Orientador: Carlos Alberto Luengo / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-04T03:18:18Z (GMT). No. of bitstreams: 1 Maestro_LuisFernando_M.pdf: 1958027 bytes, checksum: cb56dc89c2faea48a95f3fa1350d5666 (MD5) Previous issue date: 2005 / Resumo: Desde a sua descoberta em 1991, os Nanotubos de Carbono (NTC) têm atraído muito a atenção da comunidade científica, devido as suas propriedades. Neste trabalho é apresentada uma breve revisão das pesquisas em NTC e algumas definições básicas relevantes para a sua estrutura e propriedades. Em vista da utilização deste material em uma futura aplicação e devido ao interesse do grupo na área de energia, é apresentado o estado da arte do armazenamento de Hidrogênio e, em particular, no armazenamento em sólidos de grande área superficial, classe a qual os NTC pertencem. Apresentam-se as modificações realizadas em um Reator de Arco Elétrico (Forar II) para se realizar a Síntese de NTC, são relatadas as experiências e a caracterização das amostras obtidas utilizando-se Microscopia Eletrônica de Varredura e Espectroscopia Raman. / Abstract: Since their discovery in 1991 Carbon Nanotubes (CNT) have received increasing attention by the scientific community due to their properties. Here is presented a brief review of ongoing CNT research, and basic definitions useful to understand their structure and significant properties. Because of future applications in the energy area, are presented developments in Hydrogen storage, more specifically its adsorption in solids with large internal surface areas, a characteristic of CNT materials. Modifications of the existing FORAR II to obtain CNT by the electric arc method are presented, and a description of the routines employed to obtain CNT. The characterization of catalysts and CNT by Scanning Electron Microscopy and Raman Spectroscopy are presented and discussed. / Mestrado / Física da Matéria Condensada / Mestre em Física

D.Phil. / The topic of this thesis centres around the construction of carbocycles from carbohydrates involving the use of SmI2 as reagent. Carbocycles, in particular functionalised cyclopentanes, form important substructures in a plethora of natural compounds, many of which are biologically active. The conversion of carbohydrates into stereodefined cyclopentanoids represents an attractive means of generating this type of compounds. The SmI2-mediated cyclisation of hex-5-enals, derived from suitably substituted carbohydrate precursors via a zinc-mediated Grob-fragmentation reaction, was found to be a facile reaction generating stereodefined cyclopentanols in moderate yields. The effect of varying stereochemistry and substitution pattern on the outcome of the cyclisation was also investigated. In order to achieve a more efficient route to the cyclic products, a Grobfragmentation based on SmI2 was investigated. The substituent at the anomeric position of the carbohydrate was found to be crucial to the success of the fragmentation. This protocol gave rise to conditions under which fragmentation and cyclisation could be effected in a tandem fashion mediated by SmI2. Various attempts at intermolecular trapping of the organosamarium species generated after cyclisation proved fruitless. In order to circumvent these obstacles a tandem radical bicyclisation reaction was investigated for the construction of highly functionalised bicyclic ring systems. This approach proved to be a successful alternative for incorporating functionality at the exocyclic carbon atom which is otherwise difficult to achieve. The second part of this work is directed at the synthesis of stereodefined alkoxyaminocyclopentanes from carbohydrates promoted by SmI2. The intramolecular trapping of a radical by an oxime ether represents an attractive means of generating these compounds. The preparation of the iodo oxime ether starting materials from suitably substituted carbohydrates is discussed. The radical cyclisation under the action of SmI2 was found to be a facile process providing a rapid route to aminocyclopentanes which are used as key intermediates for the preparation of carbocyclic glycosidase inhibitors and analogues. In the third part of this work, the construction of functionalised cyclobutanols from carbohydrate precursors is discussed. The 4-exo-trig cyclisation holds promise as a general route for the stereoselective synthesis of highly functionalised cyclobutanols. Initial attempts at the 4-exo-trig cyclisation proved fruitless. It was subsequently established that the radical acceptor plays a crucial role in the success of the transformation. This led to the isolation of the first carbohydrate derived cyclobutanol. The final part of this thesis involves the development of a new approach for the conversion of suitably functionalised pentose derivatives into chiral cyclopentanols. Cyclisation of a ribose derived hemiacetal via its in situ formed hex-5-ynal is discussed. This approach opens new possibilities for the construction of complex carbocycles from readily available carbohydrate starting materials.

