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Item Silver nanoparticle-modified cellulose nanocrystals for fouling control in membrane distillation(University of the Witwatersrand, Johannesburg, 2023-06) Mpala, Josephine Tshepiso; Nthunya, Lebea; Richards, Heidi; Etale, AnitaA global reduction in water resources and the growing demand for fresh water has motivated the quest for the development of sustainable water-augmenting technologies. Membrane distillation (MD) is envisaged as an attractive desalination technology, surpassing cost challenges faced by conventional desalination technologies. Yet, its industrial commercialization faces multiple limitations, including the production of low water fluxes, membrane wetting and membrane fouling. This study sought to investigate the performance of silver nanoparticles (AgNPs) embedded on cellulose nanocrystals (CNCs) (CNC-capped AgNPs) to lessen the impact of biofouling in MD. This was conducted through coating the polyvinylidene fluoride (PVDF) membrane with CNCcapped AgNPs. Prior to coating with CNC-capped AgNPs, PVDF membrane properties were improved (for MD suitability) through blending with polyvinylpyrrolidone (PVP) and functionalized carbon nanotubes (f-CNTs). The resulting membrane had an improved overall porosity, and a respective increase in surface roughness (75%) and mechanical strength (45%). Pristine CNC-capped AgNPs’ characterization presented stable AgNPs with minimal leaching. Transmission electron microscopy (TEM) micrographs revealed a uniform dispersion of spherically shaped AgNPs exhibiting 13.3 ± 3.4 nm average diameter. The presence of AgNPs on the surface of CNCs afforded excellent thermal stability and good anti-microbial activity, mainly against E. coli, P. aeruginosa, S. aureus, S. epidermis, and S. saprophyticus. Following membrane modification, preliminary anti-bacterial tests conducted on the CNC/AgNP-modified PVDF membrane revealed a 98.7%, 52.3%, 78.0%, 53.9% and 93.3% reduction of E. coli, P. aeruginosa, S. aureus, S. epidermis, and S. saprophyticus cells, respectively, demonstrating its ability to control biofouling. Although the CNC/AgNP-modified PVDF membrane exhibited improved membrane properties, such as high surface roughness, high liquid entry pressure (LEP), and good hydrophobicity, its performance in MD (with artificial seawater as the feed stream) was poor, producing the lowest average water flux (0.179 ± 0.0303 kg/m2 /hr) compared to the unmodified PVDF membrane (0.528 ± 0.0838kg/m2 /hr), mainly due to pore blockage. However, upon spiking the artificial seawater with a monoculture of G. Stearothermophilus, the CNC/AgNP-modified PVDF membrane displayed the most stable water flux while the unmodified PVDF membrane’s water flux decreased by 79.3% over the 24-hour (h) period. This was attributed to the formation of a biofouling layer on the PVDF membrane which was absent on the CNC/AgNP-modified PVDF membrane. The AgNPs on the surface of the membrane afforded minimal bacterial deposition during operation. These results ascertain the possibility of biofouling minimization in MD using CNC-capped AgNPs, contributing to MD’s body of work for its ultimate realization for up-scaling.Item Carbon nitride-based catalysts for thermal carbon monoxide oxidation: Does phase matter?(University of the Witwatersrand, Johannesburg, 2023-06) Mohamed, Ahmed Gamal Abdelmoneim; Ozoemena, Kenneth Ikechukwu; Abdullah, Aboubakr M.; Eid, KamelCarbon monoxide (CO) has a poisonous effect on all living organisms as it binds to the hemoglobin of blood cells, preventing oxygen uptake. Thus, the conversion of CO to less dangerous gas such as CO2 is an essential process. This work presented the utilization of carbon nitrides (C3Nx) in different phases (βgC3N4, βC3N5, βC3N6) for thermal carbon monoxide (CO) oxidation. Herein, gC3N4, C3N5, and C3N6 were prepared by pyrolysis of their amine precursors, which were doped with Fe by two distinct methods; mechanical mixing (Fe/C3Nx-M) and polymerization (Fe/C3Nx-P). The controlled preparation of Fe/gC3N4-P allowed the formation of hierarchical porous structures with high surface area (219 m2/g) compared to the Fe/gC3N4-M (77 m2/g). This enabled the ease of reactants diffusion, enhanced the electron transfer, and maximized the atomic utilization. Accordingly, Fe/gC3N4-P (T100= 245 °C) presented higher catalytic activity than Fe/gC3N4-M (T100= 291 °C). In addition, bimetallic FeTi/gC3N4-P and trimetallic FeTiCu/gC3N4-P catalysts achieved the complete conversion of carbon monoxide (CO) at lower temperatures; 175 and 147 °C, respectively, which