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Langmuir 2008, 24, 12420-12425
Micrometer and Nanometer Scale Patterning Using the Photo-Fries Rearrangement: Toward Selective Execution of Molecular Transformations with Nanoscale Spatial Resolution Thomas Griesser,†,§ Joseph Adams,‡ Julia Wappel,† Wolfgang Kern,†,§ Graham J. Leggett,‡ and Gregor Trimmel*,† Institute for Chemistry and Technology of Materials (ICTM), Graz UniVersity of Technology, Stremayrgasse 16, 8010 Graz, Austria, Department of Chemistry, UniVersity of Sheffield, Brook Hill, Sheffield, S3 7HF, U.K., and Institute of Chemistry of Polymeric Materials, MontanuniVersita¨t Leoben, Franz Josef Strasse 18, 8700 Leoben, Austria ReceiVed July 24, 2008. ReVised Manuscript ReceiVed August 30, 2008 The photolithographic modification of monolayers provides a versatile and powerful means of fabricating functionalized nanostructured surfaces. In this contribution, we present photosensitive thiol-bearing aryl ester groups which are capable of undergoing the so-called photo-Fries rearrangement to yield hydroxyketones. Phenyl 16-mercaptohexadecanoate was prepared by a three-step synthesis. This molecule undergoes a photoisomerization reaction upon illumination with UV light at ca. 250 nm. Subsequently this molecule was applied as a self-assembled monolayer on gold. Following photochemical modification, the adsorbates were selectively derivatized to yield amino-functionalized surfaces using a simple two-step reaction. This reaction was monitored by X-ray photoelectron spectroscopy and contact angle measurements and friction force microscopy. Micrometer-scale patterned surfaces were produced using a contact mask in conjunction with a frequency-doubled argon ion laser (λ ) 244 nm). Near-field optical exposure was carried out by coupling the laser to a scanning near-field optical microscope and yielded nanometer-scale resolution. Following derivatization, the resulting structures were analyzed by friction force microscopy. Clear contrast was observed in the friction signal following surface modification.
Introduction The functionalization of inorganic surfaces by self-assembled monolayers (SAMs) is a widely applied and important technique for the fabrication of nanostructured and hierarchically organized materials.1 In this context, thiols readily chemisorb on metal surfaces such as gold, silver, or copper to form highly ordered and smooth monolayers. The driving forces are the reaction of the thiol functionality with the metal surface leading to covalent metal-sulfur bonds, combined with the formation of noncovalent interactions between long alkyl or aryl backbones. For many applications, two-dimensional patterning of surface properties and site-selective immobilization are required. Different approaches have been developed to produce patterned SAM surfaces. At the micrometer scale the most prominent is microcontact printing using a preformed PDMS stamp,2 which transfers the thiol ink onto a suitable substrate. For nanometerscale patterning, a variety of approaches based on scanning probe microscopes have been explored.3 These include dip-pen nanolithography,4 in which an atomic force microscope (AFM) probe is “inked” in a solution of a suitable adsorbate and traced across a substrate, depositing a trail of molecules (for example, * Corresponding author. E-mail:
[email protected]. Tel.: ++433168738458. Fax: ++43316-8738951. † Graz University of Technology. ‡ University of Sheffield. § Montanuniversita¨t Leoben. (1) (a) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151–256. (b) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103–1189. (c) Li, X.-M.; Huskens, J.; Reinhoudt, D. N. J. Mater. Chem. 2004, 14, 2954–2971. (d) Ulman, A. Chem. ReV. 1996, 96, 1533. (2) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550–575. (3) Kra¨mer, S.; Fuierer, R. R.; Gorman, C. B. Chem. ReV. 2003, 103, 4367– 4418. (4) Piner, R. D.; Zhu, J.; Xu, F.; Hong, S.; Mirkin, C. A. Science 1999, 283, 661.
