Oxidative stress can be defined as a state of disturbance in the pro-oxidant/antioxidant balance in favour of the former, leading to potential damage. Processes associated with oxidative stress involve reactive oxygen species and radicals and can result in elevated levels of oxidatively modified or toxic molecules that can cause cellular malfunction, and even cell death. Destruction of membrane lipids, lipid peroxidation, caused by reactive oxygen species and radicals has been coupled to many diseases and also normal ageing. The measurement of low molecular weight biomarkers of oxidative stress present in complex matrices such as brain tissue, plasma, urine or cerebrospinal fluid is a delicate and difficult task and there is a need for improved analytical tools in this field of research. The major foci of this thesis and the work underlying it are the development of analytical techniques and methods for determining biomarkers for oxidative stress and lipid peroxidation. Aspects of particular concern include the effects of sample treatments prior to analysis, evaluation of the developed methods with respect to possible artefacts, and the scope for results to be misinterpreted. The specific research goals and issues addressed are detailed in five papers, which this thesis is based upon. <b>Paper I</b> focuses on malondialdehyde, describing and evaluating two new simplified sample pre-treatment regimes for the determination of malondialdehyde in rat brain tissue by capillary electrophoresis with UV detection. The effects of sample storing and handling are also considered. <b>Paper II</b> describes the synthesis, characterization and implementation of a new internal standard for the determination of malondialdehyde in biological samples using electrophoretic or chromatographic separation techniques. The usefulness of the internal standard is demonstrated in analyses of rat brain tissue samples. <b>Paper III</b> presents a method for the determination of 4-hydroxynon-2-enal in brain tissue from rats employing micellar electrokinetic chromatography separation and laser-induced fluorescence detection. <b>Paper IV</b> is focused on the development of a new methodology for determining the stereoisomeric F2-isoprostanes in human urine samples employing chromatographic separation on porous graphitic carbon and detection by electrospray ionization-tandem mass spectrometry. The results from this study conflict with the hypothesis that peripheral isoprostanes are elevated in patients with Alzheimer’s disease. <b>Paper V</b> describes porous graphitic carbon chromatography-tandem mass spectrometry for the determination of isoprostanes in human cerebrospinal fluid. A new simplified sample pre-treatment regime, involving a column switching technique, is presented that allows direct injection of a relatively large volume of CSF into the chromatographic system.

