School of Chemistry and Physics

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A study on the atmospheric and environmental impacts of aerosol, cloud and precipitation interaction.
(2022) Yakubu, Abdulaziz Tunde.; Chetty, Naven.
Understanding the mechanisms and processes of aerosol-cloud-precipitation interactions (ACPI) is essential in the determination of the specific role of aerosols in modulating extreme weather events and climate change in the long run. Atmospheric aerosols are mainly of various types and are emitted from differing sources. Considering they commonly exist in the heterogeneous forms in most environments, they significantly influence the incoming solar energy and the general perturbation of the clouds depending on their constituents. Thus, a systemic identification and characterisation of these particles are essential for proper representation in climate models. To better understand the process of climate change, this research explores the climate diversity of South Africa to examine aerosol sources and types concerning the atmospheric aerosol suspension over the region and their role in clouds and precipitation formation. The study further provided answers to the cause of extreme precipitation events, including drought and occasional flooding experienced over the region. Also, an insightful explanation of the process of ACPI is provided in the context of climate change. Furthermore, the research found that the effective radiative forcing (RF) over South Africa as monitored in Cape Town and Pretoria is negative (i.e., cooling effect) and provided an analysis of the cause. Similarly, the validation of some satellite datasets from MISR (Multiangle Imaging Spectroradiometer) and MODIS (Moderate Resolution Imaging Spectroradiometer) instruments against AERONET (Aerosol Robotic Network) is conducted over the region. Although a significant level of agreement is observed for the two instruments, intense improvements are needed, especially regarding measurements over water surfaces. Finally, the study demonstrated the proficiency of effective rainfall prediction from satellite instrument cloud datasets using machine learning algorithms.
Assessment of antiretroviral drugs uptake by vegetables from contaminated soil and their adsorption by exfoliated graphite in river and wastewater.
(2022) Kunene, Philisiwe Nganaki.; Mahlambi, Precious Nokwethemba.
This study was directed toward vegetable uptake of the commonly used antiretroviral drugs (ARVDs), abacavir, nevirapine, and efavirenz. Antiretroviral drugs are used to treat the human immune-deficiency virus (HIV). South Africa (SA) is one of the countries with a high number of infected people on ARV therapy, therefore, the ARVDs are anticipated to be existing at high concentrations in the South African environment than in other countries worldwide. In recent years, the presence of ARVDs in the environment has drawn attention; hence studies have reported their presence in aquatic environments while very few studies have been conducted on their uptake using vegetables. This work was therefore based on the optimization and application of sensitive, simple, cost-effective, and robust techniques for quantifying ARVDs in vegetables. Based on this information, ultrasonic extraction (UE) and microwave-assisted extraction (MAE) were used to isolate target compounds from vegetable samples to the aqueous phase. Dispersive liquid-liquid microextraction (DLLME) and solid-phase extraction (SPE) were utilized to preconcentration and clean up the extracts from UE and MAE, respectively. A liquid chromatography photodiode array detector (LC-PDA) was utilized to detect and quantify the extracted compounds. The UE with and without DLLME cleanup were compared with each other, also, MAE with and without SPE cleanup were compared with each other. The methods comparison was done in terms of their detection (LOD) and quantification limits (LOQ), extraction efficiencies (%Recovery), relative standard deviations (%RSD), and concentrations of ARVDs found in vegetable samples. In comparison of UE and ultrasonic-assisted dispersive liquid-liquid microextraction (UADLLME), the LOD and LOQ obtained ranged between 0.0081 - 0.015 μg/kg and 0.027 - 0.049 μg/kg for UE and 0.0028 -0.0051 μg/kg and 0.0094 - 0.017 μg/kg for UADLLME respectively. High recoveries ranging from 93 to 113% in UE and 85 to 103% in UADLLME with less than 10% RSD in both procedures were obtained. These results indicated that UADLLME is more sensitive than the UE method, although they are both accurate and precise. The UE can be recommended for routine analysis as UADLLME showed the inability to extract analytes from root vegetables. The optimized UE and UADLLME methods were applied to extract ARVDs from vegetables bought from local fruit and veggie supermarket. Vegetables were categorized as root (carrot, potato, and sweet potatoes), leaf (cabbage and lettuce), and fruit (green paper, butternut, and tomato). The target ARVDs were quantified in most samples with concentrations up to 8.18 μg/kg. The concentrations obtained were slightly high in UADLLME than in UE as a result of its high sensitivity. Efavirenz was the most dominant drug, while the potato was the most contaminated vegetable. In the comparison of MAE and MAESPE, the obtained LOD and LOQ ranged from 0.020 to 0.032 μg/kg and 0.068 to 0.109 μg/kg for MAE and 0.019 to 0.066 μg/L and 0.065 to 0.22 μg/L for MAE-SPE. The obtained recoveries ranged from 85 to 103% for MAE and from 82 to 98 % for MAE-SPE, respectively, and the RSDs were all less than 6%. These results showed that both methods have comparable sensitivity; however, the recoveries values for MAE were slightly higher than those obtained in MAE-SPE, which signals MAE’s high accuracy. The optimized MAE and MAE-SPE methods were applied to remove ARVDs in the root (potatoes, onions, and beetroot), leaf (lettuce, and spinach), and fruit (green paper, cucumber, and eggplant) vegetables bought from local fruit and veggie supermarket. The obtained ARVDs concentration range was 1.48 ± 0.5 - 27.9 ± 1.2 μg/kg. The MAE-SPE resulted in low concentration compared to MAE without cleanup. Beetroot exhibited high concentrations of the target ARVDs, while nevirapine was found to have high concentration and as a dominant compound. The results obtained revealed that the vegetables from the studied area are contaminted with ARVDs, which could indicate their possible irrigation with wastewater effluent or the use of sludge as biosolids in the agricultural areas. This is a concern as it leads to unintentional consumption by consumers which could lead to drug resistance by the human body or have human health effects. The study was then expanded by conducting the phytoremediation approach to investigate the uptake of abacavir, nevirapine, and efavirenz by beetroot, spinach, and tomato from the contaminated soil. The three selected vegetable plants were planted and irrigated with ARVDs spiked (at 2000 and 5000 μg/L) water over a period of three months. The optimized UE and LC-PDA methods were used to extract and quantify the selected ARVDs from the target vegetables and soil. The obtained results showed that the studied vegetables have the potential to take up abacavir, nevirapine, and efavirenz from contaminated soil, be absorbed by the root, and translocate to the aerial part of the plants. Abacavir was found at high concentrations to a maximum of 40.21 μg/kg in the root, 18.43 μg/kg in the stem, and 6.77 μg/kg in the soil, while efavirenz was the highest concentrations, up to 35.44 μg/kg in leaves and 8.86 μg/kg in fruits. Spinach root accumulated more ARVDs than beetroot and tomato. The bio-accumulation factor ranged from 2.0-14 μg/kg in beetroot, 3.6 - 15 μg/kg in spinach, and 6 – 10 μg/kg in tomato. The root concentration factor range was 0.047 – 17.6 μg/kg; 0.34-5.9 μg/kg, and 0.14-2.82 μg/kg in beetroot, spinach, and tomato, respectively. The translocation factor range obtained was 0.40 – 38 μg/kg, 0.08 – 19 μg/kg, and 0.14 – 49 μg/kg in beetroot, spinach, and tomato, respectively. However, the accumulation of ARVDs in all studied plants showed that they could be used in phytoremediation. The results obtained in the phytoremediation approach revealed that the utilization of the contaminated water has an influence on the presence ARVDs in vegetables; hence this work also focused on evaluating the exfoliated graphite adsorption of ARVDs in water. Natural graphite was intercalated with acids and exfoliated with thermal shock to obtain the exfoliated graphite. The scanning electron microscopy images showed that the exfoliated graphite had increased c-axis distance between the layers with accordion-like structure which were confirmed by the lower density of exfoliated graphite material (0.0068 gmL-1) compared to the natural graphite (0.54 g mL-1). Fourier Transformed Infrared Spectroscopy results showed the C=C in natural and exfoliated graphite at 1635 cm-1 stretching. The phenolic, alcoholic, and carboxylic groups were observed from 1000 to 1700 cm-1 for the intercalated and exfoliated graphite. The Energy-dispersive X-ray results further confirmed these results, which showed carbon and oxygen peaks in the intercalated and exfoliated graphite spectrum, whereas natural graphite showed only a carbon peak. Raman spectroscopy results showed that the material’s crystallinity was not affected by the intercalation and exfoliation processes as observed from the ratios of the G and D peaks and the G' and D'. Natural, intercalated and exfoliated graphite contained the D, G, D', and G' peaks at about 1350 cm-1, 1570 cm-1, 2440 cm-1, and 2720 cm-1, respectively. The exfoliated graphite material showed the characteristic of a hexagonal phase graphitic structure by (002) and (110) reflections in the X-ray diffraction results. The exfoliated graphite adsorption method was optimized based on the pH of a solution, adsorbent dosage, and adsorption time prior to application to water samples. The optimum pH solution, adsorbent dosage, and adsorption time were 7, 30 mg, 0.01 μg/L, and 30 minutes respectively. The kinetics and isotherm studies were conducted to assess the model that best fit and explain the experimental data obtained. The kinetic model and adsorption isotherm studies showed that the experimental data fit well pseudo-second-order kinetics and is well explained by Freundlich’s adsorption isotherm. The maximum adsorption capacity of the exfoliated graphite (EG) for ARVDs ranges between 1.660-197.0, 1.660-232.5, and 1.650-237.7 mg/g for abacavir, nevirapine, and efavirenz, respectively. These results showed that under proper operating conditions, the EG adsorbent could potentially be applied as a water purifying tool for the removal of ARVDs pollutants.
