Magister Scientiae - MSc / It is well known that a lead pencil is made of graphite, a naturally form of carbon, this is important but not very exciting. The exciting part is that graphite contains stacked layers of graphene and each and every layer is one atom thick. Scientists believed that these graphene layers could not be isolated from graphite because they were thought to be thermodynamically unstable on their own and taking them out from the parent graphite crystal will lead them to collapse and not forming a layer. The question arose, how thin one could make graphite. Two scientists from University of Manchester answered this question by peeling layers from a graphite crystal by using sticky tape and then rubbing them onto a silicon dioxide surface. They managed to isolate just one atom thick layer from graphite for the first time using a method called micromechanical cleavage or scotch tape. In this thesis chemical method also known as Hummers method has been used to fabricate graphene oxide (GO) and reduced graphene oxide. GO was synthesized through the oxidation of graphite to graphene oxide in the presence of concentrated sulphuric acid, hydrochloric acid and potassium permanganate. A strong reducing agent known as hydrazine hydrate has also been used to reduce GO to rGO by removing oxygen functional groups, but unfortunately not all oxygen functional groups have been removed, that is why the final product is named rGO. GO and rGO solutions were then deposited on silicon substrates separately. Several characterization techniques in this work have been used to investigate the optical properties, the morphology, crystallography and vibrational properties of GO and rGO.

No description available.

Graphene oxide (GO) is a material of both fundamental and applied interest. Elucidating this complex material is crucial to both control its physical chemical properties and enable its applications in technology. Graphene oxide films synthesized from epitaxial graphene on silicon carbide constitute a particular -- simplified -- form of GO, suitable for fundamental physical chemistry studies of oxidized sp2 carbon materials. In this thesis work, I used density functional theory calculations and I developed a lattice-model Monte Carlo scheme to elucidate puzzling experimental observations and to gain molecular insight into the chemical composition, thermochemical and structural properties of this type of ultrathin GO films on silicon carbide substrates.