alkanethiols have been deposited onto gold surfaces,5 and nucleic acids,6 proteins,7-9 and other molecules have been deposited onto a variety of substrates), and nanoshaving/nanografting,10,11 in which adsorbates have removed, mechanically, from a monolayer and replaced by an alternative adsorbate. Many of the approaches reported to date have relied upon the removal or deposition of whole molecules, and there have been few approaches that have facilitated specific modification of the terminal group of the adsorbate molecule. Among the exceptions are the use of electron beam irradiation to convert terminal nitro groups to amines in monolayers of aromatic adsorbates12 and the use of a conducting AFM probe to oxidize the terminal methyl group in a siloxane.13,14 Although photolithographic techniques are the mainstay of electronic device manufacturing, they have remained underutilized for monolayer patterning. However, there is now rapidly accumulating evidence that photochemical methods provide a powerful and versatile tool for molecular patterning. For example, (5) Hong, S.; Zhu, J.; Mirkin, C. A. Science 1999, 286, 523–525. (6) Demers, L.; Ginger, D. S.; Park, S.-J.; Li, Z.; Chung, S.-W.; Mirkin, C. A. Science 2002, 296, 1836–1838. (7) Hyun, J.; Ahn, S. J.; Lee, W. K.; Chilkoti, A.; Zauscher, S. Nano Lett. 2002, 2, 1203–1207. (8) Lee, K.-B.; Park, S.-J.; Mirkin, C. A.; Smith, J. C.; Mrksich, M. Science 2002, 295, 1702–1705. (9) Lee, K.-B.; Lim, J.-H.; Mirkin, C. A. J. Am. Chem. Soc. 2003, 125, 5588– 5589. (10) Wadu-Mesthrige, K.; Amro, N. A.; Garno, J. C.; Xu, S.; Liu, G.-Y. Biophys. J. 2001, 80, 1891–1899. (11) Liu, M.; Amro, N. A.; Chow, C. S.; Liu, G.-Y. Nano Lett. 2002, 2, 863– 867. (12) Golzhauser, A.; Eck, W.; Geyer, W.; Stadler, V.; Grunze, M. AdV. Mater. 2001, 13, 806. (13) Maoz, R.; Frydman, E.; Cohen, S. R.; Sagiv, J. AdV. Mater. 2000, 12, 725–731. (14) Maoz, R.; Frydman, E.; Cohen, S. R.; Sagiv, J. AdV. Mater. 2000, 12, 424–429.
10.1021/la802382p CCC: $40.75 2008 American Chemical Society Published on Web 10/07/2008
Nanoscale Patterning by Photo-Fries Rearrangement
Langmuir, Vol. 24, No. 21, 2008 12421
Scheme 1. Mechanism of the Photo-Fries Rearrangement
photochemical structuring was achieved by applying an additional layer of a photoacid generator on top of a SAM which upon irradiation induces an acid-catalyzed reaction in the SAM layer.15 Another convenient approach, where no additional layer is required, is the photo-oxidation of thiols to the corresponding sulfonate, which can then be removed by washing with a polar solvent or which can be replaced by a second thiol with a different functionality.16 Contrary to expectations, nanometer-scale patterning of thiols may be accomplished readily by utilizing a scanning near-field optical microscope (SNOM) to carry out exposure in the optical near-field. A resolution of 9 nm has been obtained on microcrystalline gold surfaces.17 An alternative approach is the use of thiol-bearing photoreactive groups. Examples are photocleavable o-nitrobenzyl esters18 or azide groups.19 Other reports deal with the photoinduced Wolff rearrangement of diazoketones resulting in ketenes which can react in situ with amines and alcohols20 and the cis-trans isomerization of azobenzene21 and stilbene units.22 In the present contribution we combine thiol-SAMs with the photo-Fries rearrangement reaction of aryl esters. In this photoreaction aryl esters are transformed into hydroxyketones upon irradiation of UV light as first described by Anderson and Reese.23 The aryl ester is excited into its first excited singlet state. Then scission of the C-O bond leads to the formation of the acyl and the phenoxyl radical. The photogenerated radicals can recombine to the o- or p-cyclohexadienone derivatives as the “cage products”. Tautomerism then yields the corresponding o- and p-hydrox(15) Lee, K.; Pan, F.; Carroll, G. T.; Turro, N. J.; Koberstein, J. T. Langmuir 2004, 20, 1812–1818. (16) (a) Huang, J.; Hemminger, J. C. J. Am. Chem. Soc. 1993, 115, 3342– 3343. (b) Rieley, H.; Kendall, G. K.; Zemicael, F. W.; Smith, T. L.; Yang, S. Langmuir 1998, 14, 5147. (c) Hutt, D. A.; Cooper, E.; Leggett, G. J. J. Phys. Chem. B 1998, 102, 174–184. (17) (a) Montague, M.; Ducker, R. E.; Chong, K. S. L.; Manning, R. J.; Rutten, F. J. M.; Davies, M. C.; Leggett, G. J. Langmuir 2007, 23, 7328. (b) Leggett, G. J. Chem. Soc. ReV. 2006, 35, 1150–1161. (c) Sun, S.; Leggett, G. J. Nano Lett. 2004, 4, 