pubs.acs.org/Langmuir © 2009 American Chemical Society
Spontaneous Adsorption on a Hydrophobic Surface Governed by Hydrogen Bonding Fuquan Dang,*,† Takeshi Hasegawa,‡ Vasudevanpillai Biju,† Mitsuru Ishikawa,† Noritada Kaji,§ Takao Yasui,§ and Yoshinobu Baba†,§ † Health Technology Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Hayashi-cho 2217-14, Takamatsu 761-0395, Japan, ‡Department of Chemistry, Tokyo Institute of Technology, Ookayama 2-12-1, Meguro-ku, Tokyo 152-855, Japan, and §Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
Received March 10, 2009. Revised Manuscript Received May 3, 2009 Spontaneous adsorption from solution onto solid surface is a common phenomenon in nature, but the force that governs adsorption is still a matter of considerable debate.1,2 We found that surfactants and cellulose adsorb from solution onto a poly(methyl methacrylate) (PMMA) surface in an ordered and cooperative way governed by hydrogen bonding. The glucose rings of n-dodecyl-β-D-maltoside (DDM) and hydroxyethylcellulose (HEC) stand perpendicular to the surface, H-bond to the surface COOMe groups with their CdO and Me-O bonds parallel to the surface, and form a tight monolayer. The non-H-bonded COOMe groups orient their CdO bonds perpendicular to the surface. In contrast, the glucose rings of hydrophobically modified hydroxyethylcellulose (HMHEC) lie flat with the side chains perpendicular to the surface and H-bond to the perpendicular-oriented CdO groups. The non-H-bonded COOMe groups orient their CdO bonds parallel but Me-O bonds near-perpendicular to the surface for stabilizing HMHEC. The current work provides a detailed picture of how surface-active molecules interact with a solid surface and selfassemble into greatly different architectures.
Introduction Spontaneous adsorption from solution onto solid surface has been extensively studied for decades because of its significant impact on our daily life in areas, such as food and food science, cleaning and detergency, pharmaceuticals and cosmetics, mineral and oil industries, separation and materials sciences, and nanotechnology.1-6 It is generally assumed that spontaneous adsorption on a solid surface is governed by noncovalent interactions. However, our understanding of the chemistry of such interfacial and surface phenomena at the molecular level such as the governing force and the molecular arrangement on a solid surface is still poorly developed.1,2,5-7 Poly(methyl methacrylate) (PMMA) is a widely used material in the fabrication of disposable microfluidic chips.8-10 However, *To whom correspondence should be addressed. E-mail: fuquan-dang@ aist.go.jp. Phone: +81-87-869-4104. Fax: +81-87-869-4113. (1) Holmberg, K.; Jonsson, B.; Kronberg, B.; Lindman, B. Surfactants and Polymers in Aqueous Solution; Wiley: West Sussex, 2002; pp 357-387. (2) Norde, W. Driving forces for protein adsorption at solid surfaces. In Biopolymers at Interfaces; Malmsten, M., Ed.; Marcel Dekker: New York, 2003; Chapter 2. (3) Constantine, C. A.; Mello, S. V.; Dupont, A.; Cao, X.; Santos, D.Jr.; Oliveira, O. N.Jr.; Strixino, F. T.; Pereira, E. C.; Cheng, T.-C.; Defrank, J. J; Leblanc, R. M. J. Am. Chem. Soc. 2003, 125, 1805–1809. (4) Zhao, B.; Haasch, R. T.; MacLaren, S. J. Am. Chem. Soc. 2004, 126, 6124–6134. (5) Lehn, J.-M. Science 2002, 295, 2400–2403. (6) Barth, J. V.; Costantini, G.; Kern, K. Nature 2005, 437, 671–679. (7) Zhang, R.; Somasundaran, P. Adv. Colloid Interface Sci. 2006, 123-126, 213–229. (8) Soper, S. A.; Ford, S. M.; Qi, S.; McCarley, R. L.; Kelly, K.; Murphy, M. C. Anal. Chem. 2000, 72, 643-651A. (9) Muck, A.Jr.; Wang, J.; Jacobs, M.; Chen, G.; Chatrathi, M. P.; Jurka, V.; Vyborny, Z.; Spillman, S. D.; Sridharan, G.; Schoning, M. J. Anal. Chem. 2004, 76, 2290–2297. (10) Dang, F.; Tabata, O.; Kurokawa, M.; Ewis, A. A.; Zhang, L.; Yamaoka, Y.; Shinohara, S.; Shinohara, Y.; Ishikawa, M.; Baba, Y. Anal. Chem. 2005, 77, 2140–2146.
