M.Tech. (Chemical Technology) / Please refer to full text to view abstract

Oxytetracycline (OTC) is a broad spectrum antibiotic used extensively in the agricultural and human-health sector, and is effective against various gram positive and –negative bacteria as well as large viruses and certain pathogenic Rickettsiae. This study addresses the lack of voltammetric knowledge regarding the electroanalytical characterisation of OTC and its analysis in complex matrices. Cyclic voltammetry (CV) revealed several irreversible anodic peaks for OTC at a bare glassy carbon electrode (GCE). These current responses were improved through the selection of a diluent for OTC stock preparation, electrolyte solution and electrolyte pH, stir time and applied preconditioning potential. Under enhanced adsorptive conditions and using square wave voltammetry (SWV), a detection limit of 24.3 nM was achieved. The electrode surface could be renewed in vitro for 10 successive scans. OTC oxidation was characterised as a one electron:one proton ECiE mechanisms. Next, investigating the viability of voltammetry in various complex microbiological growth media revealed that selected growth media contained interfering redox active components, which, while simultaneously coating the electrode surface, effectively reduced GCE performance and lowered the active electrode surface area, as ascertained through CV and electrochemical impedance spectroscopy (EIS) studies. This interference lowered OTC current response in the presence of growth media which was partially recovered by appropriate growth media selection and sample dilution. In testing the use of acid functionalised multi-walled carbon nanotubes (MWCNTs) to improve anodic OTC response, charge-based attraction was observed between the MWCNT dispersal agent Nafion® and OTC, while increased surface area associated with prolonged acid functionalisation time aided in improving OTC current response.

This project was focused on the generation of porous and nanotubular anodic films on titanium and Ti-6wt.%Al alloy, and investigation of the key factors responsible for a transition between porous and nanotubular morphologies. Advanced analytical techniques were employed for characterisation of the anodic films, in particular scanning and transmission electron microscopies, including analytical transmission electron microscopy, Raman spectroscopy, nuclear reaction analysis, Rutherford backscattering spectroscopy and atomic force microscopy. Preparation of electron transparent sections for analysis by analytical transmission electron microscopy was undertaken using the focused ion beam technique. Initially, the influence of a post-anodizing rinsing treatment, using various media, on the morphology, structure and composition of anodic films generated on titanium in 0.2 M ammonium fluoride in glycerol, containing 0 and 5 vol.% added water, was investigated. Porous anodic films were formed in an electrolyte without added water followed by rinsing with ethanol. It was revealed that the oxide-rich nanotubes are embedded in a fluoride-rich matrix, with fluoride-rich material being more extensive and oxide-rich nanotubes being thinner-walled for the anodic films generated in the electrolyte with no added water followed by rinsing with ethanol compared with those for the films formed with added water to the electrolyte and rinsed similarly. However, post-anodizing rinsing of the former films transforms the porous morphology of the films to a tubular one. The contents of titanium and fluorine were reduced in the case of anodic films with the nanotubular morphology. It was suggested that dissolution of the fluoride-rich matrix occurs during rinsing of the specimens with water, leading to the transition from porous to nanotubular morphologies and subsequent loss of titanium and fluorine. Further work was undertaken to study the effect of ageing in deionised water on the morphology, structure and composition of the anodic films. It was revealed that loss of titanium and fluorine is greater for the films generated in the electrolyte with no added water followed by rinsing with water and ethanol and ageing compared with that for the films formed in the electrolyte with 5 vol.% added water followed by rinsing with water and ethanol and ageing. Finally, the anodic films generated on the Ti-6wt.%Al alloy were investigated. Porous anodic films were formed in the electrolyte without added water followed by rinsing with ethanol whereas the films treated with water disclosed nanotubular morphology. Porous anodic films contained greater amount of fluorine than nanotubular ones. Compositional analysis revealed an increased amount of fluorine for the anodic films generated on the alloy compared with those for the films formed on titanium under all investigated conditions. The difference in film compositions may be related to the difference in composition of the substrates used for anodizing, in particular, to the presence of aluminium as alloying element in the Ti-6wt.%Al alloy.