was attributed to the enhanced reducibility, and synergistic effect of Ti and Cu. Besides, FeTi/gC3N4-P and FeTiCu/gC3N4-P showed higher catalytic activity than Pd/C commercial catalyst (T100= 198 °C). In addition, the trimetallic FeTiCu/gC3N4-P showed a stable catalytic behavior without any deactivation for more than ten hours. This study showed that the C3Nx phases worked successfully in the thermal catalytic CO oxidation. However, the gC3N4 phase is the most active one when doped with metal(s), as it offered higher crystallinity, graphitization, and thermal stability than C3N5 and C3N6. This study also paves the way for the utilization of gC3N4 as a support for different metals to be used efficiently in various thermal catalytic applications, not only CO Oxidation.Item Tailored Fabrication of MXene/Chitosan Nanocomposites as Efficient Adsorbents for Heavy Metals Removal from Wastewater(University of the Witwatersrand, Johannesburg, 2023-08) Ibrahim, Yassmin Ahmed Ismail; Eid, Kamel; Ozoemena, Kenneth IkechukwuMXene (Ti3C2Tx) has been extensively utilized in water purification systems, including toxic metal ions removal, owing to the unique layered structure and abundant oxygen surface groups. However, challenges such as aggregation and solubility of Ti3C2Tx nanosheets in water have prompted the need for innovative strategies. In this study, we introduce a i3C2Tx-incorporated chitosan matrix (MX/CS) adsorbent designed to address solubility concerns during water treatment. MX/CS adsorbents are tested towards the capture of “cadmium” (Cd 2+) and “Zinc” (Zn2+) ions in aqueous solutions at varied pH values (i.e., acid, neutral and alkaline), initial ions concentrations (25, 50 and 100 ppm), and varied Ti3C2Tx loading (i.e., 1, 5 and 10), to study the optimization adsorption parameters. In addition, the Ti3C2Tx nanosheets were activated/alkalinized at ratio (2:1, i.e., 2MX:OH/CS), where more negative-ions sites are provided, thus, enhancing the preferential sorption for heavy metal ions in terms of high adsorption capacities, and kinetics than the non-activated samples (MX-10/CS). Freundlich isotherms are predominated for the Cd2+ and Zn2+ ions adsorption on both adsorbents. A selectivity study reveals that Zn2+ ions got adsorbed faster on the adsorbents than Cd2+ ions because of its low atomic radii and electronegativity. Finally, the adsorbents will be generated and prepared for additional adsorption cycles to test their stability. The second part of this work is to present the novel fabrication of multifunctional hydrophobic polymeric foam MX nanocomposites for large-scale ultrafast wastewater treatment. Likewise, the foam nanocomposite will be tested for the adoption of multi-ions solution over wide pH rage to demonstrate the applicability of the novel adsorbent for large-scale applications. Overall, this research contributes to the advancement of water treatment technologies by enhancing the stability of MXene-based adsorbents and introducing an innovative fabrication method for hydrophobic polymeric foam MX nanocomposites. The outcomes demonstrate the applicability of these novel adsorbents for efficient and scalable water purification solutions.Item The synthesis of aryl benzamides as potential HIV-1 non-nucleoside reverse transcriptase inhibitors (NNRTIs)(University of the Witwatersrand, Johannesburg, 2023-07) Mohasoa, Likhopotso Cecilia; Zimuwandeyi, Memory; Bode, Moira L.Dihydro-alkoxybenzyloxopyrimidines are heteroaryl-containing compounds that have previously been shown to exhibit excellent activity against HIV-1 reverse transcriptase (RT) enzyme. In our own laboratory, 2-chloro-N-(6-(piperidin-1-yl)pyridin-2-yl)benzamide was identified as a compound with activity against wild-type HIV-1. Using these two structural types as a guide, as part of our ongoing studies to search for anti-HIV therapeutic agents that target the RT enzyme, a library of arylbenzamide compounds bearing a pyrimidine ring as a central core was synthesized. These compounds contained an oxygen linker to allow flexible rotation of the molecule in the RT active site, with the aim of achieving activity against wild-type and mutant HIV-1. As a starting point, in order to first identify a suitable synthetic method and then apply it for our target novel compounds, four different carboxylic acids and two classes of amines were tested. Amidation reactions were carried out on unsubstituted benzoic acid, 3-methoxybenzoic acid, 3-hydroxybenzoic acid, and 3-((2,6-dichloropyrimidin-4-yl)oxy)benzoic acid. In this last case, the 3-hydroxybenzoic acid moiety had already been linked to