Langmuir 2006, 22, 6727-6729
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Compression Induced Helical Nanotubes in a Spreading Film of a Bolaamphiphile at the Air/Water Interface Peng Gao and Minghua Liu* Beijing National Laboratory for Molecular Sciences (BNLMS), CAS Key Laboratory of Colloid, Interface and Chemical Thermodynamics, Institute of Chemistry, The Chinese Academy of Sciences, Beijing, 100080 P. R. China ReceiVed February 20, 2006. In Final Form: May 27, 2006 Inspired by the elegant helical structures endowed by mother nature, we designed an L-glutamic acid terminated bolaamphiphile and obtained helical nanotubes through the manipulation on the two-dimensional Langmuir films at the air/water interface. It has been found that on the subphase with a pH value lower than 3, stable monolayers with plateau regions were obtained for the bolaamphiphile. Although a flat and uniform morphology was observed for the film deposited at a surface pressure below the plateau region, helical nanotube structures were obtained when the film was compressed over the plateau region. It was suggested that the compression of the monolayer at the air/water interface caused the one end of the bolaamphiphile to leave from the water surface and form an intermediate monolayer in which one end group attached on the water surface and the other extruded in the air. Such an intermediate monolayer subsequently rolled into a helical structure due to the chiral nature of the L-glutamic acid headgroup.
The helical structure is one of the most unique structures found in nature and plays an important role both in biological and material sciences.1 Many biologically important substances possess helical structures. For example, DNA has a double helical structure, which controls the heredity of the biospecies.2 Many proteins and polypeptides have R-helical conformations which are also important in transferring information. Helical structures are also important in material sciences3 and are suggested to show many specific properties.4 Inspired by these elegant structures and functions, many efforts have been devoted to the synthesis and/or assembly of helical structures, and various helical structures have been obtained from the assembly of small molecules,5 polymers,6 amphiphiles7 and templates.8 The helical structure is essentially a three-dimensional entity, and thus, various * To whom correspondence should be addressed. E-mail: liumh@ iccas.ac.cn. Tel: +86-10-82612655. Fax: +86-10-62569564. (1) (a) Braun, E.; Eichen, Y.; Sivan, U.; Ben-Yoseph, G. Nature 1998, 391, 775. (b) Fink, H. W.; Schoonenberger, C. Nature 1999, 398, 407. (c) Kasemo, B. Surf. Sci. 2002, 500, 656. (d) Zhang, G.; Yan, X.; Hou, X.; Lu, G.; Yang, B.; Wu, L.; Shen, J. Langmuir 2003, 19, 9850. (2) Watson, J. D.; Crick, F. H. C. Nature 1953, 171, 737. (b) Macgregor Jr., R. B.; Poon, G. M. K. Comput. Biol. Chem. 2003, 27, 461. (3) (a) Tian, Z. R.; Voigt, J. A.; Liu, J.; Mckenzie, B.; Mcdermott, M. J. Am. Chem. Soc. 2002, 124, 12954. (b) Song, J.; Cheng, Q.; Kopta, S.; Stevens, R. C. J. Am. Chem. Soc. 2001, 123, 3205. (4) (a) Tachibana, H.; Kishida, H.; Tokura, Y. Langmuir 2001, 17, 437. (b) Ikeda, S.; Nishinari, K. J. Agric. Food Chem. 2001, 49, 4436. (5) (a) Cornelissen, J.; Fischer, M.; Sommerdijk, N.; Nolte, R. J. M. Science 1998, 280, 1427. (b) Engelkamp, H.; Middelbeek, S.; Nolte, R. J. M. Science 1999, 284, 785. (c) Giorgi, T.; Lena, S.; Mariani, P.; Cremonini, M. A.; Masiero, S.; Pieraccini, S.; Rabe, J. P.; Samory´, P.; Spada G. P.; Gottarelli, G. J. Am. Chem. Soc. 2003, 125, 14741. (6) (a) Kim, J.; Kumar, J.; Kim, D. Y.; AdV. Mater. 2003, 15, 2005. (b) Yashima, E. Anal. Sci. 2002, 18, 3. (c) Schenning, A. P. H. J.; Jonkheijm, P.; Peeters, E.; Meijer, E. W. J. Am. Chem. Soc. 2001, 123, 409. (d) Cornelissen, J. L. M.; Rowan, A. E.; Nolte, R. J. M.; Sommerdijk, N. A. J. M. Chem. ReV. 2001, 101, 4039. (e) Ho, R.-M.; Chiang, Y.-W.; Tsai, C.-C.; Lin, C.-C.; Ko, B.-T.; Huang, B.-H. J. Am. Chem. Soc. 2004, 126, 2704. (f) Huang, X.; Liu, M. H. Chem. Commun. 2003, 66. (g) Li, B. S.; Cheuk, K. K. L.; Ling, L.; Chen, J.; Xiao, X.; Bai, C.; Tang, B. Z. Macromolecules 2003, 36, 77. (7) (a) Fuhrhop, J.-H.; Helfrich, W. Chem. ReV. 1993, 93, 1565. (b) Maison, W.; Arce, E.; Renold, P.; Kennedy, R. J.; Kemp, D. S. J. Am. Chem. Soc. 2001, 123, 10245. (c) Garoff, R. A.; Litzinger, E. A.; Connor, R. E.; Fishman, I.; Armitage, B. A. Langmuir 2002, 18, 6330. (d) Shimizu, T.; Masuda, M.; Minamikawa, H. Chem. ReV. 2005, 105, 1401-1444. (8) (a) Sone, E. D.; Zubarev, E. R.; Stupp, S. I. Angew. Chem. Int. Ed. 2002, 41, 1075. (b) Jung, J. H.; Kobayashi, H.; van Bommel, K. J. C.; Shinkai, S.; Shimizu, T. Chem. Mater. 2002, 14, 1445. (c) Jung, J. H.; Shinkai, S. Top. Curr. Chem. 2004, 248, 223-260.