1381–1384. (d) Sun, S.; Chong, K. S. L.; Leggett, G. J. J. Am. Chem. Soc. 2002, 124, 2414–2415. (18) (a) Ryan, D.; Parviz, B. A.; Linder, V.; Semetey, V.; Sia, S. K.; Su, J.; Mrksich, M.; Whitesides, G. M. Langmuir 2004, 20, 9080–9088. (b) Critchley, K.; Jeyadevan, J. P.; Fukushima, H.; Ishida, M.; Shimoda, T.; Bushby, R. J.; Evans, S. D. Langmuir 2005, 21, 4554–4561. (c) Critchley, K.; Zhang, L.; Fukushima, H.; Shimoda, T.; Bushby, R. J.; Evans, S. D. J. Phys. Chem. B 2006, 110, 17167–17174. (19) (a) Wollman, E. W.; Kang, D.; Frisbie, D.; Lorkovic, I. M.; Wrighton, M. S. J. Am. Chem. Soc. 1994, 116, 4395–4404. (b) Frisbie, C. D.; Wollman, E. W.; Wrighton, M. S. Langmuir 2005, 11, 2563–2571. (c) Monsathporn, S.; Effenberger, F. Langmuir 2004, 20, 10375–10378. (20) Hu, J.; Liu, Y.; Khemtong, C.; El Khoury, J. M.; McAfoos, T. J.; Taschner, I. S Langmuir 2004, 20, 4933–4938. (21) (a) Manna, A.; Chen, P.-L.; Akiyama, H.; Wie, T.-X.; Tamada, K.; Knoll, W. Chem. Mater. 2003, 15, 20–28. (b) Akiyama, H.; Tamada, K.; Nagasawa, J.; Abe, K.; Tamaki, T. J. Phys. Chem. B 2003, 107, 130–135. (c) Weber, R.; Winter, B.; Hertel, I. V.; Stiller, B.; Schrader, S.; Bremer, L.; Koch, N. J. Phys. Chem. B 2003, 107, 7768–7775. (22) Wolf, M. O.; Fox, M. A. J. Am. Chem. Soc. 1995, 117, 1845–1847. (23) (a) Anderson, J. C.; Reese, C. B. Proc. Chem. Soc., London 1960, 217. (b) Bellus, D. AdV. Photochem. 1981, 8, 109.
yketones.24 As side reactions, generation of phenol and decarboxylation can occur. The mechanism of the photoreaction is shown in Scheme 1. Besides its use in organic chemistry,25 the photo-Fries reaction has been used in polymer chemistry. For example, Guillet and co-workers investigated the photoreaction in poly(phenyl acrylate) and poly(naphthyl acrylates).26 Poly(acetoxystyrene) and poly(formyloxy)styrene have been proposed as positive resist materials for photolithography.27 Previous papers have demonstrated that this photoreaction can be used for the surface modification of poly(acetoxystyrene) and poly(phenyl(norbornenecarboxylates)).28 The photogenerated hydroxyketones offer a broad variety of different modification reactions. In combination with photolithographic techniques patterned functionalized surfaces have been obtained.29 An advantage of this photo-Fries reaction is the availability of aryl esters. In this contribution we first show the synthesis of the photoreactive thiol, phenyl 16-mercaptohexadecanoate, and its applicability for the preparation of SAMs and patterned functionalized surfaces by postmodification reactions. The photochemistry and postmodification reaction have been studied by contact angle measurements and X-ray photoelectron spectroscopy (XPS). Patterns have been obtained by photolithography using a contact mask or a SNOM and subsequently characterized by friction force microscopy (FFM).
Experimental Section All chemicals were purchased from commercial sources and were used without further purification. CH2Cl2 was distilled over CaH2 and degassed with argon. Ethanol was degassed with nitrogen. 1H NMR and 13C NMR spectra were recorded on a Varian INOVA 500 MHz spectrometer operating at 499.803 and 125.687 MHz, respectively, and were referenced to SiMe4. A relaxation delay of 1 s and 45° pulse were used for acquisition of the 1H NMR spectra. Peak shapes are indicated as follows: s (singlet), d (doublet), dd (doublet of doublet), t (triplet), m (multiplet). Solvent residual peaks were used for referencing the NMR spectra to the corresponding values given in literature.30 (24) (a) Kalmus, C. E.; Hercules, D. M. J. Am. Chem. Soc. 1974, 96, 449–456. (b) Miranda, M. A.; Galindo, F. Photo-Fries Reaction and Related Processes. In CRC Handbook of Organic Photochemistry and Photobiology, 2nd ed.; Horspool, W. M., Ed.; CRC Press: Boca Raton, FL, 2004. (c) Lochbrunner, S.; Zissler, M.; Piel, J.; Riedle, E.; Spiegel, A.; Bach, T. J. Chem. Phys. 2004, 120, 11634–11639. (25) Bellus, D.; Hrdlovic, P. Chem. ReV. 1967, 67, 599–609. (26) (a) Li, S. K. L.; Guillet, J. E. Macromolecules 1977, 10, 840–844. (b) Merle-Aubry, L.; Holden, Y. M.; Guillet, J. E. Macromolecules 1980, 13, 1138– 1143. (27) Frechet, J. M. J.; Tessier, T. G.; Wilson, C. G.; Ito, H. Macromolecules 1985, 18, 317–321. (28) Ho¨fler, T.; Griesser, T.; Gstrein, X.; Trimmel, G.; Jakopic, G.; Kern, W. Polymer 2007, 48, 1930–1939. (29) Griesser, T.; Hoefler, T.; Temmel, S.; Kern, W.; Trimmel, G. Chem. Mater. 2007, 19, 3011–3017. (30) Gottlieb, H. E.; Kotlyar, V.; Nudelman, A. J. Org. Chem. 1997, 62, 7512– 7515.
