Electrochemical Characteristics of Ferrocenecarboxylate-Coupled

Department of Chemistry, Chonbuk National University, Jeonju, Chonbuk 561-756, Korea, and Department of Chemistry, Seonam University, Namwon, ...
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Langmuir 2004, 20, 4147-4154

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Electrochemical Characteristics of Ferrocenecarboxylate-Coupled Aminoundecylthiol Self-Assembled Monolayers Kyoungja Seo,† Il Cheol Jeon,*,† and Dong Jin Yoo‡ Department of Chemistry, Chonbuk National University, Jeonju, Chonbuk 561-756, Korea, and Department of Chemistry, Seonam University, Namwon, Chonbuk 590-711, Korea Received September 23, 2002. In Final Form: January 6, 2004 The electrochemical characteristics of the modified electrodes with ferrocenecarboxylate-coupled aminoundecylthiol monolayers prepared in two sequential steps were studied. The self-assembled monolayer (SAM) was prepared through the covalent attachment of ferrocenecarboxylate in an activation solution containing N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide coupling agent to aminoundecylthiol SAMs formed on a substrate. In the ferrocenecarboxylate-coupled aminoundecylthiol monolayers, the ferrocene moieties were expected to be packed regularly with enhanced ordering compared with those in the FcCOO(CH2)11SH monolayer. As the ferrocene coverage increases, the formal potential for the ferrocene-ferricenium (Fc/Fc+) couple shifts to the positive potential and the full width at half-maximum (∆Efwhm) increases also. The maximum coverage is found to be about 3 × 10-10 mol cm-2, which is considered to be a value obtained from a well-ordered ferrocene-tethered SAM. As for the mass change, the increase in ferrocene coverage caused the enhancement in ion association between the ferricenium cations and perchlorate anions resulting in a mass increase upon oxidation; however, the mass change per mole electron decreases. The results obtained from the ferrocenecarboxylate-coupled aminoundecylthiol monolayers were explained to be due to the well-ordered packing with regular spacing compared with those of the FcCOO(CH2)11SH monolayer.

Introduction The SAMs on electrode surfaces are interesting model systems for studies of interfacial electron transfer.1-4 The terminal functional groups on SAMs are attractive for applications of sensor5 and oriented films as well as for the modification of surface properties such as nanotribology,6 wetting,6-8 and adhesion.5,9-21 Utilization of SAMs * Corresponding author. Tel.: +82-63-270-3415. Fax: +82-63270-3408. E-mail: [email protected]. † Chonbuk National University. ‡ Seonam University. (1) Molecular Design of Electrode Surface; Murray, R. W., Ed.; Techniques of Chemistry Series 22; Wiley: New York, 1992. (2) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (3) (a) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301. (b) Chidsey, C. E. D.; Loiacono, D. Langmuir 1990, 6, 682. (c) Chidsey, C. E. D. Science 1991, 251, 919. (d) Collard, D. M.; Fox, M. A. Langmuir 1991, 7, 1192. (e) Hickman, J. J.; Ofer, D.; Laibinis, P. E.; Whitesides, G. M.; Wrighton, S. Science 1991, 252, 688. (4) (a) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (b) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (c) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (d) De Long, H. C.; Buttry, D. A. Langmuir 1990, 6, 1319. (5) (a) Finklea, H. O.; Avery, S.; Lynch, M.; Furtsch, T. Langmuir 1987, 3, 409. (b) Hautman, J.; Kelvin, M. J. Chem. Phys. 1989, 91, 4994. (6) (a) Finklea, H. O.; Hanshew, D. D. J. Am. Chem. Soc. 1992, 114, 3173. (b) Bain, C. D.; Whitesides, G. M. Langmuir 1989, 5, 1370. (c) Ulman, A.; Evans, S. D.; Shnidman, Y.; Sharma, R.; Eilers, J. E.; Chang, J. C. J. Am. Chem. Soc. 1991, 113, 1499. (7) Abbott, N. L.; Folkers, J. P.; Whitesides, G. M. Science 1992, 257, 1380. (8) Chidesy, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682. (9) Rowe, G. K.; Creager, S. E. Langmuir 1991, 7, 2307. (10) Bard, A. J.; Abruna, H. D.; Chidsey, C. E. D.; Faulkner, L. R.; Feldberg, S.; Itaya, K.; Majda, M. M.; Melroy, O.; Murry, R. W.; Porter, M. D.; Soriaga, M.; White, H. S. J. Phys. Chem. 1993, 97, 7147. (11) Thomas, R. C.; Houston, J. E.; Michalske, T. A.; Crooks, R. M. Science 1993, 259, 1883. (12) Atre, S. V.; Liedberg, B.; Allara, D. L. Langmuir 1995, 11, 3882. (13) Yan, L.; Marzolin, C.; Terfort, A.; Whitesides, G. M. Langmuir 1997, 13, 6704. (14) Rojas, M. T.; Kaifer, A. E. J. Am. Chem. Soc. 1995, 117, 5883.