helical structures are mainly obtained in the three-dimensional media such as solutions or organogels.7d Here, we report the formation of a helical nanotube through a two-dimensional Langmuir technique. Langmuir as well as Langmuir-Blodgett techniques provide an effective method to assemble molecules in a two-dimensional way.9 In the long history of the investigations on the Langmuir and Langmuir-Blodgett techniques, it is the central topic to obtain a uniform and pinhole free film.10 However, recently, the technique appeared to be effective also in fabricating nanostructures such as nanochannels,11 nanopatterings,12-14 and nanotubes,15 which has aroused many interests. In the investigation on the interfacial behavior and assembly of a series of bolaamphiphiles, we have found that the bolaamphiphiles are relatively easier to form interesting nanostructures at the air/ water interface.16 In this paper, we extended our work to the interfacial assembly of a chiral bolaamphiphilic N,N′-eicosanedioyl-di-L-glutamic acid (EDGA)17 and obtained the helical nanotubes through the compression of the spreading films in a two-dimensional air/water interface. The compound was spread from a mixed EtOH/chloroform (volume ratio in 2/8 or 1/9) or DMSO/CHCl3 (1/9) solution (ca. 2 × 10-4 M) onto the aqueous subphase with different pH values. The pH values of the subphases were adjusted by adding hydrochloric acid. Figure 1 shows the π-A isotherms of the EDGA spreading films on the subphases with different pH values. At pH values (9) Gaines, G. L., Jr. Insoluble monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966. (10) (a) Roberts, G. G. Langmuir-Blodgett Films; Plenum Press: New York, 1990. (b) Ulman, A. An introduction to ultrathin Organic Films, from Langmuir to Self-Assembly; Academic Press: Boston, MA, 1991. (11) (a) Gleiche, M.; Chi, L. F.; Fuchs, H. Nature 2000, 403, 173-175. (b) Lu, N.; Chen, X.; Molenda, D.; Naber, A.; Fuchs, H.; Talapin, D. V.; Weller, H.; Muller, J.; Lupton, J. M.; Feldmann, J.; Rogach, A.; Chi, L. Nano Lett. 2004, 4, 885. (12) Pignataro, D.; Sardone, L.; Marletta, G. Langmuir 2003, 19, 5912. (13) Moraille, P.; Badia, A. Langmuir 2002, 18, 4414. (b) Moraille, P.; Badia, A. Angew. Chem. Int. Ed. 2002, 41, 4303. (c) Moraille, P.; Badia, A. Langmuir 2003, 19, 8041. (14) Chen, P. L.; Gao, P.; Zhan, C.; Liu, M. H. ChemPhysChem 2005, 6, 1108. (15) Guo L., Wu Z. K., Liang Y. Q. Chem. Commun. 2004, 1664. (16) (a) Lu, Q., Liu, Y. H., Li, L.; Liu, M. H. Langmuir 2003, 19 (2), 285. (b) Jiao, T. F.; Liu, M. H. J. Phys. Chem. B 2005, 109, 2532. (17) Zhan, C. L.; Gao, P.; Liu, M. H. Chem. Commun. 2005, 462.
