Ferrocenylalkylthiolates as a Probe of Heterogeneity in Binary Self

53, FcC6S−/HOC2S−Au, 2.8 × 10-10, 220/330 .... 0.1, 1.4 × 10-10, 0.21, 190, 1.0 × 10-10, 100, 300, 3.3 × 10-11, 91 ..... 35, n/a, 0.40a, 66, 3...
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Langmuir 2006, 22, 4438-4444

Ferrocenylalkylthiolates as a Probe of Heterogeneity in Binary Self-Assembled Monolayers on Gold Lawrence Yoon Suk Lee, Todd C. Sutherland,† Simona Rucareanu, and R. Bruce Lennox* Department of Chemistry and the FQRNT Centre for Self-Assembled Chemical Structures, McGill UniVersity, 801 Sherbrooke Street West, Montreal, Quebec, H3A 2K6, Canada ReceiVed December 7, 2005. In Final Form: February 2, 2006 The oft-cited complexity of tethered ferrocene electrochemistry in single component (FcRS-) or binary (FcRS-/ CH3R′S-) self-assembled monolayers (SAMs) on gold has been investigated. The complex voltammetry is shown to be linked to local electrostatics caused by the formation of the ferrocenium ion. This conclusion is reached by studying model effects in binary SAMs, where a cationic alkylthiolate (H3N+C11S-Au) is mixed with FcC12S-Au. A fitting procedure involving both a Gaussian and a Lorentzian distribution is used for deconvolution of the two peaks which are consistently observed in the SAMs when χsurf Fc g 0.2. The lower-potential (E°′ ) 250 mV) and higherpotential (E°′ ) 350 mV) voltammetric peaks are assigned to Fc moieties in “isolated” and “clustered” states, respectively. Use of this method to better understand SAM structure is demonstrated by distinguishing the degree of homogeneity in two binary SAMs of similar composition.

Introduction Self-assembled monolayers (SAMs) have been extensively characterized as well-organized arrays of organic molecules on metal surfaces.1-5 They provide a model system for studying interfacial electron transfer,6-9 and have been described in applications such as biosensors,10-14 corrosion protection,15-17 and molecular electronic devices.18-20 The alkylthiolate SAM on gold (RS-Au) is by far the most studied of all the SAM systems. The basis for many of the interesting RS-Au SAM applications involves mixed monolayers made up of two or more different molecules.21,22 Several methods of making these binary * To whom correspondence should be addressed. Tel: 514-398-3638. Fax: 514-398-3797. E-mail: [email protected]. † Current address: Department of Chemistry, University of Calgary, Calgary, Alberta, T2N 1N4, Canada. (1) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (2) Ulman, A. An Introduction to Ultrathin Organic Films: From LangmuirBlodgett to Self-Assembly; Academic Press: New York, 1991. (3) Bain, C. D.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1989, 5, 723. (4) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 1990. (5) Finklea, H. O.; Snider, D. A.; Fedyk, J.; Sabatani, E.; Gafni, Y.; Rubinstein, I. Langmuir 1993, 9, 3660. (6) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301. (7) De Long, H. C.; Buttry, D. A. Langmuir 1990, 6, 1319. (8) Murray, R. W., Ed. Molecular Design of Electrode Surfaces; John Wiley and Sons: New York, 1992. (9) Finklea, H. O. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; Vol. 19, p 109. (10) Campuzano, S.; Pedrero, M.; Pingarron, J. M. Talanta 2005, 66, 1310. (11) Boozer, C.; Ladd, J.; Chen, S.; Yu, Q.; Homola, J.; Jiang, S. Anal. Chem. 2004, 76, 6967. (12) Chou, H. A.; Zavitz, D. H.; Ovadia, M. Biosens. Bioelectron. 2003, 18, 11. (13) Chaki Nirmalya, K.; Vijayamohanan, K. Biosens. Bioelectron. 2002, 17, 1. (14) Motesharei, K.; Ghadiri, M. R. J. Am. Chem. Soc. 1997, 119, 11306. (15) Zamborini, F. P.; Campbell, J. K.; Crooks, R. M. Langmuir 1998, 14, 640. (16) Jennings, G. K.; Laibinis, P. E. J. Am. Chem. Soc. 1997, 119, 5208. (17) Ramachandran, S.; Tsai, B.-L.; Blanco, M.; Chen, H.; Tang, Y.; Goddard, W. A., III. Langmuir 1996, 12, 6419. (18) Kitagawa, K.; Morita, T.; Kimura, S. Langmuir 2005, 21, 10624. (19) Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Science 1999, 286, 1550. (20) Andres, R. P.; Bein, T.; Dorogi, M.; Feng, S.; Henderson, J. I.; Kubiak, C. P.; Mahoney, W.; Osifchin, R. G.; Reifenberger, R. Science 1996, 272, 1323. (21) Mrksich, M.; Sigal, G. B.; Whitesides, G. M. Langmuir 1995, 11, 4383. (22) Lopez, G. P.; Albers, M. W.; Schreiber, S. L.; Carroll, R.; Peralta, E.; Whitesides, G. M. J. Am. Chem. Soc. 1993, 115, 5877.