with terminal functional groups is usually based on the information of simple n-alkylthiol SAMs such as packing, stability, electron transfer, and so forth. However, SAMs with large terminal groups are not expected to be packed densely with a good ordering as alkylthiols because of much-reduced repulsion between backbone chains of the nearest adsorbates22 that is attributed to larger separation compared with that in n-alkylthiol SAMs. Even in alkylthiol cases, such as octylthiol and decylthiol, sulfur atoms adsorbed to form a (x3 × x3)R30° adlayer on Au(111);4a,23 however, the chains attached to sulfur atoms are known to form a c(4 × 2) adlayer to minimize repulsion between chains. Generally speaking, the terminal groups are larger than the methyl group of simple alkylthiols. For enhancement of the packing or ordering in the SAM with large terminal groups, coadsorption of small adsorbates such as nalkylthiols with proper chain length has been attempted, and it shows a definite advantage for dense packing. In fact, there are previous reports from which enhancement in packing is seemed to be achieved by coadsorption.12,24,25 (15) Rowe, G. K.; Creager, S. E. J. Phys. Chem. 1994, 98, 5500. (16) Acevedo, D.; Abruna, H. D. J. Phys. Chem. 1991, 95, 9590. (17) Acevedo, D.; Bretz, R. L.; Tirado, J. D.; Abruna, H. D. Langmuir 1994, 10, 1300. (18) Tender, L.; Carter, M. T.; Murray, R. W. Anal. Chem. 1994, 66, 3173. (19) Carter, M. T.; Rowe, G. K.; Richardson, J. N.; Tender, L. M.; Terrill, R. H.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 2896. (20) Forster, R. J.; Okelly, J. P. J. Phys. Chem. 1996, 100, 3695. (21) Smalley, J. F.; Feldberg, S. W.; Chidsey, C. E. D.; Linford, M. R.; Newton, M. D.; Liu, Y. J. Phys. Chem. 1995, 99, 13141. (22) Kim, N.; Kim, Y.-G.; Kim, S. H.; Jeon, I. C. Manuscript in preparation. (23) (a) Chidsey, C. E. D.; Liu, G.-Y.; Rowntree, P.; Scoles, G. J. Chem. Phys. 1989, 91, 4421. (b) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546. (c) Widrig, C. A.; Alves, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2805. (24) Guiomar, A. J.; Guthrie, J. T.; Evans, S. D. Langmuir 1999, 15, 1198. (25) Bang, G. S.; Jeon, I. C. Bull. Korean Chem. Soc. 2001, 22, 281.

