Reactivity Control of Carboxylic Acid-Terminated Self-Assembled

Reactions that occur at interfaces often show different behaviors from their solution analogues. In this paper, we demonstrated how proximity effect, ...
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Langmuir 2005, 21, 11765-11772

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Reactivity Control of Carboxylic Acid-Terminated Self-Assembled Monolayers on Gold: Acid Fluoride Versus Interchain Carboxylic Anhydride Young Shik Chi and Insung S. Choi* Department of Chemistry and School of Molecular Science (BK21), Center for Molecular Design and Synthesis, KAIST, Daejeon 305-701, Korea Received March 31, 2005. In Final Form: September 1, 2005 Reactions that occur at interfaces often show different behaviors from their solution analogues. In this paper, we demonstrated how proximity effect, one of the unique phenomena at interfaces, could control the product distributions of interfacial reactions. Self-assembled monolayers (SAMs) of 16-mercaptohexadecanoic acid on gold surfaces were treated with cyanuric fluoride and pyridine, which are generally used for forming acid fluorides from carboxylic acids in the solution-based reaction. After the treatment, two different products, acid fluorides (AFs) and interchain carboxylic anhydrides (ICAs), were controllably obtained at surfaces under different reaction conditions with keeping the reagents the same. Various factors, such as the concentrations of reagents, reaction time, and additives, affected the product distribution (or the reaction pathway) at surfaces. We found that one of the key factors in controlling the reaction pathway was a relative contribution from the proximity effect of adjacent carboxylic acid chains in the SAMs (kinetic control) and the equilibrium shift (thermodynamic control). The relative reactivity of AFand ICA-presenting surfaces toward primary amines, such as undecylamine and [((6-aminohexyl)amino)carbonyl]ferrocene, was also investigated, in terms of the number and the ordering of the amines coupled onto the surfaces.

Introduction Interfaces act as a place where various events, such as biological reactions or heterogeneously catalyzed chemical reactions, occur. Self-assembled monolayers (SAMs) were introduced as a suitable model for studying interfacial phenomena1 and showed a wide range of potential applications, such as biocompatible coating and bioadhesion,2-8 wettability control,9-16 corrosion prevention,17-19 and micro- and nanofabrication.8,20-22 Especially, * E-mail: [email protected]. (1) Ulman, A. Chem. Rev. 1996, 96, 1533. (2) Lo´pez, G. P.; Albers, M. W.; Schreiber, S. L.; Carroll, R.; Peralta, E.; Whitesides, G. M. J. Am. Chem. Soc. 1993, 115, 5877. (3) Mrksich, M.; Whitesides, G. M. Annu. Rev. Biophys. Biomol. Struct. 1996, 25, 55. (4) Mrksich, M. Cell. Mol. Life Sci. 1998, 54, 653. (5) Kingshott, P.; Griesser, H. J. Curr. Opin. Solid State Mater. Sci. 1999, 4, 403. (6) Mrksich, M. Curr. Opin. Chem. Biol. 2002, 6, 794. (7) Schaeferling, M.; Schiller, S.; Paul, H.; Kruschina, M.; Pavlickova, P.; Meerkamp, M.; Giammasi, C.; Kambhampati, D. Electrophoresis 2002, 23, 3097. (8) Park, T. J.; Lee, K.-B.; Lee, S. J.; Park, J. P.; Lee, Z.-W.; Lee, S. Y.; Choi, I. S. J. Am. Chem. Soc. 2004, 126, 10512. (9) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87. (10) Chaudhury, M. K.; Whitesides, G. M. Science 1992, 256, 1539. (11) Colorado, R., Jr.; Lee, T. R. Langmuir 2003, 19, 3288. (12) Abbott, S.; Ralston, J.; Reynolds, G.; Hayes, R. Langmuir 1999, 15, 8923. (13) Ichimura, K.; Oh, S.-K.; Nakagawa, M. Science 2000, 288, 1624. (14) Lahann, J.; Mitragotri, S.; Tran, T.-N.; Kaido, H.; Sundaram, J.; Choi, I. S.; Hoffer, S.; Somorjai, G. A.; Langer, R. Science 2003, 299, 371. (15) Lee, B. S.; Chi, Y. S.; Lee, J. K.; Choi, I. S.; Song, C. E.; Namgoong, S. K.; Lee, S.-g. J. Am. Chem. Soc. 2004, 126, 480. (16) Chi, Y. S.; Lee, J. K.; Lee, S.-g.; Choi, I. S. Langmuir 2004, 20, 3024. (17) Abbott, N. L.; Kumar, A.; Whitesides, G. M. Chem. Mater. 1994, 6, 596. (18) Itoh, M.; Nishihara, H.; Aramaki, K. J. Electrochem. Soc. 1994, 141, 2018. (19) Sinapi, F.; Forget, L.; Delhalle, J.; Mekhalif, Z. Appl. Surf. Sci. 2003, 212, 464. (20) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550.