As a potential solution to the global energy and environmental pollution, design and synthesis of artificial photocatalysts with high activities have attracted increasing scientific interests worldwide. In recent years, the graphitic carbon nitride (g-C3N4) has shown new possible applications in the photocatalytic field due to its unique properties. However, the photocatalytic efficiency of the pristine g-C3N4 is greatly limited by the high recombination rate of the photo-induced electron-hole pairs. In this thesis, the aim is to design and fabricate efficient g-C3N4 based photocatalysts with enhanced photocatalytic activities under a visible light irradiation. In order to achieve this goal, two strategies have been employed in the present thesis. First, the as-obtained g-C3N4 was used as the host material to construct staggered-aligned composite photocatalysts by selecting semiconductors with suitable band positions. By this method, three kinds of g-C3N4-based composite photocatalysts such as g-C3N4/ZnS nanocage, g-C3N4/m-Ag2Mo2O7 and g-C3N4/MIL-88A were successfully fabricated. Second, the microstructure of the g-C3N4 was modified by the H2O2-treatment at an elevated temperature and ambient pressure. In this study, the g-C3N4 was prepared by a simple pyrolysis of urea. As for all the as-synthesized phtocatalysts, the structures, morphologies and the optical properties were carefully characterized by the following techniques: XRD, SEM, TEM, FT-IR and DRS. Also, the band edge positions of m-Ag2Mo2O7 and MIL-88A were studied by the Mott-Schottky methods. Thereafter, the photocatalytic activities were evaluated by using a solution of rhodamine B (RhB) as a target pollutant for the photodegradation experiments performed under a visible light irradiation. The results showed that all the aforementioned g-C3N4-based photocatalysts exhibited enhanced photocatalytic activities in comparison with the pristine g-C3N4. For the case of the g-C3N4-based composite photocatalysts, the enhancement factor over the pristine g-C3N4 can achieve values ranging from 2.6 to 3.4. As for the H2O2-treated g-C3N4, the degradation rate constant can be 4.6 times higher than that of the pristine g-C3N4. To understand the key factors in new materials design, we also devote a lot of efforts to elucidate the basic mechanisms during the photocatalytic degradation of organic pollutant. Based on the results of the active species trapping (AST) experiments, the main active species in each photocatalytic system were determined. In the g-C3N4/m-Ag2Mo2O7 and the g-C3N4/MIL-88A system, three kinds of active species of ·O2-, h+ and ·OH were found to be involved in the photocatalytic reaction. Among them, the ·O2- and h+ were the main active species. In the g-C3N4/ZnS and H2O2-treated g-C3N4 photocatalytic systems, the main active species was determined as the ·O2-. The reaction pathways of these active species were also demonstrated by comparing the band edge positions with the potentials of the redox couple. In addition, the relationship between the active species and the photocatalytic behaviors of N-de-ethylation and conjugated structure cleavage were studied. Finally, possible mechanisms to explain the enhanced photocatalytic activities were proposed for each photocatalytic system. The results in this thesis clearly confirm that the photocatalytic activity of the g-C3N4 based photocatalyst can efficiently be enhanced by constructions of staggered-aligned composites and by modification of the microstructure of the g-C3N4. The enhanced photocatalytic performance can mainly be ascribed to the efficient separation of the photo-induced electron-hole pairs and the increase of the active sites for the photocatalytic reaction. / <p>QC 20150909</p>

Graphitic carbon nitride (g-C3N4) heterojunction composites with the semiconducting metal oxides, CeO2, ZnO and TiO2 are prepared in situ by co-calcination of the precursor materials or by a solvothermal method. The structural, morphological and the optical properties of the prepared materials are studied using various microscopy and spectroscopy techniques. The synthesized composite materials, CeO2/g-C3N4, ZnO/g-C3N4 and TiO2/g-C3N4 are more efficient in the photocatalytic degradation of the water pollutants indigo carmine (IC) and atrazine than the pure metal oxide, g-C3N4, or their physical mixtures. The CeO2/g-C3N4 and ZnO/g-C3N4 composites also exhibit improved degradation efficiencies of atrazine as compared to the individual metal oxide or g-C3N4 materials. The improved photocatalytic activity of the composites are attributed to the effective electron-hole charge separation within composite heterojunction, resulting from the well matched energy levels of the metal oxide and g-C3N4. This strategy could be helpful for the synthesis of other metal oxide and g-C3N4 composites for photocatalytic applications.

