Structural Characterization of (3-Mercaptopropyl)sulfonate Monolayer

We have investigated the structure of (3-mercaptopropyl)sulfonate (MPS) ... the MPS monolayer covers the entire gold surface with a surface coverage a...
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Langmuir 2005, 21, 4400-4409

Structural Characterization of (3-Mercaptopropyl)sulfonate Monolayer on Gold Surfaces Cheikh Mokrani,† Julien Fatisson, Liliane Gue´rente, and Pierre Labbe´* Laboratoire d’Electrochimie Organique et de Photochimie Re´ dox, UMR CNRS 5630, Institut de Chimie Mole´ culaire de Grenoble, FR CNRS 2607, Universite´ Joseph Fourier, BP 53, 38041 Grenoble Cedex 09, France Received November 23, 2004. In Final Form: February 16, 2005 We have investigated the structure of (3-mercaptopropyl)sulfonate (MPS) monolayer self-assembled onto gold surfaces by quartz crystal microbalance with energy dissipation monitoring (QCM-D) and various electrochemical methods. QCM-D experiments show that the MPS monolayer behaves as a thin rigid film with a surfacic mass of 166 ng cm-2. Interfacial capacitance measurements demonstrate that the MPS monolayer is a rather open structure that can be penetrated by the ionic species of the phosphate buffer electrolyte. From MPS reductive desorption experiments, MPS surface concentration corresponds to 4.6 × 10-10 mol cm-2, which represents 60% of the coverage reported for a densely packed thiol monolayer. Despite this low packing density, oxidation of catechol is strongly inhibited leading to voltammograms that are free of diffusional contribution. This unique behavior has been exploited to show that the MPS monolayer covers the entire gold surface with a surface coverage at least equal to θ ) 0.9981, which means a very low number of MPS-free pinholes and/or defects. Kinetics of electron transfer toward soluble redox species has been studied using catechol as a neutral hydrophilic probe, but also ferrocyanide as hydrophilic anion and ferrocenemethanol as neutral hydrophobic molecule. It is proposed that the MPS monolayer provides a high kinetic barrier toward permeation of these species and that electron transfer mainly occurs by electron tunneling through the MPS monolayer.

Introduction In the continuity of our previous work focusing onto the layer-by-layer electrostatic self-assembly (LbL-ESA) of cationic polyelectrolytes with anionic proteins1-3 or gold nanoparticles4 on gold surfaces, we wish to report here a structural characterization of (3-mercaptopropyl)sulfonate (MPS) monolayer self-assembled onto gold surfaces (denoted Au-MPS in the following). Indeed, Au-MPS thiolated surfaces have often been used as a pH-independent, hydrophilic, and negatively charged platform for studying ESA multilayer films, in particular by electrochemical techniques.1-10 However, as will be described in the following, Au-MPS surfaces exhibit unexpected electrochemical behavior. For example in the presence of 0.1 M phosphate buffer, they strongly inhibit electron-transfer processes toward a neutral and hydrophilic compounds * To whom correspondence may be addreessed: tel, 00 33 (0)4 76 51 47 18; fax, 00 33 (0)4 76 51 42 67; e-mail, Pierre.Labbe@ ujf-grenoble.fr. † Permanent address: Laboratoire des Mate ´ riaux Inorganiques, Universite´ M. Boudiaf, BP 166, route d′Ichbilia, 28000 M’Sila, Alge´rie. (1) Coche-Guerente, L.; Mengeaud, V.; Labbe´ P. Anal. Chem. 2001, 73, 3206. (2) Ferreyra, N.; Coche-Guerente, L.; Labbe´, P.; Calvo, E. J.; Solis, V. Langmuir 2003, 19, 3684. (3) Ferreyra, N.; Coche-Guerente, L.; Labbe, P. Anal. Chim. Acta 2004, 49, 477. (4) Ferreyra, N.; Coche-Guerente, L.; Fatisson, J.; Lopez-Teijelo, M.; Labbe, P. J. Chem. Soc., Chem. Commun. 2003, 2056. (5) Forzani, E. S.; Otero, M.; Perez, M. A.; Lopez Teijelo, M.; Calvo, E. J. Langmuir 2002, 18, 4020. (6) Calvo, E. J.; Danilowicz, C.; Forzani, E.; Wolosiuk, A.; Otero, M. Compr. Anal. Chem. 2003, 39, 327. (7) Calvo, E. J.; Forzani, E.; Otero, M. J. Electroanal. Chem. 2002, 358-359, 231. (8) Forzani, E. S.; Perez, M. A.; Lopez Teijelo, M.; Calvo, E. J. Langmuir 2002, 18, 9867. (9) Calvo, E. J.; Wolosiuk, A. J. Am. Chem. Soc. 2002, 124, 8490. (10) Forzani, E. S.; Solis, V. M.; Calvo, E. J. Anal. Chem. 2000, 72, 5300.

such as catechol whereas exhibiting poor influence toward electrochemistry of the rather hydrophobic and neutral ferrocenemethanol (FcOH) or hydrophilic and negatively charged ferrocyanide Fe(CN)63-. Since gold surfaces functionalized by self-assembled monolayers (SAMs) of MPS or more generally speaking of ω-functionalized thiols have often been used as amperometric transducers, it appears interesting to get a better understanding of this unusual electrochemical behavior. Only a few studies were reported about the electrochemistry and structure of mercaptoalkylsulfonate monolayers. Fawcett and co-workers11 studied (2-mercaptoethane)sulfonate (MES) monolayer and reported on the oxidative and reductive desorption of the thiol. Savinell and co-workers12 show that a MES monolayer decreases the electron-transfer rate related to the oxidation of cationic dopamine. The cyclic voltammetry of dopamine was peak shaped with a positive shift of the anodic peak potential of about 400 mV as compared to the bare gold electrode. Mandler and co-workers13 realized the most complete study and reported characterization and electroanalytical application of mercaptodecanesulfonic acid (MDS) monolayer on gold. They show that the MDS monolayer that is formed from a 10-carbon chain is disorganized and permeable to the electrolyte. The high negative charge of the sulfonate layer with its disorganized, i.e., “brush” type nature, results in unique electrochemical characteristics. The electrochemical reversibility of Fe(CN)63-/4- strongly decreases at a MDS modified electrode as a consequence of the excess of negative charge of sulfonate group. As well the 10-methylene alkyl chain of MDS constitutes a kind of hydrophobic barrier toward penetration of highly charged anions. In contrast, when (11) Calvente, J. J.; Kovacova, Z.; Sanchez, M. D.; Fawcett, W. R. Langmuir 1996, 12, 5696. (12) Dalmia, A.; Liu, C. C.; Savinell, R. F. J. Electroanal. Chem. 1997, 430, 205. (13) Turyan, I.; Mandler, D. Isr. J. Chem. 1997, 37, 225.

