse scan. At the modified electrode (curve b), the cathodic peak for the first reduction step merges with that for the second reduction step and the anodic component appears at a less negative potential than at the polished electrode. However, the cathodic and anodic peak potentials for the MV+/0 couple are close to those at the polished electrode. When aqueous 0.2 M [Et4N]Cl is used as electrolyte (Figure 5B), adsorption is significantly reduced and it is apparent that both the peak currents and peak potentials for the MV+/0 couple are very similar at the polished and modified electrodes. Barrier Properties of Phenyl Monolayers. The results described above demonstrate that the barrier properties of the phenyl monolayer depend on the redox species. The monolayer strongly suppresses the electrontransfer reactions of Ru(NH3)63+, IrCl62-, and Fe(CN)63and at the other extreme does not appear to have any influence on the response of MV+/0. Assuming for simplicity that the monolayer contains defects but not pinholes and that electron transfer is occurring across a monolayer of average thickness d, the modifying layer is expected to influence the observed rate constant for electron transfer according to eq 1.18 β is the tunneling parameter which depends on the chemical nature of the monolayer, ko is the standard heterogeneous rate constant at the polished electrode and koobs is the observed rate constant at the modified electrode.

koobs ) ko exp(-βd)

Figure 5. Cyclic voltammograms (ν ) 100 mV s-1) of MV2+ recorded in (A) phosphate buffer, pH 7, and (B) 0.2 M [Et4N]Cl at (a) polished and (b) phenyl-modified GC (Emod ) -1.1 V vs Fc+/0).

of the figures reveals that the responses at the modified electrodes are of two general types. For Ru(NH3)63+, IrCl62-, and Fe(CN)63- (Figures 4A-C, respectively), only very low currents are observed at large overpotentials. In contrast, for 4-MC, CPZ, FcOH, Fe(bpy)32+, and MV2+ (Figures 4D-G and 5A respectively), significant currents

(1)

For this system, only rough estimates can be made for the terms β and d. As discussed above, the ATWM is less than 5.9 Å; i.e., d < 5.9 Å. A β value of 0.20 Å-1 has been tentatively suggested for substituted phenyl layers grafted onto GC using the diazonium reduction method.13 This value was derived by assuming that electron-transfer reactions of solution probes occurred across highly ordered compact monolayers on GC. On the basis of our own results,12 we believe that the modification conditions used in that study are not likely to yield a full monolayer coverage, and hence the derived β value is an underes(18) Miller, C.; Cuendet, P.; Graetzel, M. J. Phys. Chem. 1991, 95, 877-886.