Comparison of extraction methods efficiency for the extraction of polycyclic aromatic hydrocarbons and phenolics in water matrices, sludge and sediment: sources of origin and ecological risk assessment.
(2023) Ndwabu, Sinayo.; Mahlambi, Precious Nokwethemba.; Malungana, Mncedisi.
Polycyclic aromatic hydrocarbons (PAHs) and phenolic compounds (PCs) are persistent and environmentally toxic compounds. This study therefore aimed to determine the levels of both PAHs and PCs in river water, wastewater, sludge and sediment samples. The evaluation of their origin source and ecological risk was also determined. The status of both these contaminants in South African environment is still not fully investigated, which is a gap this study intended to fill together with previous studies that have been carried-out. The PAHs and PCs were extracted using different extraction methods which include a solid phase extraction (SPE) and dispersive liquid-liquid micro-extraction (DLLME) in water matrices. The microwave assisted extraction (MAE) and Ultrasonication (UE) coupled with either filtering (F) or F + SPE as a clean-up technique was used for extraction of solid samples. The analytes extracted form water or sediment were determined using GC-MS. The PAH %recoveries obtained under optimum conditions in liquid samples were determined to be 72.1 - 118% for SPE and 70.7 – 88.4% for DLLME while the LOD and LOQ were 5.00 – 18.0 ng/L and 10.0 – 44.0 ng/L for SPE while they were 6.00 – 20.0 ng/L and 11.0 – 63.0 ng/L for DLLME. The recovery test for PAHs in solid samples gave a range of 93.7% - 121% for UE and 79.6% - 122% for MAE while the LOD and LOQ ranged from 0.0250 μg/kg to 1.21 μg/kg & 0.0800 μg/kg to 3.54 μg/kg for MAE and from 0.0840 μg/kg to 0.215 μg/kg & 0.0190 μg/kg to 0.642 μg/kg for UE respectively. The LOD and LOQs for PCs in both water and solid matrices were 0.01 – 2.00 μg/L and 0.02 – 6.07 μg/L for SPE, 0.05 – 1.20 μg/kg and 0.17 – 3.17 μg/kg for MAE and 0.09 – 1.33 μg/kg and 0.26 - 3.54 μg/kg for UE correspondingly, their %recovery test gave ranges of 75.2 – 112% (SPE), 80.9 – 110% (MAE) and 79.3 – 119% (UE).The optimization and validation of these methods indicated that they can be used for the extraction of PAHs or PCs in liquid samples, however, SPE when compared to DLLME showed to be more accurate and sensitive. Moreover, in solid samples the clean-up method was a deciding factor, with F + SPE cleaned samples giving higher concentrations of both PCs and PAHs than the filtered ones in both MAE and UE. The concentrations of PAHs ranged from nd (not detected) to 1046 ng/L in river water and nd to 778 ng/L in wastewater samples with naphthalene showing dominance over all other PAHs in both water matrices. The PC concentrations at 4.12 to 1134 μg/L for wastewater and nd to 98.0 μg/L for river water were high but still within the maximum allowable limit except for 2.4-DCP (2.4 dichlorophenol) at Wdv4. The concentrations obtained from F + SPE cleaned samples were higher for both PAHs and PCs with a range from 95.96 to 926.0 μg/kg and 1.30 to 310 μg/kg compared to concentrations from filtered only samples at 21.61 to 380.6 μg/kg and 0.90 to 266 μg/kg respectively. Pyrene showed dominance over all other PAHs in both sludge and sediments while 2.4-DCP and PCP dominated the sludge and sediment samples respectively. PAHs were determined to be of petrogenic (water matrices) and pyrolytic (solid samples) origin and on average posed low (water matrices) and a medium to high (solid matrices) ecological risk. The ILCRderm values at 4.98 x 10-1 and 2.62 x 10-1 (DahA) and 5.92 x 10-2 and 5.34 x 10-2 (PCP) were highfor adults compared to that of children at 1.92 x 10-1 and 1.01 x 10-1 (DahA) and 1.39 x 10-2 and 1.26 x 10-2 (PCP) for both sediment and sludge samples respectively. The low values of ILCRderm for children indicates that the have a high risk exposure even at low concentrations of the contaminants. The findings of this study showed that both areas (uMsunduzi river and Darvill wastewater works (WWW) of interest are polluted with PAHs and PCs therefore, more regulations such as the National Environmental Management: Waste Act (NEMWA) are needed to ensure environmental, human and animal safety.
Derivatised phenanthroline transition metal chelates : targeted chemotherapeutic agents.
(2024) Hunter, Leigh André.; Akerman, Matthew Piers.
