Fabricating Chemical Gradients on Oxide Surfaces by Means of

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Fabricating Chemical Gradients on Oxide Surfaces by Means of Fluorinated, Catechol-Based, Self-Assembled Monolayers† Mathias Rodenstein,‡ Stefan Z€urcher,‡,§ Samuele G. P. Tosatti,‡,§ and Nicholas D. Spencer*,‡ ‡

Laboratory for Surface Science and Technology, Department of Materials, ETH Zurich, Wolfgang-PauliStrasse 10, CH-8093 Zurich, Switzerland, and §SuSoS AG, Lagerstrasse 14, CH-8600 D€ ubendorf, Switzerland Received February 24, 2010. Revised Manuscript Received April 21, 2010 Catechols bind strongly to several metal oxides and can thus be used as a binding group for generating self-assembled monolayers. Furthermore, their derivatives can be used to produce well-defined, centimeter-scale surface-chemical gradients on technologically relevant surfaces, such as titanium dioxide (TiO2). A simple dip-and-rinse gradientpreparation technique was utilized to produce surface-hydrophobicity gradients from perfluoro-alkyl catechols and nitrodopamine (ND). Chemical composition, quality, and properties of the functionalized surfaces were determined by means of X-ray photoelectron spectroscopy (XPS), variable-angle spectroscopic ellipsometry (VASE), and static water contact angle (sCA) measurements. Contact angles were found to be in the range of 30°-95°, correlating well with the determined surface chemical composition and adlayer thickness.

Introduction The use of monolayer coatings that are spontaneously generated by immersion into solutions of adsorbing molecules, so-called self-assembled monolayers (SAMs), has gained a lot of popularity over the past 20 years.1 SAMs allow facile and economical tailoring of surface properties, such as hydrophobicity,2-4 protein resistance,5,6 or specific biological activity.7 Depending on the substrate to be modified, different anchoring chemistries (e.g., thiols for noble metals,2-4 silanes for silica,8 or phosph(on)ates for metal oxides9,10) have been established.11-15 In general, a limitation has been that the binding chemistry to be applied is determined by the surface chemistry of the substrate material. Hence, the synthesis and combination of different anchor and functional groups have been necessary. Recently, it was reported that mussels can stick to virtually any surface under the harshest of conditions, by means of catechol-based adhesive compounds.16-18 Catechols † Part of the Molecular Surface Chemistry and Its Applications special issue. *To whom correspondence should be addressed. E-mail: [email protected].

(1) Ulman, A. Chem. Rev. 1996, 96, 1533. (2) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358. (3) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733. (4) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365. (5) Lin, Y. S.; Hlady, V.; G€olander, C.-G. Colloids Surf., B 1994, 3, 49. (6) Tosatti, S. G. P. Ph.D. Dissertation, ETH Zurich, Switzerland, 2003. (7) Michel, R.; Lussi, J. W.; Csucs, G.; Reviakine, I.; Danuser, G.; Ketterer, B.; Hubbell, J. A.; Textor, M.; Spencer, N. D. Langmuir 2002, 18, 3281. (8) Chaudhury, M. K.; Whitesides, G. M. Science 1992, 255, 1230. (9) Hofer, R.; Textor, M.; Spencer, N. D. Langmuir 2001, 17, 4014. (10) Tosatti, S. G. P.; Michel, R.; Textor, M.; Spencer, N. D. Langmuir 2002, 18, 3537. (11) Liedberg, B.; Tengvall, P. Langmuir 1995, 11, 3821. (12) Guerrero, G.; Mutin, P.; Vioux, A. Chem. Mater. 2001, 13, 4367. (13) Bhat, R.; Fischer, D.; Genzer, J. Langmuir 2002, 18, 5640. (14) Carbonell, L.; Whelan, C.; Kinsella, M.; Maex, K. Superlattices Microstruct. 2004, 36, 149. (15) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103. (16) Dalsin, J. L.; Lin, L.; Tosatti, S. G. P.; Voros, J.; Textor, M.; Messersmith, P. B. Langmuir 2005, 21, 640. (17) Dalsin, J. L.; Messersmith, P. B. Mater. Today 2005, 8, 38. (18) Lee, H.; Dellatore, S. M.; Miller, W. M.; Messersmith, P. B. Science 2007, 318, 426.

