Hydrophobic Switch - Langmuir

Apr 2, 2005 - Supported Bilayer Lipid Membrane Arrays on Photopatterned Self-Assembled Monolayers. Xiaojun Han , Singh N. D. Pradeep , Kevin Critchley...
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A Mild Photoactivated Hydrophilic/Hydrophobic Switch Kevin Critchley,† Jeyaratnam P. Jeyadevan,‡ Hitoshi Fukushima,§ Masaya Ishida,| Tatsuya Shimoda,§ Richard J. Bushby,‡ and Stephen D. Evans*,† School of Physics and Astronomy, University of Leeds, LS2 9JT, United Kingdom, Self-Organising Molecular Systems (SOMS) Centre, University of Leeds, LS2 9JT, United Kingdom, I/O group/Technology Platform Research Centre - Seiko Epson Corporation, Fujimi Plant, 281 Fujimi, Fujimi-machi, Suwa-gun, Nagano-ken 399-0293, Japan, and Cambridge Research Laboratory of Epson, 9A Science Park, Milton Road, Cambridge, CB4 0FE, United Kingdom Received December 20, 2004. In Final Form: February 11, 2005 Surface modification using light is one of the most powerful methods for controlling the physical and chemical properties of functionalized surfaces. In this paper, we report on systems where soft UV irradiation (λ ) 365 nm) converts a “low” activity fluorocarbon to a “high” activity amine-functionalized surface. An amine-functionalized SAM (self-assembled monolayer) is first masked using a tertiary amine catalyzed reaction with an N-hydroxysuccinimidyl carbonyl reagent. This mild, room-temperature reaction introduces a hydrophobic photocleavable nitrobenzyl “protecting group” terminated with a fluorocarbon end-chain. UV irradiation (λ ) 365 nm) of this hydrophobic/fluorocarbon surface cleaves the nitrobenzyl residue, returning the surface to the original hydrophilic/amine-functionalized state. This provides a mild, generic method of producing surfaces with hydrophilic/hydrophobic patterns or patterned with amine functional residues. Two different protecting groups, one terminated with a single and the other with three fluorocarbon end chains, are compared. In the case of the more bulky protecting group, only a small proportion of the amine residues react, but the surface is equally hydrophobic and the amine residues equally well shielded from further reaction. Surfaces are characterized by X-ray photoelectron spectroscopy, ellipsometry, surface potential, and contact angle measurements. Images of the photopatterned SAMs were obtained using scanning electron microscopy.

1. Introduction Self-assembled monolayers (SAMs) have received significant interest for fundamental studies in wetting,1-3 adhesion,4-7 lubrication,8,9 and corrosion.10-12 Furthermore, the introduction of patterning has widened their application, for example, for selective metal deposition, area-specific colloidal attachment, biological immobilization, as etch resists, and potential templates for “lab-ona-chip” systems.13-16 Consequently, many methodologies * Corresponding author. Telephone: +44 113 343 3852. Fax: +44 113 343 3900. E-mail: [email protected]. † School of Physics and Astronomy, University of Leeds. ‡ SOMS Centre, University of Leeds. § Seiko Epson Corp. | Cambridge Research Laboratory of Epson. (1) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 1990-1995. (2) Bain, C. D.; Eval, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155. (3) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7164. (4) Beake, B. D.; Leggett, G. J. Phys. Chem. Chem. Phys. 1999, 1, 3345-3350. (5) Clear, S. C.; Nealey, P. F. J. Colloid Interface Sci. 1999, 213, 238-250. (6) Chaudhury, M. K. Curr. Opin. Colloid Interface Sci. 1997, 2, 6569. (7) Rozsnyai, L. F.; Wrighton, M. S. Chem. Mater. 1996, 8, 309-311. (8) Tsukruk, V. V. Adv. Mater. 2001, 13, 95-108. (9) McDermott, M. T.; Green, J. B. D.; Porter, M. D. Langmuir 1997, 13, 2504-2510. (10) Zamborini, F. P.; Crooks, R. M. Langmuir 1998, 14, 3279-3286. (11) Jennings, G. K.; Laibinis, P. E. Colloids Surf., A 1996, 116, 105114. (12) Ulman, A. Characterization of organic thin films; ButterworthHeinemann: London, 1995. (13) Kataoka, D. E.; Troian, S. M. Nature 1999, 402, 794-797. (14) Lahiri, J.; Ostuni, E.; Whitesides, G. M. Langmuir 1999, 15, 2055-2060. (15) Jenkins, A. T. A.; Bushby, R. J.; Boden, N.; Evans, S. D.; Knowles, P. F.; Liu, Q. Y.; Miles, R. E.; Ogier, S. D. Langmuir 1998, 14, 46754678.

for patterning SAMs have been investigated17-26 with the most promising being microcontact printing27-30 and photolithography.19,31 These are both capable of producing complex patterns, with high spatial resolution, and more importantly offer “parallel” processability. Photolithography of SAMs normally requires short wavelengths (e254 nm) for efficient patterning, and these harsh conditions are often associated with nonspecific photodegradation, ozone induced damage, and a poorly defined surface.25,32-36 However, while increasing the wavelength (16) Huang, Z. Y.; Wang, P. C.; MacDiarmid, A. G.; Xia, Y. N.; Whitesides, G. Langmuir 1997, 13, 6480-6484. (17) Ara, M.; Graaf, H.; Tada, H. Appl. Phys. Lett. 2002, 80, 25652567. (18) Biebuyck, H. A.; Larsen, N. B.; Delamarche, E.; Michel, B. IBM J. Res. Dev. 1997, 41, 159-170. (19) Calvert, J. M.; Chen, M. S.; Dulcey, C. S.; Georger, J. H.; Peckerar, M. C.; Schnur, J. M.; Schoen, P. E. J. Vac. Sci. Technol., B 1991, 9, 3447-3450. (20) Friebel, S.; Aizenberg, J.; Abad, S.; Wiltzius, P. Appl. Phys. Lett. 2000, 77, 2406-2408. (21) Hartwich, J.; Dreeskornfeld, L.; Heisig, V.; Rahn, S.; Wehmeyer, O.; Kleineberg, U.; Heinzmann, U. Appl. Phys. A 1998, 66, S685-S688. (22) H’Dhili, F.; Bachelot, R.; Rumyantseva, A.; Lerondel, G.; Royer, P. J. Microsc. Oxf. 2003, 209, 214-222. (23) Lee, W. B.; Oh, Y.; Kim, E. R.; Lee, H. Synth. Met. 2001, 117, 305-306. (24) Li, Y.; Ben, M.; Liu, J. Chin. J. Inorg. Chem. 2002, 18, 75-78. (25) Sugimura, H.; Ushiyama, K.; Hozumi, A.; Takai, O. Langmuir 2000, 16, 885-888. (26) Ulman, A. Organic Thin Films and Surfaces Directions for the Nineties; Academic Press: New York, 1995; Vol. 20. (27) Xia, Y. N.; Whitesides, G. M. Adv. Mater. 1995, 7, 471-473. (28) Burgin, T.; Choong, V. E.; Maracas, G. Langmuir 2000, 16, 53715375. (29) Fujihira, M.; Furugori, M.; Akiba, U.; Tani, Y. Ultramicroscopy 2001, 86, 75-83. (30) Michel, B.; Bernard, A.; Bietsch, A.; Delamarche, E.; Geissler, M.; Juncker, D.; Kind, H.; Renault, J. P.; Rothuizen, H.; Schmid, H.; SchmidtWinkel, P.; Stutz, R.; Wolf, H. IBM J. Res. Dev. 2001, 45, 870870. (31) Sugimura, H.; Hanji, T.; Takai, O.; Masuda, T.; Misawa, H. Electrochim. Acta 2001, 47, 103-107.