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Immobilization of ferrocene (Fc) units has been performed for their potential application to electrochemical sensors as an intermediate system.26 Fc-immobilized SAMs have been studied by many researchers including Chidsey et al.,3a Porter et al.,27 Uosaki et al.,28a-f Creager et al.,9,29 Kalaji et al.,30a-b Finklea et al.,31a-c and so on. Except for some cases, these Fc SAMs reported are typically single-component ones of Fc-tethered molecules, which are prepared just by soaking the substrate into the adsorbate solution. Considering interactions between chains and those between terminal groups, pure singlecomponent Fc SAMs are hard to expect to be packed like simple alkylthiol SAMs. In addition, the interactions between chains become much more complicated and the packing will be made more random if the chains have functionalities such as carbonyl or amino groups other than the simple aliphatic chain.32 Thus, for a more exact understanding of Fc SAMs, the situation must be made simpler. We prepared Fc-immobilized SAMs by attaching Fc moieties to the densely packed aminoundecylthiol SAMs. By this sequential reaction concept, the interaction between the terminal Fc moieties could be excluded during formation of aminoundecylthiol SAMs, and the interaction between chains can be disregarded during attachment of Fc. The packing of large terminal groups will be determined solely by the equilibrium between Fc molecules and amino groups, which are located at the potential minimum adsorption sites. The Fc-immobilized SAM prepared in this manner shows enhanced characteristics not only in preventing permeation of solvent and counterions but also in ordering of Fc groups. Experimental Section Materials. Ferrocenecarboxylic acid (FcCOOH), N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide (EDC) from Aldrich, and cysteamine from Sigma were dissolved in ethanol (Fisher Scientific, HPLC grade) without further purification. 11-Amino1-undecylthiol [NH2(CH2)11SH] was synthesized according to the procedure described in the literature.33 11-Ferrocenycarbonyloxy1-undecylthiol [FcCOO(CH2)11SH] was a gift from Prof. Chinkap Chung of Keimyung University. Unprocessed perchloric acid from (26) (a) Labav, M.; Katz, E.; Willner, I. Electroanalysis 1998, 10, 1159. (b) Okawa, Y.; Nagano, M.; Hirota, S.; Kobayashi, H.; Ohno, T.; Watanabe, M. Biosens. Bioelectron. 1999, 3, 229. (c) Bloner, R.; Katz, E.; Cohen, Y.; Itzhak, N.; Riklin, A.; Willner, I. Anal. Chem. 1996, 68, 3151. (d) Ihara, T.; Nakayama, M.; Murata, M.; Nakano, K.; Maeda, M. Chem. Commum. 1997, 1609. (e) Willner, I.; Riklin, A.; Shoham, B.; Rivenzon, D.; Katz, E. Adv. Mater. 1993, 5, 912. (27) Popenoe, D. D.; Deinhammer, R. S.; Porter, M. D. Langmuir 1992, 8, 2521. (28) (a) Uosaki, K.; Sato, Y.; Kita, H. Langmuir 1991, 7, 1510. (b) Shimazu, K.; Yagi, I.; Sato, Y.; Uosaki, K. J. Electroanal. Chem. 1994, 372, 117. (c) Ye, S.; Sato, Y.; Uosaki, K. Langmuir 1997, 13, 3157. (d) Sato, Y.; Mizutani, F.; Shimazu, K.; Ye, S.; Uosaki, K. J. Electroanal. Chem. 1999, 474, 94. (e) Kondo, T.; Okamura, M.; Uosaki, K. J. Organomet. Chem. 2001, 637-639, 841. (f) Ye, S.; Haba, T.; Sato, Y.; Shimazu, K.; Uosaki, K. Phys. Chem. Chem. Phys. 1999, 1, 3653. (29) (a) Rowe, G. K.; Creager, S. E. Langmuir 1991, 7, 2307. (b) Weber, K. S.; Creager, S. E. J. Electroanal. Chem. 1998, 458, 17. (c) Creager, S. E.; Rowe, G. K. J. Electroanal. Chem. 1997, 420, 291. (d) Creager, S. E.; Rowe, G. K. Anal. Chim. Acta 1991, 246, 233. (30) (a) Viana, A. S.; Jones, A. H.; Abrantes, L. M.; Kalaji, M. J. Electroanal. Chem. 2001, 500, 290. (b) Viana, A. S.; Abrantes, L. M.; Jin, G.; Floate, S.; Nichols, R. J.; Kalaji, M. Phys. Chem. Chem. Phys. 2001, 3, 3411. (31) (a) Smalley, J. F.; Finklia, H. O.; Chidsey, C. E. D.; Linford, M. R.; Creager, S. E.; Ferraris, J. P.; Chalfant, K.; Zawodzinsk, T.; Feldberg, S. W.; Newton, M. D. J. Am. Chem. Soc. 2003, 125, 2004. (b) Brevnov, D. A.; Finklia, H. O.; Ryswyk, H. V. J. Electroanal. Chem. 2001, 500, 100. (c) Finklea, H. O.; Yoon, K.; Chamberlain, E.; Allen, J.; Haddox, R. J. Phys. Chem. B 2001, 105, 3088. (32) Chapman, R. G.; Lin Wan, O.; Whitesides, G. M. Langmuir 2000, 16, 6927. (33) Takehara, K.; Takemura, H.; Ide, Y. Electrochim. Acta 1994, 39, 817.