Organic Monolayers on Glassy Carbon Electrodes

timate. For SAMs on gold electrodes, values in the range of 0.78-0.97 Å-1 have been obtained for monolayers incorporating only σ bonds,19-22 while significantly lower values (0.36-0.57 Å-1) have been reported for monolayers incorporating conjugated phenyleneethylene units.23,24 It is likely that that the phenyleneethylene monolayers are the better models for a monolayer of phenyl groups; hence, we assume that β is no greater than 0.57 Å-1. Thus we estimate that koobs/ko will be greater than 0.035 for the phenyl-modified electrodes. Note that this estimate of the minimum value of koobs/ko is also valid if the monolayer contains microscopic pinholes. Pinholes would increase redox currents at the modified electrodes and hence increase koobs/ko. Most of the probes used in this study are known to have fast electrode kinetics. Considering FcOH, Ru(NH3)63+, and Fe(bpy)32+ as examples, their voltammograms are reversible at polished GC (ν ) 100 mV s-1), consistent with a lower limit of ∼0.1 cm s-1 for ko. Assuming ko ) 0.1 cm s-1 at the polished electrode, and a minimum value of 0.035 for exp(-βd), a minimum value for koobs can be calculated as 0.0035 cm s-1. From Nicholson’s relationship between ∆Ep and ko, a maximum ∆Ep value of ∼120 mV is predicted for these redox couples at the modified electrodes.25 Clearly ∆Ep after electrode modification is significantly greater than 120 mV for all three “fast” probes, indicating that the monolayer is much more blocking than predicted. This discrepancy must arise because the value assumed for β or d is too small. The value assigned to β is speculative and certainly could be larger. However even if a typical σ-bonded value of 0.85 Å-1 is assumed for the monolayer giving a minimum of 6.6 × 10-3 for the rate constant ratio, ∆Ep, at the modified electrodes is calculated to be less than ∼250 mV, again much smaller than the values observed. It seems most likely that the discrepancy arises from the value assigned to d. Two possible sources of error to consider are the value assumed for the ATWM and the assumption that the ATWM defines the distance across which electron transfer must occur. It is difficult to prove unambiguously that the phenyl films are not multilayered; however, there is evidence from related studies that films formed by this method on GC have very limited, if any, multilayer areas. Surface coverages for electroactive 4-nitrophenyl films grafted onto GC using the diazonium reduction method have been measured in a number of studies.3-5,7,12,26 With one exception,3 the surface coverages are consistent with monolayer or submonolayer films, although the surface roughness of GC increases the uncertainty of the measurements. Despite the relatively low surface coverages, small areas of multilayer structure have been proposed to account for additional features in voltammograms and XPS spectra of these films. On the basis of these studies it is not possible to discount multilayer formation for the (19) Smalley, J. F.; Feldberg, S. W.; Chidsey, C. E. D.; Linford, M. R.; Newton, M. D.; Liu, Y.-P. J. Phys. Chem. 1995, 99, 13141-13149. (20) Becka, A. M.; Miller, C. J. J. Phys. Chem. 1992, 96, 2657-2668. (21) Finklea, H. O.; Hanshew, D. D. J. Am. Chem. Soc. 1992, 114, 3173-3181. (22) Weber, K.; Hockett, L.; Creager, S. J. Phys. Chem. B 1997, 101, 8286-8291. (23) Sachs, S. B.; Dudek, S. P.; Hsung, R. P.; Sita, L. R.; Smalley, J. F.; Newton, M. D.; Feldberg, S. W.; Chidsey, C. E. D. J. Am. Chem. Soc. 1997, 119, 10563-10564. (24) Creager, S.; Yu, C. J.; Bamdad, C.; O’Connor, S.; MacLean, T.; Lam, E.; Chong, Y.; Olsen, G. T.; Luo, J.; Gozin, M.; Kayyem, J. F. J. Am. Chem. Soc. 1999, 121, 1059-1064. (25) Nicholson, R. S. Anal. Chem. 1965, 37, 1351-1355. (26) Liu, Y.-C.; McCreery, R. L. J. Am. Chem. Soc. 1995, 117, 1125411259.

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Figure 6. Cyclic voltammograms (ν ) 100 mV s-1) recorded in 0.2 M KCl of CPZ at (s) polished GC and (--) phenyl-modified GC (Emod ) -1.1 V vs Fc+/0).

phenyl-modified electrodes; however, any regions of thick film will have little influence on the apparent ATWM. Tong and co-workers’ treatment of electron transfer at variable thickness films shows that it is the thin areas of the film, even at very low fractional coverages, that determine the apparent standard heterogeneous rate constant.15 Hence it is reasonable to assume that the ATWM is no greater than 5.9 Å. Considering the assumption that redox probes undergo electron transfer at a distance from the electrode which is defined by the ATWM, studies with SAM-coated electrodes have highlighted systems where this assumption is not appropriate. Hydrophobic/hydrophilic interactions between redox probes and the monolayer surface can influence electron-transfer rates. For example, hydrophilic probes were found to have more restricted access to methyl terminated SAMs than were hydrophobic probes,16,27,28 while the opposite effects were observed at hydroxy-terminated SAMs.29 Thus, it is expected that the hydrophobic phenyl-modified GC surface may limit the approach of redox probes in the aqueous phase. With the possible exception of MV+, it appears that all probes used in this study undergo electron transfer over a greater distance than predicted on the basis of monolayer thickness. Hence, although the monolayer is very thin, it can be very blocking toward electron transfer, even for relatively hydrophobic probes. If hydrophobic/hydrophilic interactions between redox probes and the phenyl-modified surface influence the distance for electron transfer, tuning of standard heterogeneous rate constants on the basis of probe hydrophobicity/hydrophilicity is expected. At a first glance, the cyclic voltammograms in Figures 4 and 5 which show faster kinetics at the modified electrodes for CPZ, FcOH, 4-MC, MV2+, and Fe(bpy)32+ than for Ru(NH3)63+, IrCl62-, and Fe(CN)63- seem to support the notion that hydrophilic probes are forced to undergo electron transfer over a greater distance than are hydrophobic probes. This interpretation is complicated, however, by the relative electrode kinetics of the probes. For example, under the experimental conditions employed in this study, IrCl62-/3and Fe(CN)63-/4- are quasireversible at polished GC and, on the basis of standard heterogeneous rate constants alone, are expected to have slower kinetics at phenylmodified electrodes than are FcOH and Fe(bpy)32+, which are reversible under the same conditions. However, clear evidence for hydrophobic/hydrophilic discrimination is seen by comparing the cyclic voltammograms of Ru(NH3)63+ in phosphate buffer, pH 7, and CPZ in aqueous 0.2 M KCl, Figures 4A and 6, respectively. At the polished GC electrode, the oxidation of CPZ is chemically reversible (27) Slowinski, K.; Chamberlain, R. V.; Miller, C. J.; Majda, M. J. Am. Chem. Soc. 1997, 119, 11910-11919. (28) French, M.; Creager, S. E. Langmuir 1998, 14, 2129-2133. (29) Becka, A. M.; Miller, C. J. J. Phys. Chem. 1993, 97, 6233-6239.