10.1021/la046851s CCC: $30.25 © 2005 American Chemical Society Published on Web 04/02/2005

Mild Photoactivated Hydrophilic/Hydrophobic Switch

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Figure 1. Schematic showing the formation of photocleavable surfaces (step a), and subsequent photodeprotection (step b). The NH2-PEG SAM was reacted with the active N-hydoxysuccinyl ester reagent, at room temperature. Irradiation with UV light (365 nm) caused photochemical deprotection and returned the surface to the original NH2-PEG SAM state.

reduces the likelihood of undesired photodegradation, it also limits the ultimate lateral resolution achievable (by irradiation in the far-field). In this paper, as shown in Figure 1, reagents 1 and 2 are used to mask a hydrophilic amine-functionalized SAM with a hydrophobic protecting group that can be removed by photolysis at relatively long wavelengths (λ ≈ 365 nm). This relies on well-established photochemistry of nitro-benzyl compounds.37,38 These reagents have previously been developed mainly for biofunctionalization work,39,40 conditions where the use of “soft” UV is essential to avoid damage of fragile biomolecular species.38,39,41 Similar surface photodeprotection has been used by Nevins et al. to achieve siteselective protein immobilization42 and by Nakagawa and Ichimura who have compared 365 nm photodeprotection of a nitrobenzyl protected SAM with 254 nm photodegredation.43 They suggested that 254 nm photodegradation was the more successful pattern-transfer technique, but both of their amine-protection and 365 nm photodeprotection steps seem to be quite incomplete.43 As compared to the Nevins system, we have reversed the orientation of the photocleavable group so that this is wholly detached in the photolysis step. By using a tertiary amine catalyzed reaction of an N-hydroxylsuccinyl “active ester” rather than Nakagawa’s carbonate, we hoped to obtain a cleaner protection chemistry and certainly reaction at lower temperatures. This means that thiol-on-gold SAMs can be used as well as Nakagawa’s organosilane-on-SiO2 (32) Ferris, M. M.; Rowlen, K. L. Appl. Spectrosc. 2000, 54, 664-668. (33) Rowlen, K. L.; Ferris, M. M. Abstr. Pap. Am. Chem. Soc. 2000, 219, 17-COLL. (34) Dressick, W. J.; Calvert, J. M. Jpn. J. Appl. Phys. Part 1 Regul. Pap. Short Notes Rev. Pap. 1993, 32, 5829-5839. (35) Dressick, W. J.; Dulcey, C. S.; Chen, M. S.; Calvert, J. M. Thin Solid Films 1996, 285, 568-572. (36) Calvert, J. M.; Brandow, S. L.; Chen, M. S.; Dressick, W. J.; Dulcey, C. S.; Koloski, T. S.; Stenger, D. A. Abstr. Pap. Am. Chem. Soc. 1993, 205, 11-COLL. (37) Dunkin, I. R.; Gebicki, J.; Kiszka, M.; Sanin-Leira, D. J. Chem. Soc., Perkin Trans. 2 2001, 1414-1425. (38) Yip, R. W.; Sharma, D. K.; Giasson, R.; Gravel, D. J. Phys. Chem. 1985, 89, 5328-5330. (39) Fodor, S.; Read, J.; Pirrung, M.; Stryer, L.; Lu, A.; Solas, D. Science 1991, 251, 767-773. (40) Ryan, D.; Parviz, A. P.; Linder, V.; Semetey, V.; Sia, S. K.; Su, J.; Mrkich, M.; Whitesides, G. M. Langmuir 2004, 20, 9080-9088. (41) Pelliccioli, A. P.; Wirz, J. Photochem. Photobiol. Sci. 2002, 1, 441-458. (42) Nivens, D. A.; Conrad, D. W. Langmuir 2002, 18, 499-504. (43) Nakagawa, M.; Ichimura, K. Colloids Surf., A 2002, 204, 1-7.

Figure 2. Molecular structure of the photoactive reagents used to modify amine-functionalized SAMs. The active ester reagents have identical head and photoreactive groups. Reagent 1 has a single “hydrophobic” semi-fluorinated chain, and reagent 2 has three semi-fluorinated chains.

SAMs. Furthermore, to increase the contrast in surface energies, semi-fluorinated chains are introduced.44 Reagents 1 and 2 are composed of three parts: (1) an active ester to facilitate reaction with the amine-functionalized surface, (2) a central photocleavable nitrobenzyl linker, and (3) a hydrophobic semi-fluorinated end-chain (Figure 2). The advantage of this modular structure is that all three of these functions can be separately engineered, and in this paper we compare two variants with different hydrophobic end-chain functionality. The poly(ethylene glycol) (PEG) derivative was chosen to promote biocompatibility of the system by providing a flexible linker for surface attachment. Further, the hydrophilic nature of the PEG units serves to maximize the contrast in surface free energy between the protected and deprotected regions. The protection of the preformed amine surface provides a generic methodology suitable for extension to other monolayer systems, for example, organosilanes. Notwithstanding this, it is also possible to protect the ω-functional group prior to SAM formation, and results of these studies will be presented separately. 2. Experimental Section Materials. Anhydrous methyl sulfoxide 99.8% (DMSO), dichloromethane 99.9% (DCM), ethyl alcohol (spectroscopic grade), and hydrogen peroxide (27.5 wt %) were used as received from Sigma-Aldrich. Sulfuric acid (98%) was supplied by Fisher Scientific. The amine SAM precursor 11 (NH2-PEG-SH) was supplied by Seiko Epson Corp. Triethylamine was supplied by Sigma-Aldrich and was dried by distillation from KOH and stored over 4 A molecular sieves under argon. Silicon substrates were cut into approximately 1 cm2 squares from n-type doped, (100) wafers, resistivity of 1-5 Ω cm, as supplied by Laporte Electronics. Millipore Milli-Q water with a resistivity better than 18.0 MΩ cm was used throughout. High purity (99.99%) temper annealed gold wire (0.5 mm diameter) was supplied by Advent. General Procedures and Instrumentation. Column chromatography was carried out on Merck 60 (230-400 mesh) silica (44) Fukushima, H.; Seki, S.; Nishikawa, T.; Takiguchi, H.; Tamada, K.; Abe, K.; Colorado, R. J.; Graupe, M.; Shmakova, O. E.; Lee, T. R. J. Phys. Chem. B 2000, 104, 7417.