or tertiary SAMs have been reported,23-27 including sequential or simultaneous adsorption of the components under passive or driven conditions. Control of the net composition expressed as the surface mole fraction of each component, as well as the relationship of one component to the other (i.e., whether phase separation occurs), is very important to applications involving binary SAMs. The preparation of binary SAMs under incubation conditions of the Au surface in an RSH solution is highly dependent upon specific experimental parameters. These dependencies and the limited understanding of what is important in SAM formation lead to control problems in terms of both coverage and homogeneity. We and others have previously addressed this formation problem by exploring kinetic rather than thermodynamic formation conditions.25,28 However, an assessment tool which reports the extent of the 2D phase separation of the SAM components has been lacking. Direct imaging,29,30 decoration with metal deposition followed by imaging,31 and impedance spectroscopy data fitted to an appropriate equivalent circuit5,32 have been used to determine pinholes and/or phase separation of a binary SAM. A recent approach to determine the homogeneity of binary SAMs involves the reductive desorption of the SAM.33,34 The phase-separated SAMs are reported to show two distinct desorption peaks, whereas homogeneously mixed SAMs yield a single peak. None of these techniques, however, is without (23) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155. (24) Kang, J. F.; Liao, S.; Jordan, R.; Ulman, A. J. Am. Chem. Soc. 1998, 120, 9662. (25) Ma, F.; Lennox, R. B. Langmuir 2000, 16, 6188. (26) Shon, Y. S.; Lee, S.; Perry, S. S.; Lee, T. R. J. Am. Chem. Soc. 2000, 122, 1278. (27) Imabayashi, S.; Hobara, D.; Kakiuchi, T.; Knoll, W. Langmuir 1997, 13, 4502. (28) (a) Paik, W.; Eu, S.; Lee, K.; Chon, S.; Kim, M. Langmuir 2000, 16, 10198. (b) Eu, S.; Paik, W. Mol. Cryst. Liq. Cryst. 1999, 337, 49. (29) Tamada, K.; Hara, M.; Sasabe, H.; Knoll, W. Langmuir 1997, 13, 1558. (30) Okabe, Y.; Akiba, U.; Fujihira, M. Appl. Surf. Sci. 2000, 157, 398. (31) Sun, L.; Crooks, R. M. J. Electrochem. Soc. 1991, 138, L23. (32) Xing, Y. F.; Li, S. F. Y.; Lau, A. K. H.; O’Shea, S. J. J. Electroanal. Chem. 2005, 583, 124. (33) Hobara, D.; Ota, M.; Imabayashi, S.; Niki, K.; Kakiuchi, T. J. Electroanal. Chem. 1998, 444, 113. (34) Kakiuchi, T.; Iida, M.; Gon, N.; Hobara, D.; Imabayashi, S.; Niki, K. Langmuir 2001, 17, 1599.

10.1021/la053317r CCC: $33.50 © 2006 American Chemical Society Published on Web 03/22/2006

Probe of Heterogeneity in Binary SAMs

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Table 1. Literature Examples of Nonideality in the Cyclic Voltammograms of Tethered FcRSH SAMs on Au reference 41 41 43 47 47 51 52 53 54

SAM studied

ΓFc (mol‚cm-2)

Eanodic (mV)a

FcCO2C11CONHC2S-/C10CONHC2S-Au FcCO2C11CONHC2S-/C16S-Au FcCO2C11S-/C10S-Au FcC6S-Au FcC6S-/C6S-Au FcCO2C11S-Au FcC8S-/C6S-Au FcC6S-/HOC2S-Au FcC6SH-/C6S-Au

2.4 × 10 3.3 × 10-10 2.7 × 10-10 4.5 × 10-10 1.5 × 10-10 5.8 × 10-10 2.7 × 10-10 2.8 × 10-10 2.9 × 10-10

510/590 520/600 500/580 370/430 320/410 550/630 320/400 220/330 250/360

-10

a Values converted to Ag/AgCl (saturated KCl) reference electrode. The positions of the peak potentials are as per the reference or estimated by a visual inspection of the published data.