Graphene sheets, one atom thick, two dimensional layers of carbon atoms, have gained enormous importance over the past few years due to their unique attributes - high electronic, thermal conductivities and exceptional mechanical strength. Chemical reduction of graphene oxide (GO) has been considered as a viable route for large scale production of graphene sheets. The reduced graphene oxide (r -GO) sheets although their conductivities are comparatively lower than that of graphene are nevertheless versatile material for applications in thin films and composites. An important consideration in the design of solution processing techniques for the preparation of processable graphene sheets is the dispersibility of GO and r -GO in different solvents, especially aqueous dispersibility. While GO is dispersible in water over a wide range of pH values, r -GO shows poor dispersibility and over a limited range of pH values. Graphene oxide, the oxidized form of graphene, are single atomic layers with lateral dimension that can extend to hundreds of nanometers. The sheets contain a sizable fraction of carbons that are sp3 hybridized and covalently bonded to oxygen in the form of epoxy, carbonyls as well as ionizable hydroxyl and carboxylic functional groups located on the rim of the sheets. The remaining carbons form isolated sp2 graphene like networks. On reduction the oxygen functionalities are removed and the sp2 network partially restored. This thesis focuses on the aqueous dispersibility of GO and r -GO, and describes a strategy to enhance the dispersibility of r -GO by cyclodextrin functionalization. Chapter 1 of the thesis provides a brief review of the synthetic procedures and structure of GO and r -GO while Chapter 2 describes the experimental methods and characterization techniques used in the thesis. The chemistry underlying the aqueous dispersibility of GO and r -GO at different values of pH have been investigated by zeta potential measurements, pH titrations and infrared spectroscopy (Chapter 3). These measurements show that r -GO sheets have ionizable groups with a single pKa value (8.0) while GO sheets have groups that are more acidic (pKa = 4.3), in addition to groups with pKa values of 6.6 and 9.0. Infrared spectroscopy has been used to follow the sequence of ionization events. In both GO and r -GO sheets, it is ionization of the carboxylic groups that is primarily responsible for the build up of charge, but on GO sheets, the presence of phenolic and hydroxyl groups in close proximity to the carboxylic groups lowers the pKa value by stabilizing the carboxylate anion, resulting in superior water dispersibility. Till recently GO was primarily considered only as an easily available precursor for chemical routes to r -GO but it has now been recognized as an interesting material in its own right. Two such attributes that have attracted wide spread attention are the in- trinsic and tunable fluorescence of GO as well as formation of liquid crystalline phases. Aqueous dispersions of GO exhibit strong pH dependent fluorescence in the visible region that originates, in part, from the oxygenated functionalities present. In Chapter 4, the spectral migration on nanosecond timescales of the pH dependent features in the fluores- cence spectra of GO is described. The changes in the steady state fluorescence spectra with pH have been correlated with the sequence of dissociation events that occur in GO dispersions at different values of pH described in Chapter 3, from time resolved emission spectra (TRES) constructed from the wavelength dependent fluorescence decay curves, it is shown that the migration is associated with excited state proton transfer. Both ‘intramolecular’ and ‘intermolecular’ transfers involving the quasimolecular oxygenated aromatic fragments are observed. Aqueous dispersions of GO constitute a distinctive class of 2D-anisotropic colloids with competing interactions - long range electrostatic repulsion, originating from ionized carboxylic groups located on the rim of the sheets and weak dispersive attractive interactions originating from the un-oxidized sp2 graphitic domains. In Chapter 5, it is shown that, colloidal dispersions of GO are intrinsically frustrated, exhibiting a range of arrested or metastable states, encompassing fluid, glass and gels that coexist with liquid crystalline order. These states can be accessed by varying the relative magnitudes of the repulsive and attractive forces by changing the ionic strength of the medium, by addition of salt and/or the concentration of the dispersion. At low salt concentrations, where long range electrostatic repulsions dominates, the formation of a repulsive Wigner glass is observed while at high salt concentrations, when attractive forces dominate, the formation of gels that exhibits a nematic to columnar liquid crystalline transition. These studies highlights how the chemical structure of GO - hydrophilic ionizable groups and hydrophobic graphitic domains coexisting on a single sheet - gives rise to a rich and complex phase diagram. The poor dispersibility of r -GO in aqueous media limits its use in practical applica- tions. To enhance the dispersibility, r -GO sheets have been functionalized by covalently linking -cyclodextrin ( -CD) cavities to the sheets via an amide linkage (Chapter 6). The functionalized -CD: rGO sheets, in contrast to r -GO, are dispersible over a wide range of pH values (2 - 13). Zeta potential measurements indicate that there is more than one factor responsible for the dispersibility. It is shown that planar aromatic molecules adsorbed on the r -GO sheet as well as nonplanar molecules included in the tethered -CD cavities have their fluorescence effectively quenched by the -CD: rGO sheets. The -CD: rGO sheets combine the hydrophobicity associated with r -GO along with the hydrophobicity of the cyclodextrin cavities in a single water dispersible material. Resonance Raman spectroscopy is a powerful analytical tool for detecting and identi- fying analytes, but the associated strong fluorescence background severely limits the use of the technique. In Chapter 7, it is shown that the cyclodextrin functionalized -CD: rGO sheets, described in Chapter 6, provides a versatile platform for resonance Raman detection. Planar aromatic and dye molecules that adsorb on the r -GO graphitic domains and non-planar molecules included within the tethered -CD cavities have their fluorescence effectively quenched. Using the water dispersible -CD: rGO sheets, it is possible to record the resonance Raman spectra of adsorbed and included organic chromophores directly in aqueous media without having to extract or deposit on a substrate. The Raman signal intensities show a linear dependence with the concentration of analyte present in water. This is significant, as it allows for the identification and estimation of organic analytes present in water by resonance Raman spectroscopy.

