Effect of Geometric Constraints on the Self-Assembled Monolayer

3808

Langmuir 1998, 14, 3808-3814

Effect of Geometric Constraints on the Self-Assembled Monolayer Formation of Aromatic Disulfides on Polycrystalline Gold Krisanu Bandyopadhyay, Vijaya Patil, Murali Sastry, and K. Vijayamohanan* Physical/Materials Chemistry Division, National Chemical Laboratory, Pune 411008, India Received January 23, 1998. In Final Form: April 13, 1998 The self-assembled monolayer (SAM) formation tendency of two comparatively small aromatic disulfide molecules, namely naphthalene disulfide (NDS) and diphenyl disulfide (DDS), has been investigated using quartz crystal microgravimetry (QCM), cyclic voltammetric, and impedance techniques. The typical time period for monolayer formation on polycrystalline gold has been found to be about 2 h for both the molecules from the QCM data. A significant change in double-layer capacitance values (from 21 µF/cm2 for bare gold to ∼14 and ∼8 µF/cm2 for NDS and DDS, respectively) upon monolayer formation for both the cases has been observed, which correlates well with the QCM area per molecule values (∼49 and 36 Å2 for NDS and DDS, respectively). The difference in the permeability of the two monolayers to simple ionic species was also investigated using K3Fe(CN)6 as redox probes in aqueous solution. A mixed linear/radial mode of diffusion is observed at the DDS-modified electrode in contrast to a predominant linear one at the electrode derivatized with a NDS monolayer. Impedance measurements indicate apparent surface coverages of 99.6 and 99.8% and rate constants of 9.4 × 10-5 and 4.1 × 10-5 cm/s for the Fe(CN)63-/4- couple in the case of NDS and DDS, respectively. These results strongly demonstrate the effect of geometrical constraints in controlling the microscopic structure and the packing density of the SAMs and highlight the importance of intramolecular conformational changes in controlling the monolayer packing density.

Introduction The formation of self-assembled monolayers (SAMs) by spontaneous adsorption of molecules from solution onto suitable substrates1 has been shown to be a versatile technique for surface modification with a number of applications such as chemical sensors,2 nonlinear optical materials,3 high-density memory devices,4 and photopatterning methodology5 to name just a few. Although a wide variety of substrates and functional groups are known to form SAMs,1 the thiol/disulfide monolayer on Au has received considerable attention in recent years.6 Most of the thiol/disulfide compounds investigated so far contain a long hydrophobic tail, which enables these compounds to form a compact monolayer on Au surfaces.6c The bonding pattern in both thiols and disulfides is the same except for an oxidative dissociation of the S-S bond for disulfides.7 While the nature of surface attachment is believed to involve a Au-thiolate interaction, the possibility of thiolate group dimerization to form disulfides * To whom communication should be addressed. E-mail: viji@ ems.ncl.res.in. Fax: 0091-212-337044. (1) Ulman, A. An Introduction to ultrathin organic flims: from Langmuir-Blogett to Self-Assembly; Academic Press: New York, 1991. (2) Mirkin, C. A.; Ratner, M. A. Annu. Rev. Phys. Chem. 1992, 43, 719. (3) Li, D.; Ratner, M. A.; Marks, T. J.; Znang, C. H.; Yang, J.; Wong, G. K. J. Am. Chem. Soc. 1990, 112, 7389. (4) Kawanishi, Y.; Tamaki, T.; Sakuragi, M.; Seki, T.; Swuzki, Y.; Ichimura, K. Langmuir, 1992, 8, 2601. (5) (a) Wollman, E. W.; Kang, D.; Frisbie, C. D.; Larcovic, T. M.; Wrighton, M. S. J. Am. Chem. Soc. 1994, 116, 4395. (b) Tarlov, M. J.; Burgess, D. R. F., Jr.; Gillen, G. J. Am. Chem. Soc. 1993, 115, 5305. (6) (a) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (b) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358. (c) Porter, M. D.; Thomas, B. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (d) Miller, C.; Cudent, P.; Gra¨tzel, M. J. Phys. Chem. 1991, 95, 877. (e) Sunder, V. A.; Liedberg, B.; Allara, D. L. Langmuir 1995, 11, 3882. (f) Schnider, T. W.; Buttry, D. A. J. Am. Chem. Soc. 1993, 115, 12391. (g) Bain, C. D.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1989, 5, 723. (h) Wah, S.; Chang, T.; Biebuyck, H. A.; Whitesides, G. M.; Jeon, N.; Nuzzo, R. G. Langmuir 1995, 11, 4371. (7) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87.