experimental drawbacks, such as destruction of the SAMs, or interpretive uncertainties. Controlled formation of homogeneous or heterogeneous binary SAMs is only possible if one has access to an appropriate assessment tool. We have exploited ferrocenylalkylthiol (Fc(CH2)nSH) electrochemistry to study this problem, inspired by the electrochemical features identified by Collard and Fox35 in their early electrochemical studies on exchange reactions of alkylthiolate SAMs. The ferrocenylalkylthiol system is probably the most studied of all electrochemically active SAMs, beginning with Chidsey’s work6 in 1990. The system has the advantage of having relatively straightforward electrochemistry, in the sense that every surfacetethered ferrocene can be reversibly interrogated by Faradaic electrochemistry. One can literally count the number of surfacebound ferrocenes and changes in this number, which arise in the course of an experiment. Electrochemical signatures of FcRSAu SAMs have been reported by Chidsey and many other research groups.36-40 Ideal behavior is observed when the FcRS- surface coverage is low and the FcRS- species is diluted by nonelectroactive alkylthiols. Several studies have reported abnormal peak width broadening, splitting of the peak, or the appearance of a high-potential shoulder on the FcRS-Au peak at relatively high surface coverages of FcRSH (Table 1).6,38,41-53 Although there have been many suggestions as to the origin of these voltammetric features, there has been no consensus as to which is operational or dominant. Suggestions include structural disorder in the SAM due to steric crowding of Fc groups,6,42,54 double-layer effects,55,56 differences arising from the polycrys(35) Collard, D. M.; Fox, M. A. Langmuir 1991, 7, 1192. (36) Chidsey, C. E. D. Science 1991, 251, 919. (37) Hickman, J. J.; Ofer, D.; Zou, C.; Wrighton, M. S.; Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1991, 113, 1128. (38) Uosaki, K.; Sato, Y.; Kita, H. Langmuir 1991, 7, 1510. (39) Popenoe, D. D.; Deinhammer, R. S.; Porter, M. D. Langmuir 1992, 8, 2521. (40) Rowe, G. K.; Carter, M. T.; Richardson, J. N.; Murray, R. W. Langmuir 1995, 11, 1797. (41) Chambers, R. C.; Inman, C. E.; Hutchison, J. E. Langmuir 2005, 21, 4615. (42) Walczak, M. M.; Popenoe, D. D.; Deinhammer, R. S.; Lamp, B. D.; Chung, C.; Porter, M. D. Langmuir 1991, 7, 2687. (43) Voicu, R.; Ellis, T. H.; Ju, H.; Leech, D. Langmuir 1999, 15, 8170. (44) Kondo, T.; Takechi, M.; Sato, Y.; Uosaki, K. J. Electroanal. Chem. 1995, 381, 203. (45) Ye, S.; Sato, Y.; Uosaki, K. Langmuir 1997, 13, 3157. (46) Sabapathy, R. C.; Bhattacharyya, S.; Leavy, M. C.; Cleland, W. E., Jr.; Hussey, C. L. Langmuir 1998, 14, 124. (47) Rowe, G. K.; Creager, S. E. Langmuir 1994, 10, 1186. (48) Rowe, G. K.; Creager, S. E. J. Phys. Chem. 1994, 98, 5500. (49) Yao, X.; Wang, J.; Zhou, F.; Wang, J.; Tao, N. J. Phys. Chem. B 2004, 108, 7206. (50) Quist, F.; Tabard-Cossa, V.; Badia, A. J. Phys. Chem. B 2003, 107, 10691. (51) Seo, K.; Jeon, I. C.; Yoo, D. J. Langmuir 2004, 20, 4147. (52) Kawaguchi, T.; Tada, K.; Shimazu, K. J. Electroanal. Chem. 2003, 543, 41. (53) Auletta, T.; Van Veggel, F. C. J. M.; Reinhoudt, D. N. Langmuir 2002, 18, 1288. (54) Rowe, G. K.; Creager, S. E. Langmuir 1991, 7, 2307. (55) Smith, C. P.; White, H. S. Anal. Chem. 1992, 64, 2398.

talline Au structure,57 and the differential order within the SAM.38 Very recently, theoretical simulations suggest that the “asymmetric broadening” of the voltammograms in electroactive SAMs arises from a “spatial inhomogeneity” in the SAM.58 This treatment supplements what had been proposed by White and Smith in their “plane of electron transfer” (PET) model,55 by considering multiple PETs where ferrocene groups adopt different vertical positions in the SAM. The result is a distribution of electron transfer rate constants, where the value of each rate constant is determined by the distance of the Fc moiety from the Au surface. A gradient of solvent polarity will likely lead to further complexity, given that the ferrocene E°′ is sensitive to solvent polarity.59 An additional rationale for the two Fc peaks involves a two (or multiple)-state model, where one voltammetric peak arises from Fc molecules which are isolated from other Fc molecules (by CH3(CH2)nS-, for example) and the other peak arises from Fc molecules which are proximal to one another.53 This is an interesting mechanism, as it suggests that the voltammetry may therefore reflect the extent and manner of phase-separated species in 2D SAMs. We have explored this possibility by investigating the voltammetry of binary Fc(CH2)12S-Au SAMs where the diluent thiol has a well-defined charge (-1, 0, or +1). We report here that the dual-peak voltammetry of the tethered Fc is indeed consistent with the local effects expected from phase separation. The two peaks in the cyclic voltammogram of ferrocenylalkylthiol SAMs are thus assigned to Fc(CH2)12S-Au surrounded either by neutral alkylthiols or by other Fc+(CH2)12S- species. In the following, we describe a detailed study of using this 12-ferrocenyl1-dodecanethiol oxidation peak twinning in Fc(CH2)12S-/C10SAu SAMs to report the relative levels of heterogeneity in binary SAMs. Experimental Section Materials. Decanethiol (CH3(CH2)9SH, C10SH, 96%), undecanethiol (CH3(CH2)10SH, C11SH, 98%), tetradecanethiol (CH3(CH2)13SH, C14SH, 98%), perchloric acid (HClO4, 70%), and sodium phosphate (NaH2PO4, and Na2HPO4) were purchased from Aldrich and used without any further purification. 11-Mercaptoundecanoic acid (HOOC(CH2)10SH, HOOCC10SH, 95%) and potassium perchlorate (KClO4, 99%) were purchased from Aldrich and purified by recrystallization (× 2). Polycrystalline Au pieces and Au wire (0.5 mm diameter, 99.999%) were purchased from Alfa-Aesar. 12-Ferrocenyl-1-dodecanethiol (Fc(CH2)12SH, Fc ) (η5-C5H5)Fe(η5-C5H4), FcC12SH) was prepared following a literature procedure in three steps starting from ferrocene.60 Friedel-Crafts acylation of (56) Fawcett, W. R. J. Electroanal. Chem. 1994, 378, 117. (57) Brett, D. J. L.; Williams, R.; Wilde, C. P. J. Electroanal. Chem. 2002, 538-539, 65. (58) Calvente, J. J.; Andreu, R.; Molero, M.; Lopez-Perez, G.; Dominguez, M. J. Phys. Chem. B 2001, 105, 9557. (59) Hupp, J. T.; Weaver, M. J. J. Electrochem. Soc. 1984, 131, 619. (60) Creager, S. E.; Rowe, G. K. J. Electroanal. Chem. 1994, 370, 203.