10.1021/la047125s CCC: $30.25 © 2005 American Chemical Society Published on Web 03/24/2005

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using MES, the reversibility of Fe(CN)63-/4- was not strongly affected at high ionic strength. They also observed from cyclic voltammetry experiments that the presence of a MDS monolayer has a pronounced effect on the electrochemistry of organic molecules such as catechol and its derivatives. For example, in 0.1 M H2SO4, the anodic peak potential for catechol oxidation is shifted by about 250 mV on the MDS-modified electrode as compared to bare gold. This behavior was explained by the fact that because of its high negative charge, the MDS monolayer exhibits a high kinetic barrier toward permeation of hydrophobic species due to its high hydrophilicity. The combination of these properties was exploited for the selective amperometric determination of cationic species in the presence of organic interferents. On a more general point of view, modification of gold surfaces by organic thiol SAM has received considerable attention in recent years.14-16 SAMs provide a means for controlling the chemical nature of the metal-solution interface since they offer the combination of high structural order, versatility in tailoring chain terminal functionalities, and ease of reproducible preparation and quantitative analysis. Monolayer assemblies having functional terminal groups have been recognized as a useful starting platform for building complex supramolecular assemblies that can be in situ characterized using various techniques such as ellipsometry, surface plasmon resonance (SPR), or quartz crystal microbalance (QCM). Such an approach involving SAM-functionalized gold surfaces is routinely used for studying biological phenomena in solid supported lipid membrane models17-21 or self-assembled multilayers of various kinds involving polyelectrolytes, biomolecules, or nanoparticles.1-10,22 SAM-modified gold electrodes have also stimulated considerable interest because of their applications as well to fundamental studies and to electroanalytical devices. They can be employed as insulating barriers between an electrode and a redox couple to study long-range electron transfer,23 as microarray electrodes for measuring very fast electron-transfer kinetics24 or for creating selective voltammetric detectors.25,26 It has been early recognized that pinholes and other defects may exist in the monolayer, and important work has been devoted to characterize these defects as well as the average molecular structure and structural disorder of self-assembled thiol monolayer on gold.23-30 These studies were essentially performed with (14) Ulman, A. In An introduction to ultrathin organic films from Langmuir-Blodgett to self-assembly; Academic Press: San Diego, CA, 1991. (15) Finklea, H. O. In Electroanalytical Chemistry; Bard, J. A., Rubinstein, J., Eds.; Marcel Dekker: New York, 1996; Vol 19, p 119. (16) Ulman, A. Chem. Rev. 1996, 96, 1533. (17) Ho¨o¨k, F.; Kasemo, B.; Nylander, T.; Faut, C.; Sott, K.; Elwing, H. Anal. Chem. 2001, 73, 5796. (18) Rudd, T.; Gallagher, J. T.; Ron, D.; Nichols, R. J.; Fering, D. G. Biochem. Soc. Trans. 2003, 31, 349. (19) Plunkett, M. A.; Claesson, Per M.; Rutland, M. Langmuir 2002, 18, 1274. (20) Stalgren, J. J. R.; Eriksson, J.; Boschkova, K. J. Colloı¨d Interface Sci. 2002, 253, 190. (21) Pope, L. H.; Allen, S.; Davies, M. C.; Roberts, C. J.; Tendler, S. J. B.; Williams, P. M. Langmuir 2001, 17, 8300. (22) Hicks, J. F.; Seok-Shon, Y.; Murray, R. W. Langmuir 2002, 18, 2288. (23) Terretaz, S.; Becka, A. M.; Traub, M. J.; Fettinger, J. C.; Miller, C. J. J. Phys. Chem. 1995, 99, 11216. (24) Finklea, H. O.; Snider, D. A.; Fedyk, J.; Sabatini, E.; Gafin, Y.; Rubinstein, I. Langmuir 1993, 9, 3660. (25) Steinberg, S.; Rubinstein, I. Langmuir 1992, 8, 1183. (26) Takehara, K.; Takemura, H.; Aihara, M.; Yoshimura, M. J. Electroanal. Chem. 1996, 404, 179. (27) Retna Raz, C.; Ohsaka, T. Electroanalysis 2002, 14, 679. (28) Diao, P.; Jiang, D.; Cui, X.; Gu, D.; Tong, R.; Zhong, B. J. Electroanal. Chem. 1999, 464, 61.

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hydrophobic alkanethiol or ω-hydroxyalkanethiol of alkyl chains varying from 2 to 18 methylen groups using electrochemical techniques such as cyclic voltammetry (CV), electrochemical impedance spectroscopy, or scanning electrochemical microscopy. Various outer-sphere redox couples such as ferrocenemethanol,24,30 hexacyanoferrate,24,28,31 (trimethylammonio)methylferrocene,24 rutheniumhexaamine, or octacyanomolybdate32 were used to probe electron transfer, diffusion through pinholes, or permeation kinetics at such SAM-modified gold electrodes. Electrochemical behavior of these probes is strongly dependent on the fractional surface coverage θ as well as on the structural disorder of the monolayer. When long chain monolayers essentially free of pinholes (θ ∼ 1) are created using optimized conditions,28,31,32 cyclic voltammograms that are free of mass transfer effects are observed instead of peaked CV curves recorded on the bare electrode. Under these conditions, charge-transfer reaction occurs by electron tunneling across thick layers in well-assembled domains or at thinner layers in collapsed sites.28,33 Collapsed sites correspond to structural defects where no thiol molecules are adsorbed but where adjacent thiol molecules are “lying down” giving rise to a thin layer. Another reason for the formation of collapsed sites is that differently tilted thiol molecules at the boundaries of differently oriented domains also give rise to thin layer formation. Under these conditions the standard electrontransfer rate constant k at the film-covered electrode can be related to the k° constant at the bare electrode by relation (1)

k ) k° exp(-βd)