on gold surfaces has also been recently considered.8 Most of the data obtained from different techniques are consistent with a closed packed, commensurate (x3 × x3)R30° overlayer structure with a c(4 × 2) superlattice (Scheme 1a) during the chemisorption of thiols and disulfides on Au(111) surfaces.9 In such a model, each sulfur head group may rest over a 3-fold hollow site on the underlying Au(111) surface. Detailed calculations confirm that the 3-fold hollow sites on the Au(111) surface are more stable than the possible bridge site or on-top site and that chemisorption is epitaxial.10 A recent highimpedance STM study of monolayer formation from rigid rodlike conjugated arene thiols11 also shows an ordered, commensurate SAM formation on the Au(111) surface. However, several studies are also available in which an ordered SAM has been formed but with an incommensurate overlayer structure.12 The structural and electrochemical studies of arenethiol monolayers on gold13 demonstrate that these molecules can effectively block (8) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216. (9) (a) Camillone, N., III.; Chidsey, C. E. D.; Liu, G. L.; Scoles, G. J. Chem. Phys. 1993, 98, 3503. (b) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853. (10) Sellers, H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389. (11) Dhirani, A.; Zehner, R. W.; Hsung, R. P.; Guyot-Sionnest, P.; Sita, L. R. J. Am. Chem. Soc. 1996, 118, 3319. (12) (a) Liu, G.-Y.; Fenter, P.; Chidsey, C. E. D.; Ogletree, D. F.; Eisenberger, P.; Sameron, M. J. Chem. Phys. 1994, 101, 4301. (b) Wolf, H.; Ringsdorf, H.; Delamarche, E.; Takami, T.; Kang, H.; Michel, B.; Gerber, C.; Jaschke, M.; Butt, H.-J.; Bamberg, E. J. Phys. Chem. 1995, 99, 7102. (c) Caldwell, W. B.; Campbell, D. J.; Chen, K.; Herr, B. R.; Mirkin, C. A.; Malik, A.; Durbin, M. K.; Dutta, P.; Huang, K. G. J. Am. Chem. Soc. 1995, 117, 6071. (13) (a) Tour, J. M.; Jones, L.; Person, D. L.; Lamba, J. J. S.; Burgin, T. P.; Whitesides, G. M.; Allara, D. L.; Parikh, A. N.; Atre, S. V. J. Am. Chem. Soc. 1995, 117, 9529. (b) Zehner, R. W.; Sita, L. R. Langmuir 1997, 13, 2973. (c) Sabatani, E.; Cohen-boulakia, J.; Bruening, M.; Rubinstein, I. Langmuir 1993, 9, 2974. (d) Kwan, W. S. V.; Atanasoska, L.; Miller, L. L. Langmuir 1991, 7, 1419. (e) Kwan, V. W. S.; Cammarata, V.; Miller, L. L.; Hill, M. G.; Mann, K. R. Langmuir 1992, 8, 3003. (f) Hutchison, J. E.; Postlethwaite, T. A.; Murray, R. W. Langmuir 1993, 9, 3277.

S0743-7463(98)00088-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/16/1998

Aromatic Disulfides on Polycrystalline Gold Scheme 1. (a) Atomic View of the Au(111) Plane with the Bonding Mode of Sulfur Atoms for Thiols/ Disulfides. (b) Simulated View of an Adsorbed NDS Molecule on a Au(111) Surface with One Sulfur at the 3-Fold Hollow Site and Another at the Bridge Site

Langmuir, Vol. 14, No. 14, 1998 3809 Scheme 2. Possible Dissociation Modes of Two Disulfides and the Corresponding Distance between the Two Sulfur Atoms on a Gold Surface during SAM Formation

(a)

(b)