9296 DOI: 10.1021/la900850u
its surface is highly hydrophobic and exhibits moderate electroosmotic mobility (EOF) in aqueous solution, mostly due to nonesterified carboxyl groups. Spontaneous adsorption of various analytes such as organic dyes, DNA, sugars, and proteins is common on PMMA surfaces, resulting in poor performance of PMMA chips. Such adsorption is often passivated by a coating layer of surfactants and water-soluble polymers physically preadsorbed on the channel wall, i.e., dynamic coating, a well-used method for surface modification in separation science.10-13 The effectiveness and convenience of dynamic coating are verified for various molecules, but not proteins because of their high surface affinity. Without a detailed understanding of how molecules interact with a solid surface, attempts to address the adsorption of proteins are of limited success. To better understand the adsorption mechanism on a hydrophobic surface, we systematically characterized spontaneous adsorption of surfactants and cellulose from aqueous solution onto a PMMA surface using atomic force microscopy (AFM), microchip electrophoresis ( μ-CE), and infrared external-reflection (IR-ER) spectroscopy coupled with water contact angle and EOF measurements. We found that surfactants and cellulose adsorb on PMMA surfaces in an ordered and cooperative way governed by H-bonding. The current work significantly advances our understanding of spontaneous adsorption at a solid surface, offering controlled bottom-up fabrication of self-assembled molecular architectures for a broad range of applications such as microfluidic and microelectronic devices. (11) Belder, D.; Ludwig, M. Electrophoresis 2003, 24, 3595–3606. (12) Dang, F.; Kakehi, K.; Cheng, J.; Tabata, O.; Kurokawa, M.; Nakajima, K.; Ishikawa, M.; Baba, Y. Anal. Chem. 2006, 78, 1452–1458. (13) Lucy, C. A.; MacDonald, A. M.; Gulcev, M. D. J. Chromatogr. A 2008, 1184, 81–105.
Published on Web 05/21/2009
Langmuir 2009, 25(16), 9296–9301
Dang et al.
Article
Materials and Methods Materials. Hydrophobically modified hydroxyethylcellulose (HMHEC) was synthesized from hydroxyethylcellulose (HEC) (Mw, 100 000) and glycidyl hexadecyl ether, purified, and characterized as described.14 The degree of substitution (DS) of the CH2CHOHCH2-n-C16H33 group in HMHEC is 1 mol %. N-linked glycans from human R-1-acid glycoprotein (AGP) were released with peptide N-glycosidase (ProZyme, San Leandro, CA) and labeled with 8-aminopyrene-1,3,6-trisulfonate (APTS) (Molecular Probes, Eugene, OR) as described previously.15 All chemicals and reagents were obtained from Sigma (St. Louis, MO), unless specified otherwise. Each of n-dodecyltrimethylammonium chloride (DTAC, g98% purity) (Wako, Tokyo, Japan), n-dodecyl-β-D-maltoside (DDM, g98% purity), HEC, and HMHEC (1 g apiece, Figure 1 a) was mixed with a 20 mM phosphate buffer (pH 6.98, 50 mL), and then the mixture was homogenized by stirring at room temperature (26 C) for 1 h. Each solution thus prepared was used as a stock solution (2 wt %). A sheet of PMMA (Nittou Jushi Kogyou Co., Ltd., Tokyo, Japan) (1.0 mm 15 mm 40 mm) was incubated in the DTAC, DDM, HEC, and HMHEC solutions (0.25-1.0 wt %) in a 20 mM phosphate buffer (pH 6.98) for 0.5 h at room temperature (26 C) for spontaneous adsorption. Following the spontaneous adsorption, the PMMA surfaces were rinsed copiously with water, then dried with N2 gas, and finally characterized by measurement of AFM, IR-ER, and contact angle. Because both sides of the PMMA sheets as received were covered with protective thin films, which were peeled off before use, no further procedure was employed to clean the PMMA surfaces. AFM. The AFM images of PMMA surfaces were acquired in tapping mode using a MFP-3D AFM (Asylum Research, Santa Barbara, CA) in air at ambient temperature (26 C). The measurements were performed at a scan frequency of 1 Hz using an ultrasharp (