4440 Langmuir, Vol. 22, No. 9, 2006 12-bromododecanoyl chloride in dichloromethane in the presence of AlCl3 yields an R-keto alkyl chain. Sequential reduction of the ketone group under Clemmensen conditions (Zn/HCl), followed by conversion of the bromide to the thiol by treatment with thiourea in ethanol and basic hydrolysis (NaOH) yields the 12-ferrocenyl1-dodecanethiol as a yellow-orange powder. Further purification was achieved by column chromatography on silica (hexane/ethanol 95:5) and recrystallization from hexane solution by layered addition of methanol. The product had spectroscopic properties consistent with those reported previously. 1H NMR (400 MHz, CDCl3): δ 1.27-1.39 (m, 16H, -CH2-), 1.56-1.64 (m, 4H, -CH2-CH2SH, -CH2-CH2-Fc), 2.04 (m, 2H, -CH2-Fc), 2.50-2.56 (m, 2H, -CH2-SH), 4.48 (br s, 9H, -Fc). 11-Amino-1-undecanethiol (H2N(CH2)11SH, H2NC11SH) was synthesized according to a previously published procedure.61 The structure of the final product was confirmed by 1H NMR and MS. 1H NMR (400 MHz, D O pH 3-4 (CD COOD)): δ 1.30 (m, 14H, 2 3 -CH2-), 1.66 (m, 4H, -CH2-CH2-SH; -CH2-CH2-NH2), 2.69 (m, 2H, -CH2-SH), 2.92 (m, 2H, -CH2-NH2). EI-MS m/z 202.1 (100%) [M - H]+ (calcd average mass for C11H24NS+: 202.16 (100%). ES+-MS m/z 203.34 [M]+; 406.15 [2M]+ (calcd average mass for C11H25NS: 203.17). Preparation of SAMs. A polycrystalline Au bead electrode was used throughout all electrochemical studies. The Au electrode was prepared by melting small Au pieces into a spherical shape (ca. 1.5 mm diameter) and attaching it to an Au wire. Prior to the deposition of a thiol monolayer, the Au electrode surface was flame-annealed and then quenched with Milli-Q water (Millipore Co., 18.2 MΩ‚ cm) several times. A clean surface of Au was obtained by a repetitive electrochemical sweep from -0.8 to 1.3 V (vs Ag/AgCl) in 50 mM KClO4 at a scan rate of 20 mV‚s-1 until a reproducible cyclic voltammogram was obtained. The effective surface area of an Au electrode was estimated by measuring the differential capacitance in 50 mM KClO4. Both single-component and mixed-thiol monolayers were formed by immersing the precleaned Au electrode overnight in a freshly prepared solution with a total thiol concentration of 2 mM, unless stated otherwise. Instruments and Electrochemical Measurements. Amperometric measurements were carried out using a potentiostat/galvanostat model 273 (EG&G Princeton Applied Research) controlled by customized LabView programs. A conventional three-electrode system was employed with an in-house-designed glass cell. A SAMmodified Au bead electrode was used as the working electrode, a coiled Au wire as the auxiliary electrode, and an Ag/AgCl (saturated KCl) electrode as a reference electrode. The electrolyte solution was purged with Ar gas for 30 min prior to each measurement to minimize oxygen levels. A blanket of Ar was maintained over the electrolyte solution during the entire electrochemical investigations. All measurements were carried out at room temperature (ca. 20 °C).

Results and Discussion Electrochemical Signature of Tethered Ferrocenes in Binary SAMs. Throughout the history of tethered ferrocenes in SAM research, implicit or explicit concern has been expressed as to whether the kinetics and thermodynamics of electron transfer of each redox center are identical. It is generally assumed to be identical, but probably rarely is so. Ideal electrochemical behavior of tethered ferrocenes on a gold surface is evident from the symmetrical redox peaks with a ∆Efwhm of 90.6 mV and no peak separation.6,36 This ideal voltammetric behavior has been observed when the mole fraction of Fc (χsurf Fc ) is small with the remaining portion of the SAM being made up of nonelectroactive alkylthiolates. Figure 1 shows cyclic voltammograms (CVs) obtained from the SAMs that had been formed in different C10SH/FcC12SH solutions where χsoln Fc (Fc mole fraction in solution) ranges from 0.1 to 1.0. These mixed SAMs are stable and do not show any (61) Takehara, K.; Takemura, H.; Ide, Y. Electrochim. Acta 1994, 39, 817.

Lee et al.

