Langmuir 2005, 21, 721-727
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Local Environment and Property of Water inside the Hollow Cylinder of a Lipid Nanotube Hiroharu Yui,*,†,‡ Yanli Guo,‡ Kana Koyama,‡ Tsuguo Sawada,†,‡ George John,† Bo Yang,§ Mitsutoshi Masuda,†,§ and Toshimi Shimizu†,§ CREST, Japan Science and Technology Agency, Nanoarchitectonics Research Center, National Institute of Advanced Industrial Science and Technology, Tsukuba Central 4, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8562, Japan, Department of Advanced Materials Science, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5-603 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan, and Nanoarchitectonics Research Center, National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8562, Japan Received September 7, 2004 We investigated the local environment of water confined inside the hollow cylinder of lipid nanotubes (LNTs) by time-resolved fluorescent measurements and attenuated-total-reflectance infrared (ATR-IR) spectroscopy. The LNT was obtained by self-assembly of cardanyl glucosides in water at room temperature and had an open-ended cylindrical nanospace with a diameter of 10-15 nm, a length of 10-100 µm, and hydrophilic inner and outer surfaces. We introduced a fluorescent probe of 8-anilinonaphthalene-1-sulfonate into the confined water and observed an extremely slow dynamic Stokes shift with a correlation time of 1.26 ns, which was 2-3 orders of magnitude longer than that of bulk-phase water. From the peak shift of the fluorescent spectrum, the local solvent polarity (ET(30)) of the confined water was estimated as 50 kcal/mol, which is 20% lower than that in bulk water. ATR-IR measurements showed that the hydrogenbond network of water inside the LNT was more developed than that in bulk water at room temperature, which is in contrast to the water in other self-assembled confined geometries, such as Aerosol-OT (AOT) reversed micelles.
Introduction Water in nanometer-sized restricted geometries is of much interest because the structures and properties of water in such confined geometries often show different and sometimes unexpected features compared to those of bulk water. Such characteristic structures and properties of water in a restricted environment are considered to play important roles for the activity of biological macromolecules, specific chemical and physical processes occurring at interfaces, and lubrication phenomena.1,2 To obtain fundamental information on confined water, intensive studies of water in micelles and microemulsions,3 nanometer films,4 nanoporous silica,5 and other kinds of * To whom correspondence may be addressed. E-mail:
[email protected]. † CREST, Japan Science and Technology Agency. ‡ The University of Tokyo. § National Institute of Advanced Industrial Science and Technology. (1) Chen, S. H.; Bellissent-Funel, M. C. Hydrogen Bond Network; NATO ASI Series C: Mathematical and Physical Science Vol. 435; Kluwer: Dordrecht, 1994. (2) Robinson, G. W.; Zhu, S.-B.; Singh, S.; Evans, M. W. Water in Biology, Chemistry and Physics: Experimental Overviews and Computational Methodologies; World Scientific: Singapore, 1996; Chapter 9. (3) (a) Bhattacharyya, K. Acc. Chem. Res. 2003, 36, 95. (b) Scodinu, A.; Fourkas, J. T. J. Phys. Chem. B 2002, 106, 10292. (c) Boyd, J. E.; Briskman, A.; Colvin, V. L.; Mittleman, D. M. Phys. Rev. Lett. 2001, 87, 147401. (d) Venables, D. S.; Huang, K.; Schmuttenmaer, C. A. J. Phys. Chem. B 2001, 105, 9132. (e) Pant, D.; Levinger, N. E. Langmuir 2000, 16, 10123. (f) Willard, D. M.; Levinger, N. E. J. Phys. Chem. B 2000, 104, 11075. (g) Shirota. H.; Horie, K. J. Phys. Chem. B 1999, 103, 1437. (4) (a) Raviv, U.; Laurat, P.; Klein, J. Nature 2001, 413, 51. (b) Zhang, X.; Zhu, Y.; Granick, S. Science 2002, 295, 663. (c) Berger, C.; Desbat, B.; Kellay, H.; Turlet, J.-M.; Blaudez, D. Langmuir 2003, 19, 1. (5) Ricci, M. A.; Bruni, F.; Gallo, P.; Rovere, M.; Soper, A. K. J. Phys. Condens. Matter 2000, 12, A345 and references therein.