10.1021/la0604836 CCC: $33.50 © 2006 American Chemical Society Published on Web 06/30/2006
6728 Langmuir, Vol. 22, No. 16, 2006
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Figure 1. Surface pressure-area isotherms of EDGA on aqueous subphase with different pH values.
of 6 and 7, no surface pressure was detected. When the pH value of the subphase was decreased to 4 and 5, surface pressures were detected at a small molecular area, and the isotherms could be recorded. When the pH value of the subphase was decreased to 1∼3, almost the same isotherms were obtained. In these isotherms, the onset of the surface pressure appeared at a molecular area of 1.5 nm2/molecule. Upon compression, an inflection point was observed at about 8 mN/m in the isotherm. After the inflection point, the surface pressure decreased slightly and increased again at around 0.5 nm2/molecule. It is well-known that the pKa values for the glutamic acid were pK1 ) 2.13 and pK2 ) 4.32, respectively. These data indicated that at a higher pH value of 6-7 EDGA may be deprotonated and could be soluble in the subphase. Therefore, we could not obtain the π-A isotherm. On the subphase of pH 1-3, the compound was kept in a carboxylic acid state and formed the film on water surface. In the medium pH value, the compound was partially deprotoned and smaller molecular areas were obtained. Both the spreading films before and after the inflection point were deposited onto solid substrates by a vertical up-taking speed of 2 mm/min. Figure 2 shows some of the AFM pictures of the films transferred from subphase of pH 3 before and after inflection point with different spreading solvents. At 5 mN/m below the surface pressure of the inflection, a flat and uniform film was obtained (Figure 2a). Interestingly, many spokewise fibrous structures were observed for the films transferred at a surface pressure above the inflection point (Figure 2b). An enlargement of the AFM pictures showed that these nanostructures were helically orientated in one-direction (Figure 2c,d). In addition, we have observed that these helical structures were gathered at different places. A detailed analysis on these helical structures revealed that they were nanotubes with a width of 30-70 nm, and the lengths exceeded several micrometers. In repeated experiments, helical structures were always obtained for the films spread on the subphase with pH ) 1-3. When the pH of the subphase was larger than 4, no helical structures could be obtained. On the other hand, the spreading solvent could affect the morphologies although no difference was observed in the π-A isotherms. Table 1 listed some parameters of nanotubes formed from different spreading solvents. It is clear that although nearly the same pitch was observed for all of the nanotubes the diameter of the nanotubes from DMSO/CHCl3 could be twice of the nanotubes from the other solvents. However, the diameter of the naotubes from ethanol containing solvents was very similar. It seemed that very small amount of ethanol could take part in the monolayer formation through the H-bond with EDGA. During the compression, such slight amount of ethanol could probably
Figure 2. AFM images of one-layer EDGA LB films deposited on freshly cleaved mica from the subphase of pH 3 at different surface pressures (a) 5 mN/m before inflection point (b-d) transferred after inflection point with different spreading solvents: (b,c) EtOH/CHCl3, (d) DMSO/CHCl3. Table 1. Diameter, Pitch and Height of the Helical Nanotubes Obtained from Different Spreading Solvents EtOH diameter (nm) pitch (nm) height (nm)
60 (main) 40 (few) 74 (few) ∼51 ∼10.8
EtOH/CHCl3 35 (∼50%) 60 (∼50%) ∼48 8.7∼10.5
DMSO/CHCl3 100∼110 ∼52 ∼13
Scheme 1. Illustration on the Possible Formation of the Helical Structures for the Bolaamphiphile at the Air/Water Interface through Compressiona
a (a) Monolayer anchored on water surface before the plateau region. (b) After compression over the plateau region, an intermediate monolayer with one end anchored on water and the other extruded to the air was formed. (c) The intermediate monolayer rolled into helical nanotube.
prohibit the formation of larger helical structure although we could not verify this at the present stage. Such formation of the helical nanotubes can be possibly explained as shown in Scheme 1. When the film was spread on the water surface, it formed a monolayer with two headgroups anchored on the water surface, which could be verified from the π-A isotherm. Upon compression over the plateau region, one end of the bolaamphiphile was vertically aligned to form a stretched conformation as illustrated in Scheme 1b. Such changes
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Figure 3. FT-IR spectra of EDGA in the 40-layer LB film transferred from the air/water interface at different surface pressure at 288K. The pH of the subphase was 2. The zero surface pressure meant the film was transferred at an area of 1.5 nm2/molecule.