12422 Langmuir, Vol. 24, No. 21, 2008 FTIR spectra were recorded with a Perkin-Elmer Spectrum One instrument (spectral range between 4000 and 450 cm-1). All FTIR spectra of the samples were recorded in transmission mode (films on CaF2 discs). Contact angles were measured using a Rame-Hart model 100-00 contact angle goniometer using the sessile drop method. A 2 µL droplet was placed on the sample surface and observed using a traveling telescope fitted with a goniometer scale. XPS spectra were obtained on two different instruments: an ESCALAB 250 instrument equipped with a monochromatic Al KR X-ray source, using a pass energy of 20 eV and a step size of 0.10 e and a Kratos Axis ULTRA “DLD” X-ray photoelectron spectrometer, equipped with a monochromatic Al KR X-ray source (hν ) 1486.6 eV) operating with a base pressure in the range of 10-8 to 10-10 mbar. All samples were run as insulators, unless stated otherwise. In order to minimize X-ray-induced damage, the X-ray source was operated at a relatively low output power of 60 W. Here highresolution scans of the C1s region were acquired at a pass energy of 20 eV and a step size (resolution) of 0.1 eV. Survey scans were acquired with a pass energy of 160 eV and a step size of 1.0 eV. All spectra have been normalized to the Au4f7/2 peak. Friction force microscopy measurements were carried out on a Digital Instruments Nanoscope Multimode IIIa atomic force microscope (Digital Instruments, Cambridge, U.K.). The probes used were silicon nitride Nanoprobes (Digital Instruments, Cambridge, U.K.). The nominal force constants of these probes were 0.06 or 0.12 N m-1. The calibration of normal forces involved two steps. First, the photodetector sensitivity was calibrated by measuring a force curve for a very stiff sample. Mica was used, because relative to the very flexible lever, the stiffness of the mica is sufficiently large that it may be assumed that all deflection during the force measurement will be in the lever. Under these circumstances, the photodetector sensitivity is the gradient of a plot of photodetector signal versus displacement while measuring repulsive forces. Second, the spring constants of the levers were determined from their thermal spectra using a routine implemented within the microscope software (on our instrument, it is contained within the Digital Instruments PicoForce software) and based on the method of Hutter and Bechhoefer.31 16-Bromohexadecanoic Acid (1).32 A solution of 16-hydroxyhexadecanoic acid (2.38 g; 9.17 mmol) in 15 mL of acetic acid (100%) and 15 mL of hydrobromic acid (48%) was heated under reflux for 48 h. Upon cooling, 1 crystallized as a white solid. The crude product was filtered off, washed several times with water, and recrystallized from ice-cold cyclohexane to give 3 g (98%) of 1. 1H NMR (δ, 500 MHz, 20 °C, CDCl ): 3.40 (t, 2H, C16), 2.34 3 (t, 2H, C2), 2.17 (s, 1H, COOH) 1.88-1.82 (m, 2H, C15), 1.66-1.57 (m, 2H, C3), 1.46-1.20 (m, 22H, C4-14) ppm. 13C NMR (δ, 125 MHz, 20 °C, CDCl3): 164.06 (COOH), 34.04 (C2), 32.84 (C16), 29.60-28.17 (C4-15), 24.67 (C3) ppm. FTIR (CaF2, cm-1): 3043, 2918, 2851, 1699, 1472, 1430, 1409, 1271, 1211, 921. Phenyl 16-Bromohexadecanoate (2). To a solution of 1 (2.0 g; 5.96 mmol) and phenol (0.62 g; 6.56 mmol) in 60 mL of dichloromethane 1.85 g (8.95 mmol) of N,N-dicyclohexylcarbodiimide (DCC) was added. The mixture was stirred at room temperature for 20 h. The reaction was quenched with aqueous HCl (5%, 20 mL) and stirred for 5 min. Then the reaction mixture was filtered over Celite. The filtrate was washed with saturated sodium bicarbonate solution (50 mL) and water (50 mL × 3) and dried over anhydrous sodium sulfate. After evaporation of the solvent the crude product was purified by column chromatography (ethylacetate/cyclohexane 1:50) to give 1.15 g (47%) of 2. Melting point: 50 °C. 1H NMR (δ, 500 MHz, 20 °C, CDCl ): 7.37 (t, 2H, ph3,5), 7.21 3 (t, 1H, ph4), 7.07 (d, 2H, ph2,6), 3.40 (t, 2H, C16), 2.55 (t, 2H, C2), 1.88-1.82 (m, 2H, C15), 1.78-1.71 (m, 2H, C3), 1.42-1.22 (m, 22H, C4-14) ppm. 13C NMR (δ, 125 MHz, 20 °C, CDCl3): 172.27 (CdO), 150.73 (ph1), 129.33 (ph3,5), 125.64 (ph4), 121.53 (ph2,6), (31) Hutter, J. L.; Bechhoefer, J. ReV. Sci. Instrum. 1993, 64, 1868. (32) Ihalainen, P.; Peltonen, J. Langmuir 2002, 18, 4953.