The derivatisation of 1,10-phenanthroline at the 2-position afforded two classes of compounds with two different bridging groups in this study. The first group comprised two amide-bridged tetradentate N4-donor ligands and were chelated to copper(II), nickel(II) and palladium(II). The ligand chelation occurred with concomitant deprotonation of the amide N-H, resulting in a monoanionic ligand and monocationic complexes when coordinated to the divalent metal ions. The ligands N-(quinolin-8-yl)-1,10-phenanthroline-2-carboxamide, HL1, and N-(pyridin-2-ylmethyl)-1,10-phenanthroline-2-carboxamide, HL2, were characterised by NMR, IR and UV/vis spectroscopy as well as mass spectrometry. The second class of compounds were imine-bridged copper(II) chelates. These chelates were synthesised via a templating condensation reaction between various salicylaldehyde derivates and 1,10-phenanthrolin-2-ylmethanaminium chloride, yielding eight additional copper(II) chelates. The metal chelates were characterised by IR, UV/vis and EPR spectroscopy, and mass spectrometry. HL1, [Cu(L4)(NO3)] and [Cu(L7)](NO3) were further studied by X-ray diffraction. The copper(II) chelates exhibit two different solid-state structures with the nitrate counter ion coordinated to the metal centre in [Cu(L4)(NO3)], but in the outer coordination sphere for [Cu(L7)](NO3). The paramagnetic copper(II) chelates were studied with EPR spectroscopy, which confirmed the square planar coordination geometries of these chelates in solution. The metal chelates were designed to be chemotherapeutic agents, exerting their cytotoxicity through DNA intercalation and, for the copper(II) chelates, DNA cleavage through the catalytic production of ROS. The ability of the copper(II) chelates to catalyse the production of hydroxyl radical in situ in the presence of ascorbic acid and hydrogen peroxide was studied via a hydroxyl radical assay using Rhodamine B as an analogue for the aromatic DNA bases. Competitive binding studies determined the affinity of the metal chelates towards ct-DNA, [Cu(L1)](PF6) has the highest binding constant: 5.91 × 106 M-1. DFT calculations were performed on the ligands and metal chelates to determine the geometry-optimised structures, vibrational frequencies, 1H and 13C NMR chemical shifts and electronic transitions. The B3LYP/6-311G (d,p) level of theory was used for the ligands, copper(II) and nickel(II) chelates and the B3LYP/LanL2DZ level of theory for the palladium(II) chelates. The TD-DFT method was used for the energy calculations. The experimental and calculated results were compared where possible, and a reasonable correlation was found. The cytotoxicity of five amide-based chelates was evaluated against four human cancer cell lines, namely A549, TK-10, HT29 and U251, using an MTT assay. The screened chelates exhibited favourable anticancer activity with the mean IC50 values against the four cancer cell lines ranging from ca. 12 to 35 μM. Importantly, it was found that the combination of the copper(II) ion and the ligand was essential for enhanced cytotoxicity. The complex [Cu(L1)](PF6) was identified as the lead drug candidate based on the high DNA affinity and cytotoxicity. This compound was most cytotoxic towards the glioblastoma cell line U251 with an IC50 value of 7.59 μM. The imine-based chelates were screened against three human cancer cell lines: MDA-MB, HELA, and SHSY5Y, and a healthy human cell line, HEK293. The selectivity index of these chelates for neoplastic versus the healthy cell line was calculated. The imine-based chelates showed a high selectivity towards the triple-negative breast cancer MDA-MB, an order of magnitude more toxic to the tumour cell than the healthy one. This selectivity index is significantly improved over that of cisplatin. A gel mobility shift assay investigated the interactions between the copper(II) chelates and plasmid DNA. The in vivo biodistribution of [Cu(L1)](PF6) was determined using the copper-64 radiolabelled analogue of [Cu(L1)]Cl and microPET-CT scanning. The initial biodistribution studies suggested that the complex has good serum stability and showed that there was no significant accumulation in any organs. The subsequent study involved a xenograft model using the A549 cell line and showed significant uptake and retention of the complex in the tumour. The cytotoxicity of the chelate when synthesised with the non-radioactive isotopes of copper and the uptake of the radiolabelled equivalent in a tumour model suggest that this complex could have application as a “theranostic agent”.
Design and fabrication of tissue-like phantoms for use in biomedical imaging.
(2022) Ntombela, Lindokuhle Charles.; Chetty, Naven.; Adeleye, Bamise.
The continuous need for tissue-like samples to understand biological systems and the development of new diagnostic and therapeutic applications has led to the adoption of tissue models using potential materials. This work presents a low-cost method for manufacturing PVAslime glue-based phantoms to replicate diseased and healthy biological tissues’ optical, mechanical, and structural properties. The deformable phantoms with complex geometries are vital to model tissues’ anatomic shapes and chemical composition. Absorption and scattering properties were set by adding black India ink and aluminium oxide (Al2O3) particles in varying quantities to obtain slime phantom tissues with optical properties of the brain, malignant brain tumour, lung carcinoma, and post-menopausal uterus. The phantom properties were characterized and validated using a He-Ne laser emitting at 532 nm and 630 nm wavelengths propagated through various thicknesses of the fabricated phantom. The incident and transmitted intensity were measured to determine the absorption coefficient (a) and scattering coefficient (s). Furthermore, the effective attenuation coefficient (eff ) and penetration depth () were deduced from the reduced scattering coefficient (0s) and the anisotropy factor (g) obtained through the scattering phase function and Wolfram Mathematica. The anisotropy factor demonstrated a forward scatter, typical of strongly scattering media as real tissues. Such geometrically and optically realistic phantoms would function as effective tools for developing techniques in diagnostic and therapeutic applications such as laser ablation and PDT cancer treatment.
Determination of neonicotinoid insecticides in water, soil and sediment samples: acute and chronic risk assessment.
(2022) Ngomane, Nkosinathi Chris.; Mahlambi, Precious Nokwethemba.
Neonicotinoids are a type of insecticides pesticides widely used worldwide as a result of their low vertebrates toxicity, relative environmental stabilities, good bioavailability and high level of selectiveness. These insecticides are commonly employed in agricultural activities, in grass management and horticulture as well as in households to control domestic pet flea. Due to neonicotinoids intensive usage, they are continuously introduced to the water bodies where they can adversely affect the aquatic life and accumulate in sediments. Moreover, they can end up in drinking and unintentionally consumed by human beings resulting to health effects. With this regard, this work reports for the first time on the occurrence of neonicotinoids in sediment, soil tap, sludge, wastewater and river water samples from the province of KwaZulu-Natal. Also, the ecological risk of neonicotinoids in water sources was also assessed for the first time in the samples from this province.The liquid chromatography coupled with a photo-diode array detector (LC-PDA) method was modified and applied for the simultaneous detection of neonicotinoids (clothianidin, thiamethoxam and imidacloprid). Ultrasonic extraction (UE), soxhlet extraction (SE) and solid-phase extraction (SPE) methods were developed and applied for the extraction of nitro-guanidine neonicotinoids in water, soil and sediment samples. The SPE, SE, and UE parameters that influence the recoveries of the analytes were first optimized before application to real samples for the analytes recovery improvement. The SPE was used for the extraction of neonicotinoids in sludge and water samples, while SE and UE were both used to extract soil and sediment samples. The extraction conditions optimized for SPE were conditioning solvent and sample volume. While for the UE were extraction time, extraction solvent, and the solvent volume. And for SE method, extraction solvent and the extraction solvent volume were optimized. The LC-PDA method used for detection was also first optimized to improve peak separation, retention times, detection limits and quantification limits. The optimized parameters for the LC-PDA method were the mobile phase, flow rate, and the PDA detection wavelength. Optimum water recoveries of the neonicotinoids ranged from 79 to 112%. The detection and quantification limits of the analytes in water samples were 0.013 - 0.031 μg/L and 0.041 - 0.099 μg/L, respectively. The obtained analytes concentration ranged from 0.061 - 0.10 μg/L, 0.077- 3.76 μg/L and 0.99 - 15 μg/L in tap, river and wastewater, respectively. Analyte recoveries ranged from 85 - 102% in soil and 92 - 103% in sediment for the ultrasonic extraction method. The neonicotinoid recoveries ranged from 83 to 109% in soil and between 84 to 94% in sediment samples for the Soxhlet extraction method. The method’s detection limits and quantification limits in solid samples ranged from 40 - 80 μg/kg and 140 - 270 μg/kg, respectively. The relative standard deviation was less than 4%. The concentration determined in real environmental samples were 47 to 410 μg/kg in soil and 25 to 410 in sediment. The toxicity studies showed that clothianadin pose a high risk towards daphnia species in the river. Imidacloprid, clothianidin and thiamethoxam posed medium risk against algae, daphnia and fish species in the effluent receiving water bodies. These results imply the necessity to continuously monitor these neonicotinoids in the water sources. In South Africa there is limited data concerning the environmental occurrence of neonicotinoids, therefore this work will contribute towards the information available for the analysis of neonicotinoids. This will assist the policy makers to establish the MRL values that are precise for the African continent.
Improved collection of photogenerated current using bi-metal nanoparticles.
(2024) Jili, Ncedo.; Mola, Genene Tessema.