>Magister Scientiae - MSc / Tuberculosis (TB), a deadly disease second to HIV/AIDS, is a global health problem. Diagnosis of active tuberculosis is tedious and requires expensive procedures since there is no recognizable method for sole detection of active TB. Although this is a deadly disease, treatment drug toxicity is also an issue that also causes fatalities in diagnosed patients. Therefore, a rapid sensitive and specific diagnostic method is imperative for TB drug management. In this study spectroscopic and/or electrochemical biosensors were developed for the detection and quantification of TB treatment drugs. The biosensors were constructed with electroactive layers of graphene oxide coupled to silver nanoparticles and/or zinc oxide nanoparticles. These nanoparticles coupled with graphene oxide sheets were covalently attached onto the enzymes such as Cytochrome P450-2D6 to achieve the electrochemical detection of the TB treatment drugs and obtain the required electron transfer between the electrode surface and enzyme. The surface morphology of graphene oxide, nanoparticles as well as the green synthesized nanocomposites were achieved using High-Resolution Transmission Electron Microscopy (HRTEM), Atomic Force Microscopy (AFM), and High- Resolution Scanning Electron Microscopy (HRSEM) while the elemental analysis were obtained using Fourier Transform Infrared Spectroscopy (FTIR), Energy Dispersive X-Ray (EDX), Raman spectroscopy and X-Ray diffraction (XRD). Additionally, the optical properties of the developed nanocomposites where further characterised using Small Angle X-ray Scattering (SAXS), Photoluminescence Spectroscopy (PL) and Ultraviolet Spectroscopy (UV-vis). The electrochemical studies were obtained using cyclic voltammetry (CV) and showed an increase in electron conductivity for the green synthesized zinc oxide nanoparticles coupled with graphene oxide (ZnONPs/GO) and silver nanoparticles coupled with graphene oxide (AgNPs/GO) nanocomposite which was an indication that they were suitable as platforms towards biosensor development. Furthermore, amperometric technique was also used for biotransformation of the TB treatment drugs (Ethambutol and Pyrazinamide) in standard solutions of 0.1 M phosphate buffer (pH 7.0). Furthermore, the sensitivity value of 0.0748 μA/μM was determined for the ethambutol biosensor while a value of 0.1715 μA/μM was determined for the pyrazinamide biosensors. Very good detection limits were obtained for the standard solutions of ethambutol and pyrazinamide where a value of 0.02057 nM was determined for ethambutol at concentration linear range of 50 μM – 400 μM. Additionally, a value of 0.8975 x 10-2 nM was determined for pyrazinamide at the concentration linear range of 100 μM – 300 μM. The determined limit of detections have provided a clear indication that these biosensors have potential of being used in human samples since these values are below the peak serum concentrations of these drugs in TB diagnosed patients as reported in literature. This was further confirmed by the limit of quantification values determined for each biosensor where a value of 0.8975 x 10-2 nM was determined for pyrazinamide and a value of 0.02057 nM was determined for ethambutol.

No description available.