Figure 1. Cyclic voltammograms of binary SAMs formed from 2 mM ethanolic solutions containing various ratios of C10SH and FcC12SH. Electrodes were incubated for at least 9 h at room temperature. CVs are measured in 1.0 M HClO4, and the scan rate is 20 mV‚s-1.

variation in peak height or shape over numerous scans (see the Supporting Information). Figure 1a represents the typical CV obtained from a SAM formed by incubation in a mixed solution of C10SH (1.8 mM) and FcC12SH (0.2 mM). Both oxidation and reduction peaks are slightly asymmetric and are centered at 190 mV (vs Ag/AgCl). Integration of the anodic peak, with correction for the charging current contribution, yields the charge (QFc) associated with the ferrocene oxidation. This value is used to determine the surface coverage of the FcC12S- (ΓFc) using eq 1,6

ΓFc ) QFc/nFA

(1)

where n is the number of electrons involved in the electrontransfer process (n ) 1 for ferrocene/ferrocenium), F is the Faraday constant, and A is the geometric surface area of the electrode. For the χsoln Fc ) 0.1, the FcC12S- coverage determined is 1.4 × 10-10 mol‚cm-2. This value corresponds to about 30% of the theoretical maximum value (4.5 × 10-10 mol‚cm-2), based on the assumption of hexagonal packing of the ferrocene moiety that is a sphere of diameter 6.6 Å6,62 (or the experimental maximum value of 4.6 × 10-10 mol‚cm-2 obtained here). From the molar ratio calculations which assume that 21.4 Å2 is taken up by each alkyl chain in the C10SH SAM,2,63 the surface mole fraction of the tethered ferrocene, χsurf Fc , is determined. Table 2 shows χsurf Fc values calculated from the CVs in Figure 1, revealing a deviation from χsoln Fc , as per expectations from previous reports.23,25,64 The relationship between these two mole fraction values is plotted in Figure 2. The χsurf Fc is consistently larger than for FcRSH SAMs diluted with shorter-chainlength the χsoln Fc is smaller than the χsoln alkylthiols, and the χsurf Fc Fc when the diluent 47,54 alkylthiol is longer. The more favorable adsorption of longerchain thiols compared to shorter ones is due to their relative solubilities in ethanol and the more favorable packing interactions among long hydrophobic chains. The ∆Efwhm of the oxidation peak is 117 mV, which is somewhat greater than the theoretical value of 90.6 mV.36,65,66 This nonideality and obvious unsym(62) Seiler, P.; Dunitz, J. D. Acta Crystallogr., Sect. B 1979, B35, 1068. (63) Camillone, N., III; Chidsey, C. E. D.; Liu, G. Y.; Putvinski, T. M.; Scoles, G. J. Chem. Phys. 1991, 94, 8493. (64) Laibinis, P. E.; Fox, M. A.; Folkers, J. P.; Whitesides, G. M. Langmuir 1991, 7, 3167. (65) Murray, R. W. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1984; Vol. 13, p 191. (66) Laviron, E. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1982; Vol. 12, p 53.

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Table 2. Mole Fractions, Surface Coverages, Peak Potentials, and Peak Widths of the Cyclic Voltammograms in Figure 1 χsoln Fc

total ΓFc (mol‚cm-2)

0.1 0.4 0.5 0.6 1.0

1.4 × 3.3 × 10-10 3.4 × 10-10 4.5 × 10-10 4.6 × 10-10 10-10

a χsurf Fc

0.21 0.62 0.65 0.96 1.00

peak Ib E°′ (mV) 190 230 260 250 -

ΓIFc

(mol‚cm-2)

1.0 × 1.1 × 10-10 7.6 × 10-11 2.7 × 10-11 10-10

peak IIb ∆Efwhm (mV) 100 97 92 83 -

E°′ (mV) 300 340 360 370 360

ΓIIFc

(mol‚cm-2)

3.3 × 2.1 × 10-10 2.7 × 10-10 4.2 × 10-10 4.6 × 10-10 10-11

∆Efwhm (mV) 91 74 65 70 50

a Calculated from the measured total ΓFc, assuming that the full coverage by FcC12S- is 4.6 × 10-10 mol‚cm-2. FcC12S-Au and C10S-Au were assumed to occupy 34.2 and 21.4 Å2, respectively.2,6,62,63 b Determined by peak deconvolution process using a Gaussian-Lorentzian function. The voltammetric characteristics for peak I are the fitting results using a Gaussian function, whereas those for peak II are fitting results using a Lorentzian function.

Figure 2. Plot of surface mole fraction of FcC12S-Au (χsurf Fc ) as a function of incubating solution mole fraction of FcC12SH (χsoln Fc ). Lines are to aid visualization only.

metrical nature of the peak indicate that some Fc moieties experience a different microenvironment than do the majority of the Fc moieties. When the SAM is formed from χsoln Fc ) 0.4 (Figure 1b), ΓFc increases to 3.3 × 10-10 mol‚cm-2 and χsurf Fc increases to 0.62. A more interesting observation is the presence of an additional redox couple (peak II) at 340 mV. We have extensively explored appropriate deconvolution methods for these two voltammetric peaks involving three combinations of fitting functions (see the Supporting Information). In each combination, the lower-potential peak (peak I) is fitted to a Gaussian distribution with three free parameters (peak position, peak width, peak area), whereas the higher-potential peak (peak II) is fitted to either a Gaussian, Lorentzian, or Extreme function, also with the same free parameters.67 We find that excellent fits to the data, and thus reasonable estimates of relative peak contributions to the total Fc signal, result from a Gaussian (peak I)-Lorentzian (peak II) surf fitting over all χsurf Fc values studied except for very low χFc ( 0.5. We therefore applied a Gaussian-Lorentzian (G-L) fitting to all the data described in this report. The deconvolution of Figure 1b reveals that peak II is sharper (∆Efwhm ) 74 mV) than peak I (∆Efwhm ) 97 mV) and peak I shifts to a more positive potential (E°′ peak I ) 230 mV) compared to that obtained in the χsoln Fc ) 0.1 case (E°′ ) 190 mV, Figure 1a). About 35% (1.1 × 10-10 mol‚cm-2) of the total FcC12S- coverage is linked to peak I, and 65% (2.1 × 10-10 mol‚cm-2) is linked to peak II. After observing these two voltammetric peaks in CVs where soln χsoln Fc > 0.1, we re-examined the χFc ) 0.1 case (Figure 1a). The (67) OriginPro, 7.5; OriginLab Corporation.