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and electrochemically quasireversible (∆Ep) 73 ( 2 mV at ν ) 100 mV s-1), although the voltammogram shows some distortion due to adsorption. At the same scan rate, reduction of Ru(NH3)63+ is electrochemically reversible. After electrode modification, the peak for oxidation of CPZ is shifted ∼350 mV, whereas no peak is seen for reduction of Ru(NH3)63+ even when the scan is extended ∼1 V past the potential of the reduction peak at polished GC. Hence, it appears that CPZ can approach significantly more closely to the modified electrode than can Ru(NH3)63+. Similar behavior is seen when comparing the voltammograms of Ru(NH3)63+ and 4-MC at polished and modified electrodes in phosphate buffer (Figure 4A,D); 4-MC is kinetically slower than Ru(NH3)63+ at polished GC but faster at the modified electrode. The voltammetric behavior of the MV2+/+ and MV+/0 couples in aqueous 0.2 M tetraethylammonium chloride electrolyte (Figure 5B) may also be attributed to monolayer selectivity. Both couples are electrochemically reversible at the polished electrode, and they have very similar homogeneous self-exchange rate constants (6 × 107 and 1.0 × 108 M-1 s-1, respectively, in dimethylformamide30), suggesting that their standard heterogeneous rate constants at GC are similar. However, at the phenyl-modified electrode the apparent rate constant for the MV2+/+ couple is much smaller than for the MV+/0 couple, which is reversible under the experimental conditions. The lower charge of the reduced species (and hence greater hydrophobicity) may allow more favorable interaction of the probe with the monolayer. In fact for the MV+/0 couple it is possible that the distance over which electron transfer occurs is accurately defined by the ATWM or is even less than the ATWM (due to permeation of the probe into the monolayer). The reversible (30) Fuhlendorff, R.; Lund, T.; Lund, H.; Pedersen, J. A. Tetrahedron Lett. 1987, 28, 5335-5338.

Downard and Prince

kinetics at the polished and modified electrodes do not allow these possibilities to be explored. Probe size or shape appears not to be a primary influence on electrode kinetics. The diameter of Fe(bpy)32+ is greater than that of Ru(NH3)63+, IrCl62-, and Fe(CN)63-, yet the former complex exhibits faster electrode kinetics at the modified surface. On the other hand, for the hydrophobic MV2+, MV+, and MV0 species, it is possible that size and shape, coupled with π-π interactions with the phenyl monolayer, could account for their fast kinetics at the modified electrodes. Conclusions Using the diazonium reduction method a layer of phenyl groups can be assembled on GC electrodes and the blocking behavior varied by varying the surface coverage. The surface coverage determines the blocking behavior because as coverage increases the number of defect sites and microscopic pinholes decreases, increasing the apparent ATWM (average thickness of the whole monolayer). However, with the possible exception of the MV+/0 couple, all redox couples studied in this work undergo electron transfer across a greater distance than that predicted on the basis of the maximum thickness of a monolayer. The distance of approach to the monolayer appears smaller for hydrophobic than hydrophilic probes, supporting the notion that hydrophobic/hydrophilic interactions between the probes and the monolayer surface determine the distance for electron transfer. It is not possible to unambiguously confirm that the ATWM is less than that of a highly ordered monolayer; however, even if the modifying film is exclusively multilayer in structure, our finding of more favorable interactions for hydrophobic probes is applicable. LA010499Q