Seo et al. Aldrich was then used to prepare the electrolyte solution with deionized water (resistivity > 18 MΩ cm). Apparatus. A commercial electrochemical quartz crystal microbalance (EQCM; SHIn EQCN 1000, Korea) was used for all electrochemical measurements. The electrochemical cell consisted of a quartz crystal gold electrode as the working electrode, a Pt wire as the counter electrode, and a Ag|AgCl electrode as the reference electrode. The gold electrodes on 10MHz quartz crystal (ICM Co., OK, U.S.A., or Morion, Inc., Petersburg, Russia) were chemically cleaned in a piranha solution (98% H2SO4 and 30% H2O2, 2:1 by volume; piranha solution reacts vigorously with organic materials and must be used with extreme caution) or electrochemically polished in 0.1 M H2SO4. The roughness factor of the gold electrodes was determined from the charge consumed for the reduction of surface oxide.34 A roughness factor of 1.2 was found from the ratio of the surface area determined from the surface oxide reduction to the geometric electrode area. The mass sensitivity of 10-MHz quartz crystals is calculated to be 4.42 ng cm-2 Hz-1. It was then rinsed with deionized water and ethanol and then dried with a stream of N2 gas. All electrochemical measurements were carried out at room temperature after the electrolyte solution was deaerated by purging Ar gas for at least 10 min. Preparation of Fc Monolayers in One Step [FcCOO(CH2)11SH SAM]. The FcCOO(CH2)11SH SAM was formed by immersing cleaned gold electrodes in an ethanol solution containing 1 mM of FcCOO(CH2)11SH for 2 h. It was then thoroughly rinsed with ethanol and dried with N2 gas. Preparation of Fc Monolayers in Two Steps [Aminoundecylthiol SAMs Coupled with Ferrocenecarboxylate]. Covalent attachment of ferrocenecarboxylate to the NH2(CH2)11SH SAM was induced by a coupling agent, which is a 30 mM EDC solution in ethanol and water 3:1 by volume. This procedure is widely applied for formation of amide bonds between amino groups and carboxylic functional groups in the peptide synthesis. It is also used for the adaptation of redox centers in the monolayers.35 The NH2(CH2)11SH SAM was formed by immersing a cleaned gold electrode in an ethanol solution containing 1 mM NH2(CH2)11SH for 2 h. It was then thoroughly rinsed with ethanol and dried with N2 gas. The NH2(CH2)11SH SAM-modified electrode was immersed for 24-96 h in an activation solution of 30 mM EDC along with 10 mM ferrocenecarboxylic acid. Sometimes the activation solution with ferrocenecarboxylic acid was put in a refrigerator at about 4 °C for over 1 week for sufficient activation. It was then rinsed with ethanol and dried with N2.

Results and Discussion The main purpose of this study is to characterize the NH2(CH2)11SH SAM coupled with ferrocenecarboxylate that is obviously different from the SAM of FcCOO(CH2)11SH in some important aspects. In the following sections, therefore, the general electrochemical characterization of NH2(CH2)11SH SAMs coupled with ferrocenecarboxylate will be described first comparing with the FcCOO(CH2)11SH SAM. Then the maximum coverage of the Fc moiety and finally interpretation of some characteristics of the SAM depending on the coverage of Fc groups will be discussed. General Observations. Figure 1A is a typical cyclic voltammogram (CV) with the corresponding frequency change response obtained from the ferrocenecarboxylatecoupled aminoundecylthiol SAMs. As just described, the electrode modified with a NH2(CH2)11SH SAM was immersed in an EDC coupling solution with ferrocenecarboxylic acid for about 72 h at about 4 °C. In Figure 1A, a couple of symmetrical redox peaks of the Fc/Fc+ couple was observed at around 500 mV and the coverage of the Fc moiety was estimated to be 1.4 × 10-10 (34) Angerstein-Kozlowska, H. A.; Conway, B. E.; Hamelin, A.; Stoicoviu, L. J. Electroanal. Chem. 1987, 228, 429. (35) Katz, E.; Schmidt, H. J. Electroanal. Chem. 1994, 368, 87.

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Figure 1. (A) CV and frequency change of the ferrocenecarboxylate-coupled aminoundecylthiol SAMs (two-step method). (B) Plot of ∆m versus ∆Q showing a constant slope that means the constant mpe (174 g mol-1) during redox reaction. Electrolyte: 0.1 M HClO4. Scan rate: 100 mV s-1.

mol cm-2. The peak potential was more positive than that of the 11-ferrocenyl-1-undecanethiol [Fc(CH2)11SH] SAM28a but similar to the peak potential of the 11-ferrocenycarbonyloxy-1-undecylthiol [FcCOO(CH)11SH] SAM.3a,27 The peak-to-peak separation was 15 mV at the scan rate of 100 mV s-1, with the full width at half-maximum (∆Efwhm) values of 158 and 146 mV for the oxidation and reduction processes, respectively. The redox peak current of Fc/Fc+ increased linearly with scan rates of 100-1000 mV s-1 (see Supporting Information), indicating that Fc moieties are anchored on the electrode surface. The charge under the anodic peak was 13.0 µC cm-2 equivalent to the coverage of 1.4 × 10-10 mol cm-2 (disregarding the roughness factor). The maximum coverage of the Fc(CH2)11SH SAM has been proposed to be about 4.5 × 10-10 mol cm-2,3a assuming that the packing in the Fc(CH2)11SH SAM is equivalent to the hexagonal closest-packing (hcp) of the Fc molecules that are presumed to be spheres of diameter 6.6 Å. Thus, the Fc coverage of a monolayer obtained from Figure 1A corresponds to about one-third of the proposed maximum coverage.

