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Highly Oriented, Self-Assembled Alkanephosphate Monolayers on Tantalum(V) Oxide Surfaces Dorothee Brovelli,† Georg Ha¨hner,† Laurence Ruiz,† Rolf Hofer,†,‡ Gerolf Kraus,‡ Adrian Waldner,‡,§ Johanna Schlo¨sser,‡ Peter Oroszlan,‡ Markus Ehrat,‡ and Nicholas D. Spencer*,† Laboratory for Surface Science and Technology, Department of Materials, ETH-Zu¨ rich, CH-8092 Zu¨ rich, Switzerland, and Novartis Pharma AG, CH-4002 Basel, Switzerland Received December 30, 1998. In Final Form: April 7, 1999 Octadecyl phosphoric acid ester has been found to produce oriented, well-ordered monolayers on a flat tantalum(V) oxide surface, via self-assembly from a heptane/propan-2-ol solution. By means of contact angle, optical waveguide lightmode spectroscopy (OWLS), near-edge X-ray absorption fine structure spectroscopy (NEXAFS), and X-ray photoelectron spectroscopy (XPS) measurements, it has been shown that these layers closely resemble those formed by the corresponding thiol-gold system, with respect to packing density, inclination, and order. The system shows promise as an approach to functionalizing oxide surfaces with well-ordered organic monolayers, with potential applications in the fields of biochemical analysis and sensors.
Introduction The development of methods for the generation of selfassembled monolayers (SAMs)1,2 has presented the surface scientist and engineer with a highly flexible approach for the creation of concentrated planes of functionality. While this methodology has tremendous possibilities for applications in such varied areas as biosensors,3 corrosionresistant systems,4 adhesion promotion,5 and so forth, the specific chemistries generally employed have led to certain limitations. The largest classes of SAMs investigated until now have been based either on the interaction of chlorosilanes6 with OH-terminated oxide surfaces or on the adsorption of thiols on gold.1 While the former approach offers great flexibility, it has the disadvantage of frequently producing ill-defined surfaces due to the onset of uncontrolled polymerization reactions. The latter (thiol-gold) approach can produce monolayer films with a high degree of perfection, but the necessity for a gold (or in certain circumstances silver7) surface all but rules it out in many applications, especially those in which optical transmission is a requirement for the system. A smaller number of publications has appeared where alternative chemistries have been employed to coat oxide surfaces with SAMs. These have included hydroxamic,8 * To whom correspondence should be addressed. † ETH-Zu ¨ rich. ‡ Novartis Pharma AG. § Current Address: Novartis Animal Health, Ltd., CH-4002 Basel, Switzerland. (1) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (2) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. Sellers, H.; Ulman, A.; Schnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389. Bishop, A. R.; Nuzzo, R. G. Curr. Opin. Colloid Interface Sci. 1996, 1, 127. Ulman, A. Chem. Rev. 1996, 96, 1533. (3) Hickman, J. J.; Ofer, D.; Laibinis, P. E.; Whitesides, G. M.; Wrighton, M. S. Science 1991, 252, 688. Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wanne, K. J.; Yu, H. Langmuir 1987, 3, 932. (4) Bram, Ch.; Jung, Ch.; Stratmann, M. Fres. J. Anal. Chem. 1997, 358, 108. (5) Maoz, R.; Netzer, L.; Gun, J.; Sasgiv, J. J. Chim. Phys. 1988, 85, 1059. (6) Ulman, A. Adv. Mater. 1990, 2, 573. (7) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559.
