Langmuir 2001, 17, 7047-7052
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Order and Orientation in Self-Assembled Long Chain Alkanephosphate Monolayers Adsorbed on Metal Oxide Surfaces Georg Ha¨hner,*,† Rolf Hofer,‡ and Irene Klingenfuss‡ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK, and Department of Materials, ETH Zu¨ rich, Sonneggstr. 5, CH-8092 Zu¨ rich, Switzerland Received May 14, 2001. In Final Form: July 30, 2001 Octadecylphosphoric acid ester was employed to establish monolayers on various flat oxide substrates. The films were prepared via self-assembly from a heptane/propan-2-ol solution. The alkanephosphate was found to produce oriented, well-ordered films on titanium oxide, niobium oxide, and aluminum oxide surfaces. Contact-angle measurements and near-edge X-ray absorption fine structure spectroscopy (NEXAFS) indicate that these layers closely resemble those formed by a similar thiol-gold system, with respect to packing density, inclination, and molecular order. The preparation scheme appears to be rather universally applicable to a variety of metal oxide substrates. These systems show promise as an approach to functionalizing different oxide surfaces with well-ordered organic monolayers, with potential applications in the fields of corrosion protection, adhesion promotion, biochemical analysis, and sensors.
Introduction Self-assembled monolayers (SAMs) have attracted a lot of interest over the past two decades, since they provide a highly flexible approach for the creation of well-defined monomolecular organic films.1-6 While the SA technique principally bears potential for applications in the areas of corrosion-resistant systems,7 adhesion promotion,8 or biosensors,9,10 the specific chemistries generally involved impose certain limitations. The largest classes of SAMs investigated until now have been thiols on gold11 andsto a lesser extentsthose based on the interaction of chlorosilanes with OH-terminated oxide surfaces.11 The latter approach offers some flexibility; it has, however, the disadvantage of frequently producing ill-defined surfaces due to the onset of uncontrolled polymerization reactions. The thiol-gold approach can produce monolayer films with high reliability and a satisfactory degree of reproducibility, but the necessity for a gold (or other noble metal) surface all but rules it out in many applications, for example those in which optical transmission is a requirement for the system. †
University College London. ETH Zu¨rich. * Corresponding author. E-mail
[email protected]; phone +44 20 7679 3496; fax +44 20 7679 1360. ‡
(1) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 2, 92. (2) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (3) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (4) Sellers, H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389. (5) Bishop, A. R.; Nuzzo, R. G. Curr. Opin. Colloid Interface Sci. 1996, 1, 127. (6) Ulman, A. Chem. Rev. 1996, 96, 1533. (7) Bram, Ch.; Jung, Ch.; Stratmann, M. Fresenius J. Anal. Chem. 1997, 358, 108. (8) Ulman, A. Adv. Mater. 1990, 2, 573. (9) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wanne, K. J.; Yu, H. Langmuir 1987, 3, 932. (10) Hickman, J. J.; Ofer, D.; Laibinis, P. E.; Whitesides, G. M.; Wrighton, M. S. Science 1991, 252, 688. (11) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991.