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gel and thin-layer chromatography using aluminum-backed plates coated with a 0.25 mm layer of silica gel 60 H containing fluorescer. The plates were visualized using ultraviolet light, Vanillin and Anisaldehyde stain. 1H NMR spectra were recorded on general electric QE 300, Bruker AM 500 spectrometers. Mass spectra were recorded on a VG autospec mass spectrometer. Microanalysis was carried out at the University of Leeds microanalytical laboratory. 3-Methoxy-4-(1H,1H,2H,2H-perfluorooctyloxy)-benzaldehyde (4). Vanillin 3 (1.0 g, 6.57 mmol) was added to a mixture of KOH (0.73 g, 13 mmol) and ethanol (10 mL) under nitrogen. 1-Iodo-1H,1H,2H,2H-perfluorooctane (5.0 g, 10.54 mmol) was added at 70 °C, and the mixture was refluxed for 48 h. After being cooled to room temperature (RT), the mixture was filtered and the solids were washed with Et2O. Solvent was removed in vacuo, and the residue was dissolved in water. The aqueous mixture was extracted with Et2O. The organic layer was washed with water and brine, dried over MgSO4, filtered, and the solvent was removed in vacuo. Purification by flash chromatography with nhexane/EtOAc (70/30) gave the product 4 as a white solid (0.90 g, 30%). 1H NMR (300 MHz, CDCl3) δ: 9.87 (s, 1H, Ar-H), 7.48-7.43 (m, 2H, Ar-H), 6.99 (d, J ) 8.2 Hz, 1H, Ar-H), 4.434.38 (t, J ) 7.14 Hz, 2H), 3.93 (s, 3H), 2.81-2.69 (m, 2H). 13C NMR (75 MHz, CDCl3): 191.28, 153.26, 150.35, 131.28, 126.90, 112.34, 110.02, 61.54, 56.47, 31.82, 31.54, 31.25. MS (ES+) 499 ([M + H]+, 55), 454 (20), 391 (22), 279 (5), 241 (5). 5-Methoxy 2-Nitro-4-(1H,1H,2H,2H-perfluorooctyloxy)benzaldehyde (5). Nitric acid (70%, 10 mL) was cooled to 0 °C, 3-methoxy-4-(1H,1H,2H,2H-perfluorooctyloxy)-benzaldehyde, 4 (0.3 g, 0.6 mmol), was added with stirring, and the mixture was brought to RT over 3 h and poured into ice-water (50 mL). The resultant yellow solid was collected by filtration, washed with cold water and cold ethanol, dried, and recrystallized from 95% ethanol, affording 5 (0.25 g, 80%) as a yellow solid. 1H NMR (300 MHz, CDCl3) δ: 7.64 (s, 1H, Ar-H), 7.43 (s, 1H, Ar-H), 4.45 (t, J ) 6.77 Hz, 2H), 4.01 (s, 3H), 2.86-2.69 (m, 2H). 13C NMR (75 MHz, CDCl3): 188.08, 154.01, 151.17, 126.77, 110.71, 108.91, 91.23, 62.28, 57.17, 31.78, 31.48, 31.21. MS (ES+) 544 ([M + H]+, 100), 514 (15), 410 (15), 282 (10), 178 (10). Anal. Calcd for C16H10F13O5N: C, 35.35; H, 1.85; N, 2.57. Found: C, 35.35; H, 1.95; N, 2.75. 5-Methoxy-2-nitro-4-(1H,1H,2H,2H-perfluorooctyloxy)benzyl Alcohol (6). To an ice-cooled solution of 5-methoxy-2nitro-4-(1H,1H,2H,2H-perfluorooctyloxy)-benzaldehyde, 5 (0.3 g, 0.55 mmol), in anhydrous THF (5 mL) was added NaBH4 (42 mg, 1.1 mmol), and the mixture was stirred at 0-5 °C for 4 h. The reaction was quenched by addition of water (10 mL). The organic layer was separated, the aqueous layer extracted with EtOAc (3 × 10 mL), and the combined organics dried with anhydrous Na2SO4. After filtration, the solvent was removed under reduced pressure to give a solid, which was chromatographed on a silica gel column using nhexane/EtOAc (60/40) (0.21 g, 70%). 1H NMR (300 MHz, CDCl3) δ: 7.74 (s, 1H, Ar-H), 7.22 (s, 1H, Ar-H), 4.98 (s, 2H), 4.37 (t, J ) 6.87 Hz, 2H), 3.99 (s, 3H), 2.82-2.65 (m, 2H), 2.62 (br s, 1H). 13C NMR (75 MHz, CDCl3): 154.80, 146.79, 139.89, 133.75, 111.82, 110.56, 63.18, 56.90, 31.80, 31.54, 31.26. MS (ES+) 528 ([M - OH]+, 100). Reagent 1. To a stirred solution of 5-methoxy-2-nitro-4(1H,1H,2H,2H-perfluorooctyloxy)-benzyl alcohol, 6 (0.2 g, 0.36 mmol), in anhydrous CH3CN (5 mL), under an argon atmosphere, was added N,N-disuccinimidyl carbonate (0.1 g, 0.39 mmol) followed by Et3N (0.05 mL, 0.35 mmol). The solution was stirred for an additional 5 h, and the solvent was removed in vacuo. The residue was purified by silica gel chromatography using DCM/ EtOAc (96/4), to give reagent 1 as a pale yellow solid (0.15 g, 62%, mp 46-48 °C). 1H NMR (300 MHz, CDCl3) δ: 7.80 (s, 1H, Ar-H), 7.23 (s, 1H, Ar-H), 5.99 (s, 2H), 4.39 (t, J ) 6.73 Hz, 2H), 4.05 (s, 3H), 2.87 (s, 4H), 2.80-2.68 (m, 2H). 13C NMR (75 MHz, CDCl3): 168.88, 155.07, 151.81, 147.36, 139.23, 126.89, 110.54, 109.49, 69.48, 62.09, 57.08, 31.76, 31.51, 31.23, 25.86. MS (FAB) 686 ([M]+, 8), 528 (100). HRMS (ES+) calcd. for C21H15N2O9F13Na 709.0468, found 709.0473. Anal. Calcd for C21H15N2O9F13: C, 36.73; H, 2.20; N, 4.08. Found: C, 36.45; H, 2.35; N, 4.25. Hydrolysis of Solutions of Reagent 1 on Exposure to Atmospheric Moisture. Although reagent 1 could be stored as the solid in the dark for weeks or even months without noticeable

