Supramolecular self-Assembly of lipid derivatives on carbon nanotubes

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Entry by Sandeep Koshy, AP 225, Fall 2010

Title: Supramolecular Self-Assembly of Lipid Derivatives on Carbon Nanotubes

Authors: Cyrille Richard, Fabrice Balavoine, Patrick Schultz, Thomas W. Ebbesen, Charles Mioskowski

Journal: Science

Volume: 300

Issue:

Pages: 775-778

Summary

In this work, Richard et al. aim to explore the supramolecular structures formed by surfactants on the surface of carbon nanotubes and to assess their ability to noncovalently functionalize nanotubes. They observed the formation of half-cylinders, helices and double helices on the surface of carbon nanotubes using surfactants above their critical micelle concentration. They then designed their own detergents, which are compatible with biosensing applications, and demonstrated stable supramolecular structure formation on nanotube surfaces. This noncovalent functionalization method may be useful in biosensing and bioelectronics.

Soft Matter Keywords: surfactant, self-assembly, carbon nanotube, micelle

Background

Carbon nanotubes (cylinder allotropes of carbon) have many unique properties such as extremely high tensile strength and elastic modulus, conducting and semiconducting properties, high thermal conductivity and ability to facilitate one dimensional transport. The authors aim to generate a novel approach to functionalize these materials without the use of covalent bonds which may alter the unique properties listed previously. Nanotubes are insoluble in water but have been shown to form stable suspensions in the presence of a detergent known as sodium dodecyl sulfate (SDS). The hydrophilic portion of SDS associates with the nanotube surface, while the polar part faces outwards and acts to solubilize the nanotubes in water. The authors wanted to explore the structure of the nanotube detergent complex and evaluate surfactants as a method of functionalizing nanotubes.

Experimental Summary

The general procedure for all experiments involved mixing 1 mg of carbon nanotubes with the surfactant to be studied and sonicating for 3 minutes. In the case of SDS, the mixture was then placed on a 10 nm thick carbon film and negatively stained with uranyl acetate and imaged using transmission electron microscopy (TEM). They also performed cryoelectron microscopy by snap freezing a mixture of SDS and nanotubes on a carbon film surface. The procedure was similar with all other surfactants tested, except in the case of double chain lipids which involved the addition of an extra dialysis step prior to TEM preparation.

Results

SDS organization on carbon nanotubes

Fig 1. Possible self-assembled structures formed by SDS on carbon nanotube surfaces.

The authors formulated hypotheses about the possible organization on the surface of carbon nanotubes via van der Waals attractions (Fig. 1). They suggested that (A) SDS orients perpendicular to the surface forming a monolayer, (B) SDS forms half cylinders axially on the nanotube and (C) SDS forms half-cylinders radially on the nanotube.

SDS structures on carbon nanotube surfaces

Fig 2. Transmission electron microscopy of SDS on nanotube surfaces.

TEM was performed on SDS/multiwalled carbon nanotube mixtures (Fig. 2). SDS was visualized as striations on all nanotube surfaces. The authors contradict their previous hypothesis about van der Waals interactions by stating that SDS is likely “chemically adsorbed” to the surface without providing any evidence of this. The authors analyzed (Fig. 2 B) the length of the striations (~45 nm) and their orientation (perpendicular to the tube axis) and concluded that half-cylinders oriented radially (Fig. 2 A) were formed (as suggested in Fig. 1 C). They also observed some degree of right or left helix formation (Fig. 2 C,D) or the formation of double helices (Fig. 2 E,F). The authors failed to correlate these SDS structures with the underlying structure of the graphite network. To show that these structures were not artifacts of staining, the authors performed TEM on frozen samples and observed similar striations (Fig. 2 G). Similar behavior was seen on single walled nanotube surfaces (Fig. 2 H). Two other detergents, OTAB and Triton X-100, were also tested. OTAB showed similar half-cylinder and helix formation whereas Triton X-100 formed a monolayer on tube surfaces. It was observed that striations were not formed under the critical micelle concentration (CMC) of SDS and OTAB. Also, after supramolecular structure had formed at concentrations above the CMC, striations were observed to disappear following dialysis of free surfactant. These observations suggest that structure formation is under thermodynamic control.

Formation of stable supramolecular structures with single-chain lipidic reagents

Fig 3. Functionalization of nanotube surface with single-chain lipidic reagents.

The authors then sought to synthesize surfactants which would form stable structures and could be used for biosensor applications. They created several molecules of the form shown in Fig. 3 A with varying n. Molecules of this form have been shown to be compatible with biosensing applications. They found that short chain (n=1) molecules could not solubulize nanotubes while higher chain lengths (n=3,7,9) formed striated structures (Fig 3 B). They showed that n=9 formed structures that were stable after dialysis.

Formation of stable supramolecular structures with double-chain lipidic reagents


Fig 4. Functionalization of nanotube surface with double-chain lipidic reagents.

Double-chained lipidic reagents of the form in Fig 4 A were then created. These were highly hydrophobic and formed vessicles in water. When mixed with nanotubes, no striations were formed. Striated structures were observed once again (Fig 4 B-E). The authors added SDS to the double chain reagents to convert vesicular structure to micellar structure. Mixing with carbon nanotubes and subsequent dialysis of SDS led to formation of striations, showing that micelle formation was required for supramolecular structure formation.

This study details a method to achieve noncovalent functionalization of carbon nanotubes which may have applications in biosensing or bioelectronics.