the pyrimidinyl core in order to test which order of reaction worked best: linking followed by amidation, or the reverse. Reaction of these benzoic acid derivatives with anilines and aminopyridines gave the resulting benzamides in 22-99% yields after optimization. When triethylamine was used as a base in amidation reactions involving 2-amino-3-bromopyridine, 2-amino 5-bromopyridine and 2-amino-5-methylpyridine, diacylation was favoured, while when pyridine was used, monoacylation predominated. The reactions to link benzoic acid derivatives to the pyrimidinyl core were carried out by displacement of chlorine on 2,4,5-trichloropyrimidine. The displacement of the first chloride was tested using three types of nucleophiles. The first nucleophile was methyl 3-hydroxybenzoate, effectively a protected benzoic acid, which afforded methyl 3-((2,6-dichloropyrimidin-4-yl)oxy)benzoate in 81% yield. Problems with subsequent hydrolysis of the ester made this route impractical. The second nucleophile was 3-hydroxybenzoic acid which provided 3-((2,6-dichloropyrimidin-4-yl)oxy)benzoic acid in 81% yield. The third nucleophile was N-(5-bromopyridin-2-yl)-3-hydroxybenzamide, where amidation had already been performed, which transformed into the desired compound N-(5-bromopyridin-2-yl)-3- ((2,6-dichloropyrimidin-4-yl)oxy)benzamide in 28%. The low yield obtained from reaction of the amidated nucleophile identified the most promising route to be linking of 3-hydroxybenzoic acid to 2,4,5-trichloropyrimidine first, followed by amidation. After the successful displacement of the first chlorine atom, two of the resulting analogues 3-((2,6-dichloropyrimidin-4-yl)oxy)-N-(p-tolyl)benzamide and N-(4-bromophenyl)-3-((2,6-dichloropyrimidin 4-yl)oxy)benzamide were functionalized with sulfur and nitrogen nucleophiles by displacement of a second chlorine atom. Ethanethiol proved to be highly nucleophilic, leading to pyrimidine C-O bond cleavage and sulfur disubstitution, while the nitrogen ucleophiles propylamine and piperidine afforded their corresponding derivatives in good yields without breaking the carbon-oxygen bond. The newly coupled propyl compound was further derivatized by means of hydrolysis with sodium hydroxide to yield the desired novel 3-((6-hydroxy-2-(propylamino)pyrimidin-4-yl)oxy)-N-(p tolyl)benzamide or 3-((6-oxo-2-(propylamino)-1,6-dihydropyrimidin-4-yl)oxy)-N-(p-tolyl)benzamide compound.Item Defect–engineered lithium titanate anode materials for lithium–ion batteries(University of the Witwatersrand, Johannesburg, 2023-10) Podile, Seromo; Haruna, Aderemi Bashiru; Ozoemena, Kenneth IkechukwuEnergy is one of the aspects that plays a central role in moving society forward since it is one of the most important agenda of global economic and energy forums. There is an urgent need to move to clean energy given the environmental and the health benefits resulting from implementing energy systems that utilize green energy. The proposed energy sources in these systems are primarily natural (e.g. wind and solar), which means they are beyond human control and would work better if coupled with energy storage devices (ESDs). From this emerge the importance of energy storage systems (ESSs) which mostly perform based on the materials utilized to assemble the devices. In this study, we seek to enhance the power and energy densities of two of the prominent energy storage systems, namely lithium-ion batteries (LIBs) and lithium-ion capacitors (LICs), using modified commercial lithium titanate (LTO) materials as anodes. The materials consist of the pristine LTO (LTO-p), LTO coated with cerium fluoride (LTO-CeF3) and dry and wet irradiated pristine LTO (LTO-p-md and LTO-p-mw) and LTO coated with cerium fluoride (LTO-CeF3-md and LTO-CeF3-mw). Microwave irradiation was used to study the possible defect that the radiation can bring to the materials and possibly use the microwave effect to improve their electrochemistry. The effects of the coated layer of CeF3 and microwave irradiation on the structure of the commercial LTO were extensively studied using powder X-ray diffractometer (XRD), Raman spectroscopy, transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA) and Brunauer-Emmett-Teller (BET) theory. Structural investigations of TEM micrographs revealed CeF3 was present on the surface of the coated LTO materials. Further analysis did show that some portion of the CeF3 coating layer was co-doped into the LTO nanostructures. The findings from XRD and XPS analyses showed that co doping promoted a mixed state of Ti3+ and Ti4+ resulting from charge compensation when Ce3+ and F possibly