Self Assembled Nanotubes and Methods for Preparation Thereof

Description

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims the benefit of U.S. Provisional Application Ser. No. 60/570,785, filed on May 13, 2004, which is incorporated herein by reference thereto.

FIELD OF THE INVENTION

The present invention relates to the preparation of self-assembling nanotubes and nanocarpets.

BACKGROUND OF THE INVENTION

Production of pure, well-defined, nanostructured materials is essential for advances in electronics and bioengineering. Here we disclose the synthesis of materials that fit these criteria.

The discovery of carbon nanotubes has attracted enormous attention over the past decade due to their potential significance in nanoelectronic devices (Iijima, S, Nature vol. 354, 56 58 (1991)). Micro and nano tubules produced from amphiphilic lipids have also captured the imagination of scientists in disciplines from biology through material science to chemistry and physics. Tubules of this type have promise as advanced materials in several applications ranging from small molecular wires, to drug encapsulation, to biosensors. To date only a few classes of lipids, nearly all of which are chiral, can form tubular structures under controlled conditions. Further, the tubes are generally not uniform in size. The difficulty in preparing optically active phospholipid variants is a major obstacle to the use of typical lipids and phospholipid analogues in the fabrication of lipid helices and tubules.

Various attempts have been made to overcome these problems by chemical modification of diacetylene lipids. Schoen et al. have discussed method of making lipid tubules composed of chiral diacetylenic phosphocholine by a cooling process (U.S. Pat. No. 4,990,291). The diacetylenic phosphocholines have distinctly different endothermic and exothermic transition temperatures. Because of this, lipid tubules can be formed by hydrating a diacetylenic phosphocholine at a temperature above its endothermic transition temperature then slowly lowering the temperature. Unlike spherical liposomes, lipid tubules reflect the chiral nature of the lipids used to form them. This chirality in molecular packing is reflected in helical structures, often visible in electron-micrographs of tubules, and in large peaks observed in their circular dichroism (CD) spectra. The importance of chirality is emphasized because both the helicity and the CD spectra change handedness when the opposite enantiomer lipid is used.

Tubules were observed by Schoen and Yager, Mol. Cryst. Liq. Cryst. vol. 106, 371 (1984), as having assembled in water from liposomes of the 2 chain, chiral, lipid diacetylene, 1,2-bis(tricosa-10,12-diynoyl)-sn-glycero-3-phosphocholine (abbreviated DC8,9PC). Tubules formed from DC8,9PC have an average diameter of 0.5 μm and lengths which range from 50 to 200 μm. The size and stability of the tubules formed these experiments were sensitive to preparation conditions and thermal history resulting in a non-homogenous preparation. Other work with chiral lipids consisting of two diacetylenic chains has demonstrated that it is quite difficult to generate uniform nanotube structures from these precursors (Thomas et al., Science vol. 267, 1635 (1995); Spector et al., Nano Letters vol. 1, 375 (1984); Wand et al., Langmuir vol. 15, 6135 (1999); Svenson et al., Langmuir vol. 15, 4464 (1999); Seddon et al., Angew. Chem. Int. Ed. vol. 41, 2988 (2002); Thomas et al., J. Am. Chem. Soc. vol. 124, 1227 (2002).

Cheng et al., Langmuir vol. 16, 5333 (2000), and Frankel et al., J. Am. Chem. Soc. vol. 116, 10057 (1994), found that compounds consisting of single, chiral diacetylenic chains can form tubules. In addition, Singh et al., J. Chem. Soc., Chem. Commun. 1222 (1988), found that the tubules can be formed from a non-chiral amphiphile composed of two diacetylenic chains. Finally, Lindsell et al., Chem. Mater. vol 12, 1572 (2000), prepared micrometer sized tubules (not nanotubes) from non-chiral amphiphile composed of single diacetylene chain. According to these results, neither chirality nor the presence two diacetylenic chains in one amphiphile were an absolute requirement for tubule formation. It should be emphasized however, that, while these investigators were able to generate some tubule like structures, the preparations were quite heterogeneous.

In this invention, we report the successful synthesis of nearly homogeneous monodisperse nanotubes, and related structures called nanocarpets, from an achiral single chain diaceylenic amphiphile. The remarkable self-assembly of this inexpensive and simple lipid is unprecedented and represents a real step toward the rational design of nanostructured materials for electronics, optics, biosensors, and scaffolds for time engineering.

SUMMARY OF THE INVENTION

The present invention provides nanotubes and nanocarpets from single chain, non-chiral, diacetylenic amphiphiles. Nanotubes or nanocarpets where the monomeric material is of the general formula (A):

Where a and b are from about 5 to 15, R is a linking group comprising linear or branched alkyl or aromatic chains that optionally contain 0 (oxygen) or N (nitrogen); R′ is H or a organic group comprising linear or branched alkyl or aromatic chains that optionally contain O or N; n is an integer from about 0 to 3, wherein R′ is the same or different when n is 2 or 3; and X is F, Cl, Br, I, CF₃SO₃ or CF₃CO₂, and combinations thereof. In another embodiment of the present invention, the nanotubes of this invention also include one or more salts thereof.