The energy demand has been continuously growing owing to the shortage of sources of traditional energy (such as fossil fuels), due to the growing population of the world, and increased industrialization, which prompted the need for more energy. However renewable energy (such as photovoltaics) has attained attention due to its reliance on the infinite energy source (sun) which provides an hour long energy flow that fulfil the yearly energy of the glob. Not only that, renewable energy sources offer clean energy, that is meant to contribute to decarbonization in the future and reduce environmental changes. Solar cell materials that can effectively capture photons and conduct charges are continuously investigated for the last six decades. Contrarily to silicon based solar cells, organic solar cells are among the most promising solar cells in terms of offering cheap device fabrication, flexibility, high absorption, etc. However, these solar cells still suffer from low efficiency compared to traditional silicon solar cells due to poor absorption, low mobility, and poor stability. Numerous strategies have been employed to improve the efficiency of OSC devices, these include Ternary OScs, Tandem OSCs, and the inclusion of nanoparticles in OSC devices. Nanoparticles remain the best candidate to feature in OSC devices because Tandem OSCs require multi-absorber layers which leads to high device cost, whereas nanoparticles can be produced at a small scale and still offer good results. This study takes advantage of the features offered by the nanoparticles and uses them to investigate the effect of Nickel doped with cobalt bi-metal nanoparticles(Ni/Co BMNPs) in the PEDOT:PSS buffer layers of the P3HT: PCBM-based devices. Solar cells were successfully fabricated with four different concentrations of Ni/Co BMNPs as 0.05 %(0.5 mg), 0.15 %(1.5 mg), and 0.25 %(2.5 mg). Significant improvements were achieved for the 0:05% with the Fill factor of 58:52 %, and current density of 15.31 mA/cm2, and maximum efficiency of 5:05 % which displayed 67:8 % improvement from the undoped device. The investigation was further conducted by simulation program called SCAPS to confirm the contribution of the metal nanoparticles on the device performance. The results were reproduced in SCAPS where the energy band gap of the P3HT:PCBM and the shallow conduction density of electrons of the PEDO:PSS were simultaneously varied. All results are comparable with the experimental results and found to be similar. The device that was made to mimic the 0:05 % device produced a FF of 57:76 %, Jsc of 15.76 mA/cm2, and maximum efficiency of 5:76 % which displayed 88 % improvement from the undoped device. This study further provides factors that contributed to the high/low device performance due to the inclusion of the BMNPs in the OSC device and some of the necessary background and theory are provided to support these findings.
Metal plasmonic as a mechanism to improve the performance of thin film polymer solar cell : device fabrication and characterization.
(2024) Ike, Jude Nodebechukwunso.; Mola, Genene Tessema.
This thesis discusses the results of the investigations on the use of plasmon metal nanoparticles (NPs) to enhance the performance of polymer solar cells, which are promising alternative solar energy converters to silicon-based solar cells. Polymer solar cells offer a cost-effective, flexible solar panel using a solution processing method for the generation of power from solar sources. Several key factors are considered to achieve this goal, including optical absorption, nano-morphology, and charge carrier mobility. The thesis focuses on investigating the potential of various dopants, such as solvent additives, thermal annealing, and metal nanoparticles (NPs), to improve the performance of thin-film organic solar cells (TFOSCs) by enhancing the charge transport processes. This research employed conventional device architectures to study the effectiveness of the active and buffer layers on charge transport and stability. The results have already been published in several internationally referred journals. In this thesis, the synthesized metal NPs were employed as mechanisms to improve the performance of TFOSC incorporated with poly-3-hexylthiophene (P3HT) as the donor material and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as the acceptor material. The popular metal NPs such as nickel, (Ni), zinc (Zn), silver (Ag), calcium (Ca), sulfur (S), and cobalt (Co) were used to synthesize various bimetallic composites. Bimetallic nanoparticles such as nickel-doped with zinc bimetallic (Ni/Zn), silver-doped with calcium (Ag/Ca), silver-doped with cobalt (Ag/Co), and silver-doped zinc sulfide (ZnS/Ag) were employed at different functional layers of solar cell structure. Hence, the study employed various spectrometers such as high-resolution transmission and scanning electron microscopy (HRTEM and HRSEM), X-ray diffraction, Ultraviolet-Visible (UV-Vis) spectroscopy to investigate the size, morphology, elemental mapping, and optical properties of synthesized metal NPs. HRTEM is indeed a powerful technique for characterizing nanoscale materials, and it can provide valuable insights for confirming the presence of core-shell structures in NPs. Compared to the pristine reference, the blend of metal NPs with active and buffer layers at different concentrations plays a crucial role in augmenting the optical and electrical properties in TFOSC devices. Such increment of optical and electrical properties in this thesis is due to improved short-circuit current (Jsc), fill factors (FFs), and charge carrier mobility, which are significant to enhance the power conversion efficiency (PCE) values in the polymer solar cells. These prominent improvements are due to the presence of localized surface plasmonic resonance (LSPR) effect of TFOSCs. Finally, this thesis provides a series of experimental works fabricated with several metal NPs in TFOSCs at different concentrations.
Metallophthalocyanine-based electrochemical sensors for accurate qualitative and quantitative analysis of emerging pollutants in water resources.
(2024) Shoba, Siyabonga Blessing.; Booysen, Irvin Noel.; Mambanda, Allen.
Water is a precious resource and safeguarding it from pollution is paramount to ensure the well-being of both the environment and human health. Emerging contaminants such as pharmaceuticals and heavy metals pose significant threats, necessitating vigilant monitoring and appropriate action. Traditional laboratory-based analytical techniques like Gas Chromatography, ICP-OES and HPLC have been instrumental in quantifying pollutants. However, their high operational costs, maintenance requirements and the need for specialized personnel limit their widespread use, especially in resource-constrained countries. Electrochemical sensors have emerged as a promising solution. They provide real-time, portable and cost-effective options for on-site detection of pollutants in water. Current advancements in electrochemical sensors are centred around achieving selective detection using chemical modifiers, all while maintaining electrocatalytic sensitivity and reproducibility. These sensors can be tailored to target specific contaminants, making them highly efficient tools for monitoring water quality and ensuring the sustainability of this invaluable resource. In the first experimental chapter, a glassy carbon electrode (GCE) was modified by an asymmetric metallophthalocyanine (MPc) complex, A3B-CoPc-flav (where A = flavonyloxy substituent and B = an alkynyloxy substituent/molecular mast). The modification of an electrode was achieved via electrochemical grafting followed by clicked chemistry between the diazonium-functionalized GCE and the a-CoPc-flav3 to afford the GCE|clicked-a-CoPc-flav3. The chemically modified electrodes (CME) were utilized as electrocatalytic detectors for dopamine (DA) under optimized conditions. The response of the GCE|clicked-a-CoPc-flav3 was linear in the concentration range of 2 μM to 14 μM, attaining limits of detection and quantification of 0.311 and 0.942 μM, respectively, and high reproducibility (%RSD of 2.25%, N = 3). Interference studies were conducted, revealing a marginal shift in the DA peak potential in the presence of interfering substances. Despite this shift, the peak current intensity of DA remained largely unaffected, affirming the selectivity and accuracy of the CME. The analytical capabilities of the CME were further assessed using real water samples. The obtained percentage recoveries of (97.1%) of DA by the GCE|clicked-a-CoPc-flav3 and the well-established HPLC-MS method (113%) are both within the acceptable range of 80-120%. In the second experimental chapter, a platinum electrode (Pt) was modified via the electropolymerization of polypyrrole (PPy) after its co-electrodeposition of tetra-[4-((1H benzotriazole)methoxy)phthalocyaninato]cobalt(II) (CoPc-Bzt). The electrodeposition of CoPc-Bzt was performed in 1:1 DMF/acetonitrile containing 1 M tetrabutylammonium hexafluorophosphate (NBu4PF6) electrolyte over 20 cycles using cyclic voltammetry to afford a Pt|PPy/CoPc-Bzt (Bzt = benzotriazole). The resultant CME was prone to fouling by the analyte of interest, mercury(II). Due to fouling the differential pulse anodic stripping voltammetry (DPASV) was used to detect Hg(II) using the Pt|PPy/CoPc-Bzt within 10 μM to 100 μM linear range. The LOD and LOQ were found to be 3.11 and 10.00 μM, respectively. Interference studies illustrated that the detection capabilities of the CME were not affected by the presence of other heavy metal cations. The analytical performances of Pt|PPy/CoPc-Bzt (97.4%) and Inductively coupled plasma – optical emission spectroscopy (ICP-OES) (112.3%) are both within the acceptable range of 80-120%. In the third experimental chapter, the Pb electrocatalytic sensing capability of a gold electrode modified via the adsorption of electrospun nanofibers (ENFs) and Nafion (Nf) as an annealed conductive