Oxides find extensive applications in various fields of modern technology. An organic modification of their surfacesseven with ultrathin filmsscan tailor some of their interface properties and improve certain qualities of the material. Titanium oxide, for example, plays an important role in biomedical areas and is employed for implants.12,13 An organic coating can establish an interface that is better suited to biological environments than the bare surface, thus improving its biocompatibility. Modifying aluminum oxide surfaces organically could improve corrosion inhibition or the adhesion of paints, e.g., in the automobile industry, and might replace the treatment with chromic acid.14,15 Tantalum and niobium oxide both have a high refractive index. The latter is one of the criteria of the substrate for an optical biosensor chip to ensure total internal reflection in the waveguide layer. In addition, high transparency and very low roughness of the surface and an amorphous (to semicrystalline) state of the waveguide material are needed to encounter only a minimum of light energy loss within the waveguide layer. The formation of amorphous (or polycrystalline) films by a physical vapor deposition process can provide highly transparent layers. Both oxides thus have some potential to be employed as base materials in planar-waveguidebased bioaffinity sensors. An organic modification of their surfaces might tailor the interface for sensor applications. Alternative chemistries to thiols and silanes have been utilized to coat oxide surfaces with SAMs. The former have included hydroxamic,16 carboxylic,17,18 phosphonic (12) Kasemo, B.; Lausmaa, J. CRC Crit. Rev. Biocompatibility 1986, 4, 335. (13) Teinemann, S. G.; Ma¨usli, P. A. Proc. 6th World Conf. on Titanium, France 1988, 535. (14) Maege, I.; Jaehne, E.; Henke, A.; Adler, H.-J. P.; Bram, C.; Jung, C.; Stratmann, M. Prog. Org. Coat. 1998, 34, 1. (15) Maege, I.; Jaehne, E.; Henke, A.; Adler, H.-J. P.; Bram, C.; Jung, Stratmann, M. Macromol. Symp. 1998, 126, 7. (16) Folkers, J. P.; Gorman, C. B.; Laibinis, P. E.; Buchholz, S.; Whitesides, G. M. Langmuir 1995, 11, 813. (17) Laibinis, P. E.; Hickman, J. J.; Wrighton, M. S.; Whitesides, G. M. Science 1989, 245, 845. (18) Aronoff, Y. G.; Chen, B.; Lu, G.; Seto, C.; Schwartz, J.; Bernasek, L. J. Am. Chem. Soc. 1997, 119, 259.
10.1021/la010713a CCC: $20.00 © 2001 American Chemical Society Published on Web 10/06/2001
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acids,15,19,20 and, to a very limited extent, phosphoric acids.15 In addition, several papers have appeared using alkyl phosphates and phosphonates for the purpose of building zirconium phosphate layer structures.21,22 As substrates, native oxides of copper,16 silver,16 titanium,16,20 aluminum,15-18 zirconium,16,18,20 and iron16 were used. However, the number of these studies is small compared to those of the thiol-gold system. Transition metal oxides, such as tantalum oxide (Ta2O5), niobium oxide (Nb2O5), and titanium oxide (TiO2), are known to interact strongly with phosph(on)ates and to form stable interfacial bonds. Both alkyl phosphates and phosphonates constitute systems that have been shown to be capable of forming monomolecular films on aluminum14,15 and tantalum oxide surfaces.23,24 Detailed knowledge about the film structure, in particular the order and orientation that is present in the films, is important for the fundamental understanding of these films as well as for future applications, but is available to a limited extent.14,20,23 In the present paper we describe the preparation and characterization of self-assembled monolayers of long chain alkylphosphoric acid esters (octadecyl phosphate or ODP) on the oxide surfaces of titanium, aluminum, and niobium from an organic solvent. We focus on a direct determination of the orientation and order of ODP on the oxide substrates. In an earlier study we reported on such films adsorbed on a tantalum oxide surface.23,24 The latter was chosen for reasons of its high refractive index, which renders it ideal for application in a planar-waveguidebased bioaffinity sensor. Upon appropriate ω-functionalization, alkanephosphate-based SAMs have the