Critchley et al. deterioration, it was found that dilute solutions in Et3N/DCM, exposed to the moist atmosphere, had completely decomposed within 1 or 2 days. The TLC showed absence of the starting material and that one major product had formed. This was isolated by column chromatography and shown to be the symmetrical carbonate 10. 1H NMR (500 MHz, CDCl3) δ: 7.76 (s, 2H), 7.06 (s, 2H), 5.62 (s, 4H), 4.38 (t, J ) 6.80 Hz, 4H), 3.98 (s, 6H), 2.78-2.70 (m, 4H). MS (EI+) 1116 (M+, 10), 1104 (100), 783 (50), 544 (30), 528 (100), 77 (55). 3,4,5-Tris(1H,1H,2H,2H,3H,3H-perfluoronon-1-yloxy)benzaldehyde (8). To a mixture of K2CO3 (0.7 g, 5.06 mmol) and anhydrous DMF (12 mL) under N2 was added 3,4,5trihydroxybenzaldehyde 7 (0.1 g, 0.58 mmol). 1-Iodo-1H,1H,2H,2H,3H,3H-perfluorononane45 (1.0 g, 2.04 mmol) was added at 65 °C, and the mixture was stirred at 65 °C overnight, poured into cold water (50 mL), and acidified with concentrated HCl. The aqueous mixture was extracted with Et2O (3 × 25 mL), the extracts were washed with water (2 × 50 mL) and dried over MgSO4, and the solvent was removed in vacuo. Purification by flash chromatography with ethyl acetate/nhexane (30/70) as eluent gave the product 8 as a white solid (0.53 g, 74%). 1H NMR (500 MHz, CDCl3) δ: 9.85 (s, 1H), 7.22 (s, 2H), 4.15 (t, J ) 5.98 Hz, 4H), 4.1 (t, J ) 5.87, 2H), 2.38-2.28 (m, 6H), 2.19-2.13 (m, 4H), 2.06-2.03 (m, 2H). 13C NMR (75 MHz, CDCl3): 191.13, 153.33, 143.25, 132.46, 108.40, 72.36, 68.05, 28.46, 28.17, 27.87, 21.87, 20.96. MS (FAB) 1235 ([M + H]+, 100), 874 (10). Anal. Calcd for C34H21F19O4: C, 33.06; H, 1.71. Found: C, 33.25; H, 1.9. 3,4,5-Tris(1H,1H,2H,2H,3H,3H-perfluorononan-1-yloxy)benzyl Alcohol. To an ice-cooled solution of 3,4,5-tris(1H,1H,2H,2H,3H,3H-perfluorononan-1-yloxy)benzaldehyde (1.7 g, 1.37 mmol) in anhydrous THF (20 mL) was added NaBH4 (0.11 g, 2.88 mmol), and the mixture was stirred at 0 °C-RT for 4 h. The reaction was quenched by addition of water (20 mL). The organic layer was separated and the aqueous layer was extracted with EtOAc (3 × 50 mL), and the combined organics were dried over anhydrous Na2SO4. After filtration, the solvent was removed under reduced pressure to give a white solid, which was used without further purification (1.19 g, 70%). 1H NMR (500 MHz, CDCl3) δ: 6.61 (s, 2H), 4.42 (s, 2H), 4.07 (t, J ) 5.9 Hz, 4H), 3.99 (t, J ) 5.86 Hz, 2H), 2.37-2.26 (m, 6H), 2.15-2.09 (m, 4H), 2.031.98 (m, 2H). 13C NMR (125 MHz, CDCl3): 151.21, 135.62, 135.44, 104.07, 70.40, 66.11, 63.74, 26.69, 26.40, 26.11, 19.94, 19.23. MS (FAB) 1236 (M+, 100), 859 (20), 496 (10), 108 (100). 1-[(6′-Bromohexyloxy)methyl]-3,4,5-tris(1H,1H,2H,2H,3H,3H-perfluorono nan-1-yloxy)-benzene (8). 50% aqueous NaOH solution (0.7 g), tetrabutylammonium bromide (12 mg), and hexane (1.6 mL) were placed in a round-bottom flask. 3,4,5Tris(1H,1H,2H,2H,3H,3H-perfluorononan-1-yloxy)benzyl alcohol (0.5 g, 0.4 mmol) and 1,6-dibromohexane (0.15 mL, 0.97 mmol) were added with efficient stirring. After 10 min at room temperature, the mixture was refluxed for 2 h, cooled to RT, and 10 mL of water was added. The organic layer, which was separated, was dried over anhydrous Na2SO4 and evaporated. Purification of the residue by flash chromatography with ethyl acetate/nhexane (20/80) as eluent gave the product 8 as a white solid (0.42 g, 75%). 1H NMR (300 MHz, CDCl3) δ: 6.56 (s, 2H), 4.40 (s, 2H), 4.07 (t, J ) 5.83, 4H), 3.98 (t, J ) 5.84, 2H), 3.47 (t, J ) 6.50, 2H), 3.41 (t, J ) 6.76, 2H), 2.34-2.23 (m, 6H), 2.172.07 (m, 6H), 2.05-1.96 (m, 2H), 1.91-1.82 (m, 2H), 1.68-1.59 (m, 2H), 1.51-1.39 (m, 4H). 13C NMR (75 MHz, CDCl3): 152.53, 136.83, 134.79, 106.25, 72.91, 71.82, 70.64, 70.48, 67.52, 33.86, 32.72, 29.77, 29.57, 28.18, 27.99, 27.89, 27.59, 26.22, 25.46, 21.41, 20.70. MS (ES+) 1421 & 1423 ([M + Na]+, 50), 1398 & 1400 (M+, 42). Anal. Calcd for C40H34F39O4Br: C, 34.31; H, 2.45; Br, 5.71. Found: C, 34.35; H, 2.7; Br, 5.45. 4-Benzyloxy-3-methoxybenzaldehyde.46 To a stirred solution of vanillin 3 (4 g, 26.31 mmol) in ethanol (20 mL) were added K2CO3 (5 g, 36.18 mmol) and benzyl chloride (3.4 mL, 29.54 mmol), and the mixture was maintained under a nitrogen atmosphere overnight. The mixture was filtered through Celite, the residue (45) Hajek, M.; Korora, M.; Ameduri, B.; Boutevin, B. J. Fluorine Chem. 1994, 68, 49-56. (46) Quideau, S., Pouyse´gu, L.; Avellan, A. V J. Org. Chem. 2002, 67, 3425-3436.