SAMs can offer unique opportunities to probe interfacial phenomena and to provide important information to understand the rules that govern “reactions in two dimensions”.23 The SAM-based reactions are affected by several factors that do not affect the solution-based reactions.24 For example, sterically demanding reactions may be hindered at surfaces, and this is true especially for well-packed SAMs. In this respect, in the SN2-type reactions, it would be favored that nucleophiles exist at surfaces (that is, in the form of nucleophile-terminated SAMs) rather than free in solution (with nucleofuge-terminated SAMs). The adjacent functional groups affect the SAM-based reactions electronically as well as sterically, and the local solvation at surfaces also should be considered. Generally, the surface-specific factors are reported to accelerate or decelerate the reactions at interfaces because of the wellorderedness and well-packedness of two-dimensionally self-assembled structures.25-36 There have been reports (21) Kra¨mer, S.; Fuierer, R. R.; Gorman, C. B. Chem. Rev. 2003, 103, 4367. (22) Ginger, D. S.; Zhang, H.; Mirkin, C. A. Angew. Chem., Int. Ed. 2004, 43, 30. (23) Sullivan, T. P.; Huck, W. T. S. Eur. J. Org. Chem. 2003, 17. (24) Chechik, V.; Crooks, R. M.; Stirling, C. J. M. Adv. Mater. 2000, 12, 1161. (25) Liu, M. H.; Nakahara, H.; Shibasaki, Y.; Fukuda, K. Chem. Lett. 1993, 967. (26) Oliver, J. S.; Singh, J. J. Org. Chem. 1997, 62, 6436. (27) Kumar, J. K.; Oliver, J. S. J. Am. Chem. Soc. 2002, 124, 11307. (28) To¨llner, K.; Popovitz-Biro, R.; Lahav, M.; Milstein, D. Science 1997, 278, 2100. (29) Bartz, M.; Ku¨ther, J.; Seshadri, R.; Tremel, W. Angew. Chem., Int. Ed. 1998, 37, 2466. (30) Houseman, B. T.; Mrksich, M. Angew. Chem., Int. Ed. 1999, 38, 782. (31) Templeton, A. C.; Hostetler, M. J.; Kraft, C. T.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 1906. (32) Neogi, P.; Neogi, S.; Stirling, C. J. M. J. Chem. Soc., Chem. Commun. 1993, 1134. (33) Scho¨nherr, H.; Chechik, V.; Stirling, C. J. M.; Vancso, G. J. J. Am. Chem. Soc. 2000, 122, 3679.

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that the kinetic pathway of the SAM-based reaction could be tuned by the local environment (specifically the substituent effect arising from the surrounding microenvironment in SAMs on gold).37 Mrksich and co-workers reported that the Diels-Alder reaction of soluble cyclopentadiene (Cp) and surface-immobilized quinone was changed to follow the first-order kinetics (through an effectively intramolecular reaction) with methyl-terminated SAMs as a surrounding microenvironment, from the bimolecular, second-order Diels-Alder reaction with hydroxyl-terminated SAMs as a surrounding microenvironment. They explained the change in kinetic behavior by using “the preassociation model”. Recently, they also found a similar change in kinetic behavior in the DielsAlder reaction between surface-immobilized mercaptobenzoquinones and free dienes, such as Cp, and suggested a different model, “the activated dienophile model”, for a more acceptable explanation of the experimental facts, although the apparent phenomena were similar to those in the previous results.38 Taken together, there are many interesting and challengeable phenomena in the twodimensional, interfacial reactions. In the course of developing versatile SAM-based reactions,39-41 we found that the reaction pathway also could be controlled at surfaces, and herein we report the first example of a controlled change in the reaction products (in other words, product distributions) of the SAM-based organic reactions, which is dictated by a relative contribution of kinetic control (through proximity effect) and thermodynamic control in SAMs terminating in carboxylic acids. Generally, proximity effect, resulting from structurally ordered and concentrated reactive functionalities in a two-dimensional plane, can accelerate the rates of quasi two-dimensional reactions over homogeneous reactions in solution.27 Our study shows that proximity effect also can change reaction pathways on SAMs, leading to the formation of interchain carboxylic anhydrides (ICAs) under the reaction conditions for forming acid fluorides (AFs) in solution. Materials and Methods Materials. Si〈100〉 wafers were obtained from Prolog Semicor, Ltd., Ukraine. Absolute ethanol (EtOH, 99.8+%, Merck), dichloromethane (CH2Cl2, 100.0%, J. T. Baker, HPLC grade), tetrahydrofuran (THF, 99.9+%, Merck), cyanuric fluoride (CyF, 97+%, Fluka), anhydrous pyridine (99.8%, Aldrich), 16-mercaptohexadecanoic acid (MHDA, 90%, Aldrich), dodecanethiol (DDT, 98+%, Aldrich), undecylamine (UDA, 98%, Aldrich), tetrabutylammonium fluoride hydrate (TBAF, 98%, Aldrich), hydrogen fluoridepyridine (HF-Py, 65-70%, Acros), sulfuric acid (H2SO4, 95.0+%, Junsei), acetic acid (99.0+%, Junsei), 1,6-diaminohexane (99+%, Fluka), and hydrogen peroxide (H2O2, 30-35%, Junsei) were used as received. [((6-aminohexyl)amino)carbonyl]ferrocene (Fc-NH2) was synthesized by reacting fluorocarbonylferrocene (Fc-COF) with 1,6-diaminohexane.42 23-Mercapto-3,6,9,12-tetraoxatricosanoic acid (HS(CH2)11(OCH2CH2)3OCH2COOH, HS-EG3(34) Chechik, V.; Stirling, C. J. M. Langmuir 1998, 14, 99. (35) Kwon, Y.; Mrksich, M. J. Am. Chem. Soc. 2002, 124, 806. (36) Chan, E. W. L.; Yousaf, M. N.; Mrksich, M. J. Phys. Chem. A 2000, 104, 9315. (37) Yousaf, M. N.; Chan, E. W. L.; Mrksich, M. Angew. Chem., Int. Ed. 2000, 39, 1943. (38) Gawalt, E. S.; Mrksich, M. J. Am. Chem. Soc. 2004, 126, 15613. (39) Lee, J. K.; Lee, K.-B.; Kim, D. J.; Choi, I. S. Langmuir 2003, 19, 8141. (40) Lee, J. K.; Chi, Y. S.; Choi, I. S. Langmuir 2004, 20, 3844. (41) Chi, Y. S.; Lee, J. K.; Lee, K.-B.; Kim, D. J.; Choi, I. S. Bull. Korean Chem. Soc. 2005, 26, 361.