constraints and can form a normal overlayer with a S-S distance of 5 Å, which is the distance between the two 3-fold hollow sites on the Au(111) surface16 (see Scheme 1a). Quartz crystal microgravimetry (QCM), cyclic voltammetry (CV), and impedance measurements are employed to rationalize the comparative self-assembling ability and ion permeation through the two monolayers on the basis of their structural differences. The results of the study are believed to be important for understanding the improved stability of SAMs from structurally constrained rigid disulfides.17 the electron transfer across the film and that the average tilt of the aromatic rings can range from vertical13c to partially tilted13d or parallel to the surface.13f In this article, we extend our earlier studies on monolayer formation with small aromatic disulfides14 through an investigation of the monolayer-forming ability of naphthalene disulfide (NDS) and diphenyl disulfide (DDS). The main emphasis is on understanding the role played by the fused naphthalene ring, which places a constraint on the S-S distance in NDS, on the monolayer quality (in terms of molecular packing and ion permeation) with the diphenyl disulfide molecule not facing such a constraint. More specifically in the case of NDS, the two sulfur atoms must retain the bonding distance of 2.4 Å (i.e. the actual S-S distance in the present aryl disulfide)15 even if normal dissociative chemisorption takes place unless a S-C bond is broken (Scheme 2a). For DDS, on the other hand, the dissociation of the S-S bond gives rise to two separate molecules (Scheme 2b), as observed in the case of long chain alkane disulfides. Keeping these facts in mind, it is unlikely that both the sulfur atoms in the NDS molecule will be able to occupy the normal 3-fold hollow sites, as has been proposed for long chain alkanethiols/disulfides adsorbed on gold.16 The inherent rigidity in the molecule may direct the other sulfur to coordinate to a quasi-bridge site (the distance between one 3-fold hollow site and a bridge site is equal to 2.4 Å, which is equal to the S-S distance in the case of NDS; Scheme 1b) on the gold surface, which is energetically unfavorable. In contrast, DDS is free from such structural (14) (a) Bandyopadhyay, K.; Sastry, M.; Paul, V.; Vijayamohanan, K. Langmuir 1997, 13, 866. (b) Bandyopadhyay, K.; Vijayamohanan, K.; Shekhawat, G. S.; Gupta, R. P. J. Electroanal. Chem., in press. (15) Zweig, A.; Hoffmann, K. J. Org. Chem. 1965, 30, 3997. (16) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546.

Experimental Section Materials. The gold substrates for these experiments were prepared by thermal evaporation of ∼2000 Å thick gold (purity 99.99%) onto conventionally cleaned glass substrates under highvacuum conditions (pressure better than 10-6 Torr) in an Edward E 306 coating unit. A 50 Å thick chromium buffer layer was used to improve adhesion of the Au film. This procedure produces polycrystalline Au films with strong (111) texture. All the substrates were stored in polypropylene containers until use. NDS was prepared by the reported procedure,18 and its purity was found to be 99.99% by GC-mass and NMR studies. DDS and octadecanethiol were obtained from Aldrich and used as received. All solvents were of reagent grade and used without further purification. Preparation of the SAMs. The gold-coated glass slides were cut to the required size (∼0.54 cm2) for use as electrodes. Prior to their derivatization with the aromatic disulfides, the Au substrates were cleaned through repeated exposures to 50 °C sulfochromic acid (saturated K2Cr2O7 in concentrated H2SO4) and to 3% aqueous HF. The cleaning with the sulfochromic acid was performed for only few seconds, which is sufficient to remove the organic impurities but provides inadequate exposure to form an oxide layer. The reproducible surface properties of gold electrodes were checked by performing cyclic voltammetric experiments with bare gold in an aqueous solution of the Fe(CN)63-/4- couple after holding the electrode cathodically for a few minutes at -0.8 V versus SCE.13b Extensive rinsing of the substrates with deionized water (Millipore system) followed each of these exposures. After three such cycles, the electrodes were immediately transferred into a deaerated 1 mM solution of the respective compounds in acetonitrile. The exposure time was typically ∼2 h, as determined from QCM measurements. The substrates were removed after the specified time, rinsed with the solvent and absolute ethanol, and then dried in a stream of Ar. (17) Schlenoff, J. B.; Li, M.; Ly, H. J. Am. Chem. Soc. 1995, 117, 12528. (18) Gamage, S. A.; Smith, R. A. J. Tetrahedron 1990, 46, 2111.

3810 Langmuir, Vol. 14, No. 14, 1998 QCM Measurements. QCM measurements were performed on an Edwards FTM5 microbalance. A 6 MHz AT cut quartz crystal was used with the microbalance having a resolution and stability of the order of (1 Hz. This translates into a mass resolution of 12.1 ng/cm2, which for common SAM-forming molecules is well within the monolayer coverage range. Frequency changes were converted to mass loading using the Sauerbrey formula.19 All measurements were performed ex situ after thorough washing and drying of the crystal prior to frequency measurements. Electrochemical Experiments. Impedance and cyclic voltammetric measurements were performed using a three-electrode cell comprising a gold-coated glass as the working electrode, a large area platinum flag counter electrode, and a saturated calomel reference electrode (SCE). Cyclic voltammetry and impedance measurements were carried out with a PAR 283 potentiostat/galvanostat and a PAR 5012 lock-in amplifier interfaced with a computer. Impedance measurements were performed at 5 mV rms at the formal potential of the redox couple, and readings were taken at five discrete frequencies per decade. Impedance analysis was carried out using the commercially available program EQUIVALENT CIRCUIT written by B. A. Boukamp (University of Twente), which determines the parameters of the assumed equivalent circuit by a weighted nolinear least-squares fit. Cyclic voltammetry was performed in a solution containing only one oxidation state of the redox couple, like 1 mM K3Fe(CN)6 in 0.1 M KF. The solution used for impedance measurements always contained equal concentrations of the oxidized and reduced forms of the redox couple, like 5 mM both K3Fe(CN)6 and K4Fe(CN)6 in 0.5 M KF in the frequency range 100 kHz to 0.1 Hz. All experiments were carried out at room temperature (25 ( 0.1 °C). The double-layer capacitance of the disulfide-treated Au electrodes was measured from cyclic voltammograms performed from -100 to +500 mV in a 0.1 M KF solution at a scan rate of 500 mV/s. The current from the positive and negative scan directions at +200 mV was summed and divided by twice the scan rate and normalized by the actual area of the electrode to obtain the reported capacitance. The surface roughness factor (R ) actual/geometrical surface area) of the Au was determined according to standard literature procedures.20