To compare the effect of the modification method, the CV and mass change were measured from an electrode modified with FcCOO(CH2)11SH on gold by the singlestep method. The modification was done just by soaking the electrode into a solution of FcCOO(CH2)11SH (onestep method). The modification process should be distinguished from that for Figure 1A, which was prepared in two sequential steps. Figure 2A is a CV obtained from the one-step method showing a couple of broad symmetric peaks overlapped with a couple of small sharp peaks. The peak separation in the overlapped peaks is about 20 mV; on the other hand, the main peaks seem to be quite symmetrical. Close examination of the mass change (Figure 2B) shows that there seems to be an additional contribution occurring between about 0.6 and 0.7 V to the major change. Interestingly, this potential range is thought to be identical with the potential range of sharp peaks. The nature of the sharp peak has been understood in part and is now under investigation.28a,30b The ∆Efwhm value for the main peaks is about 175 mV, which is much larger than that of Figure 1A. It implies

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Figure 2. (A) CV and (B) frequency change of the FcCOO(CH2)11SH SAM prepared by soaking the gold electrode in a solution of FcCOO(CH2)11SH (one-step method). Electrolyte: 0.1 M HClO4. Scan rate: 100 mV s-1.

that the interaction between Fc moieties in the SAM prepared by the single-step method is larger than that between Fc groups in the SAM prepared in two sequential steps. The frequency change recorded during potential cycling shows no relation with the scan rates. The frequency change observed in Figure 1A was about 6.6 Hz, which is equivalent to the mass change of 29.2 × 10-9 g cm-2. On the other hand, the mass change (∆m) per mole electron (∆m/∆Q) during the redox reaction of the immobilized Fc in a monolayer was obtained to be about 174 g mol-1 from the slope of the ∆m versus ∆Q plot (Figure 1B). The value of ∆Q is the charge obtained by integration of the voltammetric peaks. The mass change per mole electron (mpe) is equivalent to the mass change upon the redox reaction of 1 mol of Fc moiety, which must be due to the association of ferricenium cations and perchlorate anions to maintain the charge neutrality.28b,29d,36 Anyhow, 174 g mol-1 is larger than 100 g mol-1 for a free perchlorate ion, implying that perchlorate ions are accompanied by four (36) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853.

H2O molecules. For the oxidation process, the mpe value for Figure 2A,B was calculated to be about 172 g mol-1 regarding the coverage as 5.8 × 10-10 mol cm-2. This value is very close to 174 g mol-1 obtained from Figure 1. While examining the figure closely according to a reviewer’s suggestion, we found that the slope changes with the extent of oxidation. During the first half of the oxidation process, the slope value was about 149 g mol-1, while it increased to about 193 g mol-1 during the second half. The overall slope is calculated to be 172 ( 20 g mol-1, which is equivalent to the hydration number of 4 ( 1 depending on the oxidation extent. It means that a perchlorate ion moves with five water molecules at a high extent of oxidation while three water molecules accompany a perchlorate ion at a low extent. It might be interpreted as there is more room around the Fc moieties for perchlorate ions to approach at a high extent compared with the situation of the low oxidation extent. It is worth noting that E′, ∆Efwhm, and mpe seem to be related with the coverage value. Figure 3A shows the formal potential of the Fc moiety as a function of its

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Figure 3. (A) Variation of the formal potential (y ) 1 × 1011x + 485.79, R2 ) 0.7174), (B) the full width at half-maximum (y ) 1.267 × 1011x - 472.24, R2 ) 0.5579), and (C) mpe (y ) -2 × 1011x + 217.2, R2 ) 0.1083) as a function of the coverage of the Fc group for the ferrocenecarboxylate-coupled aminoundecylthiol SAMs. The coverages were controlled by immersing an electrode modified with the NH2(CH2)11SH SAM in an activation solution of 30 mM EDC along with 10 mM ferrocenecarboxylic acid for various time durations as described in the Experimental Section.