were also observed in some other bolaamphiphile systems.18 Due to the chiral nature of the terminal L-glutamic acid, these intermediate monolayers can further twist and roll into the helical nanotubes as shown in Scheme 1c. The driving forces for the formation of the helical structure are suggested to be due to both the H bonds among the carboxylic acid/amide units and the hydrophobic interactions between the alkyl spacer. Figure 3 shows the FT-IR spectrum of the LB film of EDGA transferred from the air/water interface at different surface pressures. In the LB films transferred at the surface pressures of zero and 4 mN/m, strong antisymmetric and symmetric CH2 stretching vibrations are observed at 2920 and 2851 cm-1, respectively, whereas those are observed at 2918 and 2850 cm-1 in the LB film transferred at a surface pressure of 16 mN/m. It is well-known that the appearance of the CH2 antisymmetric band at lower wavenumbers (ca 2918) indicates a highly ordered arrangement of the hydrocarbon chains and the higher wavenumber a gauche conformation.19 The present results indicate that there existed a gauche conformation at the lower surface pressure, which was due to the anchoring of the two headgroups of EDGA at the air/water interface at the beginning. The alkyl chains packed very orderly when the film was compressed over the plateau region. In addition, there showed a change of CH2 scissoring vibration mode δ(CH2) around 14601475 cm-1. The strong vibration at 1473 cm-1 at zero mN/m split into 1473 and 1466 cm-1 at a surface pressure of 4 mN/m. The split was only slightly changed when the film was compressed over the plateau region. These results indicated that the hydrocarbon chain changed from a triclinic packing (zero surface pressure) to an orthorhombic-type packing mode in the film with helical structure.15,20 Vibrations ascribed to the carboxylic acid (18) Lahann, J.; Mitragotri, S.; Tran, T. N.; Kaido, H.; Sundaram, J.; Choi, I. S.; Hoffer, S.; Somorjai, G. A.; Langer, R. Science 2003, 299, 371. (19) Terashita, S.; Ozaki, Y.; Iriyama, K. J. Phys. Chem. 1993, 97, 10445. (b) Nakagoshi, A.; Wang, Y.; Ozaki, Y.; Iriyama, K. Langmuir 1995, 11, 3614.
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and amide groups are observed at 1733, 1717, 1698, and 1684 cm-1 for the film transferred at zero surface pressure. When compressed to the higher surface pressure, the band at 1717 cm-1 disappeared and the bands at 1698 and 1684 cm-1 shifted into 1693 and 1680 cm-1, respectively. These spectral changes indicated the formation of a strong hydrogen bond between the carboxylic acids after the film was compressed. In addition, the vibrations at 1733 and 1717 cm-1 could be assigned to the sideways dimer and out-of-plane cyclic dimer formed from the carboxylic acid, respectively.21 With the surface pressure increased, the band at 1733 cm-1 became strong, whereas the band at 1717 cm-1 disappeared. One possible explanation for this is that the cyclic dimmer, which was easily formed when the two ends of EDGA were anchored on the water surface, was destroyed due to leaving of one end from water during the compression. This supported our explanation for the compressioninduced formation of the helical nanotube above. The amide I and II bands are observed at 1649 and 1544 cm-1, respectively, at zero surface pressure. When the film was compressed over the plateau region, these bands shifted to 1643 and 1541 cm-1, respectively, indicating the H-bond formation between the amide groups. It should be noted that at a lower surface pressure of zero and 4 mN/m partial of carboxylic acid was in an ionic state, as indicated by the existence of the vibration at 1558 cm-1. Such an ionic state was changed into a H-bonded one upon compression, suggesting that the H bond played an important role in the formation of the helical nanotube. Previously, we have found that EDGA could gel a mixture of water and ethanol solvent to form an organogel with a helical nanotube.13 Although both the helical nanotubes formed in the gel and the present helical structure are similar, the present Langmuir technique provided an easier way to obtain the helical nanotube. The organogel was only obtained in water/EtOH in a relative higher concentration (>2 mg/mL, 1 mL per run). In the present case, only a slight amount of sample (0.1 mg/mL, 100 µL per run) is needed to obtain the helical nanotubes through the Langmuir technique. So far, many helical structures have been reported in the organogel or the solution system.8 Here we showed an elegant example of the helical structures obtained using a two-dimensional Langmuir technique, in which a 3D helical structure seemed to be difficult to obtain. In summary, we have shown that a chiral bolaamphiphile with terminal L-glutamic acid headgroups could form helical nanotubes through the compression of the spreading Langmuir film at the air/water interface. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Nos. 20533050 and 90306002), the Basic Research Development Program (2005CCA06600), and the Fund of the Chinese Academy of Sciences. LA0604836 (20) Simon-Kutscher, J.; Gericke, A.; Hu¨hnerfuss, H. Langmuir 1996, 12, 1027. (21) Du, X. Z.; Liang, Y. Q. J. Phys. Chem. B 2004, 108, 5666.