Bandyopadhyay et al.

Figure 1. Variation of the mass uptake with time for a goldcoated 6 MHz AT cut quartz crystal during the SAM formation using 1 mM of (a) NDS and (b) DDS in acetonitrile.

Results and Discussions: QCM. Figure 1 shows the variation of mass uptake with time for both the NDS (1a) and DDS (1b) molecules in acetonitrile. Although both curves are similar in nature, the one corresponding to DDS is quite steep, suggesting a faster adsorption rate than that for NDS. Steady-state mass changes of ∼150 and ∼200 ng/cm2 are obtained for monolayers of NDS and DDS, respectively. The areas per molecule are calculated from these values to be 49 and 36 Å2 for naphthalene and diphenyl disulfide, respectively. For calibration purposes, a SAM of octadecanethiol chemisorbed onto the quartz crystal from an absolute ethanol solution yielded an area/molecule of 21 Å2, which is in good agreement with available data.21 On the basis of the QCM data shown in Figure 1, the typical time for monolayer formation is determined to be ∼2 h. Furthermore, a better closely packed molecular assembly for DDS than NDS is indicated. This could be explained due to the ease of relaxation during bond breaking for DDS while, for NDS, the fused ring may impose constraints for lateral movement. Since ex situ QCM measurements are generally problematic due to the possible gradual shift in the baseline, blank experiments were performed with only the solvent and respective disulfides. The mass uptake data after subtraction of the results of blank experiments underline the importance of intramolecular (19) Sauerbrey, G. Z. Phys. 1959, 155, 206. (20) Oesch, O.; Janata, J. Electrochim. Acta 1983, 28, 1237. (21) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Yu-Tai; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152.

Figure 2. Variation of double-layer capacitance values with potential for (a) bare gold, (b) NDS-modified gold, (c) DDSmodified gold, and (d) octadecanethiol-modified gold electrodes in 0.1 M aqueous KF.

conformational changes in controlling the monolayer packing density. Double-Layer Capacitance Study. Figure 2 shows a plot of differential capacitance in a limited range versus applied potential in a 0.1 M KF solution (pH ) 7) for bare gold and gold electrodes modified with both NDS and DDS monolayers. These double-layer capacitance values are obtained from cyclic voltammetric data at a scan rate of 500 mV/s, as discussed in the Experimental Section, and