Chi and Choi

COOH) was purchased from COS Biotech, Inc., Korea. Ultrapure water (18.3 MΩ/cm) from Human Ultrapure System (Human Corp., Korea) was used. Synthesis of [((6-aminohexyl)amino)carbonyl]ferrocene (Fc-NH2). Fluorocarbonylferrocene (Fc-COF) was synthesized by following the reported procedure.43 A solution of ferrocenecarboxylic acid (0.58 g, 2.5 mmol) and pyridine (0.41 mL, 5 mmol) in dry CH2Cl2 (25 mL) was cooled to 0 °C under an argon atmosphere. Cyanuric fluoride (0.9 mL, 5 mmol) was added to the solution, and the resulting mixture was stirred for 90 min. Crushed ice/water was then added, the suspension was filtered, and the organic layer was separated and washed with cold water. Concentration in vacuo followed by column chromatography provided Fc-COF (0.35 g, 60%) as a dark orange crystalline solid. 1H NMR (300 MHz, CDCl3): δ 4.84 (s, 2H), 4.56 (s, 2H), 4.30 (s, 5H). MS(EI) for C11H9OFFe: calcd, 231.9987; found, 231.9758. For the synthesis of Fc-NH2, Fc-COF (0.35 g, 1.51 mmol) was added to the solution of 1,6-diaminohexane (1.23 g, 10 mmol) in 200 mL of dry THF. The mixture was stirred at room temperature for 12 h and purified by column chromatography to give Fc-NH2 (0.25 g) in 51% yield. 1H NMR (300 MHz, CDCl3): 5.67 (br s, 1H), 4.63 (s, 2H), 4.30 (s, 2H), 4.17 (s, 5H), 3.34 (q, 2H), 2.67 (t, 2H), 1.56 (m, 2H), 1.43 (m, 2H), 1.37 (br s, 4H), 1.34 (br s, 2H). MS(EI) for C17H24N2OFe: calcd, 328.1238; found, 328.1474. Preparation of Self-Assembled Monolayers. The gold substrates were prepared by thermal evaporation of 5 nm of titanium and 100 nm of gold onto silicon wafers. Prior to use, gold substrates were cleaned for 1 min in piranha solution (3:7 by volume of 30% H2O2 and H2SO4; Caution: piranha solution reacts violently with most organic materials and must be handled with extreme care), rinsed with H2O and ethanol, and dried under a stream of argon. The SAMs of MHDA or HS-EG3-COOH were prepared by immersing the gold substrates in a 1 mM solution of ethanol/water/acetic acid (80/10/10, v/v/v) overnight according to the procedure reported previously.44 After the formation of SAMs, the substrates were rinsed with ethanol several times and then dried under a stream of argon. Mixed SAMs of MHDA and DDT (1:2) were prepared by immersing the gold substrates overnight in a solution of ethanol/acetic acid (95/5, v/v) containing 1 mM of MHDA and 2 mM of DDT. After the formation of SAMs, the gold substrates were rinsed with ethanol several times and then dried under a stream of argon. Reactions of the SAMs with Cyanuric Fluoride (CyF) and Pyridine. In a 20-mL scintillation vial, a 10mL solution of CyF (0.2-50 µL) and pyridine (1-100 µL) in CH2Cl2 was prepared. Precleaned substrates of the SAMs terminating in carboxylic acids were immersed in the freshly prepared solution of CyF and pyridine without stirring for 5-300 min at room temperature, taken from the solution, rinsed thoroughly with CH2Cl2, and dried in a stream of argon. Reactions of the Activated SAMs with Amines. A 1 mM solution of UDA or Fc-NH2 in CH2Cl2 was prepared in a 20-mL scintillation vial. Precleaned substrates presenting activated products (AFs or ICAs) were immersed in the solution without stirring for 1 h at room temperature, taken from the solution, rinsed thoroughly with ethanol, and dried in a stream of argon. (42) Chen, K.; Mirkin, C. A.; Lo, R.-K.; Zhao, J.; McDevitt, J. T. J. Am. Chem. Soc. 1995, 117, 6374. (43) Galow, T. H.; Rodrigo, J.; Cleary, K.; Cooke, G.; Rotello, V. M. J. Org. Chem. 1999, 64, 3745. (44) Lee, J. K.; Kim, Y.-G.; Chi, Y. S.; Yun, W. S.; Choi, I. S. J. Phys. Chem. B 2004, 108, 7665.