Mild Photoactivated Hydrophilic/Hydrophobic Switch was washed with DCM (3 × 25 mL), and the combined organic solvents were removed in vacuo. The oily residue was dissolved in 50 mL of DCM, which was washed with 5% NaOH solution, dried over K2CO3, concentrated, and crystallized from EtOH (4.90 g, 77%, mp 61-62 °C). 1H NMR (300 MHz, CDCl3) δ: 9.82 (s, 1H), 7.46-7.30 (m, 7H), 6.97 (d, 1H), 5.24 (s, 2H), 3.93 (s, 3H). 13C NMR (75 MHz, CDCl ): 191.37, 153.98, 150.45, 136.40, 3 130.67, 129.15, 128.64, 127.63, 127.06, 112.75, 109.69, 71.25, 56.46. MS (EI+) 242 (M+, 28), 151 (8), 91 (100), 65 (28). 4-Hydroxy-5-methoxy-2-nitrobenzaldehyde (9).47 The benzylated vanillin (1 g, 4.13 mmol) and 1,2-dichloroethane (5 mL) were cooled to -30 °C under argon. Fuming nitric acid (2 mL) was added slowly, and the temperature was maintained at ca. -15 °C for 3 h. The reaction mixture was poured into water and extracted with EtOAc. Removal of the solvent gave a bright yellow solid that was stirred at 60 °C in trifluoroacetic acid (10 mL) for 1 h. This was diluted with water (10 mL), neutralized with NaHCO3, and extracted with EtOAc. The solvent was removed, and the crude product was recrystallized from 95% EtOH to give the product 9 as a dark yellow solid (0.61 g, 75%, mp 205-207 °C). 1H NMR (300 MHz, CDCl3) δ: 10.40 (s, 1H), 7.67 (s, 1H), 7.45 (s, 1H), 6.25 (s, 1H), 4.06 (s, 3H). MS (FAB) 197 (M+, 85), 179 (20), 167 (8), 82 (35). Anal. Calcd for C8H7NO5: C, 48.74; H, 3.58; N, 7.10. Found: C, 48.50; H, 3.45; N, 7.10. [6′-{(3′′,4′′,5′′-Tris-1H,1H,2H,2H,3H,3H-perfluorononan1′′-yloxy)-benzyloxy}-hexyloxy]-5-methoxy-2-nitrobenzaldehyde. To a mixture of K2CO3 (0.2 g, 1.44 mmol) and anhydrous acetonitrile (20 mL) under N2 was added 4-hydroxy-5-methoxy2-nitrobenzaldehyde, 9 (80 mg, 0.4 mmol). 1-[(6′-Bromohexyloxy)methyl]-3,4,5-tris(1H,1H,2H,2H,3H,3H-perfluorononan-1-yloxy)benzene 8 (0.3 g, 0.23 mmol) was added at 70 °C, the mixture was refluxed overnight and filtered, and the filtrate was concentrated in vacuo. Purification of the residue by flash chromatography with ethyl acetate/nhexane (30/70) as eluent gave the product as a yellow sticky solid (0.35 g, 65%). 1H NMR (500 MHz, CDCl3) δ: 10.44 (s, 1H), 7.58 (s, 1H), 7.41 (s, 1H), 6.56 (s, 2H), 4.40 (s, 2H), 4.14 (t, J ) 6.68, 2H), 4.07 (t, J ) 5.83, 4H), 4.00 (s, 3H), 3.98 (t, J ) 5.85, 2H), 3.48 (t, J ) 6.48), 2.35-2.26 (m, 6H), 2.13-2.08 (m, 4H), 2.04-1.99 (m, 2H), 1.93-1.90 (m, 2H), 1.66 (m, 2H), 1.54-1.47 (m, 4H). 13C NMR (75 MHz, CDCl3): 188.16, 153.81, 152.88, 152.36, 144.26, 137.16, 135.15, 135.10, 125.69, 110.23, 108.29, 106.67, 106.58, 73.30, 72.17, 70.99, 70.90, 70.19, 67.87, 57.07, 34.23, 33.07, 30.12, 30.04, 29.91, 29.13, 28.52, 28.30, 28.22, 27.93, 26.57, 26.39, 26.15, 25.81, 21.76, 21.05. MS (ES+) 1538 ([M + Na]+, 80), 1219 (100), 402 (65), 381 (60). Anal. Calcd for C48H40F39O9N: C, 38.01; H, 2.66; N, 0.92. Found: C, 37.95; H, 2.60; N, 0.85. [6′-{(3′′,4′′,5′′-Tris-1H,1H,2H,2H,3H,3H-perfluorononan1′′-yloxy)-benzyloxy}-hexyloxy]-1-hydroxymethyl-5-methoxy-2-nitrobenzene. To an ice-cooled solution of the aldehyde (0.42 g, 0.27 mmol) in anhydrous THF (10 mL) was added NaBH4 (25 mg, 0.65 mmol), and the mixture was stirred at 0 °C-RT for 4 h. Reaction was quenched by addition of water (10 mL). The organic layer was separated, the aqueous layer was extracted with EtOAc (3 × 25 mL), and the combined organics were dried over anhydrous Na2SO4. After filtration, the solvent was removed under reduced pressure to give a white solid, which was purified by flash chromatography with ethyl acetate/nhexane (40/60) as eluent to give the alcohol as a pale yellow sticky solid (0.41 g, 72%). 1H NMR (500 MHz, CDCl3) δ: 7.69 (s, 1H), 7.15 (s, 1H), 6.60 (s, 2H), 4.95 (s, 2H), 4.62 (s, 2H), 4.07 (m, 6H), 4.06 (t, 2H), 3.99 (s, 3H), 3.48 (t, J ) 6.73, 2H), 2.35-2.26 (m, 6H), 2.13-2.08 (m, 4H), 2.04-1.99 (m, 2H), 1.93-1.90 (m, 2H), 1.68-1.64 (m, 2H), 1.54-1.47 (m, 4H). 13C NMR (75 MHz, CDCl3): 154.60, 152.87, 147.83, 135.14, 132.51, 111.56, 109.60, 106.60, 73.27, 72.17, 70.94, 69.75, 67.85, 63.28, 56.81, 30.04, 29.25, 28.23, 27.94, 26.40, 26.18, 21.05. MS (ES+) 1540 ([M + Na]+, 100), 1219 (60), 319 (30). Anal. Calcd for C48H42F39O9N: C, 37.96; H, 2.79; N, 0.92. Found: C, 37.75; H, 2.75; N, 0.90. Reagent 2. To a stirred solution of the alcohol (0.17 g, 0.11 mmol) in anhydrous CH3CN (8 mL), under argon atmosphere, was added N,N-disuccinimidyl carbonate (40 mg, 0.15 mmol) followed by Et3N (0.05 mL, 0.35 mmol). The solution was stirred (47) Murphy, B. P.; Rogers, C. B.; Blum, C. A. Heterocycl. Chem. 1987, 24, 941-943.

Langmuir, Vol. 21, No. 10, 2005 4557 for an additional 5 h, and the solvent was removed in vacuo. The residue was purified by chromatography on silica gel eluting with DCM/EtOAc (96/4), to give the reagent 2 as a pale yellow oily solid (0.18 g, 68%). 1H NMR (300 MHz, CDCl3) δ: 7.74 (s, 1H), 7.03 (s, 1H), 6.56 (s, 2H), 5.78 (s, 2H), 4.40 (s, 2H), 4.07 (m, 6H), 4.03 (s, 3H), 3.97 (t, J ) 5.50 Hz, 2H), 3.48 (t, J ) 6.45, 2H), 2.85 (s, 4H), 2.35-2.26 (m, 3H), 2.13-2.08 (m, 4H), 2.04-1.99 (m, 2H), 1.91 (m, 2H), 1.66 (m, 2H), 1.54-1.47 (m, 4H). 13C NMR (75 MHz, CDCl3): 168.87, 154.86, 152.87, 151.83, 148.40, 139.44, 137.16, 135.13, 125.55, 109.62, 109.20, 106.63, 73.28, 72.16, 70.95, 69.81, 69.62, 67.85, 57.01, 30.03, 29.21, 28.53, 28.24, 27.94, 26.38, 26.19, 25.85, 25.57, 21.76, 21.05. MS (ES+) 1681 ([M + Na]+, 65), 1484 (35), 1219 (60), 419 (100), 282 (100), 235 (55). Anal. Calcd for C53H45F39O13N2: C, 38.35; H, 2.73; N, 1.69. Found: C, 38.45; H, 2.85; N, 1.7. Substrate Preparation. The silicon substrates were cleaned by ultrasonication in dichloromethane for 15 min, dried under a stream of nitrogen, and rinsed under Milli-Q water before being immersed in piranha solution (70:30, v/v, H2SO4:H2O2; Caution: Piranha solution reacts violently with organic materials and should be treated with great care) for 10 min. The substrates were rinsed in Millipore water, dried under nitrogen, and placed in an Edwards Auto 306 thermal evaporator. A 100 nm gold layer was thermally deposited (0.1 nm s-1) onto a chromium adhesion layer (5 nm), at a base pressure of 1 × 10-6 mbar. The gold-coated samples were cleaned immediately prior to use by placing them in freshly prepared piranha solution, for 1-2 min, followed by a rinse with Milli-Q water. NH2-PEG SAM Formation. Freshly cleaned and dried gold coated samples were placed in a 1 mM solution of the NH2-PEGSH (12), in ethanol, for 12 h. On removal from solution, the substrates were rinsed sequentially with ethanol and Milli-Q water. NH2-PEG SAM Derivatization. The NH2-PEG SAMs were rinsed with DCM and dried with nitrogen before being placed in a 1% anhydrous DMSO solution of 1 (14.6 mM) or 2 (6.03 mM), both with 1% (71.6 mM) triethylamine catalyst (stored under an Ar atmosphere). The triethylamine was added immediately prior to submerging the NH2-PEG SAM-functionalized substrates. The solutions were maintained at room temperature, 23 °C, and stored in the dark. After 12 h (unless stated otherwise), the samples were removed, ultrasonicated in DCM, dried under nitrogen, and rinsed with Milli-Q water prior to characterization. UV Irradiation of SAMs. A 365 nm UV lamp (Blak-Ray model B 100 AP) with a nominal power, at the sample, of 7 mW cm-2 was used to irradiate the samples, in air, for 5000 s, unless stated otherwise. After the UV exposure, samples were rinsed with DCM, followed by Milli-Q water, and finally dried under a stream of nitrogen. The emission spectra of the lamp displayed three lines, 309, 331, and 365 nm, with integrated intensities of 0.005:0.03:1, respectively. Photopatterning was achieved by irradiation through a chromium/quartz mask for (which was in contact with the surface) 4000 s. Patterned samples were rinsed with DCM, followed by Milli-Q water, and finally dried under a stream of nitrogen. Wetting Measurements. Contact angles were measured using a home-built goniometer in ambient conditions. Milli-Q water droplets were advanced and receded across the surface from a microsyringe with a 0.5 mm square cut needle. The droplets were imaged with a Navitar zoom lens coupled via a ×2 extension tube to a Hamamatsu C3077 CCD camera. Images of at least three advancing and receding droplets were analyzed on both sides of each droplet to give a minimum of six values per surface. X-ray Photoelectron Spectroscopy. Spectra were obtained using a Thermo VG ESCA Lab 250 with a chamber pressure maintained below 5 × 10-9 mbar during acquisition. A monochromated Al KR X-ray source (15 kV 150 W) irradiated the samples, with a spot diameter of approximately 0.5 mm. The spectrometer was operated in Large Area XL magnetic lens mode using pass energies of 150 and 20 eV for survey and detailed scans, respectively. Spectra were obtained with an electron takeoff angle of 90°. High-resolution spectra were fitted using Avantage (Thermo VG software package) peak fitting algorithms. Surface Potential Measurements. Surface potential measurements were made using a home-built Kelvin probe apparatus. An x, y, z stage was used to move samples under a Platinum

