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Positioning and Alignment of Lipid Tubules on Patterned Au Substrates Yue Zhao and Jiyu Fang* AdVanced Materials Processing and Analysis Center and Department of Mechanical, Materials, and Aerospace Engineering, UniVersity of Central Florida, Orlando, Florida 32816 ReceiVed October 14, 2005. In Final Form: December 1, 2005 This paper presents a method for positioning and aligning self-assembled tubules of 1,2-bis(tricosa-10,12-diynoyl)sn-glycero-3-phosphochloline (DC8,9PC) by withdrawing a patterned Au substrate from tubule solution. The patterned Au substrates with alternating bare Au stripes and thiol monolayer stripes are formed by microcontact printing. We find that the lipid tubules selectively adsorb on the bare Au stripes but show no orientation order. By withdrawing the patterned Au substrates at the direction along the stripes from tubule solution, the lipid tubules are found to be aligned along the direction of the Au stripes. The angular distribution and the density of the aligned lipid tubules depend on the withdrawal rates and the adsorption time, respectively. We conclude that forces causing tubule alignment that originate in the surface tension associated with the moving meniscus dominate alignment forces exerted by the patterned Au substrates.
Introduction Self-assembled lipid tubules are hollow cylindrical supramolecular structures, which are formed by rolled-up lipid bilayers.1-3It has been found that a number of synthetic lipids with modified headgroups or acyl chains are able to self-assemble into tubule structures in solutions.4-15 The diameter of these self-assembled lipid tubules spans over a range from 10 nm to 1.0 µm, depending on the nature of lipids and the conditions under which molecular self-assembly occurs. The hollow cylindrical shape, the tunable size distribution, and the crystalline molecular order of the bilayer walls make the tubules potentially valuable as a template for the directed-synthesis of inorganic materials with controlled morphologies,16-20 a substrate for the * To whom correspondence should be addressed: E-mail: jfang@ mail.ucf.edu. (1) Schnur, J. M. Science 1993, 262, 1669. (2) Archibald, D. D.; Mann, S. Nature 1993, 364, 430. (3) Shimizu, T.; Masuda, M.; Minamikawa, H. Chem. ReV. 2005, 105, 1401. (4) Yager, P.; Schoen, P. E. Mol. Cryst. Liq. Cryst. 1984, 106, 371. (5) Frankel, D. A.; O’Brien, D. F. J. Am. Chem. Soc. 1994, 116, 10057. (6) Thomas, B. N.; Corcoran, R. C.; Cotant, C. L.; Lindemann, C. M.; Kirsch, E. J.; Persichini, P. J. J. Am. Chem. Soc. 1998, 120, 12178. (7) Thomas, B. N.; Lindemann, C. M.; Clark, N. A. Phys. ReV. E. 1999, 59, 3040. (8) Song, J.; Cheng, Q.; Kopta, S.; Stevens, R. C. J. Am. Chem. Soc. 2001, 123, 3205. (9) John, G.; Masuda, M.; Okada, Y.; Yase, K.; Shimizu, T. AdV. Mater. 2001, 13, 715. (10) Spector, M. S.; Singh, A.; Messersmith, P. B.; Schnur, J. M. Nano Lett. 2001, 1, 375. (11) Singh, A.; Wong, E. M.; Schnur, J. M. Langmuir 2003, 19, 1888. (12) Lee, S. B.; Koepsel, R.; Stolz, D. B.; Warriner, H. E.; Russell, A. J. J. Am. Chem. Soc. 2004, 126, 13400. (13) Lauf, U.; Fahr, A.; Westesent, K.; Ulrich, A. S. ChemPhysChem. 2004, 5, 1246. (14) Zhao, Y.; Mahajan, N.; Lu, R.; Fang, J. Y. Proc. Natl. Acad. Sci. U. S.A. 2005, 102, 7438. (15) Mishra, B. K.; Garrett, C. C.; Thomas, B. N. J. Am. Chem. Soc. 2005, 127, 4254. (16) Seddon, A. M.; Patel, H. M.; Burkett, S. L.; Mann, S. Angew. Chem., Int. Ed. 2002, 41, 2988. (17) Price, R. R.; Dressick, W. J.; Singh, A. J. Am. Chem. Soc. 2003, 125, 11259. (18) Jung, J. H.; Lee, S. H.; Yoo, J. S.; Yoshida, K.; Shimizu, T.; Shinkai, S. Chem. Eur. J. 2003, 9, 5307. (19) Yang, B.; Kamiya, S.; Yoshida, K.; Shimizu, T. Chem. Commun. 2004, 500. (20) Takahashi, R.; Ishiwatari, T. Chem. Commun. 2004, 1406.