Carbonation is a natural phenomenon observed in carbon structures, and it is a result of the combined presence of CO2 in the environment and moisture. Carbonation may lower the pH of the concrete to such an extent that the passivation layer of the steel is depleted and the onset of frame corrosion is observed, which is one of the major pathologies in reinforced concrete structures. This study aimed to assess accelerated carbonation in concrete samples prepared with rice husk ash (RHA) with different concentrations of graphitic carbon and different curing periods. Concrete samples were prepared with 5%, 10%, 20% and 30% of RHA as a substitution for high early strength Portland cement (Brazilian grade CPV-ARI), different water/binder ratios (0.35; 0.50; 0.65) and wet cured for 3 and 7 days. In addition, two mixes with 5% and 10% silica fume as substitutions for Portland cement were used for comparison. Accelerated carbonation was performed in 10x10cm test specimens (TS) that were preconditioned as determined by standard RILEM TC 116-PCD. These TSs were stored in a controlled temperature chamber (RH 65 ± 2%, temperature 23 ± 2°C, 3% CO2 b/v), where they were exposed to CO2 for 4, 8, 12 and 16 weeks. The mercury intrusion porosimetry test was performed in 4x8cm TSs, which were tested after 91 days. Overall results show improved accelerated carbonation performance in the samples prepared with rice husk ash with lower concentrations of graphitic carbon, even though figures were similar. Higher concentrations of RHA in the concrete samples resulted in increased carbonation coefficients for all mixes investigated. For samples of the same strength values, a reduction in carbonation coefficients was observed when their compressive strength increased from 40 MPa to 50 MPa, This was seen in all mixes investigated for the curing period used, which shows that the use of longer curing periods improves the service life of structures. / O fenômeno da carbonatação acontece naturalmente em estruturas de concreto, exigindo apenas certa concentração de CO2 no ambiente e determinado teor de umidade. Este fenômeno provoca a redução do pH do concreto a níveis que podem causar a eliminação da película passivadora do aço, podendo ocorrer a instalação do mecanismo de corrosão das armaduras, uma das principais patologias em estruturas de concreto armado. O presente estudo teve por objetivo avaliar a carbonatação acelerada em concretos compostos com cinza de casca de arroz com diferentes teores de carbono grafítico e períodos de cura. Os concretos foram moldados com cinza de casca de arroz com teores de 5%, 10%, 20% e 30% em substituição ao cimento Portland de alta resistência inicial (CPV-ARI), com diferentes relações água/aglomerante (0,35; 0,50; 0,65) e com períodos de cura em câmara úmida de 3 e 7 dias. Duas misturas com 5% e 10% de sílica ativa em substituição ao cimento Portland foram empregadas como parâmetro de comparação. A carbonatação acelerada foi realizada em corpos-de-prova 10x10cm que passaram pelo pré-condicionamento, conforme preconiza a norma RILEM TC 116-PCD, após serem inseridos em uma câmara climatizada com uma umidade de (65 ± 2)%, temperatura de (23 ± 2)°C e uma concentração de CO2 de 3% em volume, por períodos de 4, 8, 12 e 16 semanas de exposição ao CO2. Já o ensaio de porosimetria por intrusão de mercúrio foi realizado em corpos-de-prova medindo 4x8cm, que foram ensaiados aos 91 dias. Os resultados mostram, de maneira geral, um melhor desempenho da cinza de casca de arroz com menor teor de carbono grafítico frente à carbonatação acelerada, apesar da proximidade dos resultados. O aumento do teor substituição, nos concretos com cinza de casca de arroz, provocou um aumento dos coeficientes de carbonatação, para todas as misturas investigadas. Em igualdade de resistência, constata-se que houve uma diminuição dos coeficientes de carbonatação quando a resistência à compressão aumenta de 40 MPa para 50 MPa para todas as misturas analisadas e para o prazo de cura adotado, evidenciando que o prolongamento no período de cura do concreto reflete diretamente no aumento da vida útil da estrutura.