top-layer was evaluated. The fabricated ENFs comprised of a core polymeric nanocomposite of tetra-4-(3-oxyflavonephthalocyaninato)cobalt(II) (CoPc-flav), the carboxylic acid functionalized multiwalled carbon nanotubes (f-MWCNTs) and polyaniline (PANI) encapsulated in a polyvinyl acetate (PVA) ENFs. The resultant CME, Au|ENFs-1-Nf was not prone to fouling as was found when using the bare and the other constructed CMEs whose signal stabilities were compromised by background electrolyte currents. The Au|ENFs-1-Nf electrode could detect the Pb(II) cations in a reproducible manner (%RSD of 3.92%, N = 3) ranging from 8 to 125 μM, and limits of detection and quantification of 0.51 and 1.55 μM were obtained, respectively. However, the interference studies illustrated that the detection capabilities of the CME are severely compromised by the presence of other heavy metal cations. The analytical performance of the CME rendered a comparable percentage recovery (103%) with that of the ICP-OES (115%). In the fourth experimental chapter, the nanofabrication and characterization of new conductive materials, PANI-CoPc-fur (1) ((PANI = polyaniline and CoPc-fur = tetra-4-(2-furanmethylthiophthalocyaninato)Co(II)) and PANI-CoPc-fur-f-MWCNTs (2) are reported. Subsequently, an electrospun nanofiber (ENF) composite was fabricated where the core comprised of 2 that was encapsulated with a PVA shell. The resultant nanoconjugate, ENFs-2 was adsorbed on a glassy carbon electrode (GCE) followed by the immobilization of a permeable adhesion top layer of Nafion (Nf) to render the chemically modified electrode, GCE|ENFs-2-Nf. The classical physical properties of the electron-mediating layer for the CME synergistically aided in promoting its electrocatalytic activities. Consequently, the CME showed greater anodic and cathodic cyclic voltammetry (CV) peak currents compared to the bare GCE and other modified electrodes, indicating its higher sensitivity to acetaminophen (APAP), an emerging water pollutant of concern. Limits of detection and quantification (LOD and LOQ) values for APAP attained by squarewave voltammetry (SWV) were lower compared to those acquired using other electrochemical techniques. The detection of APAP at the GCE|ENFs-2-Nf attained by squarewave voltammetry (SWV) was linear from 10 to 200 μM of APAP and was reproducible (%RSD of 3.2%, N = 3). The respective calculated LOD and LOQ values of 0.094 and 0.28 μM were lower compared to those acquired using other electrochemical techniques. Analysis of APAP in the presence of commonly associated interferences metronidazole (MTZ) and dopamine (DA) illustrated a significant separation between the SWV peak potentials of APAP and MTZ, whereas there was some degree of overlap between the SWV current responses of APAP and DA. The analytical performance of the GCE|ENFs-2-Nf rendered a comparable percentage recovery (103.8%) with that of liquid chromatography–mass spectrometry (LC–MS) (106%).
Modelling of quantum phase transitions in Dirac materials.
(2022) Hussien, Musa Alnour Musa.; Ukpong, Aniekan Magnus.
In this thesis, first-principles computations of the electronic ground state are used to investigate the underlying nature of the quantum phase transitions in selected Dirac materials and their associated hybrid materials. Various methods are used to break the symmetry of the ground state electronic structure and to tune the emergent phases. These involve the application of an external electric, magnetic or electromagnetic fields, and the manipulation of the internal intrinsic fields of materials, such as the introduction of strong spin orbit coupling, electron (or hole) doping, including a strong short-ranged disorder potential. The findings show that applying a perpendicular electric or magnetic field with a staggered potential in the underlying lattice allows for a tunable electronic transition between trivial and nontrivial quantum states. Signatures of the near-field electrodynamics of carriers in nanoclusters reveal the appearance of a quantum fluid phase at the distinct energies where topological quantum phase transitions occur. Emergence of the field-induced carrier density wave phase shows that the collective excitation mode is a distinct potential for carrier transmission in spintronic, optoelectronic, and photonic technologies. Furthermore, two types of insulating Dirac material are used as the tunnel barrier region of the perpendicular tunnel junction architecture. The resulting heterostructure is an artificially assembled metal-insulator-metal multilayer, and this serves as a generic platform for characterizing spin transport properties in spintronic devices. The dependence of the emergent phenomenon of proximity induced magneto-electronic coupling on the tunnel barrier material is unraveled. By analyzing the effect of the changes in the electronic structure on the spin transmission properties, it is found that the metal-insulator-metal platform exhibits a quantum phase transition by responding sensitively to both the tunnel barrier material and the applied perpendicular electric field during in-service operation of a spintronic device. The results show that when the electric field approaches its critical amplitude, the spin density of states exhibits a discontinuous change from half-metallic to metallic transport character in the presence of monolayer hexagonal boron nitride as a tunnel barrier material, contrary to when the monolayer molybdenum disulphide is inserted in the tunnel barrier region. The role of the applied electric field in the phase transition is characterized in terms of a spin-flip transition and an induced interfacial charge transfer. It is also found that the abrupt discontinuity in the changes in the spin-flip energy with increase in applied electric field provides the necessary and sufficient evidence of a first-order quantum phase transition in the spin transport phase. These findings show that the material of the tunnel barrier layer creates a non-trivial function in defining the magnetoelectric couplings that occur dynamically during spin tunneling.
Syntheses of (pyridyl) pyrazine carboxamide palladium(II) complexes, their DNA/BSA interactions, and cytotoxicity studies.
(2023) Mvelase, Sabathile Thandeka.; Ojwach, Stephen Otieno.
Reaction of pyrazine-2,3-dicarboxylic acid with a respective amine in the presence of triphenyl phosphite afforded the corresponding carboxamide ligands: [N2, N3-bis(pyridin 2-yl)pyrazine2,3-dicarboxamide] (L1), [N2, N3-bis(6-methylpyridin-2-yl)pyrazine-2,3-dicarboxamide] (L2), [N2,N3-bis(4-methylpyridin-2-yl)pyrazine-2,3-dicarboxamide] (L3), and [N2, N3bis(quinoline-8-yl)pyrazine-2,3-dicarboxamide] (L4). Treatments of the corresponding (pyridyl)pyrazine carboxamide ligands with [PdCl2(NCCH3)2 afforded new mononuclear and dinuclear palladium(II) complexes with a general formula, [Pd2(L1)2Cl2] (Pd1), [Pd2(L2)2Cl2] (Pd2), [Pd2(L3)2Cl2] (Pd3) and [Pd(L4)Cl] (Pd4). The identities and coordination nature of the palladium(II) complexes were established through a combination of characterization techniques such as NMR, FT-IR spectroscopy, mass spectrometry, elemental analysis, and single X-ray crystallography. The molecular structures of the dinuclear Pd1 and Pd3 complexes reveal that the two (pyridyl) pyrazine carboxamide ligands coordinate with the palladium atom via one arm, while the other arm remains non-coordinating. The ligands are bridged by two palladium atoms to form dinuclear palladium(II) complexes. While one ligand coordinates to the palladium in a bidentate fashion via the nitrogen atoms of the pyrazine and amide groups, the other ligand unit coordinates to the palladium through the pyridine nitrogen atom to give two ligand units and two palladium atoms in the complex coordination sphere. On the other hand, ligand L4 binds to palladium atom in a tridentate fashion via the pyrazine, amide, and pyridine nitrogen atoms to give complex Pd4 as a mononuclear species. The interaction of the palladium complexes (Pd1-Pd4) with calf thymus DNA (CT-DNA) was monitored using UV-Vis, and fluorescence spectroscopy. Absorption spectroscopic studies revealed that complexes Pd1-Pd4 interact with CT-DNA via intercalative mode, and the computed intrinsic binding constant (Kb) values range from (4.28-13.12) x 106 M-1. In addition, Ksv values of (1.82-28.41) x 106 M-1 and KF values of (1.01-53.44) x 104 M-1 determined in competitive binding studies confirmed the intercalative binding mode. The interaction of the complexes with CT-DNA decrease in the order of Pd3 > Pd2 > Pd1 > Pd4. Furthermore, bovine serum albumin (BSA) interaction was evaluated using fluorescence studies and the results revealed the existence of a static quenching mechanism with bimolecular constant, kq range of (0.66-13.99) x 1014 M-1 s-1. The Ksv values of (1.48-29.67) x 106 M-1 and KF values of (0.10-16.10) x 105 M-1 confirmed the interaction between BSA and palladium complexes. The interaction follows this order Pd2 > Pd1 > Pd3 > Pd4, which is inconsistent with the CT-DNA trend. In general, ligands bearing electron-donating methyl groups (L2 and L3) contributed to higher binding constants in their respective complexes Pd2 and Pd3 compared to the unsubstituted complex Pd1. In addition, the mononuclear complex Pd4 showed the weakest interactions with both the DNA and BSA, pointing to some beneficial effects of increased metal atoms in the complexes. The cytotoxic effect of the ligands L1-L3 and complexes Pd1-Pd4 were examined against a human breast cancer cell line (MCF-7) using 3-(4,5-Dimethyl-2thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay. In general, the ligands displayed poor cytotoxicity L1 (IC50 > 400 μM), L2 (IC50 = 182.4 μM), L3 (IC50 = 80.2 μM), when compared to their respective palladium(II) complexes Pd2 (IC50 = 154.9 μM), Pd3 (IC50 = 230.1 μM). Complexes Pd1 (IC50 = 11.4 μM), and Pd4 (IC50 = 61.5 μM) displayed high and moderate cytotoxic activity which was attributed to the planarity of the complexes.