(1)

where d is the thickness of the insulating layer and β is the tunneling constant. β values of 1.05 and 0.9 per methylene unit for Au electrodes covered respectively by alkylthiol34 and ω-hydroxyalkylthiol31 have been reported. The existence of collapsed sites could be quantified by introducing a fractional constant corresponding physically to the roughness (disorder degree) of the monolayer.28 This constant is comprised between 0.5 for an ideally porous electrode and 1 for a perfect smooth electrode. In the case of Au electrodes covered by an octadecylmercaptan monolayer, the authors have shown that as this constant increases, the CV curves of ferrocyanide change from the reversible one on the bare electrode to an irreversible one and then a sigmoı¨dal one and at last a kinetically controlled one. Electron transfer at such pinhole-free thiol-modified electrodes can also occur by permeation of the redox species into the monolayer followed by their diffusion in the layer and electron transfer at the electrode. Bard29 and coworkers have developed this membrane model for studying permeation of the rather hydrophobic menadione through octadecanethiol monolayer modified gold electrodes. On the other hand, and in addition to collapsed sites, SAMs of alkylthiol on gold surfaces may have pinhole defects that are free of adsorbed thiol. At these sites whose size is higher than molecular size, the electron transfer rate is greater than where there is tunneling across the well-oriented monolayer. Since the electron transfer rate (29) Cannes, C.; Kanoufi, F.; Bard, A. J. Langmuir 2002, 18, 8134. (30) Cannes, C.; Kanoufi, F.; Bard, A. J. J. Electroanal. Chem. 2003, 547, 83. (31) Miller, C.; Cuendet, P.; Graetzel, M. J. Phys. Chem. 1991, 95, 877. (32) Miller, C.; Graetzel, M. J. Phys. Chem. 1991, 95, 5225. (33) Finklea, H. O.; Avery, S.; Lynch, M.; Furtsh, T. Langmuir 1987, 3, 409. (34) Xu, J.; Li, H. L.; Zhang, Y. J. Phys. Chem. 1993, 97, 11497.

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Figure 1. Schematic representations of possible structure and molecular organization for MPS monolayer on gold surface. d1 corresponds to the length of an elongated MPS molecule and d2 is the thickness of the associated layer of aqueous electrolyte.

constant at pinhole defects is the same as that at a bare Au electrode, the pinhole defect current will become quickly controlled by diffusion even at low overpotential. As a consequence, mass transfer effects will be observed in CV experiments. It has been demonstrated24,35 that pinhole defects at Au/SAMs electrodes can be treated as an array of ultramicroelectrodes since the electron transport across the full width of the monolayer is diminished by several orders of magnitude for long chain alkylthiols. The electron-transfer process at such a modified electrode can be simulated by a formal CEC mechanism35 where C is a chemical step and E an electron transfer step. In the present case, the E step corresponds to the electron transfer at the metallic surface of the uncovered pinhole defect sites whereas the preceding and succeeding reaction steps C correspond physically to diffusion of the reactants into the pinholes defined at the defect sites. For high surface coverage (θ ∼ 1) and according to this model, two types of CV curves (the normal peak shape and the polarogram shape) are expected depending on the size and distribution of pinhole defects, electron-transfer rate constant on the bare electrode, and time scale of the experiment. The aim of the present work consists of gaining further information about the molecular organization of MPSself-assembled monolayer on gold surface in order to rationalize its atypical behavior and to characterize electron-transfer processes toward soluble redox probes. As previously recognized and confirmed in the present work with MPS, mercaptoalkanesulfonate monolayers are not densely packed since surface coverages ranging from 35 to 60% of the value for a densely packed monolayer are recorded. At least two possible molecular organizations can be a priori possible to account for the MPS coverage as schematically shown in Figure 1. In structure A, more or less densely packed MPS domains are separated by MPS-free defects and pinholes whose size is higher than molecular size. In structure B, the MPS layer is covering the whole gold surface in a regular, although nondensely packed, monolayer without pinholes and defects. Three main different processes can be a priori considered to account for electron transfer on Au-MPS modified electrodes as shown schematically in Figure 2: (i) tunneling of electrons through the MPS monolayer at pinhole-free domains, the electron tunneling rate constant being ktunnel (Figure 2A); (ii) permeation of the redox species into the monolayer followed by diffusion in the layer and electron transfer at the electrode which corresponds to membrane (35) Amatore, C.; Saveant, J. M.; Tessier, D. J. Electroanal. Chem. 1983, 147, 39.

Figure 2. Schematic representation of various models for describing electron-transfer process across a MPS layer. P/Q is a soluble redox system: (A) the electron tunneling model, where De represents the diffusion coefficient of redox probe in the bulk and ktunnel the electron transfer tunneling constant through the MPS layer; (B) the membrane model, where Df represents the diffusion coefficient in the film, κ the partition coefficient between the film and the bulk, and kinterface the kinetic rate constant for the crossing of the MPS layer/bulk interface; (C) the pinhole model where redox probe diffuses from the bulk to the gold surface via pinholes at the MPS layer.

model (Figure 2B). The kinetics of the overall process is controlled by the partition coefficient constant κ between the membrane and the bulk, the diffusion coefficient Df in the membrane, and the kinetics of interface crossing whose rate constant is kinterface; (3) diffusion of the electroactive compounds to pinhole and defect sites and electron transfer at the electrode surface which corresponds to the pinhole model (Figure 2C). Various electrochemical techniques together with energy dissipation monitoring quartz crystal microbalance (QCM-D) experiments were used to quantify electron-transfer processes and gain structural information about the MPS layer. We are aware of previous work, by Mandler and co-workers,13 who studied essentially the electrochemistry of a longer mercaptoalkanesulfonic acid, i.e., mercaptodecanesulfonic acid. The objective of our work consists of giving complementary information for a better understanding of these special kinds of gold surfaces modified by short alkyl chain ω-functionalized thiol. Experimental Section Chemicals. All solutions were prepared with ultrapure water (18 MΩ cm-1) from a Millipore Milli Q system. Thiol solution, 20 mM, was prepared with (3-mercapto-1-propane)sulfonate sodium (MPS) from Aldrich in 16 mM sulfuric acid (Roth). Phosphate buffer (pH ) 6.5, 0.1 M) was obtained by mixing KH2PO4 and K2HPO4 (Acros Organics) with pure water. Catechol, ferrocenemethanol, and ferrocyanide were respectively purchased from ICN Biomedicals, Aldrich, and Prolabo and were of analytical quality.