Carboxylic Acid-Terminated SAMs on Gold

Grazing Angle FT-IR Spectroscopy. IR spectra were obtained in a single reflection mode using a dry N2-purged Thermo Nicolet Nexus FT-IR spectrophotometer equipped with the SAGA (smart apertured grazing angle) accessory. The p-polarized light was incident at 80° relative to the surface normal of the substrate, and a narrow band mercury-cadmium-telluride (MCT) detector cooled with liquid nitrogen was used to detect the reflected light. We averaged 2000 scans to yield the spectra at a resolution of 2 cm-1, and all the spectra were reported in the absorption mode relative to a clean gold surface. X-ray Photoelectron Spectroscopy (XPS). The XPS study was performed with a VG-Scientific Escalab 250 spectrometer (U.K.) with a monochromatized Al KR X-ray source. Emitted photoelectrons were detected by a multichannel detector at a take-off angle of 90° relative to the surface. During the measurements, the base pressure was 10-9-10-10 Torr. Survey spectra were obtained at a resolution of 1 eV from 3 scans, and high-resolution spectra were acquired at a resolution of 0.05 eV from 5-20 scans. All binding energies were determined with the Au 4f7/2 core level peak at 84 eV as a reference. Cyclic Voltammetry. Cyclic voltammograms (CVs) were acquired using a BAS 100B (Bioanalytical Systems, Inc.). The three-electrode electrochemical cell consisted of a modified Au electrode, a Pt wire counter electrode, and an Hg/Hg2SO4 (mercury sulfate electrode, MSE, saturated K2SO4) reference electrode. Experiments were carried out in degassed H2O containing 0.1 M HClO4 as a carrier electrolyte. The active area of the gold electrode was 0.283 cm2, and the surface coverage values were corrected for surface roughness, assuming a roughness factor of 1.2.45,46 Results and Discussion Interchain Carboxylic Anhydride and Acid Fluoride. Carboxylic acid-terminated SAMs have intensively been used for the attachment of biologically active molecules and others onto the surfaces, through the activation of carboxylic acid groups, such as acid chloride, N-hydroxysuccinimidyl (NHS) ester, pentafluorophenyl ester, and interchain carboxylic anhydride.47-50 There also has been a great deal of attention to the interfacial properties of carboxylic acid groups. For example, the interfacial acidity of carboxylic acids has been studied in detail, not only to understand the characteristics of interfacial phenomena but also to control the charge state of acid functional groups with direct implications for chemical binding and reactivity.24,51,52 Interchain carboxylic anhydrides (ICAs), which are readily prepared by the SAM-based reaction of carboxylic acid groups with trifluoroacetic anhydride and triethylamine, were introduced as a useful reactive intermediate by Whitesides and co-workers, and the reactivity of ICAs with various amines was reported.50 Since the introduction, ICAs have widely been used in surface sciences for the generation of functionalized surfaces, such as pat(45) Popenoe, D. D.; Deinhammer, R. S.; Porter, M. D. Langmuir 1992, 8, 2521. (46) Oh, S.-K.; Baker, L. A.; Crooks, R. M. Langmuir 2002, 18, 6981. (47) Duevel, R. V.; Corn, R. M. Anal. Chem. 1992, 64, 337. (48) Schmid, E. L.; Keller, T. A.; Cienes, Z.; Vogel, H. Anal. Chem. 1997, 69, 1979. (49) Hyun, J.; Chilkoti, A. Macromolecules 2001, 34, 5644. (50) Yan, L.; Marzolin, C.; Terfort, A.; Whitesides, G. M. Langmuir 1997, 13, 6704. (51) Gershevitz, O.; Sukenik, C. N. J. Am. Chem. Soc. 2004, 126, 482. (52) Konek, C. T.; Musorrafiti, M. J.; Al-Abadleh, H. A.; Bertin, P. A.; Nguyen, S. T.; Geiger, F. M. J. Am. Chem. Soc. 2004, 126, 11754.

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Figure 1. Reactions of carboxylic acid-terminated SAMs with cyanuric fluoride and pyridine.

terned surfaces of small molecules, polymers, and cells, and designed surfaces for studying biological interactions, such as protein adsorption and cell adhesion.53-57 In addition, the usefulness of ICAs in surface sciences was not limited to the flat surfaces. Gold nanoclusters functionalized with ICAs were prepared, and the reactivity of ICAs was utilized for the immobilization of the nanoclusters onto amine-modified surfaces.58 On the other hand, acid fluorides (AFs) have been used as a stable and highly reactive group in solution-based reactions for special purposes including peptide synthesis;59,60 they have not been investigated in the SAM-based reactions in detail, although AFs show a greater stability than the corresponding chloride toward neutral oxygen nucleophiles, such as water and methanol, but appear to be of equal reactivity toward anionic nucleophiles and amines in solution-based reactions.61,62 Rotello and co-workers reported the synthesis of AF-containing disulfide (11,11′dithiobis(undecanoic acid fluoride)) and the reactivity of mixed SAMs, composed of AF- and methyl-terminated disulfide (octyl disulfide), toward primary amines.63 AFs were also used for the activation of polymeric species via transformation of pendant carboxylic acids, and the activated polymers were used as a platform for immobilizing biopolymers, such as oligonucleotides.64 In this report, we studied the direct formation of AFs from carboxylic acid groups at surfaces with cyanuric fluoride (CyF) and pyridine and the effect of adjacent carboxylic acid groups on the product distribution (Figure 1). We formed the SAM of 16-mercaptohexadecanoic acid (MHDA) on gold by immersing the gold substrate in a 1 mM solution of ethanol/water/acetic acid (80/10/10, v/v/v) overnight and characterized the resulting SAM by grazing angle infrared (IR) spectroscopy. The IR spectrum of the SAM of MHDA showed two CdO stretching bands at 1742 and 1719 cm-1, from free and hydrogen-bonded carboxylic acids, respectively (Figure 2a).65 The presence of CsH asymmetric and symmetric stretching bands at 2919 and (53) Chapman, R. G.; Ostuni, E.; Yan, L.; Whitesides, G. M. Langmuir 2000, 16, 6927. (54) Yan, L.; Huck, W. T. S.; Zhao, X.-M.; Whitesides, G. M. Langmuir 1999, 15, 1208. (55) Chapman, R. G.; Ostuni, E.; Liang, M. N.; Meluleni, G.; Kim, E.; Yan, L.; Pier, G.; Warren, S.; Whitesides, G. M. Langmuir 2001, 17, 1225. (56) Ostuni, E.; Chapman, R. G.; Holmlin, R. E.; Takayama, S.; Whitesides, G. M. Langmuir 2001, 17, 5605. (57) Lee, K. B.; Kim, D. J.; Yoon, K. R.; Kim, Y.; Choi, I. S. Korean J. Chem. Eng. 2003, 20, 956. (58) Akamatsu, K.; Hasegawa, J.; Nawafune, H.; Katayama, H.; Ozawa, F. J. Mater. Chem. 2002, 12, 2682. (59) Carpino, L. A.; Sadat-Aalaee, D.; Chao, H. G.; DeSelms, R. H. J. Am. Chem. Soc. 1990, 112, 9651. (60) Carpino, L. A.; Mansour, E. M. E.; Sadat-Aalaee, D. J. Org. Chem. 1991, 56, 2611. (61) Swain, C. G.; Scott, C. B. J. Am. Chem. Soc. 1953, 75, 246. (62) Bender, M. L.; Jones, J. M. J. Org. Chem. 1962, 27, 3771. (63) Niemz, A.; Jeoung, E.; Boal, A. K.; Deans, R.; Rotello, V. M. Langmuir 2000, 16, 1460. (64) Milton, R. C. U.S. Patent 6,110,669, 2000. (65) Arnold, R.; Azzam, W.; Terfort, A.; Wo¨ll, C. Langmuir 2002, 18, 3980.