The carbon isotope fractionation in the coccolithophore Emiliania huxleyi constitutes the basis for the paleo-pCO₂ barometry. Under the premise that the carbon isotope fractionation is dependent on the availability of dissolved CO₂, measurements of the carbon isotope ratio of sedimentary alkenones can potentially produce a proxy record of ancient atmospheric CO₂ levels. However, recent studies, including this thesis have suggested that other factors than CO₂ may influence the carbon isotope fractionation in Emiliania huxleyi and hence the validity of the proxy. In this thesis work the effects of irradiance on carbon isotope fractionation were studied in batch cultures of non-calcifying Emiliania huxleyi. It was found that the biomass becomes more ¹³C depleted as the light intensity decreases. This is in agreement with utilization of CO₂ via passive diffusion where fractionation is a function of the rate of diffusion of CO₂ into the cell relative to the rate of carbon utilization. However, results reported in the literature for a calcifying strain show the opposite trend with a ¹³C enrichment of the biomass. These results suggest that the carbon utilization of the calcifying strain of Emiliania huxleyi differ from that of the non-calcifying strain. This is supported by observations in the literature, which indicates a connection between the process of calcification and the supply of carbon for photosynthesis. A mechanism for the effect of calcification on carbon isotope fractionation in light limited cells is presented here. The mechanism is based on the fact that the calcification and photosynthesis respond differently to light limitation. This difference leads to an imbalance in the rate of calcification to the rate of photosynthesis ratio (C/P), which ultimately affects the availability of CO₂ inside the cell. Apart from light, the availability of nutrients has also been shown to affect calcification. Nutrient starved cells will enhance calcification to the degree that the C/P ratio changes, thus affecting the internal concentration of CO₂. To study the effect of these environmental parameters on carbon isotope fractionation, C₃₇:₂-alkenones were extracted from samples of marine particulate organic matter. The particulate organic matter was collected together with information of the environmental conditions during three cruises in the North-East Pacific and during a Pacific transect from Victoria B.C. to Guam. Results from the NE Pacific show a lower carbon isotope fractionation in samples collected at the bottom of the euphotic zone compared to samples collected in the mixed layer. This may be an expression of the effect of light limitation. In this work carbon isotope fractionation shows no correlation with dissolved CO₂. Instead, a correlation with the ratio of phosphate concentration to concentration of dissolved CO₂ ([PO3/4⁻/[CO₂]aq.) was observed. Nitrate availability appears to play an important role in maintaining this relationship as in the absence of nitrate the carbon isotope fractionation is lower than can be predicted from the relationship relating carbon isotope fractionation to [PO3/4⁻/[CO₂]aq. The C₃₇:₂-alkenone based results from the Pacific transect shows a strong correlation between carbon isotope fractionation and phosphate. This correlation is independent of the concentration of dissolved CO₂, implying a nutrient dominated control of isotope fractionation. However, this control may not be typical as the transect passed through waters with very low nutrient levels. Therefore, the results seen here may be a consequence of extreme nutrient conditions. In conclusion, the results presented in this thesis challenge the classical belief that the carbon isotope fractionation in Emiliania huxleyi is a direct function of the availability of dissolved CO₂ by suggesting that the observed isotope fractionation is a result of a complex interaction between environmental factors such as irradiance and the availability of nutrients. In particular, a correlation between phosphate concentration and carbon isotope fractionation has been found. / Graduate

No description available.