Griesser et al. 34.38-24.92 (C2-16) ppm. FTIR (CaF2, cm-1): 2917, 2849, 1753, 1590, 1496, 1485, 1472, 1245, 1198, 1170, 1149, 1138, 934. Phenyl 16-Mercaptohexadecanoate (3). A stirred solution of 2 (0.9 g; 2.11 mmol) in 20 mL of tetrahydrofuran (THF) was cooled down to -10 °C, and hexamethyldisilathiane (0.45 g; 2.53 mmol) and tetrabutylammonium fluoride (TBAF) (0.73 g; 2.32 mmol in 5 mL of THF with 50 µL of water) were added. The resulting reaction mixture was allowed to warm to room temperature while being stirred. After 1 h the reaction mixture was diluted with 60 mL of dichloromethane and washed with aqueous ammonium chloride (sat., 3 × 30 mL). After evaporation of the solvent the crude product was purified by column chromatography (ethyl acetate/cyclohexane 1:20) to give 0.62 g (81%) of 3. Melting point: 46.5 °C. 1H NMR (δ, 500 MHz, 20 °C, CDCl ): 7.37 (t, 2H, ph3,5), 7.22 3 (t, 1H, ph4), 7.07 (d, 2H, ph2,6), 2.57-2.50 (4H, C2,16), 1.78-1.72 (m, 2H, C3), 1.64-1.58 (m, 2H, C15), 1.44-1.27 (m, 22H, C4-14) ppm. 13C-{1H} NMR (δ, 125 MHz, 20 °C, CDCl3): 172.30 (CdO), 150.76 (ph1), 129.36 (ph3,5), 125.67 (ph4), 121.56 (ph2,6), 34.41-24.65 (C2-16) ppm. FTIR (CaF2, cm-1): 2918, 2849, 1747, 1590, 1496, 1484, 1472, 1417, 1203, 1169, 1151, 934. Preparation of SAMs. Glass slides were first cleaned by piranha solution (70:30, v/v, H2SO4/H2O2). Caution: Piranha solution reacts violently with organic materials and should be handled with great care. The substrates were then rinsed with Milli-Q water and dried. A chromium adhesion layer of approximately 5 nm was first thermally evaporated onto the glass slides, followed by a 30 nm gold layer. Afterward the gold-coated samples were immersed in a 2 mM solution of 3 in degassed ethanol. The substrates were washed with ethanol and dried in a stream of nitrogen. Irradiation and Postmodification Reactions. Caution: UV irradiation, especially with UV lasers, causes severe eye and skin burns. Precautions (UV protective goggles, gloves) must be taken! Flood UV illumination was carried out in inert atmosphere (argon) by using an ozone-free mercury low-pressure UV lamp (Heraeus Noblelight; 254 nm) with a power density of 1.35 mW/cm2 (irradiation time typically 60 min). Photopatterning was conducted using light from a frequencydoubled argon ion laser (Coherent FreD 300C, Coherent U.K., Ely) that emits at 244 nm (40 mW output power, irradiation time 3 min). Patterned structures were obtained by placing a contact mask with lines and spaces (Cr pattern on quartz, obtained from Austria Microsystems, Unterpremsta¨tten Austria) directly onto the organic layer prior to illumination. For scanning near-field photolithography, the laser was coupled to a ThermoMicroscopes Aurora III near-field scanning optical microscope (Veeco UK, Cambridge, U.K.) fitted with a fused-silica fiber probe (Veeco, Cambridge, U.K.). The probe scan velocity was 0.1 µm s-1. Postmodification Reaction. After the illumination step, the substrates were immersed in a solution of 200 µL of adipoyl chloride and 50 µL of triethylamine in 2 mL of CH2Cl2 for 1 h. The samples were washed with CH2Cl2 and subsequently immersed in a solution of 100 µL of ethylenediamine in 2 mL of CH2Cl2. The substrates were washed with CH2Cl2 and dried in a stream of nitrogen.