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coverage. The formal potential shifts linearly to the positive potential as the coverage increases, which might be attributed to the double layer effect37 or the increased interaction due to compact and regular packing of the Fc moieties with improved stability. Figure 3B shows how ∆Efwhm varies with the potential. The peak width increases as the formal potential moves to a positive potential also implying that the high free energy or high coverage value is closely related with the increased interaction between Fc moieties. And the mass change upon oxidation increases with the Fc coverage too, which is obviously due to the increase of Fc coverage requiring counteranions for the charge neutrality condition. On the other hand, the mpe decreases as the coverage of Fc increases as shown in Figure 3C, which means the number of solvent molecules moving along with counteranions decreases. The mpe was 157 g mol-1 when the Fc coverage was 3 × 10-10 mol cm-2, which is the maximum coverage value achieved in the ferrocenecarboxylatecoupled aminoundecylthiol SAM prepared by the two-step method. The maximum coverage of 3 × 10-10 mol cm-2 was obtained from the electrode modified with the NH2(CH2)11SH SAM in an activation solution of 30 mM EDC along with 10 mM ferrocenecarboxylic acid for 96 h as described in the Experimental Section. The value of 157 g mol-1 means that three water molecules accompany the counteranion perchlorate if there is no extra contribution except counterions and solvent molecules. According to the extrapolation equation in Figure 3C, the maximum mpe value at zero coverage is 217 g mol-1, which means the movement of perchlorate anions solvated with seven water molecules. However, this value is much smaller than 16 ((2), which was reported previously in the tris(bypyridyl)osmium-tethered alkylthiol SAM.25 Coverage of the Fc Moiety. Regarding the maximum coverage of the Fc moiety, 3 × 10-10 mol cm-2 was obtained from the ferrocenecarboxylate-coupled aminoundecylthiol SAM and is a considerably smaller value compared with the value 4.5 × 10-10 mol cm-2 which is generally accepted as the full coverage value in the Fc(CH2)11SH SAM. When the reaction between ferrocenecarboxylic acid and the amine groups exposed at the terminal position of an aminoalkylthiol SAM is considered, the full coverage of Fc is determined by the size of Fc and the separation between the amino groups which are the reaction sites. To estimate the Fc coverage in the Fc(CH2)11SH SAM, the hcp of Fc-tethered surfactants is usually presumed. Because the diameter of the Fc group is 6.6 Å, it is regarded that the surfactants would adsorb on the gold surface at every 6.6 Å to form hexagonal arrays. If the surfactants are assumed to adsorb on the gold surface at every 6.6 Å with a hexagonal symmetry, it may correspond to the (4x3/3 × 4x3/3)R30° structure with about a 5% lattice mismatch because the diameter of Fc (6.6 Å) is 2.3 times larger than the distance between the nearest gold atoms (ca. 3 Å) on the Au(111) surface. This adlayer structure is presented in Figure 4, which shows every third molecule holding an adsorption position at the top site while the other two molecules can adsorb on the hollow sites. In the SAMs of n-alkylthiol such as octylthiol and decylthiol, the sulfur atoms adsorb to form (x3 × x3)R30° structures; however, the chain parts form c(4 × 2) adlayers to minimize the van der Waals interaction between the chains.36,38-41 That means the most favorable (37) Smith, C. P.; White, H. S. Anal. Chem. 1992, 64, 2398. (38) Delamarche, E.; Michel, B.; Gerber, Ch.; Anselmetti, D.; Guentherodt, H.; Wolf, H.-J.; Ringsdorf, H. Langmuir 1994, 10, 2869.

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Figure 4. Model for the single-step adsorption that is proposed in previous reports. The corners of the background triangles correspond to gold atoms on the Au(111) surface in which the distance between the nearest gold atoms is about 3 Å. The large circle represents the terminal Fc moiety in Fc(CH2)11SH SAM. Considering its size, the Fc moiety is assumed to adsorb on the hollow sites and the top sites as well. The distance between the nearest Fc moieties is calculated to be about 6.9 Å. A rhombus drawn by thick lines is a unit cell of this adsorption structure that gives the full coverage value for Fc moieties of about 4.5 × 10-10 mol cm-2. The Fc adlayer can be regarded as the (4x3/3 × 4x3/3)R30° structure.