helical crystallization of proteins,21 and an encapsulation for long-term release of chemical and biological agents in soft tissue regeneration and antifouling.22,23 The assembly of lipid tubules into designed architectures on substrates over a large area is critical in developing some of their applications. Orwar and co-workers24,25 reported the assembly and manipulation of lipid tube networks with micropipet and electrofusion techniques. Shimizu and collaborators26 described an approach in which a tubule of synthetic cardanyl-β-Dglucopyranoside could be aligned by microextruding an aqueous dispersion on glass substrates with a needle. Recently, we developed a method that combines microfluidic networks and dewetting to produce two-dimensional ordered arrays of aligned tubules of 1,2-bis(tricosa-10,12-diynoyl)-sn-glycero-3-phosphochloline (DC8,9PC) on glass substrates.27 It was found that the lipid tubules could be pulled into the microfluidic networks by the capillary force-induced channel flow and aligned along the channels. Locascio and co-workers28 reported the self-assembly of lipid tubules within microfluidic channels. The bundles of self-assembled lipid tubules were found to grow along the microchannels. In this paper, we present a method to achieve the alignment and positioning of DC8,9PC tubules on a patterned Au substrate with alternating bare Au stripes and thiol monolayer stripes by withdrawing it from tubule solution. The DC8,9PC lipid tubules are found to selectively adsorb on the bare Au stripes and then are aligned by the moving contact line during the withdrawing process. This method allows a large number of lipid tubules to be positioned and aligned on substrates. (21) Wilson-Kubalek, E. M.; Brown, R. E.; Celia, H.; Milligan, R. A. Proc. Natl. Acad. Sci. U. S.A. 1998, 95, 8040. (22) Schnur, J. M.; Price, R. R.; Rudolph, A. S. J. Controlled Release 1994, 28, 3. (23) Meilander, N. J.; Pasumarthy, M. K.; Kowalczyk, T. H.; Cooper, M. J.; Bellamkonda, R. V. J. Controlled Release 2003, 88, 321. (24) Karlsson, M.; Sott, K.; Cans A-S.; Karlsson, A.; Karlsson, R.; Orwar, O. Langmuir 2001, 17, 6754. (25) Karlsson, M.; Sott, K.; Davidson, M.; Cans, A.-S.; Linderholm, P.; Chiu, D.; Orwar, O. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 11573. (26) Frusawa, H.; Fukagawa, A.; Ikeda, Y.; Araki, J.; Ito, K.; John, G.; Shimizu, T. Angew. Chem., Int. Ed. 2003, 42, 72. (27) Mahajan, N.; Fang, J. Y. Langmuir 2005, 21, 3153. (28) Brazhnik, K. P.; Vreeland, W. N.; Hutchison, J. B.; Kishore, R.; Wells, J.; Helmerson, K.; Locascio, L. E. Langmuir 2005, 21, 10841.
10.1021/la052777h CCC: $33.50 © 2006 American Chemical Society Published on Web 01/07/2006
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Figure 1. (a) Schematic illustration of the microcontact printing technique used to form a patterned Au substrate. (b) Frictional force image of the patterned Au substrate. (c) Schematic illustration of the method used to position and align lipid tubules on the patterned Au substrate. (d) Topographic image of positioned and aligned tubules on the patterned Au substrate.
Experimental Section Lipid tubules used in our experiments were prepared by cooling a 5 mg/mL suspension of 1,2-bis(tricosa-10,12-diynoyl)-sn-glycero3-phosphochloline (DC8,9PC) (Avanti Polar Lipids, Alabaster, AL) in ethanol/water (70:30 v/v) from 60 °C to room temperature at a rate of ∼0.5 °C/min. Polymerization of the DC8,9PC tubule suspension was performed with UV irradiation (254 nm) for 20 min at room temperature. The patterned Au substrates were formed with microcontact printing technique. 1-Dodecanethiol (DDT) (Aldrich) was dissolved in ethanol with a concentration of 1 mM. A poly(dimethylsiloxane) (PDMS) stamp having parallel channels was wetted with 1 mM DDT ethanol solution and then brought to contact with an Au(111) substrate (Molecular Imaging) for forming a pattern composed of alternating bare Au stripes and DDT monolayer stripes. The channels of the PDMS stamp are 700 nm high and 2 µm or 10 µm wide. The separation of the channels is 7 or 15 µm. After 1 min contact with the surface, the stamp was carefully removed, and then the sample was rinsed thoroughly with ethanol and dried in a stream of nitrogen. Water contact angle is about 38° on bare Au(111) substrate and about 110° on DDT monolayer at room temperature. The patterned Au substrate was connected to a dipping machine of Langmuir-Blodgett trough (NIMA Technology). The samples were withdrawn vertically from the tubule solution and then dried in air. A polarizing optical microscope (BX 40 Olympus) with a digital camera (Olympus C2020 Zoom) was used to observe the adsorption and alignment of the lipid tubules on the patterned Au substrates. Atomic force microscope (AFM) (Dimension 3100, Digital Instruments) was used to characterize the patterned Au substrates and image the aligned lipid tubules. Silicon nitride cantilevers with a normal spring constant of 0.2 N/m and 30 N/m were used in contact and tapping modes, respectively. The size of the cantilever tips (radius of curvature) is about 20 nm according to the manufacturer. AFM measurements were performed in contact and tapping modes at a scan rate of 0.5 Hz in air under ambient conditions.