Pure g-C3N4 and MoS2 modified g-C3N4 materials were synthesized using a facile heating method and a low-temperature hydrothermal method, respectively. The obtained samples were characterized by XRD pattern and N2 adsorption-desorption technique at 77K. The adsorption and photocatalytic performance of all obtained samples were investigated by discoloration of direct black 38 dye in the dark and under visible light irradiation. The results showed that all obtained samples exhibited good discoloration efficiency of direct black 38 dye. The two factors including pH values and Mo loading effected mainly on elimination efficiency of direct black 38 dye. MoS2 modified g-C3N4 materials possessed the more enhanced adsorption and photocatalytic performance in comparison to pure g-C3N4 at pH value of 3.5, with adsorbent dosage of 0.1 g/L. Furthermore, it was found that the adsorption process and photo-catalysis simultaneously occurred under visible light irradiation and followed up a pseudo-second-order kinetic reaction of Langmuir - Hinshelwood model. / g-C3N4 và g-C3N4 biến tính bởi MoS2 đã được tổng hợp theo phương pháp nung đơn giản và phương pháp thủy nhiệt ở nhiệt độ thấp tương ứng. Các mẫu tổng hợp đã được đánh giá đặc trưng bởi các phương pháp hiện đại như giản đồ nhiễu xạ tia X, phương pháp hấp phụ-khử hấp phụ N2 ở 77K. Khả năng hấp phụ và quang hóa xúc tác của các vật liệu tổng hợp đã được nghiên cứu bởi quá trình phân hủy màu thuốc nhuộm direct black 38 trong điều kiện bóng tối và chiếu sáng bởi ảnh sáng nhìn thấy của đèn chiếu sáng sợi đốt wolfram (220V-100W). Các kết quả nghiên cứu chỉ ra rằng các mẫu tổng hợp đều có hiệu suất xử lý màu cao đối với thuốc nhuộm direct black 38. Hai yếu tố gồm pH dung dịch và hàm lượng MoS2 ảnh hưởng chính đến hiệu suất xử lý màu direct black 38. g-C3N4 biến tính bởi MoS2 luôn thể hiện hiệu suất hấp phụ và quang hóa cao hơn so với g-C3N4 tinh khiết. Hơn nữa, khi được chiếu sáng bởi ánh sáng nhìn thấy thì quá trình hấp phụ và quá trình quang hóa thuốc nhuộm direct black 38 trên các vật liệu tổng hợp đã xảy ra đồng thời và mô hình Langmuir - Hinshelwood động học bậc 2 đã được đề xuất cho quá trình này.

The results of this study show that novel mesoporous carbons were obtained as inverse replica of SBA-15, HMS and MCM-41 silica templates, with a large pore diameter (2-4 nm), a BET surface area of 1867, 874 and 910 m2g –1 respectively for CA_SBA-15_LPG_105, CA_HMS_LPG_80 and CA_MCM- 41_LPG_80 with bimodal pore size distribution (PSD) in the mesopores range. The results obtained show that mesoporous carbon with graphitic structures can be synthesized via the LPG route.

The results of this study show that novel mesoporous carbons were obtained as inverse replica of SBA-15, HMS and MCM-41 silica templates, with a large pore diameter (2-4 nm), a BET surface area of 1867, 874 and 910 m2g –1 respectively for CA_SBA-15_LPG_105, CA_HMS_LPG_80 and CA_MCM- 41_LPG_80 with bimodal pore size distribution (PSD) in the mesopores range. The results obtained show that mesoporous carbon with graphitic structures can be synthesized via the LPG route.

The path towards mitigation of anthropogenic greenhouse gas emissions lies in the transition from conventional to sustainable energy resources. The Hydrogen Economy, a cyclic economy based on hydrogen as a fuel, is suggested as a tool in the necessary energy transition. Photocatalysis makes use of sunlight to promote thermodynamically non-favoured reactions such as water splitting, allowing for sustainable hydrogen production. Harvesting thermal energy along with photonic energy is an interesting concept to decrease the activation energy of water splitting (i.e. ΔG = + 237.2 kJ∙mol−1). This work aims to confront this hypothesis in a gas phase photo-thermal reactor designed specifically for this study. The photocatalyst chosen is graphitic carbon nitride (g-C3N4), an organic semiconductor possessing a narrow band gap (i.e. 2.7 eV) as well as a band structure which theoretically permits water splitting. The photocatalytic performance of Pt/g-C3N4 for hydrogen evolution was tuned by altering its synthetic temperature. Electron paramagnetic resonance was used to gain insight on the evolution of the photocatalyst activity with synthesis temperature. Then, gold nanoparticles were deposited on g-C3N4 surface. Localized surface plasmon resonance properties of gold nanoparticles are reported in the literature to be influenced by temperature. Therefore Au/g-C3N4 appeared as a promising candidate for photo-thermal water splitting. X-ray spectroscopy unveiled interesting observations on the gold oxidation state. Moreover, under specific reduction conditions, gold nanoparticles with a wide variety of shapes characterized by sharp edges were formed. Finally, the development of the photo-thermal reactor is presented. The design process and the implementation of this innovative reactor are discussed. The reactor was successfully utilized to probe photoreactions. Then, the highly energy-demanding photocatalytic water splitting was proven not to be activated by temperature in the photo-thermal apparatus.