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Figure 3. IR spectra of MHDA SAMs after reactions with (a) 0.2 µL of CyF and 2 µL of pyridine, (b) 1 µL of CyF and 2 µL of pyridine, (c) 5 µL of CyF and 2 µL of pyridine, (d) 10 µL of CyF and 2 µL of pyridine, and (e) 50 µL of CyF and 2 µL of pyridine in 10 mL of CH2Cl2 for 1 h at room temperature. Figure 2. IR spectra of (a) intact MHDA SAMs and MHDA SAMs after reactions with (b) 0.2 µL of CyF and 2 µL of pyridine and (c) 4 µL of CyF and 100 µL of pyridine in 10 mL of CH2Cl2 for 1 h at room temperature. XPS spectra of SAMs presenting (d) ICAs and (e) AFs.

2850 cm-1 (not shown in Figure 2) confirmed that the monolayer was formed with a highly ordered structure.66 According to the previous report, the lateral packing density of the SAM of MHDA was calculated to be 4.32 molecules/nm2 (23.1 Å2/molecule), and the value indicated that the SAM had a relative coverage of 92.4% on Au(111) compared with that of the SAM of 1-octadecanethiol.44 After the formation of the SAM of MHDA, the gold substrate was treated with CyF and pyridine. CyF is a mild reagent that, in the presence of pyridine as a promoter, allows for the direct conversion of carboxylic acids into AFs in solution but does not affect alkene, hydroxy, or aromatic functionality.67 After the treatment of the SAM in low concentrations of reagents (0.2 µL of CyF and 2 µL of pyridine in 10 mL of CH2Cl2) for 1 h at room temperature, the two IR bands characteristic of the carboxylic acid groups (1719 and 1742 cm-1) disappeared completely and two new absorption bands appeared at 1822 and 1748 cm-1. The two IR bands are characteristic of the CdO stretching absorption of carboxylic anhydrides and are assigned as in-phase and out-of-phase stretching modes of the two coupled carbonyl groups of carboxylic anhydride, respectively (Figure 2b).53 To further confirm that the product at the surface was ICA, not AF, we characterized the surface by X-ray photoelectron spectroscopy (XPS). The XPS spectrum did not show any observable F 1s signal (Figure 2d). The IR and XPS data, therefore, prove that the product at the surface was ICA. In contrast, when we increased the concentrations of reagents (4 µL of CyF and 100 µL of pyridine in 10 mL of CH2Cl2) while keeping the other reaction conditions the same (1-h reaction at room temperature), we observed only one new absorption band at 1837 cm-1 in the IR spectrum; this peak is characteristic of the CdO stretching absorption of AFs (Figure 2c).63 The IR absorption band from ICA was not detected. The results demonstrated that the simple modulation of the amounts of the reagents (CyF and pyridine) controlled the product (ICA or AF) in (66) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145. (67) Olah, G. A.; Nojima, M.; Kerekes, I. Synthesis 1973, 487.

Figure 4. IR spectra of MHDA SAMs after reactions with (a) 4 µL of CyF and 1 µL of pyridine, (b) 4 µL of CyF and 10 µL of pyridine, and (c) 4 µL of CyF and 100 µL of pyridine in 10 mL of CH2Cl2 for 1 h at room temperature.

the carboxylic acid-terminated SAMs. In addition, the XPS spectrum of the substrate exhibited an F 1s peak centered at 687.7 eV (Figure 2e). Because the amount of the reagents affected the product distributions, we further investigated the effect of the concentration of each reagent. Figure 3 shows the dependence of the reaction products on the amount of CyF. The amount of pyridine was fixed to be 2 µL and that of CyF was varied to 0.2, 1, 5, 10, or 50 µL. After the reaction for 1 h under low concentrations (0.2 and 1 µL) of CyF, we observed IR peaks only from ICAs (1822 and 1748 cm-1, Figure 3a (or Figure 2b); 1823 and 1750 cm-1, Figure 3b) and did not observe an F 1s signal in the XPS spectra. In contrast, as the amount of CyF increased, a major IR peak appeared at 1838 cm-1 (a characteristic peak of AFs) and the intensity of peaks from ICAs decreased (Figure 3e). The observation indicated that the major product at the surface was changed to AFs from ICAs by the increased amount of CyF. The concentration of pyridine also affected the product distribution in a fashion similar to the concentration of CyF. Figure 4 shows the IR spectra indicating the dependence of the reaction products on the amount of pyridine. When the small amount of pyridine (1 µL) was used (the amount of CyF was fixed to be 4 µL), ICAs were the major product at the surface: characteristic peaks of ICAs (1829 and 1750 cm-1) appeared in the IR spectrum

Carboxylic Acid-Terminated SAMs on Gold

Figure 5. IR spectra of MHDA SAMs after reactions with 10 µL of CyF and 2 µL of pyridine in 10 mL of CH2Cl2 for (a) 5 min, (b) 1 h, (c) 3 h, and (d) 5 h at room temperature.