thiol monolayer stripes with the microcontact printing technique. A poly(dimethylsiloxane) (PDMS) stamp having parallel channels was wet with 1 mM 1-dodecanethiol (DDT) ethanol solution and then brought to contact with an Au(111) substrate for forming a pattern composed of alternating bare Au stripes and DDT monolayer stripes. Atomic force microscopy (AFM) in the frictional force mode was used to characterize the patterned Au substrate. In the frictional force mode, the lateral deflection of the AFM cantilever was measured as the tip was scanned. It has been demonstrated that the lateral deflection of the AFM cantilever is very sensitive to the deposition of self-assembled organic monolayers.29-30 Figure 1b is a frictional force image of the patterned Au substrate. As can be seen, the exposed Au stripes with a width of ∼2 µm appear brighter than the methyl-terminated DDT monolayer stripes with a width of ∼7 µm because they have higher friction force. We used the patterned Au substrate for guiding the adsorption of DC8,9PC tubules. The patterned Au substrate was immersed horizontally in tubule solution and then withdrawn at the direction along the stripes (Figure 1c). The position of the Au stripes on the patterned surface can be easily traced by observing the selective adsorption of impurities (small aggregates of lipid molecules). It is clear from Figure 1d that the tubules selectively adsorb on the hydrophilic Au stripes and aligned along the stripe direction, while the hydrophobic DDT monolayer stripes prevent the tubule adsorption. It is known that the tubules terminated by the phosphochloline groups are hydrophilic. The selective adsorption is likely due to the hydrophilic interactions between the Au stripes and the tubules. The AFM measurements also show that the positioned and aligned lipid tubules have smooth external surfaces and a semicircular profile whose height of about 458 nm is consistent with the
Results and Discussion Figure 1a shows a schematic illustration of the preparation of a patterned Au substrate with alternating bare Au stripes and
(29) Wilbur, J. L.; Biebuyck, H.; MacDonald, A.; Whitesides, G. M. Langmuir 1995, 11, 825. (30) Fang, J. Y.; Knobler, C. M. J. Phys. Chem. 1995, 99, 10426.
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Figure 2. (a) Optical microscope image of aligned lipid tubules on the patterned Au substrate. Histograms of the angles of the aligned lipid tubules away from the withdrawal direction with the fitted Gaussian distribution at the withdrawal rate of 1 mm/min (b) and 10 mm/min (c).
Figure 4. Optical microscope images of aligned lipid tubules on the patterned Au substrate by first (a) and second (b) withdrawal processes at a constant withdrawal rate of 10 mm/min. (c) Plots of the density of the aligned tubules vs withdrawal times at different withdrawal rates. Figure 3. Optical microscope images of aligned lipid tubules on the patterned Au substrate at a constant withdrawal rate of 10 mm/ min after 1 h (a) and 12 h adsorption (b). Plots of the density of the aligned tubules vs adsorption time.
expected cylinder, suggesting that there is no significant deformation of the aligned lipid tubules. Examination of the aligned tubules with an optical microscope shows that the lipid tubules are aligned over a large length scale
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Figure 5. Optical microscopy images of the lipid tubules absorbed on the patterned Au substrates. The width of the Au stripes is 2 µm (a) and 10 µm (b). These images were taken in tubule solution.
