Graphene is a single layer of graphite with many unique mechanical, electrical, and optical properties. In addition, graphene is also known to adsorb wide range of biomolecules including single-stranded DNA. On the other hand, the adsorption of double-stranded DNA was much weaker. To properly disperse in water, graphene oxide (GO) is often used due to its oxygen-containing groups on the surface. Recently, it was discovered that it could efficiently quench the fluorescence of fluorophores that were adsorbed. With these properties, it is possible to prepare DNA-based optical sensors using GO. Majority of the DNA/GO-based fluorescent sensors reported so far were relied on the complete desorption of DNA probes. Even though all these reports demonstrated the sensitivity and selectivity of the system, the fundamentals of binding between DNA and GO were hardly addressed. Understanding and controlling binding between biomolecules and inorganic materials is very important in biosensor development. In this thesis, adsorption and desorption of DNA on the GO surface under different buffer conditions including ionic strength, pH, and temperature were systematically evaluated. For instance, adsorption is favored in a lower pH and a higher ionic strength buffer. It was found that once a DNA was adsorbed on the surface, little desorption occurred even in low salt buffers. Even with high pH or temperature, only small percentage of adsorbed DNA can be desorbed. To completely desorb the DNA, complementary DNA is required. The energies and activation energies associated with DNA adsorption/desorption were measured and molecular pictures of these processes were obtained. With the fundamental understanding of the DNA/GO interaction, we demonstrated that it is possible to achieve sensor regeneration without covalent immobilization. In addition, we also achieved the separation of double-stranded DNAs from single-stranded ones without using gel electrophoresis. We also studied the fluorescence property of DNA near the GO surface using covalently attached DNA probes. It was found that the fluorophore quantum yield and lifetime changed as a function of DNA length. This study is important for rational design of covalently linked DNA sensors. This study confirmed that fluorescence quenching by GO occurs in a distance-dependent manner. Energy transfer occurred between the fluorophore and GO to result in reduced quantum yield, shorter lifetime, and lower fluorescence intensity. Although fluorescent sensors based on covalently attached DNA probes on GO have not yet been reported, the study presented here clearly supported its feasibility.

Graphene is a single layer of graphite with many unique mechanical, electrical, and optical properties. In addition, graphene is also known to adsorb wide range of biomolecules including single-stranded DNA. On the other hand, the adsorption of double-stranded DNA was much weaker. To properly disperse in water, graphene oxide (GO) is often used due to its oxygen-containing groups on the surface. Recently, it was discovered that it could efficiently quench the fluorescence of fluorophores that were adsorbed. With these properties, it is possible to prepare DNA-based optical sensors using GO. Majority of the DNA/GO-based fluorescent sensors reported so far were relied on the complete desorption of DNA probes. Even though all these reports demonstrated the sensitivity and selectivity of the system, the fundamentals of binding between DNA and GO were hardly addressed. Understanding and controlling binding between biomolecules and inorganic materials is very important in biosensor development. In this thesis, adsorption and desorption of DNA on the GO surface under different buffer conditions including ionic strength, pH, and temperature were systematically evaluated. For instance, adsorption is favored in a lower pH and a higher ionic strength buffer. It was found that once a DNA was adsorbed on the surface, little desorption occurred even in low salt buffers. Even with high pH or temperature, only small percentage of adsorbed DNA can be desorbed. To completely desorb the DNA, complementary DNA is required. The energies and activation energies associated with DNA adsorption/desorption were measured and molecular pictures of these processes were obtained. With the fundamental understanding of the DNA/GO interaction, we demonstrated that it is possible to achieve sensor regeneration without covalent immobilization. In addition, we also achieved the separation of double-stranded DNAs from single-stranded ones without using gel electrophoresis. We also studied the fluorescence property of DNA near the GO surface using covalently attached DNA probes. It was found that the fluorophore quantum yield and lifetime changed as a function of DNA length. This study is important for rational design of covalently linked DNA sensors. This study confirmed that fluorescence quenching by GO occurs in a distance-dependent manner. Energy transfer occurred between the fluorophore and GO to result in reduced quantum yield, shorter lifetime, and lower fluorescence intensity. Although fluorescent sensors based on covalently attached DNA probes on GO have not yet been reported, the study presented here clearly supported its feasibility.

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 / 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.

>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.

Graphene-based materials have generated tremendous interest in the past decade. Manipulating their characteristics using wet-chemistry methods holds distinctive value, as it provides a means towards scaling up, while not being limited by yield. The majority of this thesis focuses on the surface functionalization of graphene oxide (GO), which has drawn tremendous attention as a tunable precursor due to its readily chemically manipulable surface and richly functionalized basal plane. Firstly, a room-temperature based method is presented to reduce GO stepwise, with each organic moiety being removed sequentially. Characterization confirms the carbonyl group to be reduced first, while the tertiary alcohol is reduced last, as the optical gap decrease from 3.5 eV down to 1 eV. This provides greater control over GO, which is an inhomogeneous system, and is the first study to elucidate the order of removal of each functional group. In addition to organically manipulating GO, this thesis also reports a chemical methodology to inorganically functionalize GO and tune its wetting characteristics. A chemical method to covalently attach fluorine atoms in the form of tertiary alkyl fluorides is reported, and confirmed by MAS 13C NMR, as two forms of fluorinated graphene oxide (FGO) with varying C/F and C/O ratios are synthesized. Introducing C-F bonds decreases the overall surface free energy, which drastically reduces GO’s wetting behavior, especially in its highly fluorinated form. Ease of solution processing leads to development of sprayable inks that are deposited on a range of porous and non-porous surfaces to impart amphiphobicity. This is the first report that tunes the wetting characteristics of GO. Lastly as a part of a collaboration with ConocoPhillips, another class of carbon nanomaterials - carbon nanotubes (CNTs), have been inorganically functionalized to repel 30 wt% MEA, a critical solvent in CO2 recovery. In addition to improving the solution processability of CNTs, composite, homogeneous solutions are created with polysulfones and polyimides to fabricate CNT-polymer nanocomposites that display contact angles greater than 150o with 30 wt% MEA. This yields materials that are inherently supersolvophobic, instead of simply surface treating polymeric films, while the low density of fluorinated CNTs makes them a better alternative to superhydrophobic polymer 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.