Biofuel cells (BFCs) use either enzymes or bacteria to catalyse a fuel to generate power. Their advantages over conventional fuels is that they do not use precious metals and the high selectivity of biocatalysts mean that no separation membranes are required between the electrodes. However, the application of BFCs is limited by their low power output and poor enzyme lifetimes. This thesis addresses these limitations by investigating aligned carbon nanotubes (aCNTs) as potential electrode materials. These aCNT electrodes offer high surface areas to increase enzyme coverage and hence power output and their surface topology can stabilise the enzymes to ensure maximum lifetime and current density.A novel BFC half cell was developed using aCNTs and the fungal enzyme, Trametes versicolor laccase which catalyses the four-electron reduction of oxygen to water. Laccase was shown to communicate directly with the nanotubes enabling the oxidant reduction reaction to be monitored without the need for mediators. Initial investigations compared aCNTs with other commonly reported carbon electrodes and found that the current densities were ~30-fold higher on the aCNTs than at pyrolytic graphite edge electrodes. The high surface area of these electrodes contributed to greater electroactive coverage of enzyme and minimal loss of enzyme upon deposition. Cathodic currents increased linearly with geometric electrode area; however they did not scale with actual electrode surface area and the current density was limited to the order of μA cm-2 due to O2-transport limitations. It was also discovered that the porous contribution of these aCNT electrodes could lead to misleading interpretations on nanotube electrochemistry. This effect was observed when increments in electrode area resulted in apparently significantly faster kinetics. This improvement in catalytic behaviour was proposed to be due to a transition from mass diffusion limited to thin layer cell behaviour exhibited by porous materials. Thermal pretreatment of the aCNT electrodes in oxidative and reductive atmospheres were found to improve their performance. These treatments worked by changing the nanotube surface chemistry and purifying the nanotubes, as evidenced by various physical characterisation methods. Furthermore, laccase activity was enhanced significantly after electrodes had been treated under both atmospheres, where it was believed that the removal of contaminant material and higher defect densities increased electrochemical performance.Finally, mass transport limitations were addressed by developing micro-patterned aCNT electrodes which possessed channels in the arrays, allowing better oxygen diffusion. Fundamental studies showed higher current densities per surface area and thus represent a promising electrode for future BFC research.

The project has been concerned with structure/property relationships in a series of different carbon nanotube (CNT) composite fibres. Raman spectroscopy has been proved to be a powerful technique to characterise the CNT-containing fibres. Electrospinning has been used to prepare poly(vinyl alcohol) (PVA) nanofibres containing single-wall carbon nanotubes (SWNTs). The effect of the processing conditions including the polymer concentration, electric voltage, tip-to-collector distance, nanotube concentration and the collection method upon the morphology, diameter and the alignment of the fibres have been investigated.Raman spectroscopy of individual SWNTs dispersed in PVA electrospun fibres have been studied systematically in terms of the Raman band frequency, intensity and linewidth. The G'-band shift per unit strain during tensile deformation has been found to be dependent on the nanotube chirality. A detailed study has been undertaken of the efficiency of reinforcement in PVA/SWNT nanocomposites. The stress-induced Raman band shifts in the nanocomposites have been shown to be controlled by both geometric factors such as the angles between the nanotube axis, the stressing direction and the direction of laser polarisation, and by finite length effects and bundling. A theory has been developed that takes into account all of these factors and enables the behavior of the different forms of nanocomposite, both fibres and films, to be compared.The effects of dispersion and orientation of nanotubes and the interfacial adhesion on mechanical properties of poly(p-phenylene terephthalamide) (PPTA)/SWNTs composite fibres have been investigated. It has been shown the change of orientation of the polymer molecules upon incorporating nanotubes had direct effect on mechanical properties of the PPTA fibres. An in-situ Raman spectroscopy study during fibre deformation has revealed good stress transfer from the matrix to nanotubes in low strain range, and the interface failed when the strain exceeded 0.5%.Raman spectroscopy has also been employed to investigate the microstructure and micromechanical process of neat carbon nanotube (CNT) fibres. It has been found the fibres consisted of both SWNTs and MWNTs and varied in composition at different locations. High efficiency of stress transfer both within the fibre and in composites has been observed, suggesting the promising potential of CNT fibres in reinforcing polymers.