4558

Langmuir, Vol. 21, No. 10, 2005 Scheme 1

a

Critchley et al. Scheme 2

a

a (i) K2CO3/DMF/CF3(CF2)5(CH2)3I, 70%; (ii) NaBH4/THF, 70%; (iii) aqueous NaOH/hexane/TBAB/Br(CH2)6Br, 75%; (iv) K2CO3/EtOH/PhCH2Cl, 77%; (v) (a) fuming HNO3, (b) TFA, 75%; (vi) K2CO3/CH3CN/7/8 65%; (vii) NaBH4/THF, 72%; (viii) N,N-disuccinimidyl carbonate/Et3N/CH3CN, 68%.

Scheme 3. Formulas

a (i) KOH/EtOH/CF3(CF2)5CH2CH2I, 30%; (ii) HNO3, 80%; (iii) NaBH4/THF, 70%; (iv) N,N-disuccinimidyl carbonate/Et3N, 62%.

probe (area 2.7 mm2). The probe was vibrated at a frequency of 800 Hz using a piezo actuator (P200-16 Linos Photonics), to create an alternating current. The contact potential difference (CPD) between the probe and the sample was determined using the null technique.48,49 The surface potential was calculated as the change in CPD between a gold substrate with a film and that of a freshly cleaned gold substrate. Ellipsometry. A Jobin-Yvon UVISEL spectroscopic ellipsometer was used to measure the thickness of the SAMs. The wavelength was varied between 300 and 800 nm in steps of 15 nm. DeltaPsi2 software was used to model and fit the acquired data assuming a simple three-layer system. Values for the base layer (gold support) were obtained from a freshly cleaned gold substrate. The SAM was modeled as a transparent thin film using the Cauchy approximation, n(λ) ) A + (B × 104/λ2) + (C × 109/λ4) and k(λ) ) 0, where λ is the wavelength in units of nanometers and A, B, and C are the Cauchy parameters dependent on optical properties of the material. Parameter A was restricted to a value of 1.46, and parameters B and C were allowed to vary between 0-1 nm2 and 0-0.25 nm4, respectively.50 The ambient, air, was assumed to have n ) 1 and k ) 0. At least six ellipsometric measurements were made per sample (and on more than one sample of each type). Scanning Electron Microscope (SEM) Images. SEM images were obtained using a Thermo VG ESCA Lab 250 equipped with a FEG1000 field emission gun. The analysis chamber pressure was lower than 4 × 10-9 mbar. A primary beam energy of 7 keV was used for the imaging of the patterned surfaces.

3. Results and Discussion Synthesis. The syntheses of reagents 1 and 2 are outlined in Schemes 1 and 2. The introduction of the semifluorinated chains proved surprisingly difficult. The problem with semi-fluorinated alkyl halides of the 1-iodo1H,1H,2H,2H-perfluoroalkane type is that the fluorines on the 3-carbon promote E2 elimination, which competes (48) Rossi, F. Rev. Sci. Instrum. 1992, 63, 3744-3751. (49) Prutton, M. Introduction to surface physics; Clarendon Press/ Oxford University Press: Oxford, 1994. (50) Sharma, S.; Johnson, R. W.; Desai, T. A. Appl. Surf. Sci. 2003, 206, 218-229.

with the desired substitution at C-1.51 For the alkylation of the phenol 3, a wide range of bases and solvents was explored, but it was not possible to improve on the rather modest yield obtained using KOH/ethanol. For the equivalent trisalkylation of 3,4,5-trihydroxybenzaldehyde, even these conditions failed and it proved necessary to use 1-iodo-1H,1H,2H,2H,3H,3H-perfluorononane. With the fluorines further removed from the reactive site, competing elimination is no longer a problem,51 but because this alkyl halide is not commercially available, it had to be synthesized.45 The other issue in these syntheses is the regiospecificity of the nitration steps. It has been previously established that nitration of the 3,4-dialkoxybenzaldehyde derivative is clean and specific for the 5-position,52 so nitration of compound 4 was straightforward. However, nitration of 3-alkoxy-4-hydroxy-benzaldehyde derivatives gives mainly the 4-nitro product.53 Hence, in the conversion of compound 3 to compound 9 (Scheme 2), it was necessary to use benzyl protection of the 4-hydroxyl group. The final products (1 and 2) were stable on storage, both as solids and in solution, although, as a precaution, they were protected from ambient light. However, once triethylamine was added to a solution of the reagent 1, it deteriorated. Aged solutions showed one major product, the carbonate 10 (Scheme 3), which presumably arises through the action of atmospheric moisture. Compounds 1 shows λmax of hexane at 279(s) and 324 nm and compound 2 shows λmax at 27(s) and 327 nm, and, as expected, when a solution is photolyzed this chromophore cleanly and rapidly disappears. Characterization of the NH2-PEG SAMs. The aminefunctionalized poly(ethylene glycol) thiol (NH2-PEG-SH) (51) Brace, N. O.; Marshall, L. W.; Pinson, C. J.; Wingergen, G. v. J. Org. Chem. 1984, 49, 2361-2368. (52) Kumar, S.; Wachtel, E. J.; Kienen, E. J. J. Org. Chem. 1993, 58, 3821-3827. (53) Grenier, J. L.; Cotelle, N.; N., C.; Cotelle, P. J. Phys. Org. Chem. 2000, 13, 511-517.