Aromatic Disulfides on Polycrystalline Gold

the corresponding value for octadecanethiol is also included for comparison. The double-layer capacitance value for bare gold is found to be ∼21 µF/cm2, in good agreement with available data.22 The roughness factor of the substrate is measured to be 1.8, and this value is used throughout the study to calculate the differential capacitance values. The roughness factor of the present electrodes is high compared to the commonly observed values (1.1-1.2) for evaporated gold films, and this variation may be possible since in our case the gold surface was not annealed after evaporation. The differential capacitance for an electrical double layer is known to decrease with increasing separation between the electrode surface and the plane of closest approach for the ionic charges.23 Hence double-layer capacitance studies can provide additional insight into the average structure of the self-organized assemblies. The simple parallel plate capacitance model of the electrode-electrolyte interface was shown to be successful in the case of electrodes derivatized with long chain alkanethiol monolayers.d,24 Our capacitance measurements at several potentials demonstrate that both the disulfides are chemisorbed on the gold surface, as manifested from the decrease in double-layer capacitance compared to that for bare gold. As the differential capacitance is extremely sensitive to the interfacial composition, these changes can be commonly used to evaluate the extent of monolayer formation and quality. The measured capacitance values are higher for both the disulfides than those reported for compact monolayers of long chain alkanethiols such as octadecanethiol.6c The higher values may be a combined effect of the presence of pinholes in the monolayer or due to the smaller molecular dimensions of the disulfides with delocalized π-electrons. However, it should be noted that the observed double-layer capacitance values are reasonably close to the values reported in the literature13c for such small aromatic molecules. Among the two disulfides, the double-layer capacitance value is comparatively low for DDS, indicating a greater compactness and the presence of less pinholes, supporting the independent conclusion obtained using QCM data. Heterogeneous Electron Transfer on a Monolayer Coated Electrode. It is known that SAMs can create a lipophilic barrier for electron transfer between electrode surfaces and hydrophilic electroactive probes in solution.6d The extent of kinetic hindrance of the electron-transfer process imposed by the monolayer assembly depends on the thickness of the monolayer and the presence of pinholes in it. For a monolayer of fixed thickness, the degree of electrode blocking increases as the density of defects decreases.d,25 Consequently, the quality of the selfassembled monolayers can be investigated by their effects on the voltammetric behavior of electroactive species present in solutions. Accordingly, different cyclic voltammetric responses can be obtained which are controlled by the nature of the defects present in the SAM. Among the possibilities, there can be a limiting behavior of complete blocking of electron transfer6d or a response similar to those of microarray electrodes,26,27 depending on several factors such as defects in the monolayer, the thickness of the molecules, the nature, charge, and geometry of the probe molecules,26 and so forth. Long (22) Angerstein-Kozlowska, A.; Conway, B. E.; Hamelin, A.; Stoicoviciu, I. J. Electroanal. Chem. 1987, 228, 429. (23) Delahay, P. Double layer and electrode kinetics; Interscience publishers: New York 1965. (24) Sabatini, E.; Rubinstein, I. J. Phys. Chem. 1987, 91, 6663. (25) Bilewicz, R.; Majda, M. J. Am. Chem. Soc. 1991, 113, 5464. (26) Chailapakul, O.; Crooks, R. M. Langmuir 1995, 11, 1329. (27) Chailapakul, O.; Crooks, R. M. Langmuir 1993, 9, 884.

Langmuir, Vol. 14, No. 14, 1998 3811

Figure 3. (a) Cyclic voltammogram of (1) NDS- and (2) DDSmodified gold electrodes in a 0.1 M aqueous KF solution at the scan rate 200 mV/s. (b) Cyclic voltammogram of 1 mM K3Fe(CN)6 in 0.1 M KF at (1) bare gold and (2) NDS- and (3) DDSmodified gold electrodes after 2 h of dipping in a 1 mM solution of the respective disulfide in acetonitrile at a scan rate of 200 mV/s.

chain thiols/disulfides are reported to form compact monolayers, free from measurable pinholes, which provide substantial barriers to electron transfer and strong resistance to ion permeation. Figure 3a shows the superimposed cyclic voltammetric response of NDS- and DDS-modified gold electrodes, respectively, in 0.1 M KF (pH ) 7) without any redox probe in the solution. Comparison of the voltammograms reveals a lower charging (non-Faradaic) current in the case of the DDS monolayer, pointing toward a more compact assembly. This provides additional support for more efficient organization of the DDS molecules over the NDS ones. Figure 3b shows the superimposed cyclic voltammetric response of the Fe(CN)64-/3- couple at bare gold and NDSand DDS-monolayer-modified electrodes, respectively. In both the modified electrodes a drastic reduction of the non-faradaic current is distinct compared to that for the bare gold electrode. Moreover, for the DDS-modified electrode the reversible response of the redox couple is not seen, unlike the case of the NDS one, where small voltammetric peaks for the Fe(CN)63-/4- couple are clearly observed. These features suggest that the NDS monolayer is incapable of providing an effective barrier to electron transfer of the redox couple in solution due to the presence of pinholes. Further, among the two cases the decrease in charging current is more for the DDS-modified electrode, suggesting that the monolayer obtained from DDS is more compact (i.e., with a smaller number of pinholes) than the NDS one. The cyclic voltammograms taken here in all the cases correspond to the first scan, beginning with the positive direction, although subsequent cycles also give similar responses. To confirm the defect distribution and mode of electron transfer through SAMs of the two