confined geometries have been carried out by using various spectroscopic techniques and computer simulations.6 Among various morphologies of such restricted nanometer-sized geometries, one-dimensional hollow structures provide a variety of potential applications as channels for nanofluidic devices,7 templates for metal nanowire formation,8,9 and size-selective pores for analytical and/or storage devices.10 Carbon nanotubes are one representative, well-known nanometer-sized material with a hollow cylindrical structure.11 However, for the purposes of the study and application of aqueous solutions in such confined geometries, carbon nanotubes may be inappropriate because their inner and outer surfaces are hydrophobic. It is inherently difficult to introduce water into their hydrophobic hollows and to disperse them well in water unless their surfaces are chemically modified. On the other hand, there is another type of nanotube with hydrophilic surfaces obtained by the self-assembly of organic molecules.12-14 One example is a cardanyl glucoside-based lipid nanotube that has inner diameters (6) Beckstein, O.; Sansom, M. S. P. Proc. Natl. Acad. Sci. U.S.A. 2003, 12, 7063. (7) (a) Kopp, M. U.; de Mello, A. J.; Manz, A. Science 1998, 280, 1046. (b) Weigl, B. H.; Yager, P. Science 1999, 283, 346. (c) Kenis, P. J. A.; Ismagilov, R. F.; Whitesides, G. M. Science 1999, 285, 83. (d) Karlsson, A.; Karlsson, M.; Karlsson, R.; Sott, K.; Lundqvist, A.; Tokarz, M.; Orwar, O. Anal. Chem. 2003, 75, 2529. (8) (a) Das, G.; Talukdar, P.; Matile, S. Science 2002, 298, 16001602. (b) Yang, B.; Kaymiya, S.; Yoshida, K.; Shimizu, T. Chem. Commun. 2004, 500. (9) (a) Van Bommel, K. J. C.; Friggeri, A.; Shinkai, S. Angew. Chem., Int. Ed. Engl. 2003, 42, 980. (b) Jung, J. H.; Shinkai, S.; Shimizu, T. Chem. Eur. J. 2002, 8, 2684. (c) Jung, J. H.; Yoshida, K.; Shimizu, S. Langmuir 2002, 18, 8724. (d) Ji, Q.; Iwaura, R.; Kogiso, M.; Jung, J. H.; Yoshida, K.; Shimizu, S. Chem. Mater. 2004, 16, 250. (10) Reches, M.; Gazit, E. Science 2003, 300, 625. (11) Iijima, S. Nature 1991, 354, 56. (12) Schnur, J. M. Science 1993, 262, 1669.
10.1021/la040109a CCC: $30.25 © 2005 American Chemical Society Published on Web 12/15/2004
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products; i.e., they can be synthesized by coupling Dglucose with a natural long-chain phenol, called cardanol, obtained from cashew nut oil. The LNT is easily produced in water through the self-assembly of the cardanyl glucosides at room temperature.15 Figure 1b shows a transmission electron microscopic (TEM) image of the hollow structure of the LNT and a schematic illustration of its wall structure. The wall of the LNT consists of three to four lamellar layers of interdigitated molecules. The interdigitation means that hydrophilic sugar headgroups are present on both the inner and outer surfaces of the LNT. Thus, in contrast to carbon nanotubes, aqueous solutions can be easily introduced into the hollow cylinder of the LNT. Here, we investigated the local properties and environment of water confined in the LNT by time-resolved fluorescent spectroscopy and attenuated total reflectance infrared (ATR-IR) measurement. The dynamic property of water inside the hollow cylinders of the LNT was also investigated from the dynamic Stokes shift of fluorescent probe molecules, namely, 8-anilinonaphthalene-1-sulfonate (ANS), which were selectively introduced into the hollow cylinder of the LNT. We estimated the local solvent polarity of the water inside the hollow cylinder of the LNT from the peak shift of the time-resolved fluorescent spectra. The ATR-IR measurement was combined with a directdifference methodology to selectively obtain information on the structure and strength of the local hydrogen bond network environment. Experimental Section
Figure 1. (a) Structures of the cardanyl glucosides used for the formation of a lipid nanotube (LNT). The percentages shown are the proportion of each glycolipid in the mixture. (b) TEM image of an LNT formed by self-assembly of cardanyl glucosides and schematic illustration of the wall structure. Blue spheres and light-gray cylinders represent the hydrophilic sugar headgroups and the hydrophobic tails of the cardanyl glucoside molecules, respectively.