Results and Discussion The synthesis of phenyl 16-mercaptohexadecanoate is shown in Scheme 2. The design of this molecule was chosen such that the phenyl ester moiety represents the photoreactive group which is expected to undergo the photo-Fries reaction, whereas the thiol group binds to gold surfaces. In a first step, 16-hydroxyhexadecanoic acid was converted into 1 according to the literature protocol by using HBr and glacial acetic acid.32 By an esterification of 1 with phenol using DCC as catalyst, 2 was obtained in 47% yield. The bromide functionality in compound 2 was then substituted with hexamethyldisilathiane/tetrabutylammonium fluoride (HMDST/TBAF) to give the corresponding thiol, 3 in high yield (81%). The spectroscopic data, see the Experimental Section, are in good agreement with the proposed structure. Because of the substitution
Nanoscale Patterning by Photo-Fries Rearrangement Scheme 2. Synthesis of Photoreactive Phenyl 16-Mercaptohexadecanoate (3)
of the bromine, the chemical shifts of the ω-CH2 group changed from 3.40 ppm in the proton NMR spectrum and 32.8 ppm in the 13C NMR spectrum to 2.52 and 24.6 ppm, respectively. Figure 1 shows detailed FTIR spectra of a liquid film of 3 before and after monochromatic irradiation of 254 nm with a radiant energy per area unit of E ) 4.86 J/cm2. In the spectrum of the nonirradiated film the signals at 1751 (CdO stretch) and at 1197 cm-1 (asym C-O-C stretch) are typical of the ester units. The positions of these two bands exactly meet the expectation for esters R1-(CdO)-O-R2 with R1 being an aliphatic unit and R2 being a phenyl ring.33 After UV irradiation, significant changes were observed in the FTIR spectrum of phenyl 16-mercaptohexadecanoate. The signals of the phenyl ester group at 1751 and 1197 cm-1 were found to have decreased, as had the ester signal at 1137 cm-1. A new band was observable at 1640 cm-1. When this value is compared with the CdO stretching vibration of 2-hydroxyacetophenone at 1641 cm-1, the signal indicates the formation of a hydroxyketone which is the expected photo-Fries product. The formation of the SAM, the photoreaction in the monolayer, and a selective postillumination modification reaction was investigated by contact angle and XPS measurements. The overall reaction scheme is depicted in Scheme 3. The advancing water contact angle of the monolayer was 90°. The XPS survey spectra showed the expected signals for sulfur, carbon, and oxygen (see the Supporting Information). After illumination the contact angle of water decreased significantly, to 67°. The XPS spectrum hardly changed as the chemical composition of the monolayer was the same, and the change in binding energies of the original ester versus the photogenerated hydroxyketone was too small to be detectable within the resolution limits of the instrument used. The
Figure 1. FTIR spectra of the photoreactive thiol 3 before (a) and after (b) illumination. (Full spectra are presented in the Supporting Information.)
Langmuir, Vol. 24, No. 21, 2008 12423 Scheme 3. Photoreaction and Postexposure Chemical Modification for the Photoreactive SAM Using Compound 3
Table 1. Contact Angle of Water (Sessile Drop) on the Investigated Surfaces surface
SAM (3) pristine
SAM (3) illuminated
SAM (3) NH2-terminated
contact angle
90°
67°
38°
photoreaction was carried out under inert gas atmosphere (N2) as well as under ambient air. To examine the effectiveness of the photochemical conversion, the reaction of samples with adipoyl chloride and, subsequently, ethylenediamine was explored. It was expected that reaction (Scheme 3) would lead to the introduction of nitrogen to the sample, which could be readily identified by XPS. The introduction of diamine should also yield an increase in the surface free energy, reflected in a decrease in the contact angle and also an increase in the coefficient of friction measured by FFM. The pristine monolayer exhibited a contact angle of 90° (Table 1). Following UV exposure this decreased to 67°, and following derivatization using adipoyl chloride and ethylenediamine, it decreased further, to only 38°, indicating a substantial transformation of the surface properties. In Figure 2, the N1s and S2p(1/2, 3/2) regions of the XPS analysis are shown for the SAMs after UV illumination under nitrogen atmosphere (curve b in each graph) or under ambient conditions (curve c) and subsequent chemical modification and for a SAM which was not illuminated but also treated by the postmodification procedure (curve a). As expected, a nitrogen signal was only detected after the chemical modification step. The binding energy
Figure 2. (A) S2p region and (B) N1s region of the XPS analysis of the self-assembled monolayer (SAM) of the nonilluminated thiol layer treated by the postmodification reaction (a), after illumination under nitrogen and postmodification (b), and after illumination under ambient condition and postmodification (c).