(Figure 4a). When the amount of pyridine was increased to 10 µL, a new IR band appeared at 1836 cm-1 and the intensity of a peak at 1750 cm-1 (and a peak at 1829 cm-1) decreased (Figure 4b). The in-phase stretching band of ICAs at 1829 cm-1 was not separated from the CdO stretching band of AFs at 1836 cm-1, and we, therefore, used the out-of-phase stretching band (at 1750 cm-1) as an indicator of ICAs. The increased peak intensity at 1836 cm-1 and the decreased peak intensity at 1750 cm-1 indicated that AFs became the major product as the amount of pyridine increased. In particular, we observed the formation of pure AFs when we increased the amount of pyridine to 100 µL (Figures 4c or 2c). Taken together, both reagents (CyF and pyridine) shifted the major product from ICAs to AFs, which implies that AFs are a more stable entity than ICAs at the surface under the reaction conditions employed. Time Dependency. We reasoned that, if the formation of ICAs resulted from a relatively faster formation than the formation of AFs (“kinetic control”), the longer incubation time would yield the formation of AFs with certain ranges of the amounts of the reagents. The time dependency of relative product concentrations (or relative surface densities in our system) would also give information on the relative stability of ICAs and AFs at surfaces, because the distinction between kinetic and thermodynamic control of product distributions could be manifested by the time dependency of product distributions. To follow the time dependency of the reaction, we chose the amount of the reagents that was between the amount for the formation of ICAs (i.e., the low concentrations in CH2Cl2) and the amount for the formation of AFs (i.e., the high concentrations in CH2Cl2): 10 µL of CyF and 2 µL of pyridine in 10 mL of CH2Cl2 at room temperature. Figure 5 shows the IR spectra about the dependence of the reaction products on the reaction time when the amounts of CyF (10 µL) and pyridine (2 µL) were fixed. At the early stage of the reaction, we observed a mixture of AFs and ICAs at the surface. After the 5-min reaction, the IR peaks appeared at 1835 and 1749 cm-1 (Figure 5a), indicating the coexistence of AFs and ICAs at the surface. When the reaction was performed for a longer time, the intensity of the IR peak from AFs (1839 cm-1) increased and that from ICAs decreased. We observed the IR peak only from AFs at the surface when the reaction was conducted for 5 h (Figure 5d). The observed time dependency of the product distribution clearly showed that ICAs were formed at the early stage of the reaction and were transformed to AFs (the thermodynamic product) at the final stage under the conditions studied.

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Figure 6. IR spectra of (a) a surface covered with AFs and (b) a surface after the immersion in 10 mL of CH2Cl2 containing 1 µL of H2O and 10 µL of pyridine for 24 h at room temperature.

Additives. In solution, acid chloride is readily transformed into carboxylic anhydride by reacting even with the carbonyl oxygen of tert-butyl ester,59 and AF also, although less reactive, reacts with anionic carboxylates, such as formate or acetate.68 On the basis of these reports, we thought that the formation of ICAs at the surface was caused by the enforced reaction between AFs and adjacent carboxylate anions,69 and we investigated the in-plane reactivity of AFs toward carboxylate anions by partially hydrolyzing AFs of the AF-covered surface in the basic water-pyridine condition. When one AF was hydrolyzed to a carboxylic acid, the carboxylic acid would be converted to a carboxylate anion by pyridine under the conditions. The carboxylate anion would then react with an adjacent AF, resulting in the formation of ICA, if they reacted effectively with each other at the surface. We prepared a gold surface fully covered with AFs, and Figure 6a shows the IR spectrum of the surface covered with only AFs (characterized by the IR band at 1841 cm-1). When we partially hydrolyzed AFs in the water-pyridine solution (1 µL of H2O and 10 µL of pyridine in 10 mL of CH2Cl2 in the absence of CyF), we observed the appearance of a mixture of AF (characterized by the IR band at 1840 cm-1) and ICA (characterized by the IR bands at 1823 and 1751 cm-1) (Figure 6b). In addition, we could not detect any IR bands from carboxylic acids or carboxylate anions after the partial hydrolysis. On the basis of this observation, we thought that AFs were generated at the very early stage of the reaction and ICAs were formed by the interchain reaction between the formed AFs and the residual carboxylate anions. What then caused the transformation of ICAs into AFs in the high concentrations of reagents when ICAs were formed at the surface? We hypothesized that the transformation arose from the decomposition of ICAs into carboxylic acids (and AFs) under those conditions, and we considered the decomposition rate as one of the important factors for the control of the product distributions at the surface. In principle, the decomposition of ICAs (in other words, the opening reaction of ICAs) could be achieved by fluoride anion and pyridine (and residual water). We investigated the role of fluoride anion in (68) Bunton, C. A.; Fendler, J. H. J. Org. Chem. 1966, 31, 2307. (69) In this case, acid fluoride is the one that was formed at the very early stage of the reaction pathway. This acid fluoride group (or “initial acid fluoride” suggested by one of the referees) should be differentiated from the acid fluoride group formed as a final product. We thought that the “initial acid fluoride” group could survive to contribute to the formation of AF-rich surfaces or be converted into the interchain carboxylic anhydride group, depending upon reaction conditions. Refer to the Proposed Mechanism section.

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Figure 7. IR spectra of MHDA SAMs after the reaction with 10 µL of CyF and 2 µL of pyridine in 10 mL of CH2Cl2 containing (a) no additive, (b) 2 mM HF-Py, and (c) 2 mM TBAF for 1 h at room temperature.