substituted Ti4+ and O2-. This mixed state of titanium ions allowed the materials to have high electric conductivity than the pristine LTO (LTO-p). The spectrographs obtained from XPS analysis also showed that LTO microwave irradiated materials without coating experienced the same mixed states, which may have originated from oxygen vacancies that allowed for charge compensation when some of the LTO-p Ti4+ had to turn to Ti3+. The HRTEM analysis demonstrated changes to the lattice planes spacing of the modified LTO materials and these changes supported the (111) lattice plane shifts observed in the XRD analysis. The electrochemistry of the lithium-ion battery was studied on all the materials using cycling voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) techniques. From GCD technique, the results indicated that the modified materials had higher specific capacities than LTO-p at all rates under rate capability studies. All as prepared materials were stable with coulombic efficiency of almost 100% for 100 cycles using 1 C, with modified materials surpassing LTO-p specific capacity. Cycling at a high rate (5 C) for 1000 cycles, saw LTO-md, LTO-mw, LTO-CeF3 and LTO-CeF3-md having high-capacity degradation after 500 cycles. It was also observed that initial capacities were retained up to the 1000th cycle for LTO-CeF3-mw and LTO-p. With an aim of comparing LTO-p and LTO-CeF3 half cells, these two materials were further studied for lithium-ion capacitors (LICs). This choice was based on LTO-CeF3 having a better charge-discharge profile, high specific capacity and coulombic efficiency than LTO-p. The improvements resulted from the coating layer (leveraging the high ionic conductivity and chemically inert properties of CeF3) and the observed defects seen on the XPS studies created by the coating synthesis process. The LTO-CeF3 anode was paired with a carbon black cathode to construct a full LIC cell. The assembled cell produced a maximal energy density of 107.4 Wh kg 1 with an accompanying power density of 2000 W kg-1, and it yielded a power density of 10 000 W kg -1 with an energy density of 61.8 Wh kg -1.Item Reconstructing Late Quaternary environmental change on the southern Cape coast of South Africa: A 30,000-year geochemical record from Pearly Beach(University of the Witwatersrand, Johannesburg, 2023-11) Mey, Rachel Tess; Humphries, Marc; Quick, LynneThe southern Cape coast of South Africa has undergone complex and dynamic climate changes due to its position at the boundary of major atmospheric and oceanic systems. However, climate changes that occurred in the region during the Last Glacial Maximum (30,000 – 18,000 cal yr BP) and the deglaciation period that followed (18,000 – 11,000) are still not fully understood. Climatic change along the southern Cape coast is of archaeological, cultural and environmental significance given the ecological diversity and hominin fossil, richness of the region. Moreover, predictions of future climate change and water scarcity management issues in the southern Cape make it vital to understand and response to future climate scenarios. To address this gap, this study investigated the geochemical composition of a 5.42 m sediment record (KSV 1) extracted from a freshwater coastal wetland near Pearly Beach on the southern Cape coast. The wetland, situated in the winter rainfall zone, is influenced by the southern westerly storm track, the South Atlantic anticyclone, and Agulhas-Benguela interactions, making it an ideal location to investigate changes to large-scale atmospheric and oceanic circulation systems. Using a combination of inorganic elemental (X-ray fluorescence, XRF) and stable carbon isotope (δ13C) analysis, this study presents a 30,000 year record of palaeoenvironemental change from Pearly Beach and considers these in relation to broad-scale climatic perturbations that occurred in southern Africa over this period. A chronology for the core was derived from radiocarbon dating of 8 bulk sediment samples. Sedimentation rates over the record averaged 0.02 cm/yr, giving the XRF analyses which were conducted at 2 cm intervals, an average temporal resolution of ~ 110 years. Sediments deposited 30,000 – 15,000 cal yr BP consist primarily of quartz-rich (60 – 90% SiO2) silt and fine sand with relatively low quantities of clay material. An increase in the relative proportion of Al2O3 from ~15,000 cal yr BP, coupled with a rise in sedimentation rate, marks an increase in the accumulation of fine-grained material within the wetland that is associated with a rise in global sea levels following deglaciation. A