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represent a very promising, biomimetic approach to extend the field of surface functionalization. Their adhesion properties on a wide variety of substrates, including metals (Ag, Au),19,20 native oxide surfaces (on Ti, Cu, Fe),16,21,22 semiconductors (Si),23 and polymers such as polystyrene,18 have been reported.18 The study of their adsorption mechanisms, theoretically24,25 and experimentally,26 the influence of parameters such as pH,21 as well as their stability under a variety of conditions (solvents, temperature, and time) is thus of fundamental interest.27 The fact that, in addition to mussels, cyanobacteria also employ a catechol-based molecule, the siderophore anachelin, to covalently bind to metal ions, such as Fe3þ,28-30 further suggests the flexibility of this chemistry as a potential coating platform. Wach et al.,30 for example, have used a hybrid catechol-based molecule to generate a functional coating with active antibacterial properties. In the case of anachelin, it is possible to observe how one of the general limitations of catechol chemistry (its low stability toward oxidation, especially at elevated pH) is circumvented by the introduction of the electron-withdrawing dimethyl-ammonium group, bound to the aromatic ring of the catechol.28 To the same end, we have introduced the nitro (-NO2) group in the paraposition to the hydroxide, due to synthetic ease and relatively small additional steric bulk.27 In the present work, we further functionalize the nitro-containing species, nitrodopamine (ND), with a perfluorinated alkyl (19) Kawabata, T. Biochem. Pharmacol. 1996, 51, 1569. (20) Brooksby, P. A.; Schiel, D. R.; Abell, A. D. Langmuir 2008, 24, 9074. (21) Araujo, P. Z.; Morando, P. J.; Blesa, M. A. Langmuir 2005, 21, 3470. (22) Creutz, C.; Chou, M. H. Inorg. Chem. 2008, 47, 3509. (23) Lambert, J.; Singer, S. J. Organomet. Chem. 2004, 689, 2293. (24) Terranova, U.; Bowler, D. R. J. Phys. Chem. C. 2010, 114, 6491. (25) Li, S.-C.; Wang, J.-G.; Jacobson, P.; Gong, X.-Q.; Selloni, A.; Diebold, U. J. Am. Chem. Soc. 2009, 131, 980. (26) Lee, H.; Lee, K. D.; Pyo, K. B.; Park, S. Y.; Lee, H. Langmuir 2010, 26, 3790. (27) Malisova, B.; Tosatti, S.; Textor, M.; Gademan, K.; Z€urcher, S. Langmuir 2010, 26, 4018. (28) Z€urcher, S.; W€ackerlin, D.; Bethuel, Y.; Malisova, B.; Textor, M.; Tosatti, S. G. P.; Gademann, K. J. Am. Chem. Soc. 2006, 128, 1064. (29) Wach, J.-Y.; Bonazzi, S.; Gademann, K. Angew. Chem., Int. Ed. 2008, 47, 7123. (30) Wach, J.-Y.; Malisova, B.; Bonazzi, S.; Tosatti, S. G. P.; Textor, M.; Z€urcher, S.; Gademann, K. Chem.;Eur. J. 2008, 14, 10579.

Published on Web 05/27/2010

DOI: 10.1021/la100805z

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insights into phenomena that are not otherwise accessible, for example, taxis of cells on various scales.35-44 Numerous kinds of surface-chemical gradients have been reported, including one-, two-, or three-dimensional gradients and gradients on the micrometer or centimeter scales. Fabrication approaches have involved polymer brushes, alkanethiols on Au, micropillars, and poly(dimethylsiloxane) (PDMS) microfluidic channel systems among others.11,35,41,45,46 In the present work, in order to demonstrate the controllable adsorption and application of PFAND and ND, we have adapted the linear motion drive (LMD) gradient-fabrication protocol developed by Morgenthaler et al.42 to the technologically relevant TiO2 substrate. Changes in the adlayer thickness, its chemical composition, and surface energy have been measured for the gradient and compared to corresponding values for homogeneous adlayers.