facilitating the decomposition of ICAs and the formation of AFs by using compounds containing fluoride anion, such as HF-Py and tetrabutylammonium fluoride (TBAF). One fluoride anion would produce one AF and one carboxylate group via attacking one ICA, if the fluoride anion is reactive enough to attack the anhydride.70 The addition of fluoride anions to the reaction solution did dramatically accelerate the formation of AFs at the surface: the surfaces were fully covered with AFs after only l h of reaction with the reaction solution containing 2 mM fluoride anions (HFPy for Figure 7b and TBAF for Figure 7c) as well as 10 µL of CyF and 2 µL of pyridine in 10 mL of CH2Cl2. Only one IR peak from AFs was observed at 1840 cm-1 in Figure 7b and at 1839 cm-1 in Figure 7c, respectively. In the absence of additional fluoride anions, there was a mixture of ICAs and AFs after 1 h (Figure 7a), and 5 h was needed for the formation of the surface fully covered with AFs with the same amount of CyF and pyridine (Figure 5d). We performed another experiment, which involved a separate ICA formation step and a transformation step into AFs, for proving the effective transformation of ICAs into AFs in the reaction solution containing fluoride anions. At first, we prepared a surface presenting only ICAs by adopting the reaction condition of 1 µL of CyF and 2 µL of pyridine (Figure 3b). After the formation, the substrate was immersed in a CH2Cl2 (10 mL) solution containing 2 mM TBAF as well as 10 µL of CyF and 2 µL of pyridine for 1 h at room temperature. After the reaction, the IR peak from only AFs was detected at 1837 cm-1 from the substrate. The result indicates that ICAs effectively transformed into AFs in the reaction solution containing fluoride anions. Proximity Effect. The proximity effect of adjacent carboxylates resulted from the well-packedness of the SAM of MHDA, where the space between two adjacent carboxylates was calculated to be 5.17 Å from the packing density (4.32 molecules/nm2) of MHDA molecules. We reasoned that the spatial separation of the carboxylates would decrease the proximity effect and AFs would form as a major product at the surface. We used two methods for decreasing the proximity effect. One is the use of mixed SAMs, and the other is the use of carboxylic acids containing a flexible ethylene glycol (EG) linker (HSEG3-COOH). Alkanethiols form a well-ordered, closely packed structure with a fundamental periodicity of simple hexagonal (x3 × x3)R30° with respect to Au(111) according to STM studies.71 In the pure SAM of MHDA, (70) Olah, G. A.; Kuhn, S. J. J. Org. Chem. 1961, 26, 237.

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Figure 8. IR spectra of (a) the MHDA SAM, (b) the 1:2 mixed SAM of MHDA and DDT, and (c) the SAM of HS-EG3-COOH after the reaction with 1 µL of CyF and 2 µL of pyridine in 10 mL of CH2Cl2 for 1 h at room temperature.

one carboxylate group was surrounded by six carboxylate groups and the number of neighboring carboxylate groups could be reduced via the formation of mixed SAMs with methyl-terminated alkanethiol. We selected the 1:2 mixed SAM of MHDA and DDT (dodecanethiol) as a model of the mixed system and studied the product distribution in the system. When the reaction was performed on the 1:2 mixed SAM under the reaction conditions for forming only ICAs on the SAM of MHDA (1 µL of CyF and 2 µL of pyridine in 10 mL of CH2Cl2, 1 h of reaction at room temperature) (Figure 8a), we observed that AFs existed as a remarkable portion (Figure 8b): the CdO stretching band of AFs appeared at 1852 cm-1 with the intensity comparable to the characteristic bands of ICAs at 1829 and 1760 cm-1 in the IR spectrum after the reaction. The decrease of the proximity effect was also achieved by the formation of the SAM of HS-EG3-COOH. According to the previous report, the lateral packing density of the SAM of HSEG3-COOH was calculated to be 3.49 molecules/nm2 (28.7 Å2/molecule), and this value indicated that the SAM had a relative coverage of 80.7% compared with that of the SAM of MHDA.44 With the decrease of the packing density by the introduction of EG groups, we expected that the spatial arrangement of terminal carboxylate groups would not be suitable for the fast interchain reaction, compared with that of the SAM of MHDA. When the reaction was conducted with the SAM of HS-EG3-COOH under the same reaction conditions as those for the mixed SAM of MHDA and DDT, we observed the peak at 1840 cm-1 from AFs and the peaks at 1822 and 1775 cm-1 from ICAs in the IR spectrum (Figure 8c). From the results, we concluded that the enforced proximity played a crucial role in shifting the product distribution from AFs to ICAs. Proposed Mechanism. Figure 9a shows a proposed reaction mechanism for the formation of AFs and ICAs at the surface. We assume that, at the very early stage of reaction pathway, a small number of AFs formed at the surface, following the mechanism on the solution chemistry analogue, and then ICAs were formed by the reaction between AFs and adjacent carboxylate anions. In the low concentrations of reactants, AFs would form at the surface with a low density (Figure 9b). In that case, many reactive carboxylate anions existed next to the newly formed AF and would react with the AF spontaneously, leading to the formation of ICA, due to the positional advantage, i.e., proximity effect. In the higher concentrations of (71) Poirier, G. E. Chem. Rev. 1997, 97, 1117.

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Figure 10. IR spectra of the SAMs of MHDA presenting (a) ICAs and (b) AFs after the reaction with UDA in 10 mL of 1 mM CH2Cl2 solution for 1 h at room temperature.

Figure 9. (a) Proposed mechanism on product distributions. (b) Formation of ICAs in the low concentrations of reactants.

reactants, however, AFs formed faster than in the low concentrations of reactants. The fast formation of AFs caused a fast decrease of the number of the carboxylate anions at the surface and a decreased chance of the formation of ICAs. AFs that had not been attacked by carboxylate anions remained at the surface, and the relative density of AFs increased with the increase of the amount of the reactants. Another possible mechanism for the formation of ICAs is the involvement of the species that was suggested as an intermediate for the formation of AFs in solution-based reactions.67 The reaction between the intermediate and carboxylates would lead to the direct formation of ICAs under low concentrations of reagents. If other reactions did not occur at the surface, there should always be mixtures of AFs and ICAs at the surface. However, the decomposition of ICAs and AFs could occur under the reaction conditions. Particularly, the decomposition of ICAs is important in the transformation of products. When ICAs were decomposed into carboxylic acids, the carboxylic acid was converted into AF by the reactants existing in the reaction solution. In addition, the newly formed AF at the later stage of the reaction pathway was surrounded by chemical environments that were not the same as those at the early stage of reaction pathway: adjacent groups were not reactive carboxylate anions any more but were AFs or ICAs. Therefore, the AF formed at the later stage of the reaction pathway could exist stably at the surface. As the reaction was conducted for a longer time, the amount of ICAs decreased and AFs became predominant at the surface. Pyridine also acts as a catalyst for the hydrolysis of ICAs.68 Therefore, the hydrolysis of ICAs occurs effectively under the large amount of pyridine, and then the hydrolysis of ICAs by pyridine also leads to an “effective” formation of AFs at the surface via the reaction pathway explained previously. Reactivity of ICAs and AFs. We investigated the reactivity of the activated surfaces toward primary amines, and the number of reactive sites, by reacting the ICA- and AF-presenting surfaces with UDA and Fc-NH2. The activated surfaces were incubated in a 1 mM CH2Cl2 solution of amines for 1 h at room temperature. After the reaction between the ICA-activated surface and UDA,