The work undertaken involved the exploration of CO2 electroreduction systems, focussing heavily upon electrocatalysis utilising an array of electrochemical, spectroelectrochemical and spectroscopic techniques. The identification and characterisation of a relatively inexpensive and simple electrocatalyst for CO2 reduction was achieved, with the optimisation and development undertaken in such a manner that not just the electrocatalytic species, but also the entire electrochemical system was investigated, in order to determine and better understand the roles played by the various components. The complex of interest, Mo(CO)4bpy, represents the first molybdenum based molecular electrocatalyst reported to be active toward CO2 reduction, despite the prominence of Mo in enzymes with analogous function. The electrochemical characterisation of the complex in the both the presence and absence of CO2 was undertaken, yielding valuable information on the redox behaviour of the complex within the non-aqueous system in which it was employed and highlighting previously unreported features such as a third reduction and new reoxidation attributed to the reoxidation of a tricarbonyl anionic species. Non-aqueous solvents were chosen as they provide greater CO2 solubility than water with portions of the investigation undertaken in tetrahydrofuran, THF, then moving to the less widely used N-methylpyrrolidone, NMP. NMP is significantly less volatile than THF and has a large negative electrochemical window so is ideal for looking at reduction processes and, importantly, is also used as a commercial CO2 scrubbing solvent. Upon addition of CO2 to the Mo(CO)4bpy system there was an observable lowering of the overpotential by over 300 mV, and significant increase in CO2 associated current when compared to that for ‘direct’ CO2 reduction within the same system, at the reduction potential associated with the first reduction of the tetracarbonyl bipyridyl species. The confirmation of the anionic radical as the active species was attained through DFT calculation and EPR spectroelectrochemistry. Under inert gas the spectrum rapidly generated upon application of the first reduction potential is consistent with the expected response for the radical anionic [Mo(CO)4bpy] •−. When the system is saturated with CO2 this radical is no longer detectable. This supports the idea that the unpaired electron is transferred from the [Mo(CO)4bpy]•− to the CO2 molecule and also suggests that this transfer is rapid as no adduct is detectable via EPR even at reduced temperature (240 K). This is in keeping with the rate constants calculated from the voltammetric measurements made. The stability and activity toward CO2 reduction exhibited by Mo(CO)4bpy displayed a strong dependence on working electrode material, with gold proving optimal, indicative of adsorption being significant in the process. Optimisation of both the catalyst structure and the solvent and electrolyte system were also explored, as well as the (somewhat less directly related) comparison of various sources of diffusivity data.

No description available.