Materials and Methods Figure 1. Structures of the two molecules used for self-assembly: perfluoro-alkyl-nitrodopamine (PFAND) and nitrodopamine (ND).

chain (yielding perfluoro-alkyl-nitrodopamine (PFAND)), shown in Figure 1, in order to obtain a molecule that combines the functional properties of perfluoro-alkyls with the anchoring mechanisms of catechols in an architecture that can generate selfassembled monolayers. Fluoropolymers, such as poly(tetrafluoroethylene) (PTFE), and SAM-forming molecules, such as perfluorinated thiols, silanes, and n-alkanoic acids, are widely used for surface fluorination in academia and industry (e.g., as easy-to-clean surfaces).31-34 The modification of surfaces with PTFE is chemically challenging because of its low reactivity and solubility. Thiols, on the other hand, generally form stable layers on (noble) metals only and are thus not applicable to the functionalization of most materials. Utilizing catechol derivatives to fluorinate a wide variety of surfaces in order to render them hydrophobic or nonadhesive is therefore a promising alternative. A recent development has been the replacement of sets of discrete sample arrays of varying surface parameters, such as substrate roughness, hydrophobicity, protein resistance, or (bio)ligand densities by a continuum of these properties.35,36 Preparation and comparison of discrete arrays can be very time-consuming and yield only fragmentary results. Samples that exhibit a continuous spectrum of a surface-chemical functionality along one defined spatial dimension on a single substrate are known as surface-chemical gradients. Using surfaces that incorporate such a continuous change of a defined property can result in a multiplication and parallelization of experiments and thus a radical increase in research speed. The approach can also yield (31) Wallace, R.; Chen, P.; Henck, S.; Webb, D. J. Vac. Sci. Technol., A 1995, 13, 1345. (32) Evans, S. D.; Flynn, T.; Ulman, A.; Beamson, G. Surf. Interface Anal. 1996, 24, 187. (33) Geer, R.; Stenger, D.; Chen, M.; Calvert, J. M.; Shashidhar, R.; Jeong, Y.; Pershan, P. Langmuir 1994, 10, 1171. (34) Barriet, D.; Lee, T. R. Curr. Opin. Colloid Interface Sci. 2003, 8, 236. (35) Genzer, J.; Bhat, R. Langmuir 2008, 24, 2294. (36) Morgenthaler, S.; Zink, C.; Spencer, N. D. Soft Matter 2008, 4, 419. (37) Lee, S.-W.; Laibinis, P. E. J. Am. Chem. Soc. 2000, 122, 5395. (38) Lahann, J.; Mitragotri, S.; Tran, T.-N.; Kaido, H.; Sundaram, J.; Choi, I. S.; Hoffer, S.; Somorjai, G.; Langer, R. Science 2003, 299, 371. (39) Daniel, S.; Chaudhury, M. K. Langmuir 2002, 18, 3404. (40) Jeon, N. L.; Baskaran, H.; Dertinger, S. K. W.; Whitesides, G. M.; Water, L. V. D.; Toner, M. Nat. Biotechnol. 2002, 20, 826. (41) Wu, T.; Efimenko, K.; Vlcek, P.; Subr, V.; Genzer, J. Macromolecules 2003, 36, 2448. (42) Morgenthaler, S.; Lee, S.; Z€urcher, S.; Spencer, N. D. Langmuir 2003, 19, 10459.

16212 DOI: 10.1021/la100805z

Substrates and Chemicals. Solvents and chemicals were obtained from Sigma-Aldrich, Buchs, Switzerland unless otherwise stated. Silicon wafers with a natural SiO2 layer were obtained from Si-Mat Silicon Materials, Landsberg/Lech, Germany. Entire wafers were coated with TiO2 by physical vapor deposition (PVD, reactive magnetron sputtering) at the Paul Scherer Institute, Villigen, Switzerland, to obtain TiO2 films of about 20 nm in thickness (verified by ellipsometry). The wafers were diced into pieces of 1  1 cm2 (homogeneous samples) and 1  4 cm2 (gradients). Prior to functionalization, the substrates were sonicated twice for 7 min in toluene and then twice for 7 min in 2-propanol, and dried under a stream of nitrogen (5.0). The substrates were further cleaned for 30 min in a UV/O3 cleaner (Boekel Scientific, Feasterville, PA, model 135500), followed by overnight immersion in ultrapure water (TOC < 5 ppb, R = 18.2 MΩcm). Nitrodopamine (ND) and perfluoro-alkyl-nitrodopamine (PFAND) used for self-assembly in this study were synthesized according to the method of Tosatti and Z€ urcher.47 Synthesis of 2H,2H,3H,3H-Perfluoro-undecanoic-acidN-succinimidyl-ester. 2H,2H,3H,3H-Perfluoroundecanoic acid (1.354 g, 2.75 mmol, Fluorous Technologies Inc., Pittsburgh, PA), N-hydroxysuccinimide (348 mg, 3.02 mmol), and dicyclohexylcarbodiimide (622 mg, 3.02 mmol) were dissolved in ethyl acetate (120 mL) and stirred for 18 h at room temperature. The white precipitate formed (dicyclohexyl urea, DCU) was filtered off, and the remaining solution evaporated to dryness. The residue was recrystallized twice from ethyl acetate. Yield: 1.00 g (62%), containing some traces of DCU. 1H NMR (CDCl3, 300 MHz, ppm): 3.0 (m, 2H CH2), 2.88 (s, 4H CH2 NHS), 2.6 (m, 2H CH2).