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Table 1. Surface Properties of the NH2-PEG-SH SAM before Functionalization, after 12 h of Reaction with Reagents 1 and 2, and after 5000 s UV Irradiation at 365 nm in Air water contact angles/deg ((1°) before UV irradiation

ellipsometric thickness/Å

after UV irradiation

surface potential/mV

SAM on Au

θA

θR

θA

θR

before UV irradiation

after UV irradiation

before UV irradiation

after UV irradiation

NH2-PEG SAM functionalized with reagent 1 functionalized with reagent 2

46 112

21 74

49a 64

26a 30

25 ( 2 30 ( 2

25 ( 2a 26 ( 1

573 ( 11 -144 ( 17

165 ( 22

110

71

62

31

49 ( 2

29 ( 2

-609 ( 14

35 ( 15

a

The NH2-PEG surface was irradiated as a control experiment.

derivative 12 (Scheme 3) was used to form SAMs on gold, by adsorption from 1 mM ethanol solution, for 12 h. The monolayer characteristics are summarized in Table 1. The ellipsometerically determined thickness of 25 ( 2 Å is lower than the molecular length of 32 Å, assuming that the molecules adopt helical conformation.54-57 This thickness would therefore be consistent with a mean molecular tilt angle of 38°, or equivalently 78% of maximum possible molecular packing density. The advancing and receding water contact angles, 46° and 20°, are similar to those reported elsewhere for amine derivativatized surfaces.35,58 The large contact angle hysteresis of the NH2-PEG SAM is most likely due to water penetration into the monolayer. The surface potential of the NH2-PEG SAM was measured to be 573 ( 11 mV. This positive value indicates a decrease in work function, which is consistent with amine groups being present at air/monolayer interface.58,59 The Helmholtz equation was used to calculate a net dipole of 0.6 D per molecule, orientated with the positive end of the dipole at the air/monolayer interface. The X-ray photoelectron spectrum (XPS) of the C 1s region for a NH2-PEG SAM is shown in Figure 3a. Four peaks (labeled 1, 2, 3, and 4)

Figure 3. X-ray photoelectron spectra of the C 1s region: (a, d) for a monolayer of NH2-PEG-SH, (b) following reaction, for 12 h, with reagent 1, (c) after irradiation at 365 nm, for 5000 s, (e) following reaction, for 12 h, with reagent 2, and (f) after irradiation at 365 nm, for 5000 s. The assignment and binding energy of each peak are summarized in Table 2.

used to fit the C 1s data (Table 2) are associated with CH2-CH2, CH2-O, HNCdOCH2 units, and a shake-up satellite, respectively.60 The fractional area under peaks (54) Palegrosdemange, C. S.; Simon, E. S.; Prime, K.; Whitesides, G. M. J. Am. Chem. Soc. 1991, 113, 12-20. (55) Harder, P.; Grunze, M.; Dahint, R.; Whitesides, G. M.; Laibinis, P. E. J. Phys. Chem. B 1998, 102, 426-436. (56) Wang, R. L. C.; Kreuzer, H. J.; Grunze, M. J. Phys. Chem. B 1997, 101, 9767. (57) Pertsin, A. J.; Grunze, M.; Kreuzer, H. J.; Wang, R. L. C. J. Phys. Chem. Chem. Phys. 2000, 1729-1733. (58) Cant, N. E.; Critchley, K.; Zhang, H.-L.; Evans, S. D. Thin Solid Films 2003, 426, 31-39. (59) Campbell, I. H.; Rubin, S.; Zawodzinski, J. D.; Martin, R. L.; Smith, D. L. Phys. Rev. B 1996, 54, 14321-14324.

Table 2. Binding Energy Shifts Observed in the C 1s Spectra

a

band

chemical species

binding energy (eV)

1 2 3 4a 5 6

CH2-CH2 CH2-O HNCOCH2 HNCOO CF2 CF3

285.0 286.8 288.0 289.7 292.2 294.6

A shake-up satellite is also expected to occur close to this band.

1, 2, and 3 was measured to be 19%, 75%, and 6%, respectively, and these are comparable to the values expected based on chemical composition (14%, 83%, and 3%). The atomic percentage of each element present within the SAM is presented in Table 3, showing reasonable agreement between the experimentally determined values and those expected based on molecular structure. Protection of the Amino-Functionalized Surface. Reaction of a solution of the reagents 1 or 2 with the aminefunctionalized SAM surface in the presence of a triethylamine catalyst was followed by contact angle measurement (Figure 4). At room temperature, the reaction was rapid and complete within a little over an hour. As shown in Figure 4, both reagents yield similar limiting contact angle values, and this does not change on prolonged exposure to excess reagent. The limiting values of 112° and 74° for advancing and receding angles (Table 1) are somewhat lower than the 118° and 105-111° typically found for close-packed semi-fluorinated SAMs,1,61 suggesting that, after functionalization, the surfaces are comprised of disordered semi-fluorinated chains. The average film thickness, after functionalization with reagent 1, was observed to increase by 5 Å (Table 1).62 Reagent 1 was estimated to have a molecular length of 16 Å (excluding the N-succinimido group). Therefore, the observed layer thickness of layer 1 on the amine would be consistent with a packing density of 31% of a tightly packed layer of reagent 1 (excluding the N-succinimido group).63 SAMs functionalized by reagent 2 displayed an increase in thickness of 24 Å.64 Reagent 2 has a molecular length of ∼32 Å, which suggests that the layer was 75% (60) Beamson, G.; Briggs, D. High-resolution XPS of organic polymers: the Scienta ESCA300 database; Wiley: Chichester, New York, 1992. (61) Cheadle, E. M.; Batchelder, D. N.; Evans, S. D.; Zhang, H. L.; Fukushima, H.; Miyashita, S.; Graupe, M.; Puck, A.; Shmakova, O. E.; Colorado, R.; Lee, T. R. Langmuir 2001, 17, 6616-6621. (62) If the reagent 1 layer was modeled using a Cauchy model with A ) 1.38 (used for fluorocarbon materials), the thickness of the reagent 1 layer is found to be 6 Å. However, because the refractive index is likely to be larger than that of purely fluorocarbon materials and it made only a small change in thickness, the parameter A was maintained at 1.46 for the entire film. (63) The amine-functionalized SAM had a packing density of 78%; one can estimate the reaction yield by the increase in thickness. Thus, the reaction yield between reagent 1 and the amine groups was estimated to be 40%.

4560

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Critchley et al.

Table 3. Atomic Percentages of Each Element Determined by XPS for the Monolayer before Reaction with Reagent 1 or 2, after Functionalization, and after UV Irradiation XPS determined atomic percentage of each element (expected percentage in brackets) SAM on Au

C 1s

O 1s

N 1s

S 2p

F 1s

NH2-PEG SAM functionalized by reagent 1 functionalized by reagent 2 functionalized by reagent 1 and after UV irradiation functionalized by reagent 2 and after UV irradiation

67.0 (63.0) 48.8

27.5 (27.3) 20.4

3.9 (6.1) 3.6

1.6 (3.0) 0.4

26.8

49.7

13.5

2.0

0.8

34.0

59.0

25.8

4.2

1.4

9.6

57.9

23.3

4.7

2.9

11.3

of the maximum possible thickness. However, one must consider that reagent 2 has three tail-groups per attachment, which means that the reaction yield is substantially smaller than this value. The surface potential of the SAMs functionalized by 1 and 2 yielded -144 and -609 mV, respectively (relative to the clean gold substrate). The negative values were expected due to the strong electronwithdrawing properties of the semi-fluorinated chains. Furthermore, one would also expect reagent 2 to have a larger surface potential because there are three electronwithdrawing groups per molecule. However, SAMs of the semi-fluorinated alkanethiols CF3(CF2)7(CH2)2SH are reported to have a surface potential of approximately -800 mV, that is, significantly larger than values found here.59 This is further indication of a lower reaction yield between both reagents (1 and 2) and the amine sites. Figure 3b shows the C 1s spectra obtained from XPS following modification with reagent 1. The appearance of the new bands at 292.2 and 294.6 eV is associated with the presence of the CF2 and CF3 groups, respectively (Table 2). The reaction yields were determined by comparing the expected to the experimentally determined values for the ratio of band 5 to the sum of bands 1-4.64,65 The yield of reagent 1 per amine group was estimated to be 35%. Similarly, Figure 3e shows the results for reagent 2. In this case, the reaction yield was estimated to be 12%. In both cases, because excess reagent and long reaction times were used, and the time for saturation in the contact angle measurements long exceeded, it is clear that no reactive amine residues remain exposed (Figure 4). The fact that not all of the amine residues have reacted hence relates to the greater steric requirement/cross-sectional area of the