potential to be used as the interface that improves biocompatibility or corrosion resistance, that anchors active sensing elements, or as the basis of passive, biomolecule-resistant regions on the sensor surface provided well-ordered films are established. Here we present more data for technologically relevant materials and compare the quality of the resulting films to those of ODP adsorbed on Ta2O5 and to a similar thiol-gold system. Experimental Section Materials. Octadecylphosphoric acid ester (C18H37OPO(OH)2) was synthesized by Novartis Pharma AG according to the protocol reported by Okamoto.25 It is a stable, waxy solid and was recrystallized from hot n-hexane. Elemental analysis (wt %) yielded the following values: C, 61.72; H, 11.02; P, 8.82; O, 18.44. The atomic ratios of H/C, C/P, and O/P of 2.13, 18.04, and 4.05, respectively, calculated from these elemental analysis data, are in good agreement with the values expected for the formal stoichiometry of the compound (2.17, 18.00, and 4.00). The substrates were obtained from PSI, Villigen, Switzerland, and Crystal GmbH, Berlin, Germany (titanium dioxide, TiO2), Balzers AG, Liechtenstein (niobium pentoxide, Nb2O5), and Algroup Alusuisse, Neuhausen, Switzerland (aluminum oxide, AlOx). (19) Woodward, J. T.; Ulman, A.; Schwartz, D. K. Langmuir 1996, 12, 2, 3626. (20) Gao, W.; Dickinson, L.; Grozinger, C.; Morin, F. G.; Reven, Langmuir 1996, 12, 6429. (21) Lee, H.; Kepley, L. J.; Hong, H. G.; Akhter, S.; Mallouk, T. E. J. Phys. Chem. 1988, 92, 2597. (22) Lee, H.; Hong, H. G.; Mallouk, T. E.; Kepley, L. J. J. Am. Chem. Soc. 1988, 110, 618. (23) Brovelli, D.; Ha¨hner, G.; Ruiz, L.; Hofer, R.; Kraus, G.; Waldner, A.; Schlo¨sser, J.; Oroszlan, P.; Ehrat, M.; Spencer, N. D. Langmuir 1999, 15, 4324. (24) Textor, M.; Ruiz, L.; Hofer, R.; Rossi, A.; Feldman, K.; Ha¨hner, G.; Spencer, N. D. Langmuir 2000, 16, 3257. (25) Okamoto, Y. Bull. Chem. Soc. Jpn. 1985, 58, 3393.
Ha¨ hner et al. TiO2 was available in two modifications. In the first one 100 nm of TiO2 was deposited by a sputter-coating process on commercial glass substrates (PSI). The average roughness of the deposited films was in the nanometer range over a lateral distance of 1 µm as determined by atomic force microscopy. The second sample (Crystal GmbH) was prepared by growing epitaxially the oxide on a silicon wafer. It was subsequently cut to match a certain orientation. The specimen was then cleaned chemically and mechanically to produce a flat well-defined surface. The roughness was similar to the first TiO2 substrate. Although we do not know the exact surface structure, it is expected to be “more crystalline” than the sputtered TiO2 films. The Nb2O5 substrate was prepared by depositing 150 nm on Corning 7059 glass substrate by physical vapor deposition. The surface also had sub-nanometer average roughness. Aluminum was received in electropolished (25 V anodization polish) and in high-brilliance-rolled form with a natural oxide layer. The first species was prepared by polishing aluminum (99.99) thin strip samples (1 mm in thickness) electrolytically in phosphoric acid/sulfuric acid electrolyte and anodizing them in sulfuric acid electrolyte (200 g/L H2SO4, 20 °C, 4 min), producing an anodic oxide layer of about 2 µm thickness. AFM images over 1 µm2 regions of the untreated surfaces revealed very flat, featureless topographies with a roughness of no more than 1 nm for all substrates. The two differently prepared TiO2 and AlOx substrates were employed here in order to check for a possible influence of the substrate preparation conditions on the resulting structure of the organic phosphate adlayer. The Nb2O5 surfaces are reported by the manufacturer to be nanocrystalline (crystallite size in low-nanometer range). The non-epitax TiO2 and AlOx are largely amorphous. Cleaning of Substrates. All substrates were cleaned in an ultrasonic bath of propan-2-ol for 15 min and subsequently dried in a nitrogen stream. Surfaces were then further treated in an UV cleaner (model 135500, Boekel Ind. Inc., PA) for 30 min. Representative substrates were checked for cleanliness by photoelectron spectroscopy (XPS) and advancing water-contactangle measurements (