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Electrochemical Measurements. Cyclic voltammetry experiments were performed with a conventional three-electrode potentiostatic system. The equipment was a CHI 440 model potentiostat (CH-Instruments). Electrode potentials were referred to an Ag/AgCl/KCl (3M) reference. The working gold electrodes (diameter 2 mm, geometric surface area 0.0314 cm2) were from CH-Instruments. The cleaning procedure included successive polishing steps with alumina (1, 0.3, and 0.05 µm), each step being followed by sonication in ultrapure water for 5 min, immersion in Piranha solution for 15 min, and by a last sonication for 5 min. Caution: Piranha solution is highly corrosive and reacts violently with organic materials; precaution must be taken at all time when handled. The gold surfaces were then activated by cycling at 10 V s-1 in 0.5 M sulfuric acid between 0.2 and 1.6 V until a reproducible voltammogram corresponding to a clean surface was obtained. This procedure was repeated before each experiment. Prior to the assembling procedure, two voltammograms were recorded at 0.1 V s-1 in 0.5 M sulfuric acid between 0.2 and 1.5 V in order to check the surface conditions and to obtain information about roughness. Assuming a charge of 420 µC cm-2 for the reduction of the gold oxide monolayer,36 a roughness factor usually comprising between 2.2 and 2.7 was obtained for the gold electrode. Immediately after activation, the Au electrode was modified with sulfonate groups by immersion in the MPS solution for 30 min followed by rinsing with deionized water. Quartz Crystal Microbalance. Experiments with the energy dissipation quartz crystal microbalance (QCM-D) were performed on a Q-Sense D 300 equipment (Q-Sense AB, Go¨teborg, Sweden) by monitoring simultaneously the changes in the resonance frequencies (∆f) and energy dissipation factors (∆D) due to the assembling process. The QCM chip is excited to oscillate in the thickness-shear mode at its fundamental resonance frequency (f1 ) 5 MHz) and odd overtones (n ) 3, 5, 7) by applying a rf voltage across the electrodes. The measurements are effected by periodically disconnecting the oscillating crystal from the circuit in a computer-controlled way and measuring the decay time τ0 of the exponentially damped sinusoidal voltage signal over the crystal caused by switching of the voltage applied to the piezoelectric oscillator. The Q-Sense software then allows to acquire the dissipation factor, D, via relation (2)

D-

1 2 ) πfτ0 ωτ0

(2)

where f is the resonance frequency and τ0 is the relaxation time constant. These data give information on the adsorption process as well as on certain viscoelastic properties of the adsorbed film. In the case of homogeneous, quasi-rigid films with a thickness that is not too high, the frequency shifts are proportional to the ∆mf mass uptake per unit area that can then be deduced from the Sauerbrey37 relationship

-∆fSauerbrey

1 m nC f

(3)

where the mass sensitivity, C, is equal to 17.7 ng cm-2 Hz-1 at f1 ) 5 MHz. To go beyond the Sauerbrey approximation, the ∆Fn and Dn experimental data can be analyzed by using the framework developed by Voivona et al.38 under the hypothesis that the film is a homogeneous and isotropic viscoelastic layer. Before a QCM-D experiment was started, the gold surface was cleaned with Piranha solution during 2 min and then copiously rinsed with pure water. Atomic force microscopy (AFM) imaging demonstrates that such a chemical treatment did not modify the morphology and roughness of the crystal transducer. QCM-D experiments were run under flowing conditions by the way of a perilstatic pump operating at 50 µL min-1. All solutions were previously degassed in order to avoid bubble formation in the QCM-D measuring chamber. (36) Vela, M. E.; Salvarezza, R. C.; Arvia, A. J. Electrochim. Acta 1990, 35, 117. (37) Sauerbrey, G. Z. Phys. 1959, 155, 206. (38) Voivona, M. V.; Rodhal, M.; Kasemo, B. Phys. Scr. 1999, 59, 391.

Figure 3. AFM height mode images of a gold-covered quartz crystal transducer before (A and C) and after (B and D) a 2 min piranha treatment. The image dimensions are 5 × 5 µm2 for (A) and (B) and 1 × 1 µm2 for (C) and (D). The maximum Z-range is 10 nm for all. The rms values are 1.140, 1.118, 1.149, and 1.104 nm and the mean roughnesses are 0.896, 0.885, 0.902, and 0.866 nm for respectively A, B, C, and D, respectively.

Results and Discussion MPS adsorption onto gold surfaces has already been described by Calvo et al.5,39 using identical experimental conditions. It was shown from gravimetric measurements39 that a rigid MPS monolayer was grafted corresponding to a mass increase of 174 ng cm-2. In situ measurements in deionized water gave an ellipsometric thickness of the MPS monolayer between 1.3 and 2 nm (depending on the incident light wavelength).5 We performed AFM and QCM-D experiments in order to check under our own experimental conditions the formation of MPS monolayer onto gold-covered quartz crystals and to determine optimum self-assembling conditions. Figure 3 shows AFM height mode images of the gold-covered quartz crystal surface before and after the 2 min piranha treatment that was used to activate the gold surface prior to MPS grafting. This control experiment demonstrates that the very corrosive piranha treatment did not alter the gold surface morphology and roughness since root mean square (rms) and mean roughness were not significantly modified. Most commonly, as a precaution, possible contaminants are “removed” by piranha solution and a hydrophilic clean gold surface will be so obtained systematically. In addition, section analysis and rms values allow the conclusion that the gold surface is very smooth and we considered for QCM-D calculation a roughness factor of unity for the crystal transducer gold surface. Figure 4 shows a typical MPS grafting QCM-D experiment obtained under flowing conditions in the presence of 16 mM sulfuric acid. Frequency shifts have been normalized toward the overtone number as ∆Fn/n. Injection of 20 mM MPS sulfuric solution leads to a rapid decrease of the resonance frequency during the first minute followed by a slow decrease during the next 30 min. This agrees well with the formation of a grafted MPS layer onto the gold surface. Concomitantly the energy dissipation D remains less than 10-6 (Figure 4B), which indicates that the MPS layer is rigid so that the crystal (39) Hodack, J.; Etchenique, R.; Calvo, E. J. Langmuir 1997, 13, 2708.

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Figure 4. QCM-D experiments of MPS grafting onto goldcovered quartz crystal transducers: (A) ∆Fn/n frequency shifts and (B) Dn energy dissipation factor shifts recorded during the grafting of a monolayer of 3-mercapto-1-propylsulfonate sodium (MPS) onto a gold surface. n corresponds to the third (b), fifth (+), and seventh (×) overtones (that is 15, 25, and 35 MHz) of the fundamental quartz crystal resonance frequency (5 MHz). The experiment is conducted under flowing conditions at 50 µL/min. The MPS concentration is 20 mM in 0.016 M sulfuric acid.

oscillation dampening does not increase significantly. After the quartz is rinsed with pure electrolyte, dissipation decreases back near zero, whereas the normalized frequency shifts ∆Fn/n reach to a common value of 9.4 Hz independently of the overtone number. These experimental results indicate that the MPS layer behaves as a thin rigid film, which allows application of the Sauerbrey relation. A surface mass per unit area of 166 ng cm-2 could be deduced for MPS adsorption, which compares well with the value of 174 ng cm-2 previously reported39 and agrees with the formation of a rigid MPS monolayer. Surface concentration of MPS cannot be deduced from these QCM-D experiments since the composition of the MPS layer and in particular the relative contribution of hydration water and counterions to the mass uptake is unknown. It has been reported that the theoretical coverage of a perfectly compact hexagonal close packed thiol monolayer having 5 Å chain separation40 is 7.76 ×

Mokrani et al.