Figure 3. Peak compositions (% that each FcC12S-Au voltammetric peak contributes to the total amount of FcC12S-Au) as a function of χsoln Fc . ~: peak I (dashdot), x: peak II (dot) observed in Figure 1. Lines are to aid visualization only.

fitting process yields two peaks with maxima at 190 and 300 mV with ∆Efwhm of 97 and 74 mV, respectively. About 75% of total ΓFc comes from peak I, in which state the Fc moiety behaves ideally. The asymmetrical shape of peak I is a consequence of the contribution from peak II, which is about 25% of the total ΓFc. The CVs of χsoln Fc ) 0.5 and 0.6 (Figure 1c and d) exhibit two peaks at similar potentials to one another. Although the ΓFc in these cases are only slightly larger than the χsoln Fc ) 0.4 case (Figure 1b), the ΓFc values associated with peak II (ΓIIFc) have soln increased to 78% (χsoln Fc ) 0.5) and 94% (χFc ) 0.6) of the total surface-bound ΓFc. The contribution of peak I relative to peak II changes significantly with the increase in χsoln Fc , as illustrated in Figure 3. General trends observed with an increasing χsurf Fc value are (i) the ratio of ΓIIFc/ΓIFc increases past unity, (ii) the formal potential of each peak shifts anodically, and (iii) the ∆Efwhm values for both peaks decrease. From Figures 1 and 3, it is obvious that peak II becomes the major contributor to the total ΓFc as the χsurf Fc increases. Figure 4 shows the variations of E°′ and ∆Efwhm values with different χsoln Fc compositions. The variation in E°′ of peak II is slightly smaller than that of peak I, consistent with the observation in Figure 1 that peak II is observed at a constant potential. The two anodic peaks are observed with about 100 mV separation, where peak I is usually at ∼ 250 mV and peak II is at ∼ 350 mV. Peak I shows slightly larger ∆Efwhm values than the theoretical 90.6 mV, whereas peak II always shows narrower ∆Efwhm values. Both peaks become sharper as χsurf Fc increases, illustrating the effects of nonideal interactions. -10 The CV obtained for χsoln Fc ) 1.0 yields a ΓFc of 4.6 × 10 -2 mol‚cm , consistent with the theoretical maximum coverage for a FcRS-Au.6 In this case, however, there is now only one

4442 Langmuir, Vol. 22, No. 9, 2006

Figure 4. Plots of (a) apparent formal potential (E°′) and (b) ∆Efwhm of peak I (0) and peak II (K) observed in Figure 1 as a function of χsurf Fc .

peak, at 360 mV, and it is moderately unsymmetrical in shape. It is important to note that the origins of the unusual shape of peak II are not well understood at this time. Its form has been previously noted and commented upon.38 Both anodic and cathodic peak currents (Figure 1) exhibit a linear dependence on the scan rate, as is expected for a surfacebound redox couple.68 Figure 5a demonstrates this scan rate dependence when χsoln Fc ) 0.4. Anodic peak currents of peak I and II linearly increase with scan rate (5 to 500 mV‚s-1). Individual peak integrations, as well as the total peak integration, also show a linear dependence with scan rate (Figure 5b). ΓFc for both peak I and peak II is independent of scan rate. At all scan rates, peak I and II contribute 72% and 18%, respectively, to the total ΓFc. The same electron-transfer process is apparently operative for each peak. It should be noted that the total thiol concentration has minimal or no effect on a mixed SAM formed under long incubation times. Binary SAMs formed from the same χsoln Fc solutions with different total thiol concentrations (5, 2, 1, and 0.1 mM) yield very similar voltammetry. The ratios of ΓIFc to ΓIIFc, as well as total ΓFc, are constant with change in the incubating thiol concentration. In the case of SAMs prepared with a shorter incubation duration, the chainlength and concentration affect the composition of the SAM thus formed. Neighboring Charge Effects. As the coverage of the ferrocene thiol increases in the mixed SAM, two oxidation peaks, whose shapes, positions, and relative sizes are dependent on χsurf Fc , are consistently observed. Similar CVs, but with a lesser distinction (68) Bard, A. J.; Faulkner, L. R. Electrochmical Methods: Fundamentals and Applications, 2nd ed.; John Wiley and Sons: New York, 2000.

Lee et al.