Figure 11. Cyclic voltammograms of the ICA-activated gold surface after the reaction with Fc-NH2, with the scan rate of (a) 50 mV/s, (b) 100 mV/s, (c) 200 mV/s, (d) 400 mV/s, and (e) 800 mV/s. A plot of anodic peak currents as a function of scan rate is shown as an inset.

the two CdO stretching bands of the ICAs disappeared completely and two new absorption bands appeared at 1735 and 1563 cm-1 (Figure 10a). The IR band at 1735 cm-1 was assigned as the CdO stretching band of the carboxylic acids, and that at 1563 cm-1 was assigned as the amide II band. The amide I band did not appear because of a parallel orientation of carbonyl groups as previously reported.50 In the case of methylene vibrations, a shoulder peak at 2931 cm-1 near the CsH asymmetric stretching and a shift of the CsH symmetric stretching peak to 2854 cm-1 were observed. The positions of the peaks imply the existence of less-organized alkyl chains, which may be due to loosely packed UDA groups because the maximum surface coverage of UDA would be 50%. The new bands at 2965 and 2877 cm-1 were assigned as the asymmetric CsH stretching and the symmetric CsH stretching of the terminal methyl group of the UDA, respectively.50 In the case of AF-presenting surfaces, the CdO stretching peak of AFs disappeared completely after the reaction, and only the amide II band appeared at 1550 cm-1 (Figure 10b). The CdO stretching peak from carboxylic acids was not observed, which implies the complete conversion to the amide. The peaks from any disorganized methylene groups did not appear in the IR spectrum, and we observed only sharp peaks at 2918 and 2850 cm-1 from the well-organized alkyl chains, which implies the densely packed structure of UDA. For estimating the number of reactive sites, we measured the number of ferrocene moieties by cyclic voltammetry after the reaction with Fc-NH2. Figure 11 shows the CV obtained from the ICA-activated surface after the reaction with Fc-NH2. The inset shows the anodic peak

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current as a function of scan rate, which exhibited a linear dependence, as expected for a surface-bound species.47,72 The number of surface-bound ferrocenes was calculated by the integration of the oxidation peak. A surface coverage of 2.02 × 1014 cm-2 was calculated for the ICA-activated surface, and a surface coverage of 3.32 × 1014 cm-2 was calculated for the AF-activated surface. The number of the reactive sites of the AF-activated surface was 1.64× as many as that of the ICA-activated surface. In the ideal case, the number of ferrocenes would be twice as many at the surface presenting AF as at the surface presenting ICAs. We believe that the steric effect of the ferrocene headgroup yielded the observed smaller value. Chidsey and co-workers reported that the maximum coverage of ferrocene-terminated SAMs would be 2.7 × 1014 cm-2 from a theoretical calculation based on the diameter of the ferrocene group, and they observed larger numbers of ferrocenes from ferrocene-terminated SAMs than they expected.72 Our result obtained from the AF-activated surface is similar to that from the ferrocene-terminated SAMs. Conclusions We demonstrated that two different products could controllably be obtained at the surface from one reaction system by the simple change of reaction conditions. The key factors for this control over the reaction pathways were the proximity of adjacent chains in the SAMs (kinetic control) and the stability of products (thermodynamic control). There have been many reports on the utilization of the well-orderedness and well-packedness of selfassembled, two-dimensional structures, but little attention has been paid to the similarities and differences between the reactions in the SAMs and those in solution. A more detailed study on the chemical reactivity of the twodimensional systems would aid in the full utilization of the two-dimensional systems and the facile applicability to various technologically important areas. ICAs have been (72) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301.

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widely used in various areas of surface sciences, but one of the disadvantages in using ICAs for the attachment of molecules onto surfaces includes the 50% surface coverage of the attached molecules at a maximum; therefore, another approach is needed when a high coverage of the attached molecule is required. Acid chloride and acid fluoride would be promising intermediates for the introduction of functional molecules onto surfaces with a high density. Although highly reactive, the instability of acid chloride makes it difficult to perform the coupling reactions under ambient conditions: it was reported that acid chloride is reconverted to carboxylic acid by contact with atmospheric water at the surface.47 In contrast, AFs are relatively stable against the hydrolysis and show equal reactivity toward amines. We observed different features of molecules attached onto ICA- and AF-activated surfaces: molecules attached onto the AF-activated surface showed higher density and better-orderedness than those attached onto the ICA-activated surface. It would be, therefore, beneficial in the design and fabrication of tailormade, activated surfaces to be capable of generating either ICA- or AF-covered surfaces from one system depending upon the applications. For example, the AF-based method could be used for the cases requiring higher density and better-orderedness of attached molecules, such as immobilization of nano- or small-sized objects by multivalent interaction, and for precise tuning of surface properties such as wettability using functional groups at the other end of amino compounds. The ICA-based method would be used more effectively to overcome problems, such as severe steric hindrance and low biological binding efficiency, at highly dense surfaces. Acknowledgment. This work was supported by the Korea Research Foundation (KRF-2004-015-C00301). We thank Mi Ra Kim and Dr. Mi-Sook Won in Korea Basic Science Institute (KBSI) for the XPS analysis. We also thank Jungdon Suk, Seong Jung Kwon, and Professor Juhyoun Kwak of the Department of Chemistry at KAIST for help on cyclic voltammetry experiments. LA050847E