10-10 mol cm-2, which should correspond in the case of MPS to a mass of 121 ng cm-2. However as will be shown later from independent electrochemical measurements, the MPS surface coverage only corresponds to 4.6 × 10-10 mol cm-2 (72.6 ng cm-2), which is 60% of a densely packed monolayer. From structural consideration, the length of a fully elongated MPS molecule can be estimated to be d1 ) 0.75 nm (Figure 1). Assuming that the space between MPS molecules is filled by water and assuming that this space corresponds to 40% of surface coverage, the corresponding mass of water is 30 ng cm-2 (taking a density of 1 for water). Since QCM-D shows that the mass uptake is 166 ng cm-2, this means that QCM-D is able to sense, in addition to the MPS layer, an additional layer of water whose mass corresponds to 63.4 ng cm-2 and thickness to d2 ) 0.64 nm (Figure 1). This is not surprising since the MPS monolayer is negatively charged and should be strongly interacting with a cloud of hydrated counterions and water molecules that appears to be associated in a rigid way with the MPS underlying layer. This overlayer of “bound water and ions” is expected to influence permeation as well as electron-transfer processes. Finally, the total thickness of the hydrated MPS layer (d1 + d2) can be estimated to be 1.4 nm from QCM-D experiments. This value agrees quite well with the ellipsometric thickness reported in the literature for MPS monolayer.5 To investigate more precisely the structure of the MPS monolayer and its molecular organization, cyclic voltammetry (CV) experiments were performed using polycrystalline gold electrodes and various redox probes in aqueous electrolytic solutions. Comparisons were made between the electrochemical responses of bare Au and modified Au-MPS electrodes. All experiments were realized with carefully polished and cleaned polycrystalline Au electrodes as described in the Experimental Section. The roughness factor of the electrodes was systematically determined by cyclic voltammetry in sulfuric electrolyte in order to normalize the results toward the real surface area of a gold surface. Figure 5A compares the CV responses of Au and Au/MPS electrodes in 0.1 M phosphate buffer, pH ) 6.5. The CV shape is characteristic of interfacial capacitance charging. As expected, the capacitive current intensities are proportional to the potential sweeping rate (Figure 5B), from which the interfacial capacitances (C) can be estimated. For bare gold electrode we found a value CAu ) 1.10 µF, which corresponds to 15 µF cm-2 by taking into account a measured roughness of 2.36. A capacitance of 20.4 µF cm-2 (without roughness correction) has been reported41 for vapor-deposited gold electrodes on glass substrate in 0.1 M phosphate buffer. Assuming a roughness of the gold film42 of about 1.3, this leads to a capacitance of about 16 µF cm-2, in good agreement with our determination. Self-assembling of MPS onto the gold electrode leads to a significant decrease of the capacitive current (Figure 5A) and an interfacial capacitance CAu-MPS ) 8.7 µF cm-2 could be estimated from Figure 5B. This value is comparable to the value of 9.7 µF cm-2 that has been reported for a gold surface modified by a mercaptoethanesulfonate (MES) monolayer in the presence of 0.1 M phosphate buffer.12 In contrast, gold modified by a monolayer of mercaptobutane (whose length is comparable to that of MPS) exhibits a capacitance of only 3.0 µF cm-2 in NaF electrolyte43 as compared to the value of 22 µF cm-2 for bare gold in 0.1 M NaF. (40) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546. (41) Pierrat O.; Lechat, N.; Bourdillon, C.; Laval, J.-M. Langmuir 1997, 13, 4112. (42) Goss, C. A.; Charych, D. H., Majda, M. Anal. Chem. 1991, 63, 85.

Self-Assembled Structures on Gold Surfaces

Figure 5. (A) Cyclic voltammograms (0.1 V s-1) recorded in 0.1 M phosphate buffer pH 6.5 and (B) plots of the capacitive current intensity measured at 0.2 V as a function of the potential sweeping rate on (a) bare Au electrode and (b) Au-MPS modified electrode.

Replacement of NaF by NaCl or NaClO4 electrolytes leads to a higher capacitance of the gold-mercaptobutane interface. This has been interpreted by the fact that the highly hydrated fluoride anions could be incapable of penetrating the mercaptobutane monolayer (despite its reported open structure) whereas chloride and to a lesser extend perchlorate could penetrate to the gold. All these results and consideration demonstrate that the MPS monolayer should be a rather open structure since the interfacial capacitance of Au-MPS (CAu-MPS ) 8.7 µF cm-2) in phosphate buffer is only divided by a factor of 2 as compared to the one of bare Au (CAu ) 15 µF cm-2). As well the existence of a large amount of defects and MPS-free pinholes could be at the origin of the high capacitance of the Au-MPS surface. The surface concentration of the MPS layer could be determined from thiol reductive desorption44,45 experiments performed on Au-MPS modified electrodes in 0.5 M NaOH aqueous electrolyte. Figure 6 compares the (43) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, E. D. J. Am. Chem. Soc. 1987, 109, 3559. (44) Walczak, M. M.; Alves, C. A.; Lamp, B. D.; Porter, M. D. J. Electroanal. Chem. 1995, 396, 103. (45) Wong, S.-S.; Porter, M. D. J. Electroanal. Chem. 2000, 485, 135.

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Figure 6. (A) Cyclic voltammograms (0.02 V s-1) recorded in a 0.5 M NaOH aqueous solution on a (a) bare Au and (b) Au-MPS modified electrodes. (B) Four successive cycles recorded on the Au-MPS modified electrode.

response of bare Au electrode with the four successive one-cycle cyclic voltammograms recorded on the Au-MPS electrode at 0.02 V s-1 between 0 and -1.25 V. As expected bare gold presents a capacitive response (Figure 6A). The first cycle recorded on Au-MPS (Figure 6A) is characterized by the presence of a purely capacitive region between 0 and -0.6 V. It can be noticed that the charging current is less intense on Au-MPS than on bare Au (Figure 6A), in good agreement with the presence of the MPS monolayer on the gold surface and previous observation in phosphate buffer electrolyte. At more negative potential, one main cathodic wave at Epc ) -0.959 V is observed with two overlapping waves of lower intensity centered as shoulders at -0.92 and -1.04 V. These cathodic waves can be assigned to the reductive desorption of MPS following a process previously reported.45,45

Au-SR + e- f Au + RSThe presence of several cathodic waves has already been observed and was attributed to the chemisorption of the monolayer at different adsorbate binding sites.44 In particular the presence of two cathodic waves was shown at gold thin films having (111) crystallinity. The intensities of the two peaks were related to the relative proportion