3812 Langmuir, Vol. 14, No. 14, 1998

disulfides, voltammograms of the redox probe were taken at different scan rates with both the monolayer-modified electrodes (Figures S1 and S2 in the Supporting Information). Considering the two scan rates 10 and 200 mV/s where the voltammograms are well behaved, the current at a particular potential (+100 mV) changes nearly 90% for DDS as compared to 450% for NDS. It is known that, for a radial mode of diffusion, almost no change in current within these scan rates is expected as compared to a large current change for the linear mode of diffusion.28 Hence, it appears from the shape of the voltammograms and the observed current change with scan rates that the pinholes present in the DDS monolayer are sufficiently small in dimension and widely spaced such that their diffusion layers do not overlap, whereas the current-potential response for NDS shows that the pinholes are bigger in size and spaced closely enough to permit overlap of their diffusion layers. The current change observed in the present case suggests a mixed linear/radial mode of electron transfer at the DDS monolayer-modified electrode as compared to the NDS monolayer, where a predominant linear mode is evident. Therefore, it can be concluded that NDS forms a less ordered monolayer assembly in comparison that for DDS, which forms a more closely packed one due to the structural rigidity. These results suggest that the DDS monolayer should be more stable than the NDS one. This enhanced stability of DDS is in complete agreement with the results of temperature dependent surface-enhanced Raman spectroscopic (SERS) studies on these two monolayers, where a loss of intensity of all the major bands is obtained at 348 K for NDS and at 448 K for DDS monolayers, respectively.29 This significant difference in the decomposition temperatures of two monolayers strongly demonstrates that the DDS monolayer is more thermally stable and therefore supplements the conclusions drawn from the electrochemical studies. Impedance Analysis in the Presence of [Fe(CN)6]3-/ [Fe(CN)6]4-. Impedance measurements are based on the response of an electrochemical cell to a small amplitude alternating signal. The response is often analyzed in terms of complex impedance diagrams and interpreted using a suitable electrical circuit. According to Randle’s equivalent circuit,30 two frequency regions can be distinguished to understand the effect of the monolayer on the response of electroactive species at the electrode-SAM-electrolyte interface. In the low-frequency region, mass transfer via diffusion has to be taken into account, where the microarray behavior of the pinholes within the passivating monolayer disturbs the ω-1/2 dependence of the Warburg impedance.31 At higher frequencies, the electrode reaction is purely kinetic-controlled and the heterogeneous chargetransfer resistance (Rct) is expected to increase from that at the bare electrode due to the inhibition of electrontransfer by the monolayer present on the electrode surface.32 It is known that in this frequency region the diameter of the semicircle, to some extent, can indicate the charge-transfer resistance of the monolayer-coated electrodes.24 A comparison of the complex impedance plots of bare gold and monolayer-coated gold in Figure 4 shows (28) Whitman, R. M.; Wipf, D. O. In Electroanalytical Chemistry; Bard, A. J., Ed.; Dekker: New York, 1989; Vol. 15. (29) Bandyopadhyay, K.; Vijayamohanan, K.; Venkatramanan, M.; Pradeep, T. Communicated to J. Phys. Chem. (30) Sluyters-Rehbach, M.; Sluyters. J. H. In Electroanalytical Chemistry; Bard, A. J., Ed.; Dekker: New York, 1989; Vol. 15. (31) Finklea, H. O.; Sinder, D. A.; Fedyk, J.; Sabatani, E.; Gafni, Y.; Rubinstein, I. Langmuir 1993, 9, 3660. (32) Amatore, C.; Saveant, J. M.; Tessier, D. J. Electroanal. Chem. 1983, 147, 39.

Bandyopadhyay et al.

Figure 4. Complex impedance plots at a dc bias of 0.22 V versus SCE in 5 mM K3Fe(CN)6 + 5 mM K4Fe(CN)6 in a 0.5 M aqueous KF solution for a (a) bare gold electrode, (b) gold electrode derivatized with an NDS monolayer for 2 h, and (c) gold electrode derivatized with a DDS monolayer for 2 h. The frequency range used is 100 kHz to 100 mHz with a 5 mV rms signal at 5 steps per decade.

the effect of adsorbed NDS and DDS monolayers on the response of gold electrodes, respectively. From the analysis of the spectra shown in Figure 4b and c, we obtain charge-transfer resistances of about 567 and 1300 Ω cm2. It should be noted that in all the cases well-defined semicircles are absent in the high-frequency region. The extraction of the Rct value at a bare gold electrode is not tried, since the electron transfer of the Fe(CN)63-/4- couple is very fast. On the other hand, the impedance data for NDS- and DDS-monolayer-modified electrodes have been analyzed using a nonlinear regression procedure to obtain the respective Rct values. Since the semicircles for both the cases are not well-defined, the quality of the fit is estimated by the χ2 (chi-squared) value, which was found to be less than 10-5, confirming a good fit. The Rct value obtained from the complex impedance plot can be used to estimate the apparent rate constant of the Fe(CN)63-/4couple at the monolayer-modified electrodes, since the monolayer assemblies provide a barrier for electron transfer, leading to an expected decrease in rate constant. Under these conditions, the charge-transfer resistance Rct can be expressed as

Rct ) RT/[F2(1 - θ)k0c] ) RT/[F2kappc]

(1)

assuming cox ) cred ) c, unit area of the electrode, and a one-electron first-order reaction, where kapp ) (1 - θ)k0 and θ is the surface coverage of the monolayer and k0 and