adsorption for sulfur would occur at the hollow sites forming a (x3 × x3)R30° adlayer on Au(111). In this adlayer structure, the adsorption sites are separated by about 5 Å forming hexagonal arrays on the gold surface on which the nearest gold atoms are separated by about 3 Å. For the same reason, it is likely that the sulfur atoms of Fc(CH2)11SH adsorb on the hollow sites to form mainly a (x3 × x3)R30° adlayer instead of a (4x3/3 × 4x3/3)R30° structure regardless of the size of the Fc groups. Moreover, as just described, the (4x3/3 × 4x3/3)R30° structure for Fc(CH2)11SH needs less favorable adsorption on the top positions as well. Therefore, it might not be reasonable to expect that the Fc(CH2)11SH SAM is a wellordered (4x3/3 × 4x3/3)R30° adlayer with an all-trans conformation. The resultant Fc(CH2)11SH SAMs would not be regularly packed with good ordering. Because the Fc moieties are confined in a limited surface, Fc moieties are not able to hold positions at an equal height from the gold surface. Consequently, there should be kinks or gauche conformations in the chains rather than an all-trans conformation.22 As for the maximum coverage of the ferrocenecarboxylate-coupled aminoundecylthiol SAM, an endeavor was made to eliminate the effects of sulfur adsorption and van der Waals interaction between chains that compete with the interaction between the terminal groups. For this purpose, the gold surface has been coated with aminoundecylthiol resulting in the formation of a densely packed, highly ordered monolayer. Any noticeable mass change due to permeation of the counterions and solvents into the SAM was not observed in the EQCM experiments (see Supporting Information) at the electrode coated with the aminoundecylthiol SAM. Then ferrocenecarboxylic acid was reacted with amino groups in the presence of EDC to immobilize Fc moieties through peptide bonds. The Fc (39) Bucher, J.-P.; Santensson, L.; Kern, K. Appl. Phys. A 1994, 59, 135. (40) Fenter, P.; Eisenberger, A.; Liang, K. S. Phys. Rev. Lett. 1993, 70, 2447. (41) Camillone, N.; Chidsey, C. E. D.; Liu, G.-Y.; Scoles, G. J. Chem. Phys. 1993, 98, 3503.

Aminoundecylthiol Self-Assembled Monolayers

Figure 5. Model representing the two-step adsorption adapted in this article. The corners of the background triangles correspond to gold atoms on the Au(111) surface in which the distance between the nearest gold atoms is about 3 Å. Small circles represent the amine terminal groups in NH2(CH2)11SH SAM. Large circles stand for the terminal Fc moieties attached to the NH2(CH2)11SH SAM through peptide bonds. Regardless of the size of the Fc moiety, the Fc moieties are assumed to adsorb only on the hollow sites. The distance between the nearest Fc moieties is calculated to be about 10 Å. A rhombus drawn by thick lines is a unit cell of this adsorption structure that gives the full coverage value for Fc moieties of about 1.9 × 10-10 mol cm-2. The Fc adlayer can be regarded as the (2x3 × 2x3)R30° structure.

coverage will be governed just by the interaction between the terminal Fc moieties and the reactivity of ferrocenecarboxylate toward the amine groups as well. If the amine groups are regarded to form the (x3 × x3)R30° adlayer like sulfur atoms, the nearest amine groups are separated by 5 Å, which is smaller than the size of Fc moieties. Therefore, it is obvious that Fc groups could react with the second nearest amine groups resulting in the (2x3 × 2x3)R30° adlayer structure to form the maximum coverage. According to this model as presented in Figure 5, the maximum coverage is calculated to be 1.9 × 10-10 mol cm-2, and it will be 2.2 × 10-10 mol cm-2 when the roughness factor is regarded to be 1.2 as usual. As just described, the maximum coverage of about 3.0 × 10-10 mol cm-2 achieved in the ferrocenecarboxylate-coupled aminoundecylthiol SAM matched comparatively well with this value. It can be considered as a binary and homogeneous SAM of FcCONH(CH2)11SH and NH2(CH2)11SH with the mole fractions of 0.25 for FcCONH(CH2)11SH and 0.75 for NH2(CH2)11SH. If the coverage value is larger than 1.9 × 10-10 mol cm-2, Fc moieties cannot be ordered at a constant height and then chains have to have kinks or gauche conformations to reduce the overlap of the terminal groups. In some cases, the multilayers might be responsible for the larger coverage value than 1.9 × 10-10 mol cm-2. In fact, the multilayers of ferrocenealkylthiol instead of monolayers have been observed when SAMs are prepared in concentrated thiol solutions.30b The hydrogen bonding between sulfur and hydrogen is known to be responsible for maintaining the multilayer structure. The overlapping small sharp peaks shown in Figure 2A might represent the existence of multilayers. The maximum coverage value for the ferrocenecarboxylate-coupled aminoundecylthiol SAM will be the same as the similar cases such as the Fc(CH2)11SH SAM and FcCOO(CH2)11SH SAM if all the surfactant molecules can be adsorbed with the hcp keeping chains in a linear alltrans conformation or forming (2x3 × 2x3)R30° adlayers.