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of (111) terraced surface sites and step sites. It was also shown that the adsorbate at step sites are bound by as much as 25 kJ mol-1 more strongly than at terrace sites. In our study, the working electrode is made of a polycrystalline gold rod that has been mechanically polished and electrochemically activated (see Experimental Section). The surface of the polycrystalline gold electrode consists of a large number of grains of different types, the complicated topography resulting from grain boundaries and other defects. This renders difficult and uncertain any model for describing crystallographic nonuniform polycrystalline gold surfaces. However it has been assumed46 that the starting polycrystalline gold surface consists of a certain distribution of the three simplest index faces, that is (100), (110), and (111) interacting with water and anions. MPS molecules are expected to bound at these various sites with different energies and thus to reductively desorb at various potentials, which is experimentally observed (Figure 6A). On the reverse scan of the first cycle (Figure 6A), an anodic wave of low intensity is observed corresponding to the readsorption of MPS that has been desorbed during the cathodic scan.11 The following cycles (Figure 6B) show a progressive decrease of the cathodic waves as a consequence of the diffusion in solution of the reductively desorbed MPS. The charge due to the reductive desorption of MPS was determined by integrating the voltammetric peak of the first negative scan after correction of the baseline, a value of 44.5 µC cm-2 (average of five different experiments) being obtained by taking into account the roughness factor of the gold electrode. This charge corresponds to a surface concentration of 4.6 × 10-10 mol cm-2 and a molecular area for MPS of 36 Å2. It has been reported that the surface concentration for a densely packed monolayer of alkanethiol is 7.76 × 10-10 mol cm-2, which corresponds to a molecular area of 21.4 Å2. From a molecular point of view, this means that the surface coverage of gold by MPS sulfur atom is about 0.60 (21.4/36). If large defects and pinholes are absent, this means that the MPS monolayer is not densely packed, even by considering that the packing density is controlled by the sulfonate headgroups whose molecular area can be estimated to be 28.1 Å2 from published data.11 This low packing density is similar to the one obtained by Fawcett et al.11 concerning the surface coverage of mercaptoethanesulfonate (MES) monolayer on a single crystal gold electrode (111). In their work it was also shown that reductive desorption of MES occurs in only one voltammetric wave, as expected for this type of surface and in contrast with our observation of several waves on a polycrystalline gold electrode. To get further insights about the MPS layer structure, we investigated the cyclic voltammetric response of three redox probes. Catechol was selected as a hydrophilic compound that can be electrooxidized into the corresponding quinone via a two-electron and two-proton exchange process. Ferrocenemethanol (FcOH) and ferrocyanide Fe(CN)63- are outer sphere monoelectronic redox species. FcOH is neutral and rather hydrophobic whereas Fe(CN)63- is hydrophilic and highly negatively charged. As previously reported with MDS,13 we have checked that the MPS monolayer is completely permeable toward the diffusion of a cationic and hydrophilic redox probe such as Ru(NH3)63+ and that identical voltammograms are recorded on Au and Au-MPS electrodes. Figure 7 shows that the cyclic voltammetry (0.1 V s-1) of FcOH and Fe(CN)63- is only slightly perturbed by the presence of (46) Perdriel, C. L.; Arvia, A. J.; Ipohorski, M. J. Electroanal. Chem. 1986, 215, 317.

Mokrani et al.

Figure 7. Cyclic voltammograms (0.1 V s-1) recorded on (a) a bare Au electrode and (b) a modified Au-MPS electrode in 0.1 M phosphate buffer pH 6.5 solution containing 0.4 mM of (A) FcOH, (B) Fe(CN)63-, and (C) catechol.

the MPS layer whereas the response of catechol is strongly inhibited. It is necessary to record the voltammograms at higher potential sweeping rates in order to show that electron transfer toward FcOH and Fe(CN)63- occurs at a lower rate on Au-MPS than on bare Au. To the best of our knowledge, this extreme dual behavior exhibited by Au-MPS electrodes has never been reported. The fact that electrochemistry of catechol is strongly depressed suggests as a first conclusion the absence of large pinholes, as those depicted in Figure 1A, and a rather homogeneous distribution of MPS, although non densely packed, over the

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whole gold surface as depicted in Figure 1B. To rationalize experimental observations, we have performed a detailed kinetic study of the various redox probes on bare Au and modified Au-MPS electrodes and tentatively interpreted the results on the basis of the various structural models that have been reported to account for the properties of self-assembled monolayers (Figure 2). One of the structural models indicates that the MPS layer is not perfect, so that MPS-free pinholes of size larger than molecular size exist where electron transfer only occurs (Figure 2C). The MPS surface coverage percentage can be quantified by the parameter θ, whereas (1 - θ) represents the corresponding percentage for pinholes. Assuming that all the current is passed via the bare spots on the electrode, it has been shown35 that at high surface coverage (θ ∼ 1), the decrease of the apparent heterogeneous charge-transfer constant is related to θ and is given by the following expression

k°Au-MPS ) (1 - θ)k°Au

(4)

where k°Au and k°Au-MPS are the heterogeneous charge transfer constant measured on the bare Au and the modified Au-MPS electrodes, respectively. Figure 7C shows that the presence of the MPS layer dramatically slows the rate of catechol oxidation relative to bare Au electrode. This decrease in the rate of electron transfer causes the kinetically limited region of the voltammogram to extend to the full extent of the voltammetric scan. Mass transfer limitations are therefore not significant for these data. When the charging current was subtracted from the measured current (the charging current was recorded in pure electrolyte), the resulting faradaic current of catechol oxidation was found to be independent of the potential scan rate (Figure 8A). The data of Figure 8A could be presented in the form of a Tafel plot with excellent correlation coefficients (Figure 8B). The general mechanism of quinonic compounds electrochemical behavior can be decomposed into four elementary steps involving sequential transfer of either one electron or one proton. Thus, there are six possible different mechanisms involving nine different possible quinonic species that are commononly described in a square scheme47 (Scheme 1). By use of the model of Laviron,48 the electrochemistry of the orthoquinone/catechol (Q/H2Q) system can be formally described as two successive electron-transfer steps E1E2 with apparent electron-transfer rate constants k°1 and k°2 (symmetry coefficient R ) 0.5) and formal potentials E°1 and E°2 that reflect pure electron transfer and protonation or deprotonation steps (Scheme 1). Such an E1E2 apparent mechanism was recently used by Bard et al.29 for studying the permeation process of menadione through SAM-modified gold electrodes. In the present work we used a simpler model also based on the work of Laviron.48 This model considers the electrochemical oxidation of catechol as a direct two-electron exchange process occurring at a formal potential E° equal to 1/2(E°1 + E°2) as shown in Scheme 1. Depending on the conditions, the overall two-electron electrochemical process is kinetically controlled by one of the two steps E1 or E2. This model was applied in the present work to characterize the electrochemistry of catechol on bare Au and modified Au-MPS electrodes and to determine the apparent limiting electron-transfer rate constants on each of these surfaces. On bare Au electrode, a cyclic voltammogram of catechol (47) Chambers, J. Q. In The chemistry of the quinonoid compounds; Patai, S., Ed.; John Wiley, New York, 1974, pp 737-791. (48) Laviron, E. J. Electroanal. Chem. 1984, 164, 213.