Aromatic Disulfides on Polycrystalline Gold

Langmuir, Vol. 14, No. 14, 1998 3813

kapp are the real and apparent standard heterogeneous rate constants, respectively. Thus kapp is calculated to be 9.37 × 10-5 and 4.09 × 10-5 cm/s from the Rct values in Figure 4b and c for NDS and DDS, respectively. A comparison of these measured apparent rate constants indicates that, at both the monolayer-modified electrodes, the rate constant of the redox couple has been decreased by 2 orders of magnitude (the rate constant of the redox couple at the bare gold electrode is 2.6 × 10-2 cm/s).24 This reduction is expected, as the organized assemblies form a barrier for electron transfer at the electrode-electrolyte interface, and the amount of decrease in the rate constant can be correlated with the degree of compactness. Interestingly, the rate constant calculated for the DDSmodified electrode is smaller than that for the NDSmodified one, suggesting a more compact and organized SAM structure in the former case. The impedance response of the monolayer-coated electrode can be explained as an array of microelectrodes, and a related theory has been developed by Finklea et al.31 For a total defect area (pinholes) less than 0.1, both the real and the imaginary parts of the Faradaic impedance can be related to frequency (as ω-1/2). From this formulation it is evident that the high-frequency domain of Faradaic impedance is less affected due to nonuniformity of the defects as a result of uneven size and spacing of the pinholes. Consequently, this portion can be analyzed to obtain the surface coverage under the assumptions that (i) 1 - θ is less than 0.1, (ii) the electrolyte contains equal concentrations of the oxidized and reduced forms of the redox couple, and (iii) the diffusion coefficients are equal. At higher frequencies corresponding to nearly isolated diffusion profiles for each microelectrode, the following expressions should be used.

Zf′ ) [Rct/(1 - θ)] + [σ/xω] + [σ/(1 - θ)xω]

(2)

Zf′′ ) [σ/xω] + [σ/(1 - θ)xω]

(3)

Similarly at low frequency,

Zf′ ) [Rct/(1 - θ)] + [σ/xω] + [σRa(0.72/D)1/2/1 - θ] (4) Zf′′ ) [σ/xω]

(5)

where Rct is the charge-transfer resistance, θ is the coverage, σ is the Warburg coefficient, ω is the frequency, D is the diffusion coefficient, Ra is the radius of the pinhole, and Zf′ and Zf′′ are the real and imaginary components of the Faradaic impedance, respectively. The essential requirement of the above set of equations is that the diffusion layers of the individual microelectrodes do not strongly overlap. In Figure 5 the real part of the Faradaic impedance (Zre) has been plotted as a function of ω-1/2 for the data shown in Figure 4 after subtracting the solution resistance and interfacial capacitance from the measured total impedance. The values for the uncompensated resistance and capacitance are obtained from the impedance data corresponding to the highest frequency. The Faradaic impedance plot shows features similar to those of a microarray electrode. The simulated behavior for an ideal microarray electrode indicates two nearly linear domains at high and low frequencies31 although our curves resemble more a monotonically increasing function with ω-1/2. This deviation from the ideal microarray behavior may be due to the nonuniform distribution of pinholes on the monolayer surface. The slope of the Zf′ versus ω-1/2 plot in the higher frequency region can also give an

Figure 5. Superimposed plot of the real part of the Faradaic impedance (Zre) versus ω-1/2 in 5 mM K3Fe(CN)6 + K4Fe(CN)6 in a 0.5 M aqueous KF solution for bare gold (a) and a gold electrode modified with NDS (b) and DDS (c).

approximate estimate of the surface coverage of the NDS and DDS monolayers using known values of the Warburg coefficient (σ). The value of σ can be obtained experimentally with a bare electrode in the same solution. Analysis of Figure 5 yields 1 - θ as 0.0046 and 0.0027 for NDS and DDS, respectively, which qualitatively confirm the greater compactness of DDS. The analysis of the Zf′ versus ω-1/2 plot at lower frequency for both the monolayers shows that the slope of the curve is decreased as ω-1/2 is increased (for bare gold it remains constant). Considering eqs 2 - 5 for NDS and DDS monolayers, one may expect the slope to be different at high ω due to differences in θ, whereas at low ω the slopes should be equal but the intercept should be different due to the possible difference in Ra. These equations also suggest that the Warburg coefficient (σ) can be obtained from the low-frequency region of the curve. It gives the values 216 and 594 Ω‚cm2/s1/2, respectively, for NDS and DDS monolayers while that of bare gold is 39.5 Ω‚cm2/s1/2. This increase in the apparent value of the Warburg coefficient for monolayer-coated electrodes suggests a lower diffusion coefficient or that the pinholes are distributed in patches31 over the surface instead of being uniformly distributed or perhaps both. An approximate determination of the monolayer dielectric constant is possible from the impedance results with a few additional assumptions. For example, if the measured total double-layer capacitance is assumed to be a parallel combination of two capacitors24