12424 Langmuir, Vol. 24, No. 21, 2008
had a value of approximately 398.6 eV and may be attributed to an alkyl amine. (cf., e.g., ethylamine 398.9 eVsNIST XPS database). In addition, a shoulder at higher binding energy (ca. 400.6 eV) was also detected (see also the Supporting Information). We attribute this to the amide nitrogen which shows a lower intensity due to the fact that it is not directly at the surface. The photo-oxidation of SAMs of alkanethiols has previously been studied extensively.16c,34 The headgroup is converted to a weakly bound alkylsulfonate species on exposure to light with a wavelength of ca. 250 nm, most likely as a result of the formation of hot electrons at the gold surface.34c However, the rate of photo-oxidation is known to be strongly dependent on the alkyl chain length and terminal functional group of the adsorbate.34c Although short-chain, carboxylic acid terminated adsorbates oxidize at low doses (ca. 1-2 J cm-2), methyl-terminated adsorbates oxidize much more slowly, and the rate of oxidation decreases as the length of the alkyl chain increases (as a result of closer packing and reduced alkyl chain mobility). In view of these considerations, it was important to ensure that the exposure process did not lead to oxidation of the thiol headgroups on a shorter time scale than the photo-Fries rearrangement. For alkanethiol-SAMs, there are clear changes in the S2p spectrum as a consequence of photochemical conversion of the thiolate to a sulfonate.34a Alkanethiolates yield a peak at 161-162 eV, and photo-oxidation leads to the appearance of a new peak some 7 eV higher in binding energy. S2p spectra were thus acquired (Figure 2), to determine whether oxidation of the headgroup was occurring. The S2p spectrum of the pristine material exhibits a single main feature at ca. 161 eV. This peak is composed of two overlapping components arising from spin-orbit coupling (S2p(1/ 2) and S2p(3/2) components) in the ratio 2:1. They are not resolved in the spectra in Figure 2a because of the low intensity of the peak. Following UV exposure under both, nitrogen and ambient atmospheres, the spectrum remains unchanged. No oxidized sulfur species which can normally be detected at a higher binding energy (approximately 169 eV)16b were evidenced. It may be concluded that, under the conditions used here, the exposure was not sufficient to cause photo-oxidation of the adsorbate head groups. Although the exposure was large enough to cause oxidation of some thiols, the long alkyl chain and the presence of the phenyl ring likely reduce the rate of penetration of oxygen species from the air-monolayer interface to the metal-sulfur bond, making the extent of oxidation slow enough to be negligible under the conditions used. Patterned functionalized surfaces were prepared by two different photolithographic methods, the use of a contact mask (chromium on SiO2) and a laser with a wavelength of 244 nm as UV source, and scanning near-field photolithography (SNP) using a near-field scanning optical microscope coupled with a UV laser (244 nm).17 These processes are depicted schematically in Figure 3. After this patterning step, the same postmodification procedure as described above was applied to create a highly hydrophilic surface in the illuminated areas. To reveal a materials contrast between modified and unmodified regions of the patterned samples, FFM was performed under ambient conditions. In this mode a soft cantilever is scanned perpendicular to its long axis. Lateral forces resulting from the interaction of the tip with the substrate lead to a twist of the cantilever depending on the friction of the surface. (33) Socrates, G. Infrared Characteristic Group Frequencies, 2nd ed.; Wiley: Chichester, U.K., 1994. (34) (a) Hutt, D. A.; Leggett, G. J. J. Phys. Chem. 1996, 100, 6657. (b) Brewer, N. J.; Beake, B. D.; Leggett, G. J. Langmuir 2001, 16, 735–739. (c) Brewer, N. J.; Janusz, S. J.; Critchley, K.; Evans, S. D.; Leggett, G. J. J. Phys. Chem. B 2005, 109, 11247.
Griesser et al.
Figure 3. Scheme of the patterning processes: left side, patterning by contact lithography; right side, pattern using SNOM.