Nanomaterials have been studied extensively in the last decade in the context of many applications such as polymer composites, energy harvesting systems, sensors, ‘transparent’-like materials, field-effect transistors (FETs), spintronic devices, gas sensors and biomedical applications. Graphene, a recently discovered two-dimensional form of carbon has captured the interest of material scientists, and physicists alike due to its excellent electrical, mechanical and thermal properties. Graphene has also kindled a tremendous interest among chemists and cell biologists to create cellular-electronic interface in the context of bio-electronic devices as it can enable fabricating devices with enhanced potential as compared to conventional bio-electronics. Graphene’s unique electronic properties and sizes comparable with biological structures involved in cellular communication makes it a promising nanostructure for establishing active interfaces with biological systems. In the recent past Field effect transistors (FETs) have been successfully fabricated using carbon nanotubes (CNTs) and nanowires (NWs) and electrical characterization of these FETs were done by interfacing them with various cell cultures, tissues and muscle cells. In these cases, exceptionally high surface area to thickness ratio of FETs provides high percentage of collectible signals and the cells that are used for the study are typically placed on the FET. In this thesis, we examine a different approach towards forming bio-electronic interfaces by covering the graphene oxide (reduced) sheets on the yeast cells. Graphene oxide and reduced graphene oxide sheets as two-dimensional electronic materials have very high charge carrier mobility, extremely high surface area to thickness ratio, mechanical modulus and elasticity. We report the synthesis of graphene oxide using wet chemistry method, reduction of graphene oxide using different reducing agents and electrical characterization of graphene oxide’s conductivity. Micro-meter sized graphene sheets are used to encapsulate the yeast cells with the aid of calcium and gold nanoparticle chains. We also demonstrate that graphene sheets form electrically conductive layers on the yeast cells and developing an electromechanical coupling with the cell. The mechanical and electrical characteristics of graphene sheets are highly dependent on the cell volume and structure which are in turn related to the environment around the cell. Furthermore, using the same principle of electromechanical coupling we study the dynamics of cell surface stresses and cell volume modification, which are of importance in processes such as cell growth, division, and response to physiological factors such as osmotic stresses.

This master thesis report evaluatesthe lubricating effect of graphene (G)and graphene oxide (GO). Thesematerials have been added, in particlecondition, in Ag-based slidingcontacts and lubricating greases. Thework focuses on the tribologicalevaluation of these materials,especially friction, wear and contactresistance analyses. The friction andwear behaviors of Ag-based contactscontaining of a wide concentrationrange of graphene and graphene oxideare tested against pure silver using atest load of 2 and 10 N at a constantspeed of 5 cm/s. It was revealed thatsmall amounts of G and GO are able tosignificantly reduce the frictioncoefficient and wear rate. Contactresistance measurements revealed thatresults similar to pure Ag can beachieved with G content up to 10vol%.Possible mechanisms, which maycontribute to this tribologicalbehavior are the Ag-C interactions andthe lubricating nature of graphene.Friction tests with G and GOcontaining lubricating greases showinconsistent results, and both greasesand corresponding test methods forevaluation require furtheroptimization. The overall, promising,tribological behavior of G and GOholds for the implementation invarious industrial applications. Thereis no doubt that these kinds ofmaterials can play an important rolein ABBs future work. This masterthesis report shows yet anotherapplication area for theseextraordinary materials.

Nanomaterials have been studied extensively in the last decade in the context of many applications such as polymer composites, energy harvesting systems, sensors, ‘transparent’-like materials, field-effect transistors (FETs), spintronic devices, gas sensors and biomedical applications. Graphene, a recently discovered two-dimensional form of carbon has captured the interest of material scientists, and physicists alike due to its excellent electrical, mechanical and thermal properties. Graphene has also kindled a tremendous interest among chemists and cell biologists to create cellular-electronic interface in the context of bio-electronic devices as it can enable fabricating devices with enhanced potential as compared to conventional bio-electronics. Graphene’s unique electronic properties and sizes comparable with biological structures involved in cellular communication makes it a promising nanostructure for establishing active interfaces with biological systems. In the recent past Field effect transistors (FETs) have been successfully fabricated using carbon nanotubes (CNTs) and nanowires (NWs) and electrical characterization of these FETs were done by interfacing them with various cell cultures, tissues and muscle cells. In these cases, exceptionally high surface area to thickness ratio of FETs provides high percentage of collectible signals and the cells that are used for the study are typically placed on the FET. In this thesis, we examine a different approach towards forming bio-electronic interfaces by covering the graphene oxide (reduced) sheets on the yeast cells. Graphene oxide and reduced graphene oxide sheets as two-dimensional electronic materials have very high charge carrier mobility, extremely high surface area to thickness ratio, mechanical modulus and elasticity. We report the synthesis of graphene oxide using wet chemistry method, reduction of graphene oxide using different reducing agents and electrical characterization of graphene oxide’s conductivity. Micro-meter sized graphene sheets are used to encapsulate the yeast cells with the aid of calcium and gold nanoparticle chains. We also demonstrate that graphene sheets form electrically conductive layers on the yeast cells and developing an electromechanical coupling with the cell. The mechanical and electrical characteristics of graphene sheets are highly dependent on the cell volume and structure which are in turn related to the environment around the cell. Furthermore, using the same principle of electromechanical coupling we study the dynamics of cell surface stresses and cell volume modification, which are of importance in processes such as cell growth, division, and response to physiological factors such as osmotic stresses.