distinct shift to higher δ13C values around this time points to a change in vegetation community, with an increase in contribution from C4 drought-tolerant plants. The presence of calcium carbonate in sediments provides a strong indicator for marine intrusions at site and a pronounced increase in marine influence is inferred from ~8,500 cal yr BP, corresponding with the stabilisation of sea levels near to present-day levels. Variations in elements typically associated with heavyminerals (titanium and zirconium) are used to infer changes in depositional energy at the site. Enrichments in Ti and Zr suggest a period of enhanced transport energy between 18,800 and 14,500 cal yr BP. This is coupled with an increase in the Si/Al ratio and linked to enhanced aeolian activity. An increase in wind intensity during this period is attributed to the intensification in the Southern Hemisphere westerly winds at this time. This corresponds with available fossil pollen records, which suggest an increase in moisture at Pearly Beach 18,500 to 15,000 cal yr BP that was possibly linked to a slowdown in the Atlantic Meridional Overturning Circulation (AMOC) and build-up of heat in the southeast Atlantic. It is possible that a build-up of heat in the southeast Atlantic brought on by a slowdown in the AMOC increased moisture uptake in the frontal storms, resulting in stronger low-pressure systems and intensified storms, as reflected in the KSV-1 record. Comparisons with other records from South Africa, including Mfabeni, Cold Air Cave and Cango Caves, indicate that the central and eastern parts of the country experienced a cooling event ~18,500 and 14,500 cal yr BP. New geochemical data from Pearly Beach suggests that this cooling may have been linked to intensification of westerly winds at this time. An apparent lag effect in the warming of South Africa after deglaciation may be attributed to the enhanced influence of westerly winds during the glacial-interglacial period, which pushed cold air masses across the interior of the country. The complex interplay between local sea level changes, westerly wind dynamics, and regional climatic conditions underscores the significance of the geochemical record from Pearly Beach in understanding past environmental changes and their broader implications for southern Africa.Item Diastereoselective conjugate addition reactions using diverse nucleophiles on a variety of Morita-Baylis-Hillman (MBH) adducts(University of the Witwatersrand, Johannesburg, 2023-09) Bhom, Nafisa; Bode, Moira L.The Morita-Baylis-Hillman (MBH) reaction involves the formation of a new carbon-carbon bond, generating an MBH adduct. These MBH adducts are multi-functional molecules, which can be used as synthons for the generation of complex and diverse compounds. The first part of the work described here involved the synthesis of a series of diverse ester and nitrile MBH adducts obtained as racemic mixtures. The MBH adducts were protected using different protecting groups, which could potentially control the diastereoselectivity and the formation of alternative products in the subsequent conjugate addition reaction. Conjugate addition reactions were performed on the protected MBH adducts using different nucleophiles to obtain the product as diastereomers. These reactions were monitored to detect whether diastereomers were obtained or not. The diastereomeric ratios obtained using different substrates, protecting groups and nucleophiles were determined. The best diastereomeric ratio was 3:1, obtained for the piperidine and benzylamine addition on the TBDMS protected nitrile adducts 192a/b and 196a/b. The addition of sulfur nucleophiles gave the conjugate addition product only and the addition of nitrogen nucleophiles gave both conjugate addition and allylic substitution products. It was found that the protecting groups did not have an effect on the diastereomeric ratio obtained, nor on the formation of alternative products. The last step performed in the sequence was the deprotection of the conjugate addition products. The configuration of the major and the minor diastereomers were determined, the major product was assigned as the syn diastereomer. The major:minor diastereomeric ratio for compound 208a/b was 3:1 and for compound 209a/b, a ratio of 2:1 was obtained. The next part of the work involved the synthesis of MBH adducts with amide as the electron withdrawing group. The originally proposed route involved the synthesis of MBH esters and their conversion into amides. The conjugate addition reactions were attempted on these amide adducts, but were unsuccessful. A