Synthesis and characterization of magnetic iron oxide nanoparticles.
(2022) Ndlovu, Nkanyiso Linda.; Masina, Colani John.
Three high-purity cubic spinel-type crystalline magnetic iron oxides i.e. Fe3O4, CoFe2O4, and NiFe2O4 nanoparticles were successfully synthesized by co-precipitation method. X-ray diffraction (XRD) showed the formation of stoichiometric phases with average particle size of 11.7 nm, 23.6 nm, and 16.4 nm for the as-prepared Fe3O4, CoFe2O4, and NiFe2O4 nanoparticles, respectively. Transmission electron microscopy (TEM) observation for all three samples revealed spherical morphology with single magnetic domain structure. From high resolution TEM (HR-TEM) imaging, lattice fringes with d-spacing of 0.473 nm and 0.248 nm corresponding to (111) and (311) reflections planes, were observed for both the Co-doped and Ni-doped samples. Energy-dispersive x-ray spectroscopy (EDX) analysis showed the presence and homogeneous distribution of main elements Fe, O, Co, and Ni in the samples. Quantitative EDX results confirmed the formation of stoichiometric CoFe2O4 and NiFe2O4 phases with the experimentally measured weight wt% of the samples closely equal to the theoretical calculated wt% values i.e. Fe = 46.35 wt%, O = 26.79 wt%, and Co = 26.87 wt% for CoFe2O4, and Fe = 47.02 wt%, O = 27.27 wt%, and Ni = 24.75 wt% for NiFe2O4. The magnetic properties of these nanoparticles were investigated by 57-Fe Mossbauer spectroscopy (MS) and Vibrating Sample Magnetometer (VSM) techniques. Room temperature MS spectrum for the pure Fe3O4 phase consist of two superimposed sextets with isomer shifts (0.321, 0.463) mm/s and hyperfine field (57.3, 43.4) T attributed to tetrahedral (A-sites) and octahedral (B-sites). The CoFe2O4 and NiFe2O4 samples both showed room temperature MS spectra consisting of two sextets and a single central paramagnetic doublet. The two sextets in each sample had almost equal isomer shifts for both A- and B-sites i.e. 0.2956 & 0.3247 mm/s and 0.3784 & 0.2761 mm/s for each of the sites of the CoFe2O4 and NiFe2O4 sample, respectively. The paramagnetic doublet was fitted with isomer shift of 0.3272 mm/s for the CoFe2O4 sample and 0.3249 mm/s for the NiFe2O4 sample. Temperature dependence M-T magnetization curves measured at H = 500 Oe inthe zero-field-cooled (ZFC) and field-cooled (FC) conditions showed the superparamagnetic nature of all three particles. The MZFC magnetization curve showed a maximum (cusp) at 225 K, 300 K, and 228 K corresponding to blocking temperature (TB), for Fe3O4, CoFe2O4, and NiFe2O4, respectively. For the CoFe2O4 sample the irreversibility temperature (Tirr) was equal to the blocking temperature (TB). While measured Tirr for Fe3O4 and NiFe2O4 was 300 K for both samples. The M-H magnetization curves at 300 K for all three samples revealed the coexistence of ferrimagnetic and superparamagnetic behaviour of the nanoparticles. At 300 K all three samples exhibit symmetrical and almost "closed" hysteresis loops with coercivity approximately 36, 70, and 117 Oe and remanence magnetization of approximately 5, 3, and 4 emu/g, for Fe3O4, NFe2O4, and CoFe2O4, respectively. Furthermore, M-H measurements at 300 K showed a high saturation magnetization of 89 emu/g for the Fe3O4 sample compared to 37 emu/g and 26 emu/g for the CoFe2O4, and NiFe2O4, respectively. M-H measurements recorded at low temperatures showed rather "opened" hysteresis loops compared to loops measured at 300 K. In contrast to saturated magnetization M-H curves for the Fe3O4 and NiFe2O4 nanoparticles, unsaturated M-H loops were observed for CoFe2O4 sample in the temperature range 10 - 100 K. A significant increase in coercivity to 102 Oe, 391 Oe, and 2.4 kOe was observed for Fe3O4, NiFe2O4, and CoFe2O4, respectively, when the temperature was reduced from 300 K to 10 K. For the CoFe2O4 sample, a highest coercivity of 2.7 kOe was measured at 100 K. And finally, M-H data at 10 K showed high saturation magnetization of 100 emu/g, 51 emu/g, and 31 emu/g, for the pure magnetite, CoFe2O4, and NiFe2O4 samples, respectively.
Synthesis and the application of molecularly imprinted polymers as solid-phase extraction and dispersive solid-phase extraction sorbents in the extraction of antiretroviral drugs in water.
(2022) Xolo, Thabiso.; Mahlambi, Precious Nokwethemba.
A multi-template molecularly imprinted polymer was synthesized, characterized, and applied to real water samples for the specific extraction of antiretroviral drugs (ARVDs), abacavir, nevirapine, and efavirenz. A MIP was synthesized by bulk polymerization at 60 ºC using abacavir, nevirapine and efavirenz (templates), 2-vinylpyridine (functional monomer), 1,1’- azobis-(cyclohexanecarbonitrile) (initiator), ethylene glycol dimethacrylate (cross-linker) and toluene: acetonitrile (9:1, v/v) (porogenic solvent mixture) for 16 hours. The temperature was then increased to 80 ℃ for 24 hours to ensure complete polymerization. The synthesized MIP was washed with acetic acid: acetonitrile (1:9, v/v) via soxhlet extraction until all three ARVDs were undetected in the washing solvent using high performance liquid chromatography coupled with a photo-diode array detector. A non-imprinted polymer (NIP) was synthesised using the same reagents and quantities except for the templates. Both MIP and NIP were characterized using Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), scanning electron microscope (SEM), N2 physisorption analysis, and elemental analysis. The FTIR results showed that the polymers were similar in structure and BET showed that they were mesoporous. The SEM showed that the MIP surface was rougher when compared to the NIP and characterization with TGA showed that they were both thermally stable. The synthesized MIP was used to study its adsorption kinetics and isotherms. Kinetics modelling revealed that the Pseudo-second-rate order kinetics was the best fitting model with correlation coefficient of 1 compared to Pseudo-first-rate order kinetics which had a correlation efficient of 0.81-0.983 for all target analytes. The best fitting adsorption isotherm was the Freundlich model with a correlation range of 0.9451-0.986 for compared to the Langmuir model which had correlation efficient of 0.6692-0.93390.0198-0.6782 for all target analytes. The traditional solid phase extraction and the MIP-based solid phase extraction methods were applied in distilled water samples spiked with 1 mg.L-1 of ARVDs and recoveries obtained were ranging from 91.68-94.59% and 97.20 to 99.68%, respectively. The MIP-based dispersive solid phase extraction method was successfully optimized with recoveries ranging from 100.28% to 102.60% for all three analytes. Selectivity studies were conducted using both the NIP and MIP with lamivudine and diclofenac as competitors. The recoveries obtained for the MIP ranged between 92% to 98% for the target analytes while they were 63% to 79% for competitors. These results showed good selectivity and strong affinity of the polymer towards the target analytes than the competitors. This is justified by the presence of imprinted recognition sites that have the same functional groups, size, and shape as the target analytes/templates hence recoveries were low for competitors. The MIP was more selective towards analytes of interest compared to the NIP (recoveries ranged from 87.9% to 91%) for the analytes of interest which indicates successful imprinting on the MIP. Reusability studies showed that the MIP can be reused for up to 8 cycles with recoveries above 92% for all target analytes. The developed, adopted, and validated methods were then applied to wastewater, tap water and river water samples from around KwaZulu-Natal. The concentrations obtained for abacavir, nevirapine and efavirenz were 10.65-295.90 μg.L-1 in wastewater, 1.95-13.15 μg.L-1 in river water, and 2.17-6.27 μg.L-1 in tap water. Efavirenz was the most dominant and consistently detected ARVD in all samples. The MIP-DSPE was the most sensitive and selective extraction technique compared to SPE and MIP-SPE. Umhlathuzana and Amanzimtoti were the most ARVD’s polluted wastewater treatment plants, whilst Northern wastewater water works was the least polluted. Camps Drift was the most ARVD’s polluted sampling point in Msunduzi river. Napierville and Scottsville showed the most contaminated tap water samples from suburbs around Pietermaritzburg.