on the patterned Au substrate (Figure 2a). Again, the position of the bare Au stripes on the patterned surface can be identified from the selective adsorption of impurities from tubule solution in the optical microscopy image. Several experiments were carried out for understanding the factors effecting the alignment of the lipid tubules on the patterned Au substrate. First, we find that the degree of the alignment of the lipid tubules depends on the withdrawal rate. Figure 2b shows the angular distribution of the aligned tubules at the withdrawal rate of 1 mm/min; 58% of the tubules are aligned within (10° of the withdrawal direction. When the withdrawal rate increases to 10 mm/min, the angular distribution of the aligned tubules becomes substantially narrower (Figure 2c). More than 90% of the tubules are aligned within (10° of the withdrawal direction. Second, we find that the number of the aligned tubules at a constant withdrawal rate of 10 mm/ min increases as the adsorption time increases (Figure 3a,b). As can be seen from Figure 3c, the density of the aligned tubules increases quickly with the adsorption time and saturates with about 330 tubules per 500 µm × 500 µm after 8 h adsorption. It is clear that a large number of lipid tubules can be aligned at a high density on patterned Au substrate. Third, we observe that the aligned tubules can be gradually added on the patterned Au substrate by multiple withdrawals from the tubule solution. In this process, the tubules aligned on the patterned Au substrate by the first withdrawal (Figure 4a) were dried in air at room temperature for 5 min and then immersed in the tubule solution. The tubules that are aligned on the patterned Au substrate by the second withdrawal are circled in Figure 4b. The aligned tubules by the first withdrawal are stable during the second withdrawal
Figure 6. (a) Optical microscopy image and (b) topographic AFM image of lipid tubules on the patterned Au substrate. The withdrawing direction is perpendicular to the direction of the stripes.
process. Plots of the density of the aligned tubules vs withdrawal times at different withdrawal rates are shown in Figure 4c. We should point out that the patterned Au substrate alone does not provide good control of the tubule orientation. Figure 5a shows an optical microscopy image of the adsorption of lipid tubules on the patterned Au substrate. This image was taken in tubule solution. Some long lipid tubules tend to orient along the Au stripe direction due to the minimization of the lateral and rotational translations by the patterned surface. However, they show a large angular distribution with respect to the direction of the Au stripes. There is no orientation order observed for the short lipid tubules. This becomes clearer when the width of the Au stripes increases. As can be seen from Figure 5b, the short lipid tubules are preferentially adsorbed on the bare Au stripes with a width of about 10 µm, but the orientation of the adsorbed short tubules is random. The withdrawal-induced alignment of the lipid tubules on the patterned Au substrate can be explained in the frame of the moving contact line. When the patterned Au substrate is immersed into tubule solution, a meniscus of the solution is formed due to the surface tension (Figure 1b). The contact line moves when the
Positioning and Alignment of Lipid Tubules
patterned Au substrate is withdrawn from tubule solution. The lipid tubules adsorbed on the bare Au stripes are expected to be aligned by the moving contact line (Figure 1c). Due to the surface tension effects, the aligned tubules should be perpendicular to the contact line, consistent with what we observed here. The alignment of linear macromolecules including DNA,31-32 peptide tapes,33 and protein tubules 34 by moving contact lines and flows have been reported in the literature. To further test the effect of the withdrawing direction on the alignment of lipid tubules on the patterned Au substrate, we withdraw the patterned Au substrate at the direction which is perpendicular to the direction of the Au stripes. The moving contact line during the withdrawing process generates sufficient force to reorient the tubules (Figure 6a). (31) Bensimon, A.; Simon, A.; Chiffaudel, A.; Croquett, V.; Heslot, F.; Bensimon, D. Science 1994, 265, 2096. (32) Deng, Z.; Mao, C. Nano Lett. 2003, 3, 1545. (33) Whitehouse, C.; Fang, J. Y.; Aggeli, A.; Bell, M.; Brydson, R. M.; Fishwick, C. W. G.; Knobler, C. M.; Henderson, J.; Owens, R. W.; Thomson, N. H.; Boden, N. Angew. Chem., Int. Ed. 2005, 44, 1965. (34) Hirst, L. S.; Parker, E. R.; Abu-Samah, Z.; Li, Y.; Pynn, R.; MacDonald, N. C.; Safinya, C. R. Langmuir 2005, 21, 3910.
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From the AFM image shown in Figure 6b, the reoriented lipid tubules are across the hydrophobic monolayer stripes and aligned at an angle of 45°∼80° with respect to the stripe direction. Here, the helical markings, which represent the edges of the lipid bilayer, are visible on the external surface of some aligned tubules. In conclusion, we report a method for positioning and aligning the self-assembled lipid tubules by withdrawing a patterned Au substrate with alternating bare Au stripes and thiol monolayer stripes from tubule solution. We find that the tubules selectively adsorb on the bare Au stripes. The moving contact line during the withdrawing process is responsible for the alignment of the lipid tubules. The angular distribution and the density of the aligned tubules depend on the withdrawal rate and the adsorption time, respectively. This method allows us to rapidly align a large number of lipid tubules on substrates, which is critical in developing some of the applications of the lipid tubules. Acknowledgment. This work was supported by University of Central Florida and National Science Foundation. LA052777H