Figure 4. Friction force images after structured illumination and postmodification reaction yielding NH2-terminated surfaces: (A) photolithographic patterning using a contact mask (15 µm lines/15 µm spaces and 2 µm lines/2 µm spaces) and inert gas conditions; (B) patterning under ambient conditions (15 µm lines/15 µm spaces). In the images the bright contrast indicates a high friction force, and dark contrast indicates low friction. (C) Section analysis of the friction force in image A (white line).
Representative FFM images of patterned functionalized surface obtained by using a contact mask during the illumination step are shown in Figure 4. The exposed regions, in which the adsorbates have undergone the photo-Fries rearrangement and subsequently been derivatized with amines, yield bright contrast (high friction), whereas the masked areas exhibit darker contrast. This may be understood in terms of the tip-sample adhesive interactions: the surface of a silicon nitride probe, such as the one used here, consists of a layer of polar silicon dioxide which will interact strongly with polar regions of the sample, leading to strong adhesion and hence a high rate of energy dissipation (hence high friction) as the tip slides across the surface. In contrast, for the virgin regions, a phenyl ring is presented to the probe, and this interacts much less strongly with the polar surface of the tip leading to a reduced rate of energy dissipation and hence a smaller friction force. Both illuminations under inert atmosphere (Figure 4, parts A and C) as well as under ambient conditions (Figure 4B) followed
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photochemistry for nanofabrication, involving, as it does, a highly specific molecular transformation. Although there are other lithographic techniques that have yielded high resolution in monolayer patterning, there are few that can offer the combination of high spatial resolution and molecular specificity that characterize the process used here. One of the biggest challenges in nanoscience is the integration of top-down (lithographic) methods with bottom-up (synthetic) ones, and this approach illustrates the potential that near-field optical methods offer in bridging this divide.
Conclusions
Figure 5. (A) Friction force images after patterning using SNP under ambient atmosphere and postmodification reaction yielding NH2terminated surfaces. (B) detail of (A) showing a fwhm of 250 nm. (C) Section analysis of the friction force in image A (white line). In the images A and B dark contrast indicates high friction force (the NH2terminated regions), whereas bright contrast indicates low friction.
by the postmodification reaction yield a stripe pattern with a high contrast in friction. In order to investigate the possibility of using the photo-Fries rearrangement to fabricate nanometer-scale structures, SNP was used to carry out near-field UV exposure. In order to facilitate imaging of the resulting patterns by FFM, the exposure was followed by the same postmodification reaction employed for the micrometer-scale patterns. The resulting structures were characterized using FFM, and line widths in the range of 200-250 nm were achieved. The FFM images in Figure 5 show a representative structure that exhibits a full width at half-maximum (fwhm) of 250 nm (see line section). The clear contrast and the line section demonstrate clearly the high difference in friction between unmodified and modified regions of the surface. It is significant to note that little effort was made to optimize the procedure (by varying the writing rate and exposure power), and improvements to this resolution are thus likely to be achievable. Previous studies of thiol patterning by SNP have focused on the oxidation of the adsorbate headgroup, followed by replacement of the oxidation products by contrasting adsorbates. However, the present study illustrates the breadth of capability offered by
In this contribution, we have introduced the photo-Fries rearrangement reaction to SAMs by synthesizing a new photoreactive thiol-bearing aryl ester end groups. Upon illumination with UV light, the aryl ester groups undergo an isomerization reaction to the corresponding hydroxyketones which are chemically more reactive than the ester units in nonirradiated areas. The enhanced reactivity was used to produce amino-functionalized surfaces by applying a two-step postmodification reaction. This reaction was proven by XPS, and in addition, high differences in the contact angle and the friction between the pristine surface layers and the modified layers were obtained. Patterned functionalized surfaces were obtained by two different photolithographic techniques, by using either a contact mask or the SNOM. Without optimization a resolution of approximately 2 µm could by obtained with the contact mask and 250 nm with SNP. The simplicity of the photoreaction as well as the possibility to introduce different functionalities by postmodification reactions makes these photoreactive SAMs interesting candidates for tuning the surface properties, e.g., metal electrodes, and for modern immobilization techniques. Acknowledgment. Financial support by the Austrian Science Fund (FWF) in the framework of a national research network (NFN Interface controlled and functionalized organic filmssProject S9702-N08 “Design and application of tunable surfaces based upon photoreactive molecules”) is gratefully acknowledged. T.G. thanks the Graz University of Technology and the University of Sheffield for financing the research visit at Sheffield. The authors thank Anna Track for helpful discussions and Petra Kaschnitz for NMR measurements. J.A. and G.J.L. thank Research Councils U.K. for support (Grant No. EP/C523857/1). Supporting Information Available: 1H and 13C NMR spectra and additional XPS data. This material is available free of charge via the Internet at http://pubs.acs.org. LA802382P