Figure 5. Plots of (a) peak current and (b) peak area as a function of scan rate. Cyclic voltammograms are measured from a binary SAM that was prepared from a 2mM FcC12SH/C10SH solution (χsoln Fc ) 0.4). Inset shows the ratio of the two peaks as a function of scan -1 rate. Scan rates are varied from 5 to 500 mV‚s . All CVs are measured in 1.0 M HClO4.

between the two peaks, have been observed in a number of previous reports.6,38,41-53 The two-peak nature of the voltammetry immediately suggests that there are (at least) two states of the FcC12S-Au in each of the binary SAMs formed from a solution where χsoln Fc g 0.4. A thorough review of the literature dealing with these types of tethered ferrocenes (Table 1) indicates that this dual-peak electrochemical signature is frequently observed surf over a range of χsurf Fc values, mostly in relatively high χFc SAMs. When commented upon, this electrochemical signature has been attributed to at least one or more of four effects: (i) steric crowding of the ferrocene,40 (ii) different solvent environments of the ferrocene in different regions of the SAM,69 (iii) double-layer effects,48,55,56 and (iv) local electrostatic effects.38,53 We have undertaken some new experiments which specifically address the viability of effect (iv). First, we note that Beulen et al. reported a pH-dependent peak shift in an asymmetric acid-ferrocene sulfide monolayer (HOOCC7SC6Fc-Au).70 The negative charge associated with the carboxylate anion leads to a cathodic shift in the redox couple, consistent with a stabilization of the Fc+ species. We have explored the generality of this by comparing the properties of three different mixed XCnS-/FcC12S-Au SAMs, where X is either CH3-, NH2-, or HOOC-. Each mixed SAM was prepared from a 9:1 XCnSH/FcC12SH solution for a relatively short (69) Creager, S. E.; Rowe, G. K. J. Electroanal. Chem. 1997, 420, 291. (70) Beulen, M. W. J.; Van Veggel, F. C. J. M.; Reinhoudt, D. N. Chem. Commun. 1999, 503.

Probe of Heterogeneity in Binary SAMs

Langmuir, Vol. 22, No. 9, 2006 4443

Figure 7. Schematic illustration of a binary SAM prepared from FcC12SH and C10SH. I and II represent isolated FcC12S-Au molecules and domain-formed FcC12S-Au molecules, respectively.

Figure 6. Effect of pH on the CV of FcC12S-Au SAMs mixed with different diluent thiols. Total thiol concentration is 2 mM, and each χsoln Fc is 0.1. Diluent thiols are (a) C11SH, (b) H2NC11SH, and (c) HOOCC10SH. All CVs are measured in 0.05 M phosphate buffer pH 7.5 (solid) and 2.5 (dashed). The scan rate is 20 mV‚s-1.

incubation time (2 h). This ensures that the Au surface is fully covered and that the Fc moieties in each sample are sufficiently diluted so as to have few or no mutual interactions. The resulting CVs, obtained in 0.05 M phosphate buffer of pH 2.5 and 7.5, exhibit similar χsurf Fc values (0.09). FcC12SH that was codeposited with C11SH (Figure 6a) was unaffected by pH changes in the supporting electrolyte, consistent with the methyl terminal groups being insensitive to pH changes. The anodic E°′ shift (cf. Figure 1a) results from the different ion-pairing properties of the ferrocenium cation to either phosphates or perchlorates.71 In the case of a H2NC11S-/FcC12S-Au SAM (Figure 6b), a 130 mV anodic shift in E°′ is observed on decreasing the pH to 2.5 from 7.5. This shift is of similar magnitude to that highlighted in the two-peak voltammograms of Figure 1. The positive charge created by a protonation of the terminal amine headgroup yields a (71) Ju, H.; Leech, D. Phys. Chem. Chem. Phys. 1999, 1, 1549.

positively charged environment in the vicinity of the FcC12SAu, resulting in an anodic shift of oxidation potential of neighboring ferrocenyl groups. An opposite but complementary effect is observed in a HOOCC10S-/FcC12S-Au SAM (Figure 6c) in basic conditions. In this case, a shift of 120 mV in the cathodic direction results when the pH is increased to 7.5 from 2.5. The negatively charged carboxylate ion headgroup stabilizes the neighboring ferrocenium ions at higher pH (>7). In a related experiment, where a mixed SAM (χsurf Fc ) 0.45) was formed from a 3:1 HOOCC7SH/FcC6SH mixture, two broad peaks (instead of a complete peak shift) were observed upon the formation of the carboxylate anion in the SAM.70 One peak shifts 180 mV cathodically, whereas the other remains at a constant potential. soln surf At the χCOOH used, the eventual χCOOH value is likely insufficient to ensure that each FcC12S-Au has a neighboring carboxylate. Binary SAMs involving alkylammonium and alkylcarboxylate diluents clearly show that there exist two redox states of the ferrocenylthiolates. If ferrocenes can interact with acid alkylthiols and also with other ferrocenes, the pH-induced charge effect would be canceled out, and the formal potential would remain constant. This supports the argument that, when the ferrocenes are clustered, positive charges from the neighboring ferrocenium ions will make the oxidation of ferrocene unfavorable, pushing the E°′ to a higher potential. On the basis of these results, we conclude that peak II is the redox signature of FcC12S-Au that have clustered to form domains. On the other hand, peak I, which predominates at low χsurf Fc values, is assigned to tethered FcC12S molecules that are isolated from one another (Figure 7) by diluent or unreacted ferrocenylalkylthiolates. We cannot at present comment about the size and shape of the domains that are formed. The utility of the tethered Fc as a probe of SAM heterogeneity is clearly illustrated in Figure 8. In this example, two binary SAMs of the same composition (ΓFc ) (2.4 ( 0.1) × 10-10 mol‚cm-2) have been prepared in a C14S- matrix. Despite their similar ΓFc values, it is obvious from a visual inspection of CVs that two different distributions of Fc states are present. Two peaks at 270 and 380 mV are identified by the peak deconvolution process (Figure 8b and c). The formal potential values of these peaks parallel those of the FcC12S-/C10S-Au system. SAM (A) is prepared via a 5 s incubation in a solution of χsoln Fc ) 0.6 and results in 36% of its FcC12S- being in a phase-separated state (peak II), whereas SAM (B) prepared via 4 h incubation in a χsoln Fc ) 0.9 solution has ∼70% of its Fc in a phase-separated state. This change in the ΓIIFc-to-ΓIFc ratio with preparation conditions suggests that the short incubation time prevents SAM (A) from rearranging its thiolates (either via 2D migration, or more likely, repetitive on-off exchange) and being phase separated. In the SAM (B) case, the two thiol species are not being deposited under kinetic control and the binary SAM phase separates. Interestingly, the CVs of SAM (B) at shorter incubation periods (not shown) correspond to a higher ΓIFc-to-ΓIIFc ratio and sum to