Figure 8. (A) Faradaic current intensity and (B) Tafel plots for 0.4 mM catechol oxidation recorded in 0.1 M phosphate buffer pH 6.5 on a modified Au-MPS electrode at a potential sweeping rate of (b) 0.1, (+) 0.01, and (×) 0.001 V s-1. The faradaic current was obtained by subtracting the capacitive current recorded in pure electrolyte to the current measured in the presence of catechol. Tafel plots were obtained by assuming E° ) 0.196 V and k ) i/nFAC. The full lines in part B represent the linear fittings. Intercepts (correlation coefficients) were -5.22 (0.9989), -5.12 (0.9965), and -5.28 (0.9963) for 0.001, 0.01, and 0.1 V s-1, respectively.

at v ) 0.1 V s-1 (Figure 7 C) exhibits one oxidation peak at Epa ) 0.243 V and one reduction peak at Epc ) 0.144 V and ∆Ep ) Epa - Epc ) 0.099 V. As the potential scan rate increases from 0.1 to 8 V s-1, the anodic and cathodic peak current intensities are proportional to v1/2 whereas Epa and ∆Ep increases. E1/2 ) 1/2(Epa + Epc) remains constant and equal to 0.196 V. At high potential scan rates, oxidation of catechol on bare gold was thus considered to occur following an irreversible direct two-electron exchange process with apparent E° ) 0.196 V. The apparent rate-limiting electron-transfer constant k°Au(catechol) was determined assuming the following relation for an irreversible system49 with n ) 2:

[(

ip ) 0.227nFACk° exp Rappn

F (E - E°) RT p

)]

(5)

(49) Bard, A. J. and Faulkner, L. R. In Electrochemical methods, fundamentals and applications; John Wiley: New York, 1980.

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Scheme 1. Reaction Scheme for Catechol (QH2)/ Orthoquinone (Q) Redox System

at MPS-free pinholes or defects where the redox probe can diffuse, that is, pinholes or defects whose size is a priori larger or equal to the probe molecular size. It is also assumed that electron transfer at the MPS-covered surface is inefficient. In fact, electron transfer could also occur at the MPS-covered surface by electron tunneling effects through the MPS layer, the corresponding electrontransfer rate constant being k°tunnel (Figure 2A). Miller and co-workers31,32 have investigated tunneling effects at ω-hydroxythiol coated electrodes by using outer-sphere monoelectronic redox systems. They show that the electron transfer rate constant decreases exponentially with chain length of the alkanethiol with a tunneling factor of 0.9 per methylene unit

k°tunnel ) k°Au exp(-0.9nMe)

A plot of ln(ip) versus (Epa - E°) gave a straight line (r ) 0.984), and from slope and intercept we could determine k°Au(catechol) ) 3.4 × 10-3 cm s-1 and Rapp ) 0.37. It has been shown48 that when the two-electron process is kinetically controlled by the second equivalent electron exchange (Scheme 1), i.e., by the ladder on the right of the reaction scheme, the anodic apparent transfer coefficient is equal to 0.25, while it is equal to 0.75 when the first equivalent electron exchange (i.e., the ladder on the left of the reaction scheme) is rate limiting. Since in the case of catechol we determined a Rapp value of 0.37 that is near to 0.25 as compared to 0.75, this suggests that oxidation of catechol is controlled by the kinetic of the ladder on the right of the reaction scheme (Scheme 1). Moiroux et al. obtained a similar conclusion concerning oxidation of reduced ubiquinone.50 The apparent rate-limiting electron-transfer constant k°Au-MPS (catechol) for catechol oxidation on modified Au-MPS electrode was determined from the Tafel plots of Figure 8B. This figure presents for three different potential scan rates a plot of ln(k) as a function of (E - E°) with

k)

[(

i nFAC

k ) k° exp Rappn

F (E - E°) RT

(6)

)]

(7)

From the average value of intercept (Figure 8B) we determine a rate constant k°Au-MPS(catechol) ) 6.3 × 10-6 cm s-1. Then by using the pinhole model and eq 4, the MPS surface coverage was determined as θ ) 0.9981, which is a quite high and unexpected value. Independently of the MPS packing density, this high θ value indicates that concerning catechol electrochemistry, a quasi-perfect surface coverage of gold is achieved by MPS. In this pinhole model, it is assumed that electron transfer can only occur (50) Marchal, D.; Boireau, W.; Laval, J. M.; Bourdillon, C.; Moiroux, J. J. Electroanal. Chem. 1998, 139, 144.

(8)

Although catechol does not correspond to a monoelectronic outer-sphere system, it is interesting to estimate the hypothetical tunneling factor if it is assumed that electrooxidation of catechol on Au-MPS only occurs by electron tunneling through the MPS layer (that is k°Au-MPS ) k°tunnel). Taking a number of equivalent methylenes, nMe ) 4, to account for the chain length of MPS and thus for the MPS layer thickness, eq 8 gives a much higher value, i.e., 1.58, for the tunneling constant than the one observed with ω-hydroxythiol, i.e., 0.9. This agrees well with the fact that electron transfer to catechol is strongly inhibited by the MPS layer. This high value as compared to values obtained with the outer-sphere monoelectronic redox system could also be a consequence of a more complicated mechanism for catechol oxidation that involves the sequential transfer of two electrons and two protons. Taking into account that electrooxidation of catechol on Au-MPS electrode could occur at the same time via pinhole and via electron tunneling through the MPS layer, this means that the determined MPS fractional surface coverage, θ ) 0.9981, is a value by defect and should be even higher. On the other hand, oxidation of catechol could also occur via a permeation process through the MPS layer, acting thus as a membrane, as depicted in Figure 2B. The low current observed for catechol oxidation on Au-MPS could be the consequence of several limiting steps: (i) An unfavorable partition coefficient κ. This appears improbable because catechol is higly hydrophilic despite its organic character and the MPS layer is highly hydrated as a consequence of its nondensely packed structure. (ii) A very low diffusion coefficient Df in the film. This also appears highly improbable because the MPS layer is not densely packed. (iii) A high kinetic barrier for crossing the MPS layer/bulk interface, which is a low value of the kinterface rate constant. This step appears to be the rate-limiting step of the eventual overall permeation process. Since the MPS layer is very thin, about 1.4 nm, it is presently difficult to assess if electron transfer occurs preferentially on the gold surface after permeation or by electron tunneling at the MPS layer/ bulk interface or if both processes occur simultaneously. It is striking to observe that in contrast to catechol, electrochemistry of neutral ferrocenemethanol and anionic ferrocyanide is only slightly perturbed by the presence of the MPS layer (Figure 7). To get better insight, we performed a kinetic analysis of the electrochemical behavior of FcOH and Fe(CN)63- on bare Au and modified Au-MPS electrodes. Cyclic voltammograms were recorded for scanning potential rates ranging from 0.1 to 25 V s-1. Both FcOH and Fe(CN)63- behave as quasireversible monoelectronic redox systems on Au and Au-MPS electrodes and the CVs were analyzed following the method