Ph. D., Faculty of Science, University of the Witwatersrand, 2011 / Nitrogen doped carbon nanotubes (N-CNTs) have become a topic of increased importance in the study of carbonaceous materials. This arises from the physical and chemical properties that are created when N is embedded into a CNT. These properties include modified chemical reactivity, modified conductivity and changed mechanical, electronic and magnetic properties. This thesis covers the analysis of the catalytic growth of N-CNTs under well defined conditions and the optimization of reaction conditions to produce N-CNTs. Herein, a range of methodologies have been devised to synthesize N-CNTs. One of the procedures used in this work uses a floating catalyst in which an organometallic complex is decomposed in the gas phase in the presence of a nitrogen containing reactant to give the N-CNTs. This thesis focuses on the use of ferrocene and ring substituted ferrocenes in the formation of N-CNTs and other shaped carbon nanostructures. It talks of the effects that physical parameters such as temperature, pressure, gas flow rates and the type and concentration of N source have on the N-CNT type, size and yields as well as the nitrogen content incorporated into the tubes that are produced using the organometallic complexes. Proposed growth models for N-CNT synthesis are also reported. This work reveals that the N-CNTs produced are less stable (thermal gravimetric analysis measurements), less graphitic and more disordered (transmission electron microscope measurements) than their undoped counterparts. The ratio of the Raman D- and G-band intensities increase with the nitrogen concentration used during the CNT growth. Furthermore, the transmission electron microscopy (TEM) studies reveal that the CNTs are multi-walled, and that the diameters of the N-CNTs can be controlled by systematically varying the concentrations of the nitrogen source. Furthermore, X-ray photoelectron spectroscopy (XPS) and CHN analysis demonstrate that substitutional N is indeed present in the CNTs mainly as pyridinic and pyrrolic xiii N (and is sp2 and sp3 coordinated). The TEM analysis also revealed that when ferrocenylaniline and ferrocene/aniline reactions are compared at similar Fe/N molar ratios, higher N doping levels are achieved when ferrocenylaniline is the catalyst. Investigations of surface and interior imaging of N-CNTs was carried out by high resolution TEM (HRTEM) and identification of N-rich regions were performed by Energy filtered TEM (EFTEM). We also investigated the solid state pyrolysis of ferrocenylmethylimidazole or a mixture of ferrocene (FcH)/methylimidazole at 800 oC at different ratios in sealed quartz tubes. TEM studies showed bamboo compartments are present in the CNTs. An investigation of the bamboo structures revealed that three methylimidazole structural isomers led to tubes with different individual bamboo compartment distances and different morphologies including different N contents. It was observed that when diverse N containing hydrocarbons were used the amount of N in the nitrogen containing reagent is more important than the source and type of the N atoms used as revealed by trends in the morphology of the N-CNTs produced. We have also studied the effect of arylferrocene ring substituents on the synthesis of CNTs and other shaped carbon nanomaterials in subsequent chapters. Magnetic properties of different N doped carbon structures produced in the earlier chapters were investigated using electron spin resonance (ESR) spectroscopy. Most importantly, we observed a large g-factor shift in samples of N-CNTs from that of the free electron. Further, the shift is temperature dependant. A facile method for attaching Au nanoparticles to the surface of pristine N-CNTs and functionalized N-CNTs has been developed. The Au nanoparticles incorporated in the N-CNTs have a wide range of diameters (10 – 35 nm) and possess different shapes. The method offers certain advantages, such as providing Au nanoparticles in good yields and ease of use. The Au/N-CNT nanohydrids are being employed in catalytic reactions e.g. the oxidation of styrene.

