The essence of this technology is to replicate what nature does: to use molecular recognition processes to form ordered assemblies of building blocks capable of conducting biochemical activities.
Peptides can serve as sturdy building blocks for a wide range of materials as they can be designed to combine with a range of other building blocks such as lipids, sugars, nucleic acids, metallic nanocrystals, and so on; this gives the peptides an edge over carbon nanotubes, which are another popular nanomaterial, as the carbon structure is unreactive.
They also exhibit biocompatibility and molecular recognition; the latter is particularly useful as it enables specific selectivity for building ordered nanostructures.
[6] The ability of peptides to perform self-assembly allows them to be used as fabrication tools, which will continue to grow as a fundamental part of nanomaterials production.
This approach is needed for nanoscale structure, as the top-down method of miniaturizing devices using sophisticated lithography and etching techniques has reached a physical limit.
Along these chains, the alternating amine (NH) and carbonyl (CO) groups are highly polar, and they readily form hydrogen bonds with each other.
Peptide synthesis can be easily conducted by the established method of solid-phase chemistry in gram or kilogram quantities.
If a thiol is introduced into the diphenylalanine then nano-spheres can be formed instead; nanospheres with diameters of 10–100 nm can also be made this way, from a diphenylglycine peptide.
[7] Surfactant peptides have been studied using a quick-freeze/deep–etch sample preparation method which minimizes effects on the structure.
[11] Dipeptides have also been shown to self-assemble into hydrogels, another form of nanostructures, when connected to the protecting group Fluorenylmethyloxycarbonyl chloride.
[15] Surfactant–like peptides that undergo self-assembly in water to form nanotubes and nanovesicles have been designed using natural lipids as guides.
[16] In water, surfactant peptides undergo self-assembling to form well-ordered nanotubes and nanovesicles of 30–50 nm through intermolecular hydrogen bonds and the packing of the hydrophobic tails in between the residues,[7] like micelle formation.
Transmission electron microscopy examination on quick-frozen samples of surfactant-peptide structures showed helical open-ended nanotubes.
[7] These types of molecular "paint" or "carpet" peptides are able to form cell patterns, interacting with or trapping other molecules onto the surface.
[7] Extensive research has been performed on nanotubes formed by stacking cyclic peptides with an even number of alternating D and L amino acids.
The stacking occurs through intermolecular hydrogen bonding, and the end product is a cylindrical structure with the amino acid side chains of the peptide defining the properties of the outer surface of the tube[11] and the peptide backbone determining the properties of the inner surface of the tube.
Indentation-atomic-force-microscopy experiments showed that dry nanotubes on mica have an average stiffness of 160 N/m and a high Young's modulus of 19–27 GPa.
[11] Apart from those made by cyclic peptides, the nanotubes' inner and outer surface properties have not yet been successfully independently modified.
Carbon nanotubes (CNTs) are another type of nanomaterial that have attracted much interest for their potential to serve as building blocks for bottom-up applications.
[17] Furthermore, CNTs exhibits strong hydrophobicity which results in a tendency to clump in aqueous solutions[17] and therefore have limited solubility; their electrical properties are also affected by humidity, and the presence of oxygen, N2O, and NH3.
Lastly, CNTs are expensive, with prices in the range of hundreds of dollars per gram, rendering most applications commercially unviable.
[7] Self-assembling and surfactant peptides can be used as targeting delivery systems for genes,[18] drugs[19] and RNAi.
[11] Self-assembling LEGO peptides can form biologically compatible scaffolds for tissue repair and engineering,[17] which should be of great potential, as a large number of diseases cannot be cured by small molecule drugs; a cell-based therapy approach is needed and peptides could potentially play a huge role in this.
Peptide can be designed to preferentially form on bacterial cell membranes and thus these tubes can perform as antibacterial and cytotoxin agents.
Researchers have also successfully developed multi-layer nanocables with a silver core nanowire, a peptide insulation layer, and a gold outer coat.
[7] It has also been found that the catalytic abilities of the lipase enzyme is greatly improved when encapsulated in a peptide nanotube.
[11] Although well ordered nanostructures have already been successfully formed from self-assembling peptides, their potential will not be fully fulfilled until useful functionality is incorporated into the structures.
Further knowledge could prove to be very useful in facilitating rational designs and precise control of the peptide assemblies.