The present invention provides uniform diameter nanotubes that assemble from molecules via non-covalent self assembly.

EXAMPLES

Example 1

Nanotubes from Mixtures of Compounds 2. 3. 4, and 5 (FIG. 1) by Quaternization with Bromoethane

The synthetic scheme is summarized in panel A of FIG. 1. Compound 2 is produced when 3.0 g of 10, 12-pentacosadiynoic acid (PDA) is converted to a succinimidyl ester by reaction with 2.77 g of N-hydroxysuccinimide (NETS) and 4.61 g of 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide (DEC). The resulting compound is added over the course of 30 minutes to a 100 fold excess of ethylene diamine in chloroform. After reaction overnight at room temperature, the mixture is extracted with chloroform. The organic phase is dried with NaSO4 and rotary evaporation to yield a white powder (compound 2). Compound 2 is then subjected to a quaternization reaction at room temperature by mixing with excess ethyl bromide in a 1:1 mixture of chloroformand/nitromethane. Reaction solvents are removed by a rotary evaporation and the obtained white solid is dissolved in a small amount (about 1-10 milliliters) of choroform. Nanotubes are formed by first slowly adding a large excess (about 3 to 4 fold) of hexane to the chloroform solution and then drying in vacuum oven at room temperature. The dried preparations are resuspended in water or hexane followed by sonication for 5-30 minutes in a sonic water bath at room temperature. The sonicated solution is spread on glass slides and dried for 3 hours at room temperature.

Mass spectroscopy and NMR analysis reveals a mixture of compounds composed of compounds 2, 3, 4, and 5.

Example 2

Nanotubes from the Secondary Amine Salt of PDA Alkylated with an Ethyl Head Group (Compound 3, FIG. 1)

PDA modified with NHS in the presence of DEC (as above) is slowly added to a 10 fold excess of N-ethylethylenediamine in dichloroethane. After the reaction, the mixture is washed with an excess of water. The organic phase is dried with sodium sulfate and rotary evaporated to yield a white powder (compound 6 FIG. 1 panel B). Pure compound 3 was prepared from compound 6. Compound 6 is dissolved in chloroform and an equal volume aqueous HBr is added. The mixture is shaken vigorously to transfer the HBr to the organic phase. The organic phase is removed and concentrated in a rotary evaporator. Hexane is added to the chloroform solution to precipitate compound 3 and the precipitate is dried in vacuuo at room temperature. To prepare nanotubes, the dried precipitate is suspended in water or hexane, dropped on glass surface, and allowed to dry.

Under SEM the nanotubes are absolutely monodisperse in wall thickness (31 run) and internal diameter (41 nm) (FIG. 2C).

The precise make up of these structures is provided by transmission electron microscopy (TEND. TEM of naked nanotubes and after staining with phosphotungstic acid reveal a hollow inner core and a wall consisting of 5 lipid bilayers (each bilayer is 43.1 Å across) (FIG. 3A). The structure in solution was further characterized by SAXS. The results (FIG. 313) suggest that the equilibrium spacing of the tubule bilayers in excess water is 57.9 Å. Although the diameter of the tubes is uniform throughout the sample, the length varies, with a mean of approximately I μg/m.

Example 3

Control of Nanotubes Diameter: Nanotubes from the Secondary Amine Salt of PDA Alkylated with N-Propyl Head Group

PDA modified with NHS in the presence of DEC was slowly added to 10 times excess of N-propylethylenediamine in dichloroethane. After the reaction, the mixture is washed with water. The organic phase is dried with sodium sulfate and rotary evaporated to yield a white powder. The HBr amine salt of PDA alkylated with n-propyl head group and nanotubes of this material are prepared as described above.

Under SEM the nanotubes are absolutely monodisperse in wall thickness (31 nm) and internal diameter (34 nm). The diameter of the nanotubes is uniform throughout the sample and the length varies from 200 nm to 1.8 μm.

Example 4

Control of Nanotubes Diameter: Nanotubes from the Secondary Amine Salt of PDA Alkylated with N-Butyl Head Group

PDA modified with NHS in the presence of DEC was slowly added to 10 times excess of N-butylethylenediamute in dichloroethane. The amine salt and nanotubes are prepared as above.

Under SEM the nanotubes are absolutely monodisperse in wall thickness (33 nm) and internal diameter (16 nm). The diameter of the nanotubes is uniform throughout the sample and the length varies from 200 nm to 1.8 μm.