Recently the theoretical discovery of single channel quantum dragons has been reported. Quantum dragons are a class of nanodevices that may have strong disorder but still permit energy-independent total quantum transmission of electrons. This thesis illustrates that multi-channel quantum dragons also exit in rectangular nanotubes and provide an approach to construct multi-channel quantum dragons in rectangular nanotubes. Rectangular nanotube multi-channel quantum dragons have been validated by matrix method based quantum transmission calculation. This work could pave the way for constructing multi-channel quantum dragons from more complex nanostructures such as single-walled zigzag carbon nanotubes and single-walled armchair carbon nanotubes.

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Single-walled carbon nanotubes (SWNTs) are envisioned as one of the most promising materials for next-generation electronic devices such as field-effect transistors, photovoltaics, new power sources and bio/chemical sensors. In particular, solution processable SWNT networks are of great interest for flexible and stretchable electronics. However, most of these applications specifically require pure semiconducting (sc-) or metallic (m-) SWNTs. However, large scale synthetic methods for SWNTs always produce a mixture of semiconducting and metallic carbon nanotubes. In recent years, several biochemical separation techniques such as DNA assisted separation, density gradient ultracentrifugation, and gel chromatography techniques have been utilized to separate semiconducting and metallic SWNTs. Although these methods can be used for sorting SWNTs according to their chiralities, they are either time-consuming or not easily scalable. In addition, the supramolecular functionalization of SWNTs with conjugated polymers has received a great deal of attention due to its capability to extract sc-SWNTs via simple sonication and centrifugation steps within a few hours. Furthermore, π-conjugated polymers can be modified by suitably changing monomers and/or comonomers, and it is also easy to control molecular weight and solubility of resulting polymers in organic solvents and aqueous media. There is also the possibility for selectively extracting specific chirality (n, m) nanotubes using specifically designed macromolecular structures. Except for its application to the separation of SWNTs, the supramolecular complexes of π-conjugated polymer and SWNTs have potential applications in many research areas such as new composite materials. After a brief overview of the current work related to the investigation of the supramolecular interaction between various conjugated polymers and SWNTs (chapter 1), synthesis of a series of different types of fluorene-based conjugated copolymers and their supramolecular complex formation properties with SWNTs are described (chapter 2, 3, 4, 5 and 6). In order to understand the effect of conjugated polymer backbone and side-chain structure on formation of supramolecular complexes with SWNTs, several crucial factors were investigated by: (1) altering the polymer backbone composition; (2) introducing different solubilizing (functional) groups while the polymer backbone remains the same; (3) changing the side-chain functional groups, and (4) introducing different polymer repeat units with varying physical and chemical properties. The experimental results indicated that all of the resulting polymer-SWNTs complexes possess excellent (or moderate) solution stability in organic solvents such as tetrahydrofuran (THF), toluene, and xylene. It was also demonstrated that the interaction between the conjugated polymers and SWNTs is strongly influenced by polymer structure; even minor changes on side-chains have a significant effect on the selectivity of the polymers in dispersing specific SWNT structures. This investigation highlights the potential importance of incorporating different types of heterocyclic aromatic rings (electron rich or electron poor), and introducing side chains with varying electronic and geometric structure on selective solubilization of SWNTs. Polymer molecular weight and solvent properties also strongly influence the π-conjugated polymer assisted dispersion of specific chirality SWNTs. Although some progress has been made, the search for a conjugated polymer that selectively solubilizes specific SWNT chiralities on large scale remains a challenge. / Thesis / Doctor of Philosophy (PhD)

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