CNTNW Templated Bioassembly

It is of great interest to modify the external surface of CNTs or NWs with biological macromolecules, such as oligonucleotides (109), proteins (41,94), and peptides (96). Such bio-nanoassembly could promote the development of new biosensors and bioelectronic nanomaterials, which could take advantage of the specific biomolecular recognition properties associated with the bound macromolecules. For such purposes, the specific recognition function has to be densely packed on the outer surface of CNTs/NWs and the biomolecules such as proteins have to remain functional. A good criterion for the conservation of the functional properties of the protein is its ability to form ordered arrays.

Balavoine et al. (41) reported on a study of streptavidin assembly on CNTs to form helical crystallization. Streptavidin is very useful in many biochemical assays, such as labeling and affinity chromatography, owing to its high affinity for (+) biotin (Ka ~ 1015). The assembly was carried out in solutions by spontaneous adsorption. MWCNTs were prepared by the arc-discharge method and stored as a suspension in methanol (approx 2 mg/mL). One hundred microliters of MWCNT suspension was dried under an ethane gas flow and resus-pended in 20 mL of a 40% aqueous solution of methanol. This suspension was sonicated to disperse the MWCNTs prior to the addition of 20 ^L of streptavidin solution (approx 10 ^g/mL) in a buffer containing 10 mM Tris (pH 8.0) and 50 mM NaCl, and then it was allowed to stand at room temperature for 45 min. Such a sample was deposited on a carbon film-covered grid and was negatively stained with a 2% uranyl acetate solution for TEM imaging. In appropriate conditions, the MWCNT surface was found almost completely covered with streptavidin, presumably owing to the interaction with its hydrophobic domains. Even though most assemblies are disordered, some regular-spaced helical structures were observed at proper conditions. Another water-soluble protein, HupR, was also studied and showed ordered arrays on a wider range of MWCNT diameters than streptavidin.

A 29-residue amphiphilic a-helical peptide was also specifically designed to coat and solubilize CNTs as well as control the assembly of the peptide-coated CNTs into macromolecular structures through peptide-peptide interactions (96). Figure 8 shows a cross-sectional view of the molecular structure and a perspective view of the helical backbones of the model illustrating the assembly of such molecules on an SWCNT surface. Six such a-helices are sufficient to surround the circumference of an individual SWCNT, while maintaining typical interhelical interactions. The hydrophobic face of the helix with apolar amino acid side chains (Val and Phe) presumably interacts non-covalently with the aromatic surface of CNTs, and the hydrophilic face extends outward to promote self-assembly through charged peptide-peptide interactions. Electron microscopy and polarized Raman studies reveal that the pep-tide-coated CNTs assemble into fibers with CNTs aligned along the fiber axis. The size and morphology of the fibers can be controlled by manipulating solution conditions that affect peptide-peptide interactions.

While the aforementioned studies are based on nonspecific adsorptions, Wang et al. (98) have used phage display to identify peptides with selective affinity for CNTs. Binding specificity has been confirmed by demonstrating direct attachment of nanotubes to phage and free peptides immobilized on microspheres. Consensus binding sequences show a motif rich in histidine and tryptophan at specific locations. Analysis of peptide conformations shows that the binding sequence is flexible and folds into a structure matching the geometry of CNTs. The hydrophobic structure of the peptide chains suggests that they act as symmetric detergents. An IgG monoclonal antibody against the fullerene C60 (110) was also studied to show binding to CNTs with some selectivity (111).

Fig. 8. Model of amphiphilic peptide helices assembled on an SWCNT surface. (A) Cross-section view showing six peptide helices wrapped around an SWCNT. A helical ribbon denotes the backbone of each peptide. The hydrophobic Val and Phe side chains are packed against the SWCNT surface. A 5-A-thick water shell was used in the energy refinement. (B) View of a peptide-wrapped SWCNT with 12 peptide helices. The peptide formed two layers with head-to-tail alignment. (Reprinted from ref. 96 with permission. Copyright 2003 American Chemical Society.)

Fig. 8. Model of amphiphilic peptide helices assembled on an SWCNT surface. (A) Cross-section view showing six peptide helices wrapped around an SWCNT. A helical ribbon denotes the backbone of each peptide. The hydrophobic Val and Phe side chains are packed against the SWCNT surface. A 5-A-thick water shell was used in the energy refinement. (B) View of a peptide-wrapped SWCNT with 12 peptide helices. The peptide formed two layers with head-to-tail alignment. (Reprinted from ref. 96 with permission. Copyright 2003 American Chemical Society.)

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