Synthesis of ketones through aerobic transition metal-catalyzed cross-coupling of thioesters.
(2021) Mthombothi, Zabenguni Zakithi.; Sithebe, Siphamandla.
Asymmetric diaryl ketones are an important class of compounds in organic chemistry due to their presence in natural products, synthesis, cosmetics as well as in biological active compounds. The aim of this project was to expand the scope of thiophilic metal carboxylates catalysts that can be applied to the aerobic Liebeskind-Srogl cross coupling reaction between various thioesters and phenylboronic acids. Thioesters bearing electron neutral, withdrawing and donating groups were successfully synthesized in yields ranging from 35 to 54%Xcv. The electron neutral thioester was used in the optimization of the aerobic Liebeskind-Srogl reaction and CuMeSal proved to be the most effective catalyst in this protocol. CuMeSal was applied in the aerobic synthesis of the asymmetric diaryl ketones bearing a wide range of functional groups including CF3, SMe, OMe and F yielding up to 65% in 24hrs. Aerobic Liebeskind-Srogl cross-coupling reaction was applied in the successful synthesis of chalcones, through the coupling of commercially available phenylboronic acids with previously synthesised thioesters catalysed by CuMeSal in DMF for 24hrs,furnishing desired products in poor to excellent yields 26-89%.
Synthesis, characterization and substitution reaction studies of pyridyl N,N’-bidentate palladium(II) complexes. A kinetic and mechanistic study.
(2023) Mjwara, Pinky Ncomela.; Sithebe, Siphamandla.; Papo, Tshephiso Rose.
The influence of structural as well as electronic properties of bidentate N,N chelates with different substituents on the mononuclear Pd(II) complexes were investigated. The complexes were synthesized and characterized by various spectroscopic methods such as 1H & 13C NMR, FT-IR, LC-MS, CHN and single x-ray crystallography. For the first set of complexes (Chapter 3), we studied the unexplored kinetics and mechanistic behaviour of N,N’-pyridyl Pd(II) complexes, viz. dichloro-(N-((pyridin-2-yl)methyl)aniline)palladium(II) (PdL1), dichloro-(4-fluoro-N-((pyridin-2-yl)methyl)aniline)-palladium(II) (PdL2), dichloro-(4-bromo-N-((pyridin-2-yl)methyl)aniline)-palladium(II) (PdL3), dichloro-(4-methoxy-N-((pyridin-2-yl)methyl)aniline)-palladium(II) (PdL4) and dichloro-(4-ethyl-N-((pyridin-2-yl)methyl)aniline)-palladium(II) (PdL5). The substitution behaviour of coordinated chloride atoms by three bio-relevant thiourea nucleophiles, viz. thiourea (TU), N,N’-dimethylthiourea (DMTU) and N,N,N’,N’-tetramethylthiourea (TMTU), of different steric demands was studied in a 0.1 M solution of ultra-pure water under pseudo-first order conditions. The reactions were studied as a function of concentration and temperature using standard Stopped-Flow and UV-Vis spectrophotometric technique. The substitution of the chloride atoms from the Pd metal by thiourea nucleophiles was a two-step reaction where the chloride trans to the pyridine ligand was substituted first, since the pyridine has a stronger trans effect compared to the amine group. The reactivity of mononuclear Pd(II) complexes containing bidentate N,N’-donor ligands with different substituents depends on the electronic effects of the complexes. The reactivity of the complexes increased with the presence of electron withdrawing substituents and decreased when an electron donating group was attached on the para position of the aniline moiety. The electron withdrawing groups influence the pull of electrons from the electron deficient amine that is coordinated to the metal center which results in the loss of electron density from the ligand moiety and increases the electrophilicity of the metal center and thus the substitution reaction. The reactivity of the nucleophiles depends on steric effects, with the bulky TMTU being the least reactive. The negative entropies and second order kinetics for all the substitution reactions support an associative mode of substitution mechanism. DFT calculations were performed to account for the observed reactivity of all the complexes studied. For the second set of novel Pd(II) complexes (Chapter 4), viz. bis[N-(4-bromophenyl)pyridine-2-carboxamidato] Palladium (Pd1) and Palladium(II) [N-(4-bromophenyl)-2-pyridinecarboxamide), pyridine chloride (Pd2), crystals were obtained and the structures were studied. Pd1 crystallizes in the monoclinic crystal system and in the P21/c space group, and Pd2 crystallizes in the orthorhombic system, with the space group Pbca.
Targeting the tumour extracellular environment through rational modification of the SNX class of HSP90 inhibitors.
(2023) Mathenjwa, Gciniwe Sindiswa.; Akerman, Matthew Piers.; Veale, Clinton Garenth Lancaster.; Bode, Moira Leanne.
HSP90 remains a valuable target for cancer therapy. Unfortunately, targeting intracellular HSP90 has proven not to be a viable chemotherapeutic approach. Compensatory HSR induction and HSP70 overexpression are the main limitations of this approach. A growing body of evidence suggests that targeting the extracellular environment would be of advantage and devoid of the drawbacks observed with intracellular HSP90 inhibition. As a result, the development of extracellular HSP90 inhibitors represents a novel opportunity for cancer therapeutics. In view of this hypothesis, we aimed to design and synthesise extracellular inhibitors and to assay these compounds against HSP90. To develop extracellular HSP90 inhibitors, cell-impermeable analogues of the well-developed benzamide HSP90 inhibitor (SNX 2112) were designed, synthesised and biologically evaluated. The desired target compounds were synthesized using developed methodology, as well as modified methodology. In Chapter 3 we compared and evaluated a variety of reported synthetic methods to deliver the analogues of SNX 2112. Interested in a general procedure for the synthesis of our analogues, we initially attempted to afford both the methyl and the trifluoromethyl containing analogues via a β- triketone mediated procedure. Despite the success observed with the methyl analogues, the instability of a trifluoromethyl containing β-triketone, deemed this procedure not feasible for this class of compounds. Our continued effort towards a general procedure led to the investigation of a tosylhydrazone mediated tetrahydroindazolone condensation; unexpectedly attempts to synthesise the methyl containing analogues via this procedure led to a 1—5 nitrogen to carbon tosyl migration, which was further investigated for varying substrates, and these results are explained in detail in this thesis. It then became apparent that each of the reported methods had its merits and shortcomings, there was no one best method, rather the synthetic approach was mainly determined by the C-3 substituent. The key intermediates were then converted into the desired targeted compounds by tethering the HSP90 pharmacophore to flexible alkyl groups, attached to polar sulfonate and phosphonate functionalities. Hypothetically, introduction of polar alkyl groups, would inhibit cell penetration thus limiting them to the extracellular environment. Based on the goals of our study we were interested in three biological evaluations; to confirm that our modified compounds were still capable of inhibiting HSP90s ATPase activity, to evaluate if our modifications reduced intracellular HSP90 activity, whether they stimulated the pro-oncogenic HSR, and to evaluate their cytotoxicity. Preliminary biological assessment of our compounds was consistent with our hypothesis. Here we showed that our compounds did not inhibit intracellular HSP90, and did not stimulate HSP70 expression, a marker of induction of the compensatory HSR. Furthermore, our analogues displayed cytotoxicity in the nanomolar range against the HeLa cell line. These preliminary data support the feasibility of targeting extracellular HSP90 as a novel anticancer strategy.