Ctotal ) Cdl + Cm ) C°gold(1 - θ) + Csθ

(6)

where Cdl is the capacitance at the pinholes, Cm is the charge across the monolayer C°gold is the capacitance of a pure gold electrode, and Cs is the capacitance of the complete disulfide monolayer assuming 100% coverage. In general, the capacitance is determined by the dielectric permittivity, which is determined by the packing density of the molecules and the thickness of the adsorbed layer, and these parameters have to be determined separately. Other than the capacitance C, independent measurements of the thickness and the electrode coverage are necessary to evaluate the dielectric permittivity of the adsorbed disulfide monolayers. The Cs values (listed in Table 1 along with other parameters obtained from impedance and QCM measurements) obtained for both the monolayers can be used to calculate the dielectric permittivity of the monolayer film (ns) by using the equation ns ) [Csd]/0, where d is the monolayer thickness (∼7 Å)

3814 Langmuir, Vol. 14, No. 14, 1998

Bandyopadhyay et al.

Table 1. Different Parameters Obtained for NDS and DDS SAMs on a Gold Surface parameter charge-transfer resistance, Rct (Ω cm2) apparent electrode coverage, θ (%) double-layer capacitance, C (µF/cm2) double-layer capacitance,a C (µF/cm2) defect-free monolayer capacitance, Cs (µF/cm2) monolayer dielectric constant,  apparent rate constant of the Fe(CN)63-/4- couple, kapp (cm/s) area/molecule,b (Å2)

NDS

DDS

567 99.6 14.9 14.6 14.8

1300 99.8 8.03 8.0 7.9

11.7 9.3 × 10-5

6.23 4.1 × 10-5

49

36

a

Obtained from cyclic voltammetric measurement in a 0.1 M KF solution without any redox probe in solution. The double-layer capacitance of bare gold is 21 µF/cm2, and that of octadecanethiolmodified gold is 1.4 µF/cm2. b From QCM measurement. The area/ molecule for octadecanethiol is 21 Å2.

obtained from a space-filling model and 0 is the permittivity of free space ()8.85 × 10-12 C2 N-1 m-2). The dielectric permittivity values obtained are 11.7 and 6.3 for NDS and DDS monolayers, respectively, which are in fairly good agreement with the values obtained by Rubinstein et al.13c for p-biphenyl mercaptan and pterphenyl mercaptan. Nevertheless they are significantly high compared to the values reported for pure benzene (2.28) and naphthalene (2.52),33 indicating the importance of geometric constraints. Summary and Conclusions In the present investigation two structurally different disulfides with comparable molecular dimensions have been used to form SAMs on polycrystalline gold in order to understand the significance of geometrical constraints. (33) CRC Handbook of Chemistry and Physics, 64 ed.; West, R., Ed.; CRC Press: Boca Raton, FL, 1984.

The primary conclusion from the study may be summarized as follows: (1) QCM measurements show a faster mass uptake and a smaller area/molecule for DDS. (2) Reduction of double-layer capacitance is higher for monolayers of DDS. (3) Heterogeneous electron transfer at the monolayer-modified electrodes indicates a mixed radial/ linear diffusion of the redox probe at the DDS-modified electrode while predominantly linear diffusion occurs for the NDS monolayer. (4) Impedance measurements in the presence of a redox couple show a greater apparent surface coverage and charge-transfer resistance and a larger reduction in the rate constant for DDS compared to the one with the NDS monolayer. (5) The DDS monolayer is thermally more stable than NDS one. All the above results indicate a more compact monolayer in the case of DDS in comparison to NDS although the apparent coverages are similar. The difference in organization of the two disulfides with comparable molecular dimensions is attributed to geometrical constraints present in the NDS molecule due to the presence of a rigid naphthalene ring. The results discussed here suggest the possible use of simple and small aromatic disulfide molecules to effectively modify electrode surfaces for several applications such as corrosion prevention, electrochemical sensors, and electrocatalysis. Acknowledgment. Acknowledgment K.B. thanks the University Grants Commission, New Delhi, Government of India, for a research fellowship while V.P. gratefully acknowledges financial support from the Council of Scientific and Industrial Research, Government of India. Supporting Information Available: Supporting Information Available: Scan rate dependent cyclic voltammograms of the Fe(CN)64-/3- couple at NDS- and DDS-modified gold electrodes at different scan rates (2 pages). Ordering information is given on any current masthead page. LA980088I