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Variation of mpe and ∆Efwhm. By interpreting the EQCM experimental data, we attempted to understand the variation of mpe values with the coverage and to propose a model for the ion pair between the ferricenium ion and the perchlorate anion, which is suggested to be located in front of the cyclopentadienyl (Cp) ring of the Fc moiety. Regarding ion-pair formation28b,29d,42 between ferricenium ions and perchlorate ions, Ye et al. suggested perchlorate anion to be located on the outside the monolayer when the ion pair is formed.28c Considering the changes of Fc moieties upon oxidation that the orientation of Cp rings toward a more perpendicular position and that there is sufficient room for perchlorate (ionic diameter of perchlorate ion ca. 2.4 Å) ions to locate in the middle of three nearest Fc moieties even in the full coverage case, perchlorate ion can reside facing the ferricenium ion unlike in the previous cases. Moreover, the positive charge after oxidation partially developed on the Cp ring can provide an advantage for perchlorate ions to locate near the Cp rings. In addition, the pKa values of free surface amine groups are known to vary from 4 to 7.43 Thus, all surface amine groups probably including the amide groups forming the peptide bonds are protonated in 0.1 M HClO4, and the positively charged plane under the Fc groups will help perchlorate ions to reside near the Cp ring of ferricenium ions but not outside. The variation of mpe value with the coverage can be explained by this model. At low coverage, there is sufficient space for a fully hydrated perchlorate ion to locate in front of the Cp ring because the Fc moieties are separated sparsely. As the coverage increases, the average free space between ferricenium ions gets smaller and the perchlorate ion is forced to reduce its hydration shell to move into the narrow gap space. It is worth noting that the hydration numbers determined in this experiment are relatively small when compared with the values in our previous report.25 According to Figure 3C, the hydration number for the perchlorate ion at zero coverage is 7 and the perchlorate ion is accompanied by just two water molecules at the full coverage. Because the protonated amino groups very close to Cp rings have been hydrated already,44 the perchlorate ion may not have to carry all the solvent molecules when it resides near the ferricenium cation. Thus, the net change in the hydration number for perchlorate anions looks much smaller. Unlike the suggestion of a previous article that the shape of the voltammetric wave is the function of the thickness of and dielectric constant of the monolayer,37 the increase of peak widths with the coverage in the present case might be ascribed to the increased interaction between Fc moieties because the average separation between Fc groups is decreased. As shown in Figure 3B, ∆Efwhm increases from about 140 mV at zero coverage to 196 mV at the maximal coverage in the ferrocenecarboxylatecoupled aminoundecylthiol SAM. Ideally, the interaction between the electroactive moieties of the adsorbates is minimal while ∆Efwhm ) 3.53RT/nF (90.3/n mV at 24 °C).45 The ∆Efwhm value for the monolayer shown in Figure 3B (42) De Long, H. C.; Donohue, J. J.; Buttry, D. A. Langmuir 1991, 7, 2196. (43) Wallwork, M. L.; Smith, D. A.; Zhang, J.; Kirkham, J.; Robinson, C. Langmuir 2001, 17, 1126. (44) Nihonyanagi, S.; Ye, S.; Uosaki, K.; Electrochim. Acta 2001, 46, 3057.

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was greater than the theoretical value of 90.3 mV. It means that Fc moieties even in the small coverage are experiencing considerable repulsion. It is not clear at the moment why the Fc groups of the reduced forms experience such a large repulsion and it is larger than the repulsion between the oxidized forms. Conclusions For the study of well-ordered Fc SAMs, the ferrocenecarboxylate-coupled aminoundecylthiol SAMs prepared by two steps are advantageous over the SAM of FcCOO(CH2)11SH prepared in one step. By reacting the aminoundecylthiol SAM with ferrocenecarboxylate in the presence of a coupling reagent, the disordering due to competition between the effects of the size of the terminal (45) Finklea, H. O. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1990; Vol. 19, p 109.

Seo et al.

groups and adsorption energy of sulfur atoms could be reduced. In the SAM prepared by two sequential steps, the maximum coverage was 3 × 10-10 mol cm-2. By interpreting the EQCM experimental data, a model for the ion pairs between ferricenium ions and perchlorate anions located in front of the Cp rings rather than outside the Fc monolayer is suggested. The variation of mpe and ∆Efwhm as a function of the Fc coverage was explained successfully. Acknowledgment. This work was supported by Grant R01-1999-000-00038-0 from the Basic Research Program of the Korea Science & Engineering Foundation. Supporting Information Available: EQCM result at the gold electrode modified with SAM and relation between scan rates and peak currents for Figure 1. This material is available free of charge via the Internet at http://pubs.acs.org. LA0208068