Example 5

Nanocarpets from the Mixtures of Compounds 2. 3. 4, and 5 by Quaternization with Bromoethane as Synthesized in Example 1

The polydiacetylene nanocarpet composed of well-aligned nanotubes and its lamella structures is prepared without any external template. In the first experiment, using same method as used for the formation of nanotubes, 20 ml (milliliter) of primitive microstructure aqueous solution (0.05 mg/ml) was sonicated for 5 min (minutes) at 25° C. (Centigrade). The diacetylene monomers were then polymerized by UV exposure at 254 nm for 30 min at 5° C. 0.5 ml of the resultant solution was spread on a glass slide followed by drying for 1 h (hour) at room temperature. At this time, which was before complete drying, one drop of chloroform was added on the surface and the slide was allowed to dry for an additional 2 h. The drying was followed by observation with the SEM.

The nanocarpets in FIGS. (2D, 2E, and 2F) were prepared without any external template. Microscopy shows that the pillars of the nanocarpet erupt from lamellar structures, (FIGS. 2D and 2E) perhaps formed by the melting of one of the ends of the nanotubes. We have observed the ability of chloroform at the junction of water/chloroform to “melt” nanotube ends in single tubes. We know that before exposure to chloroform the outer surface and inner surface of the nanotubes are hydrophilic and open-ended. Although not being bound to any particular theory, our current working hypothesis is that the addition of a small amount of chloroform to a disordered surface of nanotubes first melts the top surface of the tubes, creating a lamellar structure from which disoriented pillars can emerge. It is, perhaps, the gradual removal of chloroform that then causes the tubes to become aligned. Whatever the mechanism, the pillars of the resulting nanocarpet are approximately 100 nm in diameter and 1 gm in length (FIG. 2F). Each pillar consists of a cluster of 3-4 nanotubes of exactly the same diameter observed for the disordered nanotube systems described above. The carpet backing is 120 nm thick (FIG. 2F).

Example 6

Nanocarpets from the Secondary Amine Salt of PDA (Compound 3) Alkylated with Ethyl Head Group at FIG. 1

The polydiacetylene nanocarpet is prepared without any external template. In the first experiment, the compound 3 was dissolved in chloroform. The solution was spread on a glass slide followed by drying for 1 h at room temperature and water was added on the surface and the slide was allowed to dry for an additional 61 L The drying was followed by observation with the SEM.

The nanocarpets in FIGS. (4A and 4B) were prepared on glass surface. Microscopy shows that the pillars of the nanocarpet erupt from lamellar structures, the pillars of the resulting nanocarpet are approximately 5 μm in thickness.

Example 7

Control of Nanotubes Length

Secondary amine salt of PDA (compound 3) (1 mg/ml) alkylated with ethyl head group at FIG. 1 was placed in a glass test tube. The insoluble sample was carefully heated to a boil with a heat gun. At about 80° C., the secondary amine salt of PDA solution was clear. The solution was allowed to cool to room temperature and then placed in a chamber maintained at 4° C. for 1 week before characterization. Under SEM the nanotubes are absolutely monodisperse in wall thickness (27 nm) and internal diameter (41 nn). The diameter of the nanotubes is uniform throughout the sample, the length varies from 15 μm to 20 μm as seen FIG. 5.

Example 8

Antimicrobial Activity as Function of the Nanotubes

Several experiments were performed to assess the interaction of nanotubes with bacteria. The antimicrobial activity was tested by incubation of 2×105 Escherfchia coli in a solution containing 10 μg/ml nanotubes. E. coli K₁₂ were grown overnight in Luria broth, diluted in 0.3 mM potassium phosphate (pH 7.2), and used for either microscopy or in antimicrobial assays. For the antimicrobial assay 1 ml of a suspension containing 5×105 cells was mixed with 1 ml of a solution of nanotubes. The mixture was shaken at 37° C. for 1 hour at which time samples were serially diluted and plated on Luria-agar plates to obtain viable counts. This treatment killed 99.98% of the cells within one hour.

Exposure of diacetylene molecules to W light results in the formation of cross links between the molecules forming a polymeric chromogenic material. UV Exposure of nanotubes in solution results in a color change from white to dark blue. When cross-linked nanotubes are mixed with bacteria, the material acts as a flocculent precipitating the cells and the nanotubes and the changing the color from dark blue to light blue.

The reason for the flocculation behavior was investigated by electron microscopy. A solution of nanotubes was prepared and mixed with a suspension of E. coli. TEM grids were dipped in the mixture, excess liquid was wicked off, and the grids were observed with the TEM. In these preparations the majority of the nanotubes were seen associated with the outer surface of the bacteria. FIG. 6 shows a striking example of a nanotube that is fused with the outer surface of the bacterial cell and a cell that has been enveloped by nanotube material.