The effect of metal sulfides on hole and electron transport buffer layers in organic photovoltaics: experimental and numerical device simulation investigations.
(2022) Adedeji, Michael Adepelumi.; Mola, Genene Tessema.
Organic solar cells (OSCs) are promising alternative renewable energy sources that often suffer from insufficient absorption of solar radiation, short exciton lifetimes and small diffusion length of their charge carriers. Several strategies are being investigated to overcome these challenges in a move towards the commercialization of this solar cell technology. Increasing the path-length of incident electromagnetic radiation within the photo-absorbing layer of the solar cell, may elongate the time that light spends within the solar cell, thereby increasing the light-matter interaction time and consequently the photo-absorption within the photo-active material of the solar cell. The process described may be accomplished if suitable plasmonic metal nano-structures are added into the solar cell matrix. This intervention may also enhance the collection of the photo-generated charge carriers. The effects of metal sulphide nanoparticles incorporation in organic solar cells were studied and presented in this thesis. The metal sulphide nanoparticles were characterized and introduced into the hole- and electron- transport layers of fullerene and non-fullerene electron acceptors based solar cells, to elicit improved photoabsorption via the localized surface plasmon effects and facilitate better charge collection at the electrodes. Devices were fabricated both in an ambient environment and in a controlled environment (Nitrogen filled glovebox). The metal sulphide nanoparticles were incorporated in the fabricated solar devices using both the conventional and inverted device architectures. The power conversion efficiencies of the devices improved significantly after the incorporation of these nanoparticles. Device numerical simulation studies were also performed to reproduce some of the experimental results with a view to further investigating the devices and discussing their charge transport characteristics. The simulation results show improved charge carrier characteristics from the metal-sulphide doped devices by way of improved conductivity and shifted Fermi level offsets which were aided by the presence of the metal-sulphides. Asides from successfully achieving improved device performances in the investigations and simulations carried out in this thesis, this thesis successfully demonstrated the incorporation of nano-composites in non-fullerene acceptors-based organic solar cells for the first time to the best of our knowledge.
Vinyl-addition polymerization of norbornene catalyzed by (pyridyl)imine Ni(II), Pd(II), Co(II), and Fe(II) complexes.
(2023) van der Westhuizen, Barend Danielle.; Ojwach, Stephen Otieno.
The thesis reports the syntheses, and structural characterization of (pyridyl)imine transition metal complexes and their applications as pre-catalysts in the vinyl-addition polymerization of norbornene. The bidentate ligand (E)-N-(1 phenylethyl)-1-(pyridin-2-yl)methanimine (L1) was synthesized by reactions of 2-pyridine carboxaldehyde with (R)-(+)-α-methyl benzylamine in the presence of p-TsOH and obtained in moderate yields of 64%. On the other hand, the tridentate ligand (E)-1-(pyridin-2-yl)-N-(pyridin-2-ylmethyl)methanimine (L2) was obtained by reacting 2-pyridine carboxaldehyde with 2-picolylamine in the presence of p-TsOH in high yields of 90%. Reactions of bidentate ligand L1 and tridentate ligand L2 with NiCl2, [Pd(COD)Cl2], FeCl2, and CoCl2 metal salts gave the corresponding complexes [Ni(L1)2Cl2] (Ni1), [Ni(L2)Cl2] (Ni2), [Pd(L1)Cl2] (Pd1), [Co(L1)3][2PF6] (Co1), [Co(L2)Cl2] (Co2), [Fe(L1)3][2PF6] (Fe1), [Fe(L2)Cl2] (Fe2) in low to moderate yields (18% - 60%). The identities of the isolated complexes were confirmed by characterization with FT-IR spectroscopy, mass spectrometry, elemental analysis, single crystal X-ray crystallography for Co1, and nuclear magnetic resonance where applicable. The solid state structure of complex Co1 was established as tris-chelated, containing three ligands to give an octahedral coordination environment. The metal complexes were evaluated as pre-catalysts in the vinyl-addition polymerization of norbornene to produce poly(2,3-bicyclo[2.2.1]heptene) using modified methyl aluminoxane (MMAO) as the co-catalyst. Complex Ni1 was the most active with catalytic activity of 22.7 g×10³(PNBE).mol(M)-¹.h-¹ followed by complex Pd1 which showed catalytic activity of 17.6 g×10³(PNBE).mol(M)-¹.h-¹ whereas complex Co1 showed 0.7 g×10³(PNBE).mol(M)-¹.h-¹ and complex Fe1 showed catalytic activity of 0.3 g×10³(PNBE).mol(M)-¹.h-¹ concluding that the choice of metal center is of absolute importance to achieve high catalytic activity. The number of electron donor atoms in the ligand structure influenced catalytic activity as bidentately chelated complex Ni1 showed catalytic activity of 22.7 g×10³(PNBE).mol(M)-¹.h-¹ whereas the tridentately chelated Ni2 showed catalytic activity of 81.9 g×10³(PNBE).mol(M)-¹.h-¹. The influence of reaction parameters were investigated using Ni1 and Ni2 as pre-catalysts and it was concluded that monomer/metal ratios, co-catalyst/metal ratios, reaction temperatures, reaction times, and solvent choice influenced catalytic activity. A higher monomer/metal ratio of 1250 resulted in catalytic activity of 17.5 g×10³(PNBE).mol(M)-¹.h-¹ compared to the value of 6.8 g×10³(PNBE).mol(M)-¹.h-¹ obtained from a lower monomer/metal ratio of 625. An optimum co-catalyst/metal ratio of 1500 was established and recorded catalytic activity of 33.7 g×10³(PNBE).mol(M)-¹.h-¹. Polymerization reactions at room temperature gave higher monomer conversions of 70% as opposed to lower conversions of 17% obtained at 50 °C. The choice of solvent influenced catalytic activities of the complexes, with the more polar o-chlorobenzene solvent giving the highest monomer conversion of 70% in comparison to 31% obtained in toluene solvent. Polymers formed from all complexes were of the vinyl type with possible ring-opening metathesis polymeric inserts present in the polymer backbone. Thermal gravimetric analysis and differential scanning calorimetry of the formed polymers demonstrated that polymers formed from complex Co2 and complex Fe2 displayed degradation temperatures of 492 °C and 478 °C respectively opposed to polymers formed from complex Ni2 and complex Pd2 which gave values of 478 °C and 462 °C.
Wigner functionals and ghost imaging.
(2023) Durgapersadh, Akshay.; Konrad, Thomas.; Roux, FiIippus Stefanus.
This dissertation discusses Wigner functionals and an application, namely ghost imaging. Wigner functionals aim to provide more accurate measurement results due to the inclusion of all the degrees of freedom of light. The main concepts discussed are spontaneous parametric down-conversion (SPDC), the evolution equation for light through the SPDC crystal, the probability distribution for the ghost image, conditional probability distribution, and the point spread function. The ghost imaging calculation is done for the rational thin crystal and extreme thin crystal limits. It is shown that the probability distribution produced is the same for the rational thin crystal and extreme thin crystal limits, and consequently, the point spread function, and conditional probability distributions are the same.

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