number of alternative routes were attempted for the synthesis of amide adducts and conjugate addition products resulting from these adducts. From all the alternative routes, the best route was the originally proposed route.Item Manganese-Rich Nickel-Manganese-Cobalt Oxides as Hybrid Supercapacitor Electrode Materials(University of the Witwatersrand, Johannesburg, 2023-09) Tshivhase, Funanani; Ozoemena, Kenneth IkechukwuFossil fuels used as the conventional energy source play a substantial negative role in climatic changes and global warming. Their reservoirs on earth keep getting constrained, thus limiting their reliability. These issues make renewable energy sources an excellent alternative due to their abundance, environmental safety, affordability, and renewability. However, renewable energy is subjected to geographic limitations, and some sources are intermittent, which can be solved by applying energy storage devices. Asymmetric hybrid supercapacitors are an excellent choice due to the safety of aqueous electrolytes, exploitation of abundant metals in the metal oxides used, improvement of power and energy density and simple assembly and application. In this work, manganese-rich nickel-manganese-cobalt (MR-NMC) was studied and applied in asymmetric hybrid supercapacitors as a cathode material, and reduced graphene oxide (rGO) was used as an anode. Synthesis was done using co-precipitation-(Conv), laminar Taylor vortex flow reactor-(Lam), and microwave irradiation-(MW) approaches. Physical characterization was performed using XRD and TEM. Electrochemical studies were done using CV, GCD and EIS. Three full cells/two electrode systems were assembled and studied. Those cells were rGO//Conv MR-NMC, rGO//Lam MR-NMC and rGO//MW MR-NMC. The data obtained from electrochemistry tests was used for the calculations of specific capacitance, energy and power densities. rGO//MW MR-NMC cell had the highest specific capacitance response compared to rGO//Conv MR-NMC and rGO//Lam MR-NMC over the entire current density range used, where at the current density of 0.2 A g-1, rGO//MW MR-NMC had 44.77 F g-1, followed by rGO//Lam MR-NMC with 15,89 F g-1, then rGO//Conv MR-NMC with 13.68 F g-1. There was no significant difference in the specific capacitance responses of rGO//Conv MR-NMC and rGO//Lam MR-NMC. rGO//MW MR-NMC also exhibited higher energy density for the entire range of power density over rGO//Conv MR-NMC and rGO//Lam MR NMC. At the power density of 678,08 W kg-1, rGO//MW MR NMC had a specific energy density of 65 Wh kg-1, followed by rGO//Lam MR NMC with 23.45 Wh kg-1, then rGO//Conv MR-NMC with 19.82 Wh kg-1. Overall, the electrochemistry and the calculated perimeters thereafter showed that microwave irradiation is a reliable approach that can be used in the preparation of metal oxides used in energy storage devices for the improvement of electrochemical performance, which potentially enables the commercialization of these systems and management of energy crisis in South Africa, Africa and the world as a whole, hence the rGO//MW MR-NMC material performed better than the other two.Item Magnetic enhancement of a high entropy spinel oxide electrocatalyst for rechargeable zinc-air batteries(University of the Witwatersrand, Johannesburg, 2024) Hechter, Ernst Heznz; Ozoemena, KennethThe exploration of high entropy materials (HEMs) as electrocatalyst materials has only recently begun to accelerate. Similarly, the effect of magnetic fields on the oxygen evolution and reduction reactions has recently begun to attract great interest. In this work nanoparticles of the high entropy oxide (CuCoFeMnNi)3O4 were synthesized and supported on Vulcan carbon for use as a bifunctional OER/ORR catalyst in a rechargeable zinc-air battery (RZAB). The products were characterized to confirm and investigate the solid solution high entropy phase, and the electrochemistry was investigated with and without an external magnetic field. The HEMs demonstrated moderate intrinsic electrochemical properties, with overpotentials and current densities comparable to commercial platinum on carbon catalysts even at low loadings. Here is reported the most significant magnetic enhancement in RZAB power profile in literature at the time of writing, as well as improved RZAB stability and areal energy. This work offers insight into the mechanism of magnetic enhancement in the case of high entropy materials, and pioneers the use of combined strategies to achieve stable, cost-efficient and effective bifunctional OER/ORR electrocatalysis.Item Imputation of missing values and the application of