Langmuir 2005, 21, 2971-2980
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Synthesis and Self-Assembly of Galactose-Terminated Alkanethiols and Their Ability to Resist Proteins Markus Hederos,† Peter Konradsson,† and Bo Liedberg*,‡ Divisions of Chemistry and Molecular Physics, IFM, Linko¨ ping University, SE-581 83 Linko¨ ping, Sweden Received November 15, 2004. In Final Form: January 14, 2005 The synthesis of two galactose-terminated alkanethiols with the structural formula X-OC2H5NHCO(CH2)15SH (X ) 2,3,4,6-tetra-O-methyl-β-D-Gal or β-D-Gal) is described. Single-component and mixed selfassembled monolayers (SAMs) of the methylated and nonmethylated compounds were prepared on gold and subsequently characterized with ellipsometry, contact angle goniometry, and infrared reflectionabsorption spectroscopy. Studies of the irreversible protein adsorption onto the SAMs using ex-situ ellipsometry revealed very low levels of fibrinogen and lysozyme adsorption onto mixed SAMs displaying advancing water contact angles between 24° and 45° and below 45°, respectively. A monomethylated compound (X ) 6-O-methyl-β-D-Gal) was also synthesized and assembled on gold. This particular compound was found to possess wettability properties corresponding to the low adsorption regime of the mixed SAMs, and the results from the same set of fibrinogen and lysozyme adsorption experiments showed very low levels of protein adsorption. Our findings suggest that the protein rejecting properties rely on a fine balance between the surface energy and/or hydrogen bond donating/accepting properties of the SAM surface.
Introduction Thin organic layers on solid surfaces have been extensively employed to mimic structures, interfacial phenomena, and processes.1 One way to prepare such layers is to utilize spontaneous chemisorption of long-chain organosulfur compounds from dilute solutions onto noble metals like gold.2 The advantage of using gold as the substrate is that a single molecular layer is formed, a so-called selfassembled monolayer (SAM). The strong pinning to gold via the sulfur headgroup not only contributes to the stability and organization but also to the chemical/ biological variability and utility of such SAMs. Thereby, organosulfur SAMs can find applications in chemically demanding environments, as well as for studies of complex interactions occurring at biological interfaces.3 Nonspecific adsorption of biomolecules onto solid surfaces is a critical issue that must be carefully addressed in a range of technologically relevant areas, for example, in biomaterials development,4,5 tissue engineering,6-8 drug delivery,9,10 and biosensing.11,12 The use of high-molecularweight poly(ethylene glycol), PEG, to obtain protein- and * Author to whom correspondence should be addressed. Phone +46-13-281877. Fax: +46-13-288969. E-mail:
[email protected]. † Division of Chemistry. ‡ Division of Molecular Physiscs. (1) Ulman, A. Chem. Rev. 1996, 96, 1533-1554. (2) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 44814483. (3) Mrksich, M. Curr. Opin. Chem. Biol. 2002, 6, 794-797 and references therein. (4) Tirell, M.; Kokkoli, E.; Biesalski, M. Surf. Sci. 2002, 500, 61-83. (5) Kasemo, B. Surf. Sci. 2002, 500, 656-677. (6) Niklason, L. E. Science 1999, 286, 1493-1494. (7) Tampieri, A.; Celotti, G.; Landi, E.; Sandri, M.; Roveri, N.; Falini, G. J. Biomed. Mater. Res. 2003, 67A, 618-625. (8) Lee, L. K.; Roth, C. M. Curr. Opin. Biotechnol. 2003, 14, 505511. (9) Santini, J. T.; Cima, M. J.; Langer, R. Nature 1999, 397, 335338. (10) Stayton, P. S. Trends Biotechnol. 2003, 21, 465-467. (11) Lo¨fås, S.; Johnsson, B. J. Chem. Soc., Chem. Commun. 1990, 21, 1526-1528. (12) Holland, N. B.; Qiu, Y.; Ruegsegger, M.; Marchant, R. E. Nature 1998, 392, 799-801.
cell-repulsive properties is well known.13,14 Pale-Grosdemange et al. exploited this idea further by introducing SAMs presenting short oligomers of ethylene glycol (EGn, n ) 3-6) on gold.15 These compounds have proven to be one of the most effective monolayer candidates in the search of surfaces resisting nonspecific adsorption of proteins. It should be emphasized, however, that the EG tail is not unique in its inertness. SAMs on gold terminated with a variety of functional groups have been used to study the adsorption of proteins.16-22 Whitesides and co-workers found that SAMs terminated with tripropylene sulfoxide groups were equally protein resistant as SAMs bearing EG3 tails.20 Moreover, SAMs on gold presenting different carbohydrate structures on the surface have been explored with good results. Prime et al. showed that a maltoseterminated SAM was protein resistant,16 and Luk et al. reported that SAMs of alkanethiolates presenting mannitol groups were resistant to both proteins and cells.22 Thus, identification, design, and synthesis of alternative functional tail groups that resist protein adsorption are important research topics as there is no universal surface chemistry that renders a surface inert. It is important to stress, also, that one should generally avoid making too far-reaching predictions about the overall protein resistance of a SAM surface from single component protein adsorption experiments. More complex activation-ag(13) McPherson, T.; Kidane, A.; Szleifer, I.; Park, K. Langmuir 1998, 14, 4, 176-186. (14) Desai, N. P.; Hubbell, J. A. Biomaterials 1991, 12, 144-153. (15) Pale-Grosdemange, C.; Simon, E. S.; Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1991, 113, 12-20. (16) Prime, K. L.; Whitesides, G. M. Science 1991, 252, 1164-1167. (17) Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1993, 115, 5, 10714-10721. (18) Holmlin, R. E.; Chen, X.; Chapman, R. G.; Takayama, S.; Whitesides, G. M. Langmuir 2001, 17, 2841-2850. (19) Ostuni, E.; Chapman, R. G.; Liang, M. N.; Meluleni, G.; Pier, G.; Ingber, D. E.; Whitesides, G. M. Langmuir 2001, 17, 6336-6343. (20) Deng, L.; Mrksich, M.; Whitesides, G. M. J. Am. Chem. Soc. 1996, 118, 5136-5137. (21) Chapman, R. G.; Ostuni, E.; Yan, L.; Whitesides, G. M. Langmuir 2000, 16, 6927-6936. (22) Luk, Y.-Y.; Kato, M.; Mrksich, M. Langmuir 2000, 16, 96049608.
10.1021/la047203b CCC: $30.25 © 2005 American Chemical Society Published on Web 03/02/2005
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Hederos et al. Table 1. Ellipsometric Thickness d, Advancing (θa) and Receding (θr) Contact Angles of Water (w) and the Hysteresis of the Mixed SAMs of 1 and 2 on Gold χsol2 a
d (Å)b
θa,w (deg)b
θr,w (deg)b
hysteresis (deg)c
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
28.0 ( 0.3 28.0 ( 0.6 28.2 ( 0.7 28.1 ( 0.6 28.6 ( 0.8 28.1 ( 0.6 28.4 ( 0.7 28.0 ( 0.5 28.7 ( 0.7 28.6 ( 0.8 28.4 ( 0.7