Synthesis of 6-Nitro-3-hydroxytyramine Hemisulfate Salt (Nitrodopamine, ND). 3-Hydroxytyramine hydrochloride (1.90 g, 10 mmol) and sodium nitrite (1.52 g, 22 mmol) were dissolved in water (25 mL) and cooled to 0 °C. Sulfuric acid (17.4 mmol in 10 mL of water) was added slowly to the mixture, and a yellow precipitate was formed. After stirring at room temperature overnight, the precipitate was filtered and recrystallized from water. The product was dried under high vacuum to yield ND as a hemisulfate salt.48 Yield: 1.389 g (58%). 1H NMR (D2O, 300 MHz, ppm): 7.62 (s, 1H ND), 6.83 (s, 1H ND), 3.24 (t, 2H CH2), 3.12 (t, 2H CH2). Elemental Analysis for C16H22N4O12S in % (43) Yu, X.; Wang, Z.; Jiang, Y.; Zhang, X. Langmuir 2006, 22, 4483. (44) Morgenthaler, S.; Zink, C.; St€adler, B.; V€or€os, J.; Lee, S.; Spencer, N. D.; Tosatti, S. G. P. Biointerphases 2007, 1, 156. (45) Yoshimitsu, Z.; Nakajima, A.; Watanabe, T.; Hashimoto, K. Langmuir 2002, 18, 5818. (46) Dertinger, S. K. W.; Chiu, D.; Jeon, N.; Whitesides, G. M. Anal. Chem. 2001, 73, 1240. (47) Tosatti, S. G. P., Z€urcher, S., Patent Appl. PCT/CH2007/000603 2007. (48) Ranganathan, S.; Tamilarasu, N. J. Indian Chem. Soc. 1999, 76, 727.

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Rodenstein et al. found (calculated): C, 38.01 (38.87); H, 4.48 (4.48); N, 11.10 (11.33); S, 6.89 (6.49).

Synthesis of N-(4,5-Dihydroxy-2-nitro-phenethyl)2H,2H,3H,3H-perfluoro-undecanamide (Perfluoro-alkylnitrodopamine, PFAND). 6-Nitro-3-hydroxytyramine hemisulfate salt (170 mg, 0.84 mmol) and N-methylmorpholine (150 μL) were dissolved in dimethylformamide (DMF, 5 mL). 2H,2H,3H,3H-Perfluoro-undecanoic-acid-N-succinimidyl-ester (500 mg, 0.85 mmol) was added, and the mixture was stirred under an atmosphere of nitrogen at room temperature overnight. Hydrochloric acid (1 N, 25 mL) was added, and the yellow precipitate formed was filtered off and washed with water. The solid was dissolved in ethyl acetate, and the organic phase dried over magnesium sulfate. The solvent was evaporated, and the residue recrystallized from a 1:1 chloroform/ethyl acetate mixture (20 mL, 4 °C). Yield: 279 mg (48%). 1H NMR (DMSO-d6, 300 MHz, ppm): 11-9 (broad, 2H OH), 8.2 (t, 1H NH), 7.48 (s, 1H nitro-dopamine), 6.7 (s, 1H nitro-dopamine), 2.9 (2H CH2), 2.7-2.2 (m, 6H CH2). Elemental Analysis for C19H13N2F17 in % found (calculated): C, 33.79 (33.94); H, 2.08 (1.95); N, 4.07 (4.17). Solutions for Self-Assembly. ND is very hydrophilic due to its hydroxyl and amine groups and therefore soluble in pure H2O (TKA-GenPure UV-TOC/UF, Huber & Co AG, Reinach BL, Switzerland). PFAND is strongly hydrophobic because of its perfluorinated alkyl tail. It was dissolved in 2-propanol (SigmaAldrich, Switzerland) first and then diluted with H2O in a 2:1 ratio of H2O/2-propanol. Sample Preparation. Homogeneous Samples. Substrates were immersed in 1.5 mL of solution of different concentrations (between 0.4 and 440 μM) for different durations (2 min, 4 min, 8 min, 16 min, 32 min, 64 min, and 48 h). ND samples were subsequently immersed in pure H2O and sonicated for 1 min, while PFAND samples were immersed and sonicated in 2-propanol for 3 min after adsorption, to remove loosely bound molecules. Subsequently, samples were blown dry under a stream of nitrogen. Gradient Samples. The gradient-preparation method was adapted from that of Morgenthaler et al.42 The immersion protocol was a logarithmic z-position-versus-time program (see the Supporting Information, part 1). Bare substrates were thus immersed in PFAND solution in a controlled manner. The following parameters were chosen for the preparation of gradient samples: (i) concentration of PFAND, 220 μM; (ii) immersion time at high-coverage end, 1 h; low coverage end, 4 s; (iii) logarithmic immersion depth versus time dependence, immediate sonication of samples for 3 min in 2-propanol after gradual immersion, followed by blow-drying with N2 gas. Backfilling was carried out by immersing one-component PFAND gradients in ND (500 μM) solution for 1 h and then rinsing them thoroughly with ultrapure water, followed by blowdrying with N2.