carboxylic,9 and phosphonic acids10,11 and, to a very limited extent, phosphoric acids.11 In addition, several papers have appeared using alkyl phosphates and phosphonates in a tails-down configuration, for the purpose of building zirconium phosphate layer structures.12 In this paper we describe, for the first time, a selfassembly technique that employs alkyl phosphoric acid esters to produce dense, highly ordered monolayers in a “tails-up” configuration, on a Ta2O5 surface. Tantalum oxide was chosen because of its high refractive index, which renders it ideal for application in a planar-waveguidebased bioaffinity sensor.13 Upon appropriate ω-functionalization, alkanephosphate-based SAMs have the potential to be used as the interface that anchors active sensing elements or as the basis of passive, biomolecule-resistant regions on the sensor surface. Experimental Section Synthesis of n-Octadecyl Phosphoric Acid Esters. nOctadecyl phosphoric acid ester (ODP) was prepared according to the protocol reported by Okamoto14 (Scheme 1). ODP is a stable, waxy solid and was recrystallized from hot n-hexane. 1H NMR spectra (CDCl3) δ: 0.83-0.93 (CH3, 3H, t), 1.18-1.45 ((CH2)15, 30H, m), 1.62-1.76 (P(O) CH2CH2, 2H, quintet), 3.97-4.13 (P(O) CH2, 2H, quintet). Elemental anal (C, H, N: Leco CHN-900. P via photometry). Calc for C18H39O4P: C, 61.69; H, 11.22; P, 8.84. Found: C, 61.72; H, 11.02; P, 8.82. The C/P ratio estimated from the elemental analysis results is 18.05, in agreement with expectations. Cleaning of Materials and Containers for Substrate Handling. Glass bottles were used for storing the amphiphile (8) Folkers, J. P.; Gorman, C. B.; Laibinis, P. E.; Buchholz, S.; Whitesides, G. M. Langmuir 1995, 11, 813-824. (9) Aronoff, Y. G.; Chen, B.; Lu, G.; Seto, C.; Schwartz, J.; Bernasek, S. L. J. Am. Chem. Soc. 1997, 119, 259. Laibinis, P. E.; Hickman, J. J.; Wrighton, M. S.; Whitesides, G. M. Science 1989, 245, 845. (10) Woodward, J. T.; Ulman, A.; Schwartz, D. K. Langmuir 1996, 12, 3626. Gao, W.; Dickinson, L.; Grozinger, C.; Morin, F. G.; Reven, L. Langmuir 1996, 12, 6429. (11) Maege, I.; Jaehne, E.; Henke, A.; Adler, H.-J. P.; Bram, C.; Jung, C.; Stratmann, M. Macromol. Symp. 1997, 126, 7-24. (12) Lee, H.; Kepley, L. J.; Hong, H. G.; Akhter, S.; Mallouk, T. E. J. Phys. Chem. 1988, 92, 2597-2601. Lee, H.; Hong, H. G.; Mallouk, T. E.; Kepley, L. J. J. Am. Chem. Soc. 1988, 110, 618-620. (13) Duveneck, G. L.; Pawlak, M.; Neuscha¨fer, D.; Ba¨r, E.; Budak, W.; Pieles, U.; Ehrat, M. Sens. Actuators B 1997, 38-39, 88. (14) Okamoto, Y. Bull. Chem. Soc. Jpn. 1985, 58, 3393.
10.1021/la981758n CCC: $18.00 © 1999 American Chemical Society Published on Web 05/22/1999
Letters
Langmuir, Vol. 15, No. 13, 1999 4325 Scheme 1
stock solutions. SAM formation was performed in PTFE containers. Glass bottles and PTFE containers were immersed for 2 h in piranha solution (a 1:2 mixture of 30% H2O2 and 98% H2SO4sCaution, this solution can explode in contact with organic matter!), followed by extensive rinsing in ultrapure water prior to first use. During the investigation, all materials were cleaned with propan-2-ol after use and stored in a class-1000 laminar flow hood. Preparation of Stock Solutions of Amphiphiles. Stock solutions for SAM formation were prepared by dissolving the ODP in a 100:0.4 (v/v) mixture of n-heptane and propan-2-ol at concentrations of 5, 50, and 500 µM, followed by sonication for 10 min. For comparison purposes, a 500 µM solution was also prepared in pure propan-2-ol (ODP was found to be insoluble in pure n-heptane). The resulting solutions were filtered through a 200 nm cellulose nitrate membrane and stored at ambient temperature in glass bottles until use. No deleterious effects were observed upon storage of these solutions for up to several weeks. Substrate Cleaning and Formation of Self-Assembled Monolayers. SAM formation was studied on tantalum(V) oxide films deposited by physical vapor deposition (ion plating) on Corning glass 7059 (16 mm × 16 mm, 150 nm thickness,