Figure 4. Contact angle variation during the formation of photoactive surfaces. The symbols 9 and 0 represent the advancing and receding water contact angles of surfaces modified by reagent 1. The symbols b and O represent the advancing and receding water contact angles of surfaces modified by reagent 2.

substituted nitro-aryl as compared to the PEG residues and the degree of disorder within the SAM. In this respect, the lower “yield” when three semi-fluorinated end-chains are present is to be expected. UV Deprotection. Soft UV light (365 nm) was used to photocleave the protecting groups and to regenerate the NH2-PEG SAM (Figure 1). The water contact angles, measured as a function of irradiation time (Figure 5), show that the surface energies of surfaces functionalized by reagents 1 and 2 increased rapidly and reached a limiting value that remains unchanged on prolonged irradiation. The limiting values attained (Table 1) were higher than that of the NH2-PEG SAM, suggesting that the photoreaction is not completed. Although contamination could occur during the course of the photoreaction, when a NH2PEG-SH SAM was irradiated under the same condition, as a control, it displayed only a slight increase in water contact angle (Table 1). Assuming there was no significant contribution from contamination, we estimate from the contact angles measurements that ∼20% of the hydrophobic residues remained attached to the NH2-PEG SAM following irradiation. Table 1 summarizes the ellipsometerically determined thickness change after 5000 s exposure. These data also show that the photoreactions were “incomplete” and also suggest that approximately 20% of the thickness associated with the protecting groups remains attached to the surface. Figure 3c and f shows the C 1s spectra of the UV exposed SAMs. If we consider Figure 3a-c, relating to the use of reagent 1, we see further evidence (bands 5 and 6) for the incomplete removal of

Figure 5. Contact angle variation during photodeprotection using soft UV light (365 nm, 7 mW cm-2). The advancing and receding water contact angles, represented by 9 and 0, respectively, monitor the deprotection of SAMs modified with reagent 1. The advancing and receding water contact angles, represented by b and O, respectively, monitor the deprotection of SAMs modified with reagent 2. The inset shows the variation over the first 6000 s.

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4. Conclusions

Figure 6. Photopattern formation with soft UV light. (a) A SAM protected with reagent 1 was exposed to UV (365 nm, 7 mW cm-2) through a photomask for 4000 s (not to scale). (b) The photoactive groups were removed in the regions exposed to UV light (not to scale). (c) A scanning electron microscope (SEM) image of the chemically patterned surface. Bright regions correspond to areas where photodeprotection has occurred, yielding hydrophilic, amine-functionalized surfaces. (d) An SEM image of a dodecanethiol (DDT) SAM irradiated under conditions identical to those used to obtain the pattern shown in (c).

the fluorocarbon. By modeling these spectra, the relative decrease of the reagents 1 and 2 has been estimated to be 80% (i.e., the residues per NH2-PEG SAM decreased from 35% to 7%). Figure 3d-f presents a similar picture for the use of reagent 2, suggesting a relative decrease of 83% (i.e., per NH2-PEG SAM reduced from 12% to 2%). Patterns can readily be formed by irradiation through a mask. Figure 6a and b shows how an NH2-PEG SAM, protected with reagent 1, can be photopatterned by irradiation through a photomask. Such patterned SAMs can be readily imaged by SEM.66,67 Figure 6c demonstrates that surfaces modified by reagent 1 can be photopatterned using soft UV light. The deprotected (UV exposed) regions are both thinner and have a lower work-function, both which serve to increase electron yield, and hence these regions appear brighter. A dodecanethiol (DDT) SAM was irradiated through a photomask under identical conditions and showed no evidence of soft-UV photopatterning. This demonstrates that the “photoactive” functionalized surfaces were not patterned as a consequence of nonspecific photodegradation. (64) If the reagent 2 layer is modeled using a Cauchy model with A ) 1.38 (used for fluorocarbon materials), the thickness of the reagent 2 layer is found to be 26 Å. However, because the refractive index is likely to be larger than that of purely fluorocarbon materials and it made only a small change in thickness, the parameter A was maintained at 1.46 for the entire film. (65) The inelastic mean free path was assumed to be 25 Å. Practical Surface Analysis, 2nd ed.; Briggs, D., Seah, M. P., Eds.; John Wiley and Sons: Chichester, 1996; Vol. 1. (66) Saito, N.; Wu, Y.; Hayashi, K.; Sugimura, H.; Takai, O. J. Phys. Chem. B 2003, 107, 664-667. (67) Lopez G. P.; Biebuyck H. A.; Whitesides, G. M. Langmuir 1993, 9, 1513-1516.

Gold surfaces have been functionalized with an NH2 terminated PEG SAM and used as a base to assemble semi-fluorinated functionalized photoreactive SAMs. The attachment of the protecting groups using a tertiary amine-catalyzed reaction of an N-hydroxysucinnyl ester was rapid. Although “cartoons” drawn of such chemistry usually show reaction at every available site, this is clearly not the case. Not withstanding this, both protected surfaces were reproducibly hydrophobic, and the remaining unprotected amino residues were shielded from further chemical reaction (Figure 4). The reason for the low chemical yield is most likely steric factors because it is lower for the more bulky protecting group (12% as compared to 35%, per amino residue). The surface potential for the protected SAMs was negative, indicating an increased work function and a net dipole directed out of the plane of the surface (with respect to the normal). The increase in work function was much larger for reagent 2, suggesting a very large net dipole moment that could be used for tuning work functions over a large range. UV irradiation (λ ) 365 nm) led to the removal of a significant fraction (ca. 80%) of the hydrophobic protecting groups. However, approximately 20% consistently remained on the surface irrespective of the irradiation time. The reason for this is not yet understood, but there are two obvious explanations: (1) a competing photoreduction of the nitrogroup68 or (2) imine formation between the aldehyde product and the amine SAM.69,70 Notwithstanding this, our data show that photocontrol over “wettability” can be attained using these systems and that these should produce “rewritable” surfaces. The method can be used to create photopatterned surfaces using conventional lithographic processes. Most significantly surfaces with a welldefined chemical functionality are produced in the areas that have been irradiated enabling further chemical modification, and thus the approach has application for the directed assembly of nanoparticles and biological molecules. The potential advantages of this approach to the patterning of surfaces, as compared to micro-contact printing methods, are the ability to exploit the “gray scale”, to produce gradients on the hydrophilic/hydrophobic character or well-defined chemical functionality, and to produce SAMs that are free from phase separation. This route allows one to produce patterned amine surfaces that will not be limited to thiol on gold systems. Acknowledgment. J.P.J. and K.C. thank the Seiko Epson Corp. for funding of this study. K.C. is also grateful to the EPSRC for financial support. LA046851S (68) Kim, E.; Lee, I. Langmuir 2004, 20, 7351-7354. (69) Cameron, J. F.; Frechet, J. M. J. Am. Chem. Soc. 1991, 113, 4303-4313. (70) Patchornik, A.; Amit, B.; Woodward, R. B. J. Am. Chem. Soc. 1970, 92, 6333-6335.