Variable-Angle Spectroscopic Ellipsometry (VASE). VASE was used to measure surface coverage/layer thickness. We utilized a Woollam MF-2000F (WOOLLAM Co., Inc., Lincoln, NE) ellipsometer at three incident angles of 65°, 70°, and 75° with respect to the surface normal, averaging 50 measurements at each point. The spectral range was λ = 350-750 nm. The calculation of the thickness of the surface layer was performed using a four-layer model (Si/SiO2/TiO2/Cauchy), where Si and SiO2 were assumed to be constant for all wafers. The TiO2 layer was fitted before adsorption, and the Cauchy layer (organic layer) after adsorption with norg = n þ ik = 1.45 þ 0.01i, where n is the refractive index and k is the extinction coefficient. Refractive index variations depending on the order of the SAM (estimated to be approximately 10%) were not taken into account. The positioning of the gradient samples was adjusted manually with an estimated error of (0.5 mm. Due to the elliptical beam size, the measured thickness corresponds to an average over 2 mm  5 mm (65°) to 2 mm  8 mm (75°). If not stated otherwise, the error bars in VASE Langmuir 2010, 26(21), 16211–16220

Article graphs are standard deviations from repeated experiments (three or more). X-ray Photoelectron Spectroscopy (XPS). A Sigma2 instrument (Thermo Fisher Scientific, Loughborough, Great Britain) was utilized for routine XPS experiments. The Sigma2 is equipped with a UHV chamber (pressure < 10-6 Pa during measurements). The X-ray source is a non-monochromated 300 W Al KR source (hυ = 1486.6 eV) that illuminates the sample at an angle of 54° to the surface normal. The hemispherical analyzer is mounted at 0° with respect to the surface normal, thus operating at the magic source-analyzer angle, which eliminates the need for angulardistribution correction.49 A detector consisting of seven channeltrons is used. The spot size of the analyzed area (large-area mode) was 400 μm, and the results therefore represent a laterally averaged chemical composition. For this setup, the full width at half-maximum (FWHM) of Ag 3d5/2 is 1.4 eV using a pass energy of 25 eV. Standard measurements comprised averages over nine (for C, F, and N) or three (for Ti and O) scans for each element plus survey scans with pass energies of 25 and 50 eV, respectively. The dwell time was left at 100 ms at all times, resulting in 3-5 min measurement time per spot for each element, accumulating to about 30 min for a complete elemental scan on each measuring position. In order to assess whether it was necessary to account for the degradation of the coatings during XPS measurements, the degradation kinetics on homogeneous samples were iteratively measured (C, F, and N spectra only (see Results section)). For peak modeling, 30 (PFAND) or 10 (ND) high-resolution XPS spectra were used, recorded using a PHI5000 Versa probe (ULVAC-PHI, INC., Chigasaki, Japan) on homogeneous samples. The spectrometer is equipped with a 180° spherical capacitor energy analyzer and a multichannel detection system with 16 channels. Spectra were acquired at a base pressure of 5  10-8 Pa using a focused scanning monochromatic Al KR source (1486.6 eV) with a spot size of 200 μm and 47.6 W power. The instrument was run in the FAT analyzer mode with electrons emitted at 45° to the surface normal. Pass energies used for survey scans was 187.85 and 46.95 eV for detail spectra. The FWHM of this setup is