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Table 1. Electrochemical Characteristics Recorded on Au and Au-MPS Electrodes electrolytic medium

Au

phosphate buffer 0.1 M, pH ) 6.5 catechol 0.4 mM in phosphate buffer 0.1 M, pH ) 6.5

CAu ) 15.0 µF k°Au(catechol)b ) 3.4 × 10-3 cm s-1 symmetry coefficient ) 0.37 E° ) 0.196 V k°Au(FcOH)b ) 0.16 cm s-1 E° ) 0.207 V k°Au[Fe(CN)63-]b ) 0.04 cm s-1 E° ) 0.231 V

FcOH 0.4 mM in phosphate buffer 0.1 M, pH ) 6.5 Fe(CN)63- 0.4 mM in phosphate buffer 0.1 M, pH ) 6.5

a

Au-MPS

cm-2

CAu-MPS ) 8.7 µF cm-2 k°Au-MPS(catechol)b ) 6.3 × 10-6 cm s-1 tunneling constantc ) 1.58 k°Au-MPS(FcOH)b ) 6.9 × 10-3 cm s-1 tunneling constantc ) 0.79 k°Au-MPS[Fe(CN)63-]b ) 4.4 × 10-3 cm s-1 tunneling constantc ) 0.55

a The interfacial capacitance is determined from a plot of charging current at 0.2 V versus potential sweeping rate. b See text for the determination of electron-transfer rate constant. c The tunneling constant is determined from relation 8, the length of MPS molecule being considered equivalent to four methylene units (see text).

of Nicholson and Shain,49 which allowed determining the electron-transfer rate of these two redox probes on Au and Au-MPS electrodes (Table 1). It was satisfying to observe that rate constants measured on bare gold electrodes are in good agreement with values reported in the literature. Assuming that MPS surface coverage is independent of the nature of the redox probe and thus equal to 0.9981 as determined with catechol, it can be seen that the experimental values k°Au-MPS(FcOH) and k°Au-MPS[Fe(CN)63-] are not compatible with an electron transfer occurring only at pinholes in the MPS layer. For example in the case of FcOH, (1 - θ)k°Au(FcOH) is equal to 3.0 × 10-4 cm s-1, which is 25-fold less than the measured value k°Au-MPS(FcOH) ) 6.9 × 10-3 cm s-1. Although the pinhole process could account for a low contribution, it is obvious that electron transfer toward FcOH mainly occurs either by electron tunneling or by permeation (membrane model) through the pinhole-free portion of the MPS layer that corresponds at least to a coverage θ ) 0.9981. It has been recognized that the mercaptoalkanesulfonated surfaces constitute a kinetic barrier toward the permeation of organic compounds because of its high hydrophilicity.13 Since neutral FcOH is characterized by an important hydrophobicity (as evidenced by a low aqueous solubility and in contrast with catechol), its permeation through the thin and highly hydrophilic MPS layer is expected to be a slow process. As a consequence, it is anticipated that electron transfer will essentially occur by electron tunneling. Assuming the tunneling model of Miller is available (equation 8), and taking again nMe ) 4, a tunneling constant of 0.78 could be determined for FcOH, which agrees well with the values of 0.9 and 1.1 reported for ω-hydroxythiol and alkanethiol, respectively, with other outer sphere and monoelectronic redox systems. Considering Fe(CN)63-, the apparent electron-transfer constant k°Au-MPS[Fe(CN)63-] is as well not compatible with diffusion at MPS free pinholes. Although highly hydrophilic, permeation of negatively charged Fe(CN)63- through the MPS layer should be also a slow kinetic process as a consequence of electrostatic interactions with sulfonate headgroups. Thus electron transfer is proposed to occur mainly by tunneling effects, the corresponding constant being ktunnel ) 0.55 (Table 1). Although the determination of the tunneling constants supports some uncertainties, the general trend is that this constant is higher for hydrophobic FcOH than for hydrophilic Fe(CN)63-. This means that tunneling electron transfer is less efficient toward FcOH than FeCN63-. This behavior can be rationalized by the fact that hydrophilic

Fe(CN)63- can approach the highly hydrophilic MPS surface at a shorter distance than hydrophobic FcOH. Since we are working at high ionic strength, electrostatic repulsion between sulfonate headgroups and Fe(CN)63is expected to occur only at a short distance such as that involved for the permeation of Fe(CN)63- through the MPS layer. Conclusion MPS molecules self-assemble onto gold surfaces in the form of a nondensely packed monolayer. The average surface area occupied by one MPS molecule is about 36 Å2 as compared to 21.4 Å2, which has been reported for a densely packed monolayer of alkanethiol. Despite this low packing density, the MPS monolayer covers the entire gold surface with a fractional surface coverage at least equal to θ ) 0.9981. This high surface coverage indicates a very low amount of MPS-free defects and pinholes. As previously reported the MPS monolayer is completely permeable toward a cationic and hydrophilic probe such as Ru(NH3)63+. In contrast the MPS monolayer acts as a kinetic barrier toward the electrochemistry of neutral and anionic redox species. Oxidation of catechol is strongly inhibited, leading to voltammograms that are free of diffusional contribution. This unique behavior has been used to estimate the θ fractional surface coverage of MPS. Neutral and hydrophobic FcOH as well as anionic and hydrophilic Fe(CN)63- present peak-shaped voltammograms that are characteristic of diffusionnal effects. It is proposed that the MPS monolayer is not permeable to these species and that electron transfer occurs mainly by electron tunneling effects through the MPS layer, the contribution of MPS-free pinholes and defects being low. At high ionic strength, hydrophilic Fe(CN)63- can approach the highly hydrophilic MPS surface at a shorter distance than hydrophobic FcOH, thus leading to a faster tunneling electron transfer. QCM-D experiments demonstrate that a layer of aqueous electrolyte is rigidly associated with the MPS monolayer. This aqueous overlayer could also influence to some extend electron kinetics at the Au-MPS interface in addition to the high negative charge of the MPS monolayer and its high hydrophilicity. Acknowledgment. Financial support from CMEP (project no. 03MDU575) is greatly acknowledged. Philippe Lavalle, Unite´ 595 INSERM, Strasbourg, is acknowledged for the realization of AFM imaging experiments as well as Vincent Ball, Pierre Schaaf, and Bernard Senger for scientific discussion. LA047125S