The Rational Design of Diazenyl Amphiphiles for Self-Assembly into Nanotubes within Aqueous Systems

Author: Thomas Geoffrey Barclay

Barclay, Thomas Geoffrey, 2011 The Rational Design of Diazenyl Amphiphiles for Self-Assembly into Nanotubes within Aqueous Systems, Flinders University, School of Chemical and Physical Sciences

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A combinatorial library of amphiphiles were synthesised with amino acid hydrophilic head groups (glutamic acid, serine or aspartic acid) linked by an amide bond to a hydrophobic tail having a diphenyldiazenyl (azo) group incorporated between the proximal alkoxy chain (6, 10, 11, or 12 methylene units long) and the distal alkyl chain (2, 4, 5, 6, 7, 8, 10, 12 or 14 methylene units long). All synthetic products were analysed by nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopic methods, while differential scanning calorimetry, elemental analysis, and isotope ratio mass spectrometry were used for selected compounds. These techniques affirmed the identity and purity of the compounds. The synthesised amphiphiles were specifically designed to self-assemble into nanotubes based on the tight, helical winding of a bilayer ribbon. Transmission Electron Microscopy (TEM), Ultra Violet and Visible light spectroscopy (UV-Vis), scanning electron microscopy, atomic force microscopy and circular dichroism were used to analyse the self-assembled structures. TEM analysis of the entire library of amphiphile final products self-assembled from an aqueous methanolic solution (3:1 v/v) using a heated procedure showed that nanotubes, and chiral ribbon precursors to nanotubes, were commonly produced. The nanotubes observed were of two distinct varieties. The first variety of nanotubes were the result of helical assembly of a partially interdigitated bilayer ribbon driven by chiral symmetry breaking and also influenced by chiral molecular packing. Amphiphiles with a serine head group and an eleven methylene unit proximal chain assembled most efficiently into these helically based tubes, with the conversion to nanotubes increased with increasing distal chain length. There was no evidence that the second variety of nanotubes self-assembled from the synthesised amphiphiles had a helical basis, instead they were predicted to form from flat aggregates that rolled up to create the tubes. TEM and UV-Vis spectroscopy were conducted on selected amphiphiles self-assembled with a room temperature method that used solvent mixtures that varied in ratios of methanol and water (100:0 - 10:90 v/v). TEM showed that for similar solvent ratios the heated and room temperature methods of self-assembly generated similar self-assembled morphologies, while solvent mixtures lower in water concentration using the room temperature method enabled more organised assembly for some amphiphiles. The UV-Vis spectroscopy results showed that the nanotubes observed in TEM were the result of H- and H*-aggregation azo groups. The isomerisation of the azo group was investigated for both dissolved and aggregated amphiphiles. An amphiphile dissolved in deuterated dimethyl sulphoxide was isomerised to the cis form by UV radiation (λmax 355 nm). The thermal reversion to the trans isomer was monitored by NMR spectroscopy and the activation energy for this transformation was 92.8 kJ.mol-1. UV irradiation (λmax 355 nm) of self-assembled nanotubes led to the destruction of the tubular structure upon isomerisation of the component amphiphiles from the trans to cis forms. Thermal and photo reversion to the cis form did not result in reconstruction of the nanotubes.

Keywords: Self-Assembled Nanotubes,Diazenyl Amphiphiles,Rational Amphiphile Design

Subject: Nanotechnology thesis

Thesis type: Doctor of Philosophy
Completed: 2011
School: School of Chemical and Physical Sciences
Supervisor: Dr Stephen Clarke / Dr Janis Matisons