Guided by increasing legislation, the analysis of food borne toxins, including mycotoxins, seeks to address market related demands for the development of analytical systems to monitor this threat to food security and human health. This Thesis is directed at the assessment of the application of electrochemistry for direct electroanalysis and characterisation of the mycotoxin citrinin (CIT) in aqueous media as well as fundamental investigations of the surface of polished and oxidised glassy carbon electrodes (GCE). This study provides the first known account of CIT detection through electrochemical methods. Although electrochemically active, CIT current responses (Ip) were highly irreproducible at polished GCE with a coefficient of variation (C.V.) of 20.16 %. As stability of Ip across multiple electrode preparations is a key requirement in electroanalysis, investigations were directed at attaining stability in CIT Ip. Achieving stability in CIT Ip was investigated via two approaches, including: accounting for Ip variability between electrode preparations as a result of variable GCE surface conditions as a post-data-acquisition analysis and secondly, removing Ip variability through modification of GCE. Accounting for variability in Ip was investigated through the application of double layer capacitance as an indicator of the activity of an electrode, and in so doing serving as a relative mediator of Ip responses between electrodes. Application of this procedure dropped CIT C.V. to a third of starting value across polished GCE (C.V. = 7.18 %), chemically oxidised GCE (Pi-GCE, C.V = 8.47 %) and functionalised multi-walled carbon nanotube modified GCE (fMWCNT, C.V. = 25.79 %) and was effective with analysis of structurally distinct molecules, 2,4-dimethylaniline (2,4-DMA) and 1,2,4-trihydroxybenzene (Triol). Furthermore, it afforded the ability to determine discreet solution overlapping data sets of Ip. Stabilising Ip through GCE surface modification was achieved by anodic electro-oxidation of GCE and allowed for direct electroanalysis of CIT and subsequent characterisation and analysis of CIT in complex media as it reduced C.V. of CIT Ip to 0.73 %. Fundamental investigations of the electrode surface condition are described such that the source of variability could be identified and the interactions of CIT with the electrode understood. Two surface oxidation techniques were applied in modification of GCE; anodic electro-oxidation (EOx GCE) and chemical oxidation using piranha solution (Pi-GCE), analysis of which has previously not been reported. Fundamental analyses to determine surface morphology and chemistry of Pi-GCE, EOx-GCE and polished GCE were conducted using high resolution scanning electron microscopy (HRSEM), scanning electrochemical microscopy (SECM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), fourier transform infrared spectroscopy (FTIR) and via electroanalytical methods. These studies showed that both oxidation procedures introduced a variety of oxide species at GCE surface, and further that the extent of those species was similar with total % O being 27.67 % and 33.47 % at Pi-GCE and EOx-GCE respectively. Although chemically similar, each surface was morphologically distinct. Electrochemical analyses at the surfaces revealed Pi-GCE to behave more similarly to polished GCE than EOx-GCE. As CIT responses were found to be stable at EOx-GCE (C.V. = 0.73 %) as opposed to Pi-GCE (C.V. = 22.87 %), stability of CIT Ip was likely to be as a result of a physical interaction with electrode morphology rather than interaction on a chemical basis. Morphological analyses revealed polished GCE and Pi-GCE to be highly morphologically irregular at the micro-scale. Although comparatively smooth, the surface morphology of EOx-GCE does not account for the stability of Ip. This study thus proposed a theory to describe the mechanism by which the limited conductivity and porosity of EOx-GCE allow for it to provide a relatively stable surface area within the oxide layer, adjacent to the electrode surface, and thus provided a stable platform for electroanalysis. Voltammetric characterization of CIT at EOx-GCE revealed that anodic oxidation in aqueous media involved an uneven number of electrons to protons via an ECE mechanism. This was illustrated to be nt = 2e- accompanied by the transfer of 1H⁺ per molecule oxidised. A proposed reaction scheme for the initial stages of CIT oxidation was suggested to involve both hydroxyl and carboxyl moieties of the CIT molecule. CIT oxidation was shown to arise as a result of a relatively complex mass transport regime which included both adsorptive and diffusive derived Ip₁. The LOD in buffered aqueous media was found to be 16 nM, a highly competitive result in relation to chromatographic techniques. Further application of EOx-GCE in complex media illustrated that CIT associates non-specifically with the components of food samples, primarily proteins. As a result of this, extraction of CIT from such media is mandatory. Liquid-liquid extraction illustrated a recovery in CIT Ip₁ and in so doing provided a means of accurately and sensitively detecting CIT from food samples with an LOD of 20 nM. These responses were corroborated by HPLC analyses on the same extractions and illustrate the applicability of electroanalysis as an analytical technique.

A computational investigation of some optoelectronic applications of carbon nanotubes (CNT) is presented, including CNT-based solar cells and biosensors. The results could be used to evaluate the performance of CNT devices and clarify the necessity of further experimental research in this area. A coaxially-gated CNT field-effect transistor (CNFET) forms the basic structure of the devices modeled in this thesis. Diffusive transport is present in long-channel devices, as in our case, while the quantum mechanical effects are mainly present in the form of tunneling from Schottky-barrier contacts at the metal-CNT interfaces. Band-to-band recombination of electron-hole pairs (EHP) is assumed to be the source of electroluminescence. In a first-order approximation, protein-CNT interactions are modeled as the modification of the potential profile along the longitudinal axis of CNTs due to electrostatic coupling between partial charges, in the oxide layer of the CNFET, and the nanotube. The possibility of electronic detection is evaluated. The electroluminescence of the CNT is proposed as an optical detection scheme due to its sensitivity to the magnitude and the polarity of the charge in the oxide. The validity of the model is argued for the given models. A value for the minimum required size of a computational window in a detailed simulation is derived. The structure of an electrostatically gated p-i-n diode is simulated and investigated for photovoltaic purposes. The absorbed power from the incident light and the interaction between the nanotubes is modeled with COMSOL. The results are interpreted as a generation term and introduced to the Drift-Diffusion Equation (DDE). We have observed behavior similar to that in an experimentally-realized device. The performance of CNT-based solar cells under standard AM 1.5 sunlight conditions is evaluated in the form of an individual solar cell and also in an array of such devices. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

