AN INVESTIGATION OF THE CONDUCTIVITY OF PEPTIDE NANOSTRUCTURED HYDROGELS VIA MOLECULAR SELF-ASSEMBLY

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Abstract

Nanoscale, conductive wires fabricated from organic molecules have attracted considerable attention in recent years due to their anticipated applications in the next generation of optical and electronic devices. Such highly ordered 1D nanostructures could be made from a number of routes. One route of particular interest is to self-assemble the wires from biomolecules due to the wide range of assembly methods that can be adapted from nature. For example, biomolecules with aromatic motifs can be self-assembled so that good  stacking is achieved in the resultant nanostructure. An additional advantage of using biomolecules is that it enables the interface of the electronic materials with biological systems, which is important for many applications, including nerve cell communication and artificial photosynthesis. In this study, nanowires were prepared by the molecular self-assembly of oligopeptides that were coupled to aromatic components. In order to achieve charge transport though the nanowires, it was imperative that the aromatic components were arranged so that there was stacking with very few structural defects. Therefore, enzymes were used to control the formation of the hydogelators which subsequently self-assembled to produce nanowire networks. Two main systems were studied in this thesis.In the first system, hydrogelators were produced from aromatic peptide amphiphiles via the enzymatic hydrolysis of the methyl ester of fluorenylmethoxycarbonyl (Fmoc)-di/tripeptides. These hydrogelators formed nanostructures due to -stacking between the Fmoc groups and H-bonding between the peptides. The nanostructures in turn produced macroscale gel networks. The nanostructures were analyzed by wide angle X-ray diffraction and fluorescence spectroscopy. A combination of Fourier transform infra-red (FTIR), Transmission Electron Microscopy (TEM), Cryo-TEM, and Atomic Force Microscopy (AFM) was used to characterize the networks. The charge transport properties of the dried networks were studied using impedance spectroscopy. Fmoc-L3 was found to assemble into nanotubes whose walls consisted of 3 self-assembled layers and possessed inner and outer diameters of ~ 9 nm and ~ 18 nm, respectively. The Fmoc-L3 networks were structurally stabile and were electronically conductive under a vacuum. The sheet resistance of the peptide networks increased with relative humidity due to the increasing ionic conductivity. The resistance of the networks was 0.1 MΩ/sq in air and 500 MΩ/sq in vacuum (pressure: 1.03 mbar) at room temperature. The networks had a band gap of between 1 to 4 eV as measured by UV-Vis spectroscopy and the temperature-impedance studies. Possible routes for aligning the Fmoc-L3 networks were studied in an attempt to improve their conductivity in one direction. In particular, the peptides were assembled under an electric field (0 to 3.75 kV/cm). Random networks were produced at low field strengths, whereas a degree of alignment was obtained at a field strength of 3.75 kV/cm. The conductivity of the aligned networks in the direction of alignment was a factor of three times higher than that of the random networks.The second system studied was Fmoc-dipeptide-OMe hydrogels produced by the enzymatic condensation of an Fmoc-amino acid and an amino acid ester. Preliminary results found that Fmoc-SF-OMe assembled into nanosheets, nanoribbons and spherulites, depending on the temperature at which self-assembly occurred. The Fmoc-XY-OMe films possessed an extremely high resistance (1012 Ω).

Keyword(s)

self-assembly, peptide nanotubes, hydrogel, nanostructures, conductivity, alignment

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