Philosophiae Doctor - PhD / The results of this study show that novel mesoporous carbons were obtained as inverse replica of SBA-15, HMS and MCM-41 silica templates, with a large pore diameter (2-4 nm), a BET surface area of 1867, 874 and 910 m2g–1 respectively for CA_SBA-15_LPG_105, CA_HMS_LPG_80 and CA_MCM- 41_LPG_80 with bimodal pore size distribution (PSD) in the mesopores range. The results obtained show that mesoporous carbon with graphitic structures can be synthesized via the LPG route. / South Africa

M. Tech. (Department of Chemistry, Faculty of Applied and Computer Sciences), Vaal University of Technology. / The presence of organic and inorganic pollutants in aqueous environments is one of the major challenges confronting man. It is therefore important to develop sensitive, versatile and cheap techniques for their detection. Arsenic (III), 2-chlorophenol (2-CP) and sulfamethoxazole (SMX) are priority pollutants that pose health threats to humans and animals. This study was thus aimed at exploring two promising carbon nanomaterials as electrode modifiers for the electrochemical sensing of arsenic (III), 2-CP and SMX in water. Glassy carbon electrode (GCE) was modified with a nanocomposite of carbon dots (CDs) and graphitic carbon nitride (g-C3N4) and used as a sensor for the analytes in aqueous media. The CDs was prepared by a facile one-pot hydrothermal method using pine cone as the carbon source; g-C3N4 and g-C3N4/CDs nanocomposite were prepared via the microwave irradiation heating method. CDs, g-C3N4 and g-C3N4/CDs were dropped-dried on the surface of bare GCE. Transmission electron microscopy (TEM), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) were used to characterize the prepared materials. GCE, g-C3N4/GCE, CDs/GCE and g-C3N4/CDs/GCE electrodes were electrochemically investigated by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) using a ferrocyanide [Fe (CN) 6]3-/4- redox probe. The current and the reversibility of the redox probes were enhanced in the presence of modifiers. The electrochemical behavior of arsenic (III), 2-CP and SMX on different electrodes (GCE, CDs/GCE, g-C3N4/GCE and g-C3N4/CDs/GCE) were investigated by differential pulse voltammetry (DPV) under optimized conditions in a phosphate buffer solution (pH 7.6, 6 and 5 for 2-CP, As (III) and SMX respectively). The results demonstrated that the g-C3N4/CDs/GCE electrode significantly enhanced the oxidation peak current of all three analytes. The detection sensitivity of the analytes was greatly improved, suggesting that this new modified electrode has great potential in the determination of trace level of arsenic (III), 2-CP and SMX in water. The oxidation peak currents displayed a linear relationship to concentrations for 2-CP (0.5 - 2.5 μM, R2=0.958, n=5), arsenic (III) (2 - 10 μM R2=0.978, n=5) and SMX (0.3 - 1.3 μM R2=0.9906, n=5). The detection limits of 0.62 μM, 1.64 μM and 0.10 μM were obtained for 2-CP, arsenic and SMX, respectively. Phenol and 4-chloro-3-methyl-phenol were found to interfere with the detection of 2-CP, while, Cu2+, Zn2+, Pb2+ and Cd2+ were the only significant ions that interfered with the electrochemical detection of arsenic (III). EDTA was used as a ligand to mask the interference effects of copper, cadmium, lead and zinc on arsenic sensing. The modified electrode (g-C3N4/CDs/GCE) was used to determine arsenic, 2-CP and SMX in spiked tap and effluent water samples by the standard addition method and the results showed percentage recoveries varying from 93-118% for 2-CP, 98-100% for arsenic and 80-105% for SMX. The outcomes of this study established that the nanocomposite material represents an easy and sensitive sensing platform for the monitoring of arsenic (III), 2-CP and SMX in aqueous media.