Hydroxyapatite (HA) is a biologically active ceramic that is used in surgery to replace bone. While HA promotes bone growth along its surface, its mechanical properties are not sufficient for major load-bearing medical devices. Carbon nanotubes (CNTs), as one of the strongest and stiffest materials available, have the potential to strengthen and toughen HA, thus expanding the range of clinical uses for the material. Furthermore, studies have suggested that the nanotubes themselves possess some bioactive properties. This work sought to develop and characterise HA-CNT composites in four main areas: 1) production and characterisation of green materials, 2) investigation of appropriate sintering atmospheres, 3) evaluation of mechanical properties, and 4) assessment of biological response to in vitro cell culture. HA was synthesised by a precipitation reaction between Ca(OH)2 and H3PO4, and multi-walled CNTs were produced by chemical vapour deposition. Composites were produced by adding the CNTs to the Ca(OH)2 solution as the HA was precipitating. Both as-made (nfCNTs) and acid-treated CNTs (fCNTs) were used to make composites with loadings of 0.5-5 wt.% CNTs. The resulting slurry was shear mixed and then processed to make a powder. The powder was then uniaxially pressed into tablets of ~45% theoretical density. Characterisation of the green material with XRD and FTIR found that the primary phase was HA which was well hydroxylated. The powder particles were found to have a bimodal size distribution, and all materials had similar surface areas, as determined by BET. Composites made with fCNTs were found to have a better dispersion of CNTs in the HA matrix and better interaction between the HA and CNTs compared with nfCNT composites. CNTs oxidise at the high temperatures needed to sinter HA, yet water is necessary to prevent dehydroxylation and decomposition of the HA. Using 5 wt.% fCNT composite, fourteen sintering atmospheres were investigated to determine their effect on phase purity, hydroxylation, sintered density, and remaining CNT content after sintering. An atmosphere of CO + H2 bubbled through ice water resulted in optimal properties. Additionally, it was found that increasing the gas flow rate and the number of samples sintered in one batch increased CNT retention. However, this came at the expense of the density of the sintered samples, as composites with a higher CNT content were more porous. To optimise the composite microstructure for mechanical studies, six different sintering time/temperature profiles were examined to determine their effect on density (balanced with CNT retention) and grain size. HA and both nfCNT and fCNT composites with CNT loadings of 0.5, 1, 2 and 5 wt.% were produced using the optimised atmosphere and profile, and then tested to determine tensile strength (using diametral compression) and hardness, and to look for evidence of toughening. It was found that CNTs had little reinforcing effect; instead, mechanical behaviour results were mainly attributed to differences in porosity, due at least in part to the CNTs' presence. The in vitro cellular response to the materials was examined by culturing human osteoblast-like cells on HA and nfCNT (0.88 wt.%) and fCNT (3.3 wt.%) composites for 12 days. Cells were found to attach and grow well on HA and the nfCNT composite, with slightly enhanced response on the composite. The fCNT composite, on the other hand, showed a decrease in cell viability between days 1 and 12. These results were mainly attributed to the effects of a lower local pH due to remnant acid on the fCNTs and differences in material characteristics, such as CNT loading and surface roughness. This systematic study of the production and properties of HA-CNT composites has resulted in improved understanding of the production and processing of these materials and the effects of a wide range of sintering atmospheres on their characteristics. Additionally, it has yielded interesting preliminary results of their mechanical reinforcement potential and biological behaviour.

No description available.