transfer machine learning to predict water quality in acid mine drainage treatment plants(University of the Witwatersrand, Johannesburg, 2024) Hasrod, TaskeenAccess to clean water is one of the most difficult challenges of the 21st century. Natural unpolluted water bodies are becoming one of the most dramatically declining resources due to environmental pollution. In countries like South Africa which has a mining-centred economy, toxic pollution from mine tailing dumps and unused mines leach into the underground water table and contaminate it. This is known as Acid Mine Drainage (AMD) and poses a grave threat to humans, animals and the environment due to its toxic element and acidic content. It is, therefore, imperative that sustainable wastewater treatment procedures be put in place in order to decrease the toxicity of the AMD such that clean water may be recovered. An efficient circular economy is created in the process since original wastewater can be recycled to not only provide clean water, but also valuable byproducts such as sulphur (from the elevate sulphate content) and other important minerals. Traditional analytical chemistry methods used to measure sulphate are usually time-consuming, expensive and inefficient, thereby, leading to incomplete analytical results being reported. To address this, this study aimed at imputing missing values for sulphate concentrations in one AMD treatment plant dataset and then using that to conduct transfer learning to predict concentrations in two other AMD treatment plants datasets. The approach involved using historical water data and applying geochemical modelling as a thermodynamical tool to assess the water chemistry and conduct preliminary data cleaning. Based on this, Machine Learning (ML) was then used to predict the sulphate concentrations, thus, addressing limited data on this parameter in the datasets. With complete and accurate sulphate concentrations, it is possible to conduct further modelling and experimental work aimed at recovering important minerals such as octathiocane, S8 (a commercial form of sulphur), gypsum and metals. Historical data obtained from the three AMD treatment plants in Johannesburg, South Africa (viz., Central Rand, East Rand and West Rand) were obtained and the larger Central Rand dataset was split into smaller untreated AMD (Pump A and Pump B) subsets. Thermodynamic and solution equilibria aspects of the water were assessed using the PHREEQC geochemical modelling code. This served as a preliminary data cleanup step. Eight baseline as well as three ensemble machine learning regression models were trained on the Central Rand subsets and compared to each other to find the best performing model that was then used to conduct Transfer Learning (TL) onto the East Rand and West Rand datasets to predict their sulphate levels. The findings pointed to a high correlation of sulphate to temperature (°C), Total Dissolved Solids (mg/L) and most importantly, iron (mg/L). The linear correlation between iron and sulphate substantiated pyrite (FeS2) as their source following weathering. Water quality parameters were found to be dependent on factors such as weather and geography this was evident in the treated water that had quite different chemistry to that of the untreated AMD. Neutralisation agents used were based on those parameters, thus, further delineating the chemistry of the treated and untreated water. The best performing ML model was the Stacking Ensemble (SE) regressor trained on Pump B’s data and combined the best performing models namely, Linear Regressor (LR), Ridge Regressor (RD), K-Nearest Neighbours Regressor (KNNR), Decision Tree Regressor (DT), Extreme Gradient Boosting Regressor (XG), Random Forest Regressor (RF) and Multi-Layer Perceptron Artificial Neural Network Regressor (MLP) as the level 0 models and LR as the level 1 model. Level 0 consisted of training heterogenous base models to obtain the crucial features from the dataset. These individual predictions and features were then fed to a single meta-learner model in in the next layer (level 1) to generate a final prediction. The stacking ensemble model performed well and achieved Mean Squared Error (MSE) of 0.000011, Mean Absolute Error (MAE) of 0.002617 and R2 of 0.999737 in under 2 minutes. This model was selected to be used for TL to the East Rand and West Rand datasets. Ensemble methods (bagging, boosting and stacking) outperformed individual baseline models. However, when comparing stacking ensemble ML that combined all the baseline models with stacking ensemble ML that only combined the best performing