Since its isolation in 2004, the research landscape around graphene and other 2D materials has expanded rapidly and now encompasses fields as diverse as electronic engineering and drug delivery. For biomedical applications, one of the most desirable properties of the graphene family of nanomaterials (GFNs) is their 2D geometry; the high surface area to volume ratio that is characteristic of nanomaterials is taken to its extreme in a material that can be viewed as being entirely surface. This particular property alongside the versatility with which they may be functionalised both makes GFNs well positioned to function as the foundation of highly tailored and multifunctional therapeutics platforms. In this project, two GFN types, namely pristine graphene and graphene oxide, were prepared to form suspensions suitable for application to therapeutics delivery. Firstly, experiments using four essential amino acids with pristine graphitic material were undertaken to assess whether graphene flakes could be suitably exfoliated and suspended using sonication in the presence of aqueous solutions of these biocompatible molecules. A positive correlation was found between the hydrophobicity of the amino acid and the presence of one or more aromatic rings in the amino acid, and the efficacy of exfoliation both in terms of concentration achieved in suspension and flake thinness. However, the system itself was found to be highly complex, both with regards to the sonication used to exfoliate the graphitic flakes, and the interactions between the amino acids and the flakes. These considerations limited the wider applicability of this form of graphene preparation for therapeutics delivery applications. Secondly, work was performed on graphene oxide (GO), a GFN far more studied in the literature, but notoriously heterogeneous. Therefore much of the work completed focused on its characterisation. A combination of established and novel fluorescence-based characterisation methods were used to fully characterise three preparations of GO, before preliminary experiments were undertaken to test their interactions with cell components. The work showed that the inherent fluorescence of GO can be exploited to improve suspension characterisation; raster image correlation spectroscopy (RICS) was used to measure the apparent hydrodynamic radii of the flakes and flow cytometry was used to provide insight into the interactions between GO flakes and serum components. Preliminary cellular experiments confirmed that flow cytometry could be also employed to assess particular graphene characteristics in the context of cell culture, demonstrating the relatively low toxicity of PEGylated GO compared to unfunctionalised GO. Finally, as the therapeutics target for this project was leukaemia, a targeting ligand was designed and synthesised that could bind to CXCR4 - a receptor that is overexpressed on CLL B-cells, as well as many other cancer types. The ligand was synthesised such that it could easily be attached to GO, however its molecular structure is flexible enough that it can be attached to a number of different therapeutics materials. It was confirmed using both competition and functional assays that the molecule was antagonistic, and was able to deliver a conjugated fluorescent molecule specifically to the CXCR4 receptors on primary CLL B-cells. The work presented in this thesis illustrates the complexity that affects the use of GFNs in biomedicine, but also confirms the potential for their future development. The field is still young, and therapeutics delivery is likely to benefit from advances in the preparation of pristine graphene, and from methods to minimise the heterogeneity of GO. These steps will support a route towards clinical application. In addition, as the field of 2D materials expands, other materials with enviable surface area to volume ratios may come to the fore. Furthermore, this thesis has shown the value of exploring novel approaches to the characterisation of GFNs, and has identified approaches that may be exploited to improve applications of GFNs in biomedicine. Additionally, the aim of using GFNs as a platform for a multifunctional therapeutics delivery vehicle was developed with regards to the attractive CXCL12/CXCR4 axis, which is relevant in a large number of disease states including over 20 cancers, by demonstrating a flexible targeting ligand that could be used to exploit the CXCR4 receptor as a drug delivery target.