of 10-15 nm, outer diameters of 40-50 nm, and axial ratios of >1000.15 We have identified this as a sugar-based lipid nanotube (LNT) and have described its physical and chemical characteristics elsewhere.16 The dimensions of this hollow nanocylinder are the smallest among various types of lipid nanotubes and are similar to those of inorganic multiwall carbon nanotubes (MWCNTs)11 and microtubules in cells.17 Cardanyl glucosides are amphiphilic molecules with sugar headgroups as a hydrophilic part and a phenyl group attached to a long alkyl chain as a hydrophobic part (Figure 1a). It is worth noting that cardanyl glucosides are all renewable-resource-based (13) Fuhrhop, J.-H.; Koning, J. Membrane and Molecular Assemblies: The Synkinetic Approach; The Royal Society of Chemistry: London, 1994. (14) Bong, D. T.; Clark, T. D.; Granja, J. R.; Ghadiri, M. R. Angew. Chem., Int. Ed. Engl. 2001, 40, 988 and references therein. (15) John, G.; Masuda, M.; Okada, Y.; Yase, K.; Shimizu, T. Adv. Mater. 2001, 13, 715. (16) (a) Shimizu, T. Macromol. Rapid Commun. 2002, 23, 311. (b) John, G.; Jung, J. H.; Minamikawa, H.; Yoshida, K.; Shimizu, T. Chem. Eur. J. 2002, 8, 5494. (c) Jung, J. H.; John, G.; Yoshida, K.; Shimizu, T. J. Am. Chem. Soc. 2002, 124, 10674. (d) Frusawa, H.; Fukagawa, A.; Ito, K.; John, G.; Shimizu, T. Angew. Chem., Int. Ed. Engl. 2003, 42, 72. (17) Dustin, P. Microtubules, 2nd ed.; Springer-Verlag: Berlin, 1984.
The detailed procedure for synthesizing the LNT has been reported elsewhere.15 The cardanyl glucoside mixture was dispersed by refluxing in boiling water for about 30 min. This dispersion was then gradually cooled to room temperature and allowed to stand for several days. The cardanyl glucoside monomers self-assembled into a characteristic helically coiled ribbon structure, which finally converted into open-ended tubular structures. We prepared the LNT which contains fluorescent probe molecules within their hollow cylindrical interiors. After confirming the formation of tubular LNT morphologies by highmagnification TEM, we freeze-dried the LNT for 3 days to remove water from the cylindrical core of the LNT. We then immersed the LNT in 1.0 × 10-4 M ANS aqueous solution. The ANS solution was sucked into the cylindrical hollow space of the freeze-dried LNT by capillary action. Then, ANS solution remaining on the outside of the LNT was washed away with pure water using a filtration unit (Millipore Stirred Ultrafiltration Cell, filter pore size, 0.2 µm) until its concentration dropped to 450 nm (width 100-120 nm, fwhm) (Table 1). The former component was denoted as I, and the latter, as II. The peak wavelength of component II showed a gradual but definite red shift with time after excitation. Since neither component was observed in the absence of ANS molecules in the LNT, they were attributed to fluorescence from ANS molecules probing different environments in the LNT. The fitted result is also compared in the case of the water pool in the AOT reversed micelle in the same table in Table 1. While the fluorescent bandwidth of component II and of the water pool in the AOT were similar (∼100 cm-1), the peak wavelength was remarkably shorter in the LNT (467 nm, at the steady state) than that in the AOT reversed micelles (490 nm). Figure 4shows the peak shift of the two components, obtained by curve fitting. The peak position of component I was not remarkably different, while that of component II increased remarkably and then remained steady about 4 ns after excitation. In general, a time-dependent red shift of the fluorescence spectrum is described in terms of the Franck-Condon principle and solute-solvent interactions. The particular solvent configuration favored by the ground state of the fluorescent solute molecule is still preferred at the moment of excitation because the photoexcitation process is too fast for the solvent molecules to immediately reorient. If the excited state has a different dipole moment from the ground state, surrounding solvent molecules reorient to reduce the electrostatic energy of the newly formed dipole. If the fluorescence lifetime is longer than the solvent reorientation time, a decrease in the energy level of the excited state can be observed as a gradual red shift in the fluorescence spectrum, namely, a dynamic Stokes shift. The extent of stabilization depends on the local polarity and mobility of the surrounding solvent, so the physicochemical properties of the local environment can be inferred from the extent of the red shift and its relaxation time. From the peak position and its time course, we assigned the fluorescence components I and II. When ANS is dissolved in nonpolar or highly viscous solvents or fixed
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in a solid lattice, blue-fluorescence emission occurs around 400 nm.22 For example, as is shown in Figure 2d, ANS in pure heptane gives a fluorescent peak at 410 nm. The peak wavelength of component I was ∼403 nm and did not show a dynamic Stokes shift induced by the reorientation of the surrounding water. Thus, we concluded that component I derived from ANS molecules strongly adsorbed at the inner and outer surfaces of the LNT and/ or intercalated into gaps between the lipid molecules. On the other hand, the peak wavelength of component II showed a gradual but remarkable increase from