4444 Langmuir, Vol. 22, No. 9, 2006

Lee et al. Table 3. Deconvolution of Dual Peak CVs from Various χsoln Fc Found in the Literature reference 6 6 35 51 45

a χsoln Fc

0.10 0.25 n/a 1.00 1.00

χsurf Fc

% peak Ic

% peak IIc

65 55 66 16 8

35 45 34 84 92

b

0.12 0.35b 0.40a 1d 1d

a The value is as per the reference. b Calculated from CVs shown in the literature c Determined by peak deconvolution process using a Gaussian-Lorentzian function. Peak I is fitted to a Gaussian, and peak II is fitted to a Lorentzian. d Because the reference cites >100% coverage, a χsurf Fc ) 1 is used to simply denote that the only species present is the Fc thiolate.

in the Introduction. Table 3 summarizes three literature cases where the Fc surface coverage and contributions from Peaks I and II are a function of incubating solution compositions. The relationship between χsurf Fc and the relative contribution of peak II is consistent with the observations reported here in Table 2. It is important to recognize that there are several literature examples where a SAM of χsurf Fc ) 1 is accompanied by two peaks in the voltammogram.45,51 At first, this appears to be inconsistent with the attribution of the two peaks to isolated and clustered Fc species. However, peak I contributions of 8% and 16% suggest that voltammetry is able to report Fc moieties that simply have no Fc+ neighbors. The ΓIFc values that exceed ca. 16% of the total ΓFc (statistical likelihood of nearest-neighbor status)53 suggest that the two states might be further distinguished in terms of packing density and order. Those details would most likely only be refined via auxiliary spectroscopic studies.

Conclusions

Figure 8. Example of the peak deconvolution process used. (a) Two CVs of the SAMs prepared from a 2 mM C14SH/FcC12SH mixed solution show similar ΓFc of (2.4 ( 0.1) × 10-10 mol‚cm-2 soln (χsurf Fc ) 0.53). Conditions for SAM (A) χFc ) 0.6, 5 s incubation, soln (B) χFc ) 0.9, 4 h incubation. (b) and (c) are deconvolution details of anodic peaks of CVs in (a). Solid lines represent GaussianLorentzian fittings, and dotted lines are individual Gaussian and Lorentzian fittings. CVs are measured in 1.0 M HClO4, and the scan rate is 20 mV‚s-1.

a larger total ΓFc value than that in Figure 8a. An increased incubation period allows the FcC12S-/C14S-Au to order and phase-separate, but the competitive exchange reaction with II solution C14S- leads to a smaller χsurf Fc and a greater ΓFc value. The stronger packing interactions of a longer alkyl chain in C14Scause this difference. These kinetics experiments demonstrate how controlled conditions (time, temperature, χsoln Fc , etc.) can yield a SAM whose composition and phase separation state can be rationally prescribed. The information derived from these and related FcCnS-Au studies will thus assist in efficiently selecting the conditions to make a given binary SAM. The assignment of the two voltammetric features of tethered ferrocenylalkylthiolates clarifies many of the reports described

The voltammograms derived from tethered ferrocenylalkylthilolates and charged diluent thiolates establish that the ferrocene redox signature is shifted 130 mV anodically (H3N+C11S-/ FcC12S-Au case) and cathodically 120 mV (-OOCC10S-/ FcC12S-Au case) in low-ferrocene-coverage (χsurf Fc e 0.1) SAMs. The anodic shift closely corresponds to a voltammetric feature observed in Fc-rich binary SAMs (χsurf Fc g 0.4; diluent ) C10S-Au) and is thus assigned to Fc oxidation in the presence of proximal Fc+ species. The voltammetry persistently involves two peaks at 0.2 < χsurf Fc < 1.0. These two peaks, after a deconvolution process, are described in terms of the relative contributions from isolated and clustered Fc moieties in the binary SAMs. This methodology is of use in relating SAM preparation and conditioning to phase separation in binary SAMs. Acknowledgment. This work was supported by Natural Science and Engineering Research Council (NSERC) of Canada, Fonds Que´be´cois de la Recherche sur la Nature et les Technologies (FQRNT) via research and Centre funding, and Genome Quebec. We thank Prof. G. Ulibarri for the H2NC11SH synthesis. Supporting Information Available: Cyclic voltammogram of multiple sweeps, and comparison of three fitting methods for the deconvolution of two peaks in Figure 1. This material is available free of charge via the Internet at http://pubs.acs.org. LA053317R