Bilayerbased Superstructures Of Fullereneamphiphiles In Water

A variety of artificial lipids are known to form liposomal lipid bilayer membranes in water [43-44]. Some of the bilayer aggregates show the growth to superstructures, such as helical bilayers and tubes [45-48]. In this section, molecular bilayer-based superstructure formations from water-soluble fullerenes are summarized. Cassell et al. described that C60-N,N-dimethylpyrrolidinium iodide 1 (Fig. 2) forms supramolecular nanorods (Fig. 3) in dimethyl-sulfoxide with one part water and then adding one part benzene, and that an aqueous solution of the compound treated with ultrasonication and then filtered gives vesicles with diameters of 10-70 nm [49]. Sano and coworkers described the vesicle formation by a bola-amphiphilic fullerene 2 (Fig. 2), although the bilayer structure is not evident [37]. Hirsch et al. revealed by transmission electron microscopy (TEM) of a freeze fracture replica that a C60-carrying globular amphiphile 3 (Fig. 2) forms a vesicular structure in water [34, 38]. Zhou et al. reported that an amphiphile 4 (Fig. 2), a potassium salt of pentaphenylfullerene with charged cyclopentadienide units, forms spherical vesicles in water (Fig. 4) [41]. The average hydrodynamic radius and aggregation number of the vesicle evaluated by laser light scattering were about 17 nm and 1.2 x 104, respectively.

We synthesized a C60-carrying ammonium amphiphile 5 (Fig. 2), which is soluble in DMF, DMSO, hot chloroform, and hot alcohols, but was insoluble in THF, acetonitrile, ethylacetate, hexane, and toluene [17, 20]. Negative-stained transmission electron microscopy (TEM) revealed that the aqueous solution of 5 forms both fibrous aggregates (Fig. 5) and disk-like aggregates with 10-12 nm of thickness. This superstructure is formed via self-organization of 5 in aqueous solution. A possible molecular model for the disklike aggregate is shown in Figure 6. The wavenumber of the asymmetric and symmetric methylene stretching vibrations in the FTIR spectra of cast films of 5 from both methanol and aqueous solutions appeared at 2922 ± 0.1 and 2850 ± 0.1 cm-1, respectively, over a temperature range of 10-40 °C, indicating that the methylene chain in 5 contains a gauche conformation. The UV-visible absorption maximum for an aqueous solution of 5 appeared at 271 nm; this

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Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 3: Pages (545-556)

Langmuir-Blodgett (LB) films lipid bilayers

Figure 1. Schematic drawing of self-assembled fullerenes.

maximum is shifted to longer wavelength by 3 and 5 nm, respectively, compared to those in micellar solutions of hex-adecyltrimethylammonium bromide or sodium dodecylsul-fate. These shifts are due to the electronic interaction of the fullerene moieties in 5 in an aggregated state.

The phase transition between a crystal phase and a liquid-crystal phase is the most fundamental characteristic of lipid bilayer membranes [50]; however, aqueous aggregates of 1-5 do not possess a phase transition. The results described above indicate that the introduction of a hydrophilic moiety to fullerene enables the formation of bilayer-based assemblies in water. This kind of fullerene-based biomembrane-like carbon nanosuperstructures in aqueous solution is of

Figure 2. Chemical structure of compounds 1-7.

Figure 3. TEM micrograph of the nanorod formed from compound 1. Reprinted with permission from [49], M. Cassell et al., Angew. Chem. Int. Ed. 38, 2403 (1999). © 1999, Wiley-VCH.

interest from aspects of both fundamental and biological applications [51]. The construction of electroactive fullerene nanoarchitectures with an ordered structure based on self-assembled molecular organization would possess many chemical and biochemical applications.

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