models, it was found that there was no significant improvement in excluding bad models from the stack as long as the good models were included. In one case, it was actually beneficial to include the bad performing models. All models were trained in under 2 minutes which proved the benefit of using ML approaches compared to traditional approaches. The treated water data was highly uncorrelated such that model training was unsuccessful with the highest achievable R2 value being 0.14, thus, no treated water model was available for TL. TL was successfully conducted on the cleaned and modelled East Rand AMD dataset using the Central Rand (Pump B) stacking regressor and a high level of accuracy with respect to Mean Square Error (MSE), Mean Absolute Error (MAE) and R2 (MSE:0.00124, MAE:0.0290 and R2:0.963) between the predicted and true sulphate values was achieved. This was achieved despite a marked difference in the distributions between the Central Rand and East Rand datasets which further proved the power of utilizing ML for water data. TL was successful in imputing missing values in the West Rand dataset following prediction of sulphate levels in the cleaned and modelled West Rand AMD and treated water datasets. No true values for sulphate levels in the West Rand dataset were given, as such, accuracy comparisons could not be made. However, a general baseline idea of the amount of sulphate present in the West Rand treatment plant could now be understood. The sulphate levels in all three treatment plants (Central Rand, East Rand and West Rand) were found to greatly differ from each other with the Central Rand having the most normal distribution, the East Rand having the most precise distribution and the West Rand having the most variable distribution. Whilst the sulphate levels in the treated effluent waters could not be reliably predicted due to inherent issues (e.g., analytical inaccuracies and inconsistences) and poor correlations within the treated water datasets, sulphate levels in all three of the untreated AMD datasets were successfully predicted with a high degree of accuracy. This underpinned the observation made previously about the discrepancies between treated and untreated water. The study has shown that it is possible to impute missing values in one water dataset and use transfer learning to complete and consolidate another similar, but scarce, dataset(s). This approach has been lacking in the water industry, resulting in the reliance and use of traditional methods that are expensive and inadequate. This has caused water practitioners to abandon scarce datasets, thus, losing potentially valuable information that could be useful for water remediation and recovery of valuable resources from the water. As a spin off from the study, it has been indicated that automation of such data analysis is possible. This was achieved by developing a Graphical User Interface (GUI) for ease of use of the SE-ML model by those with little to no programming background nor ML knowledge e.g., the laboratory staff at the AMD treatment plants. This can also be used for teaching purposesin academia.Item Design and synthesis of triazine derivatives as non-nucleoside reverse transcriptase inhibitors(University of the Witwatersrand, Johannesburg, 2024) Munetsi, Wendy; Bode, Moira; Ngwira, KennedyThis research work was carried out to investigate the properties of different groups that can be used to modify the triazine core with the aim of designing a new library of possible HIV non- nucleoside reverse transcriptase inhibitors (NNRTIs). Triazine derivatives have been used extensively in the synthesis of numerous classes of drugs due to their significant biological activity. In this project, the specific focus was to synthesize 1,3,5-triazine derivatives by successive nucleophilic substitution reactions of the Cl atoms from cyanuric chloride. In the first step of the substitution reactions, 2,4,6-trichloro-1,3,5-triazine was reacted with various anilines, phenols and thiophenols which acted as nucleophiles to displace one of the Cl atoms upon reaction completion. The yields varied from 28% -90% with the best yields being observed when the anilines were used as a nucleophile and most of the substituents in this first step were anilines. The substituents used at each step of the substitution were vital in terms of determining the order of the reaction to enable a successful reaction. The introduction of different linkers to the triazine core such as -NH, -S, -O yielded compounds with different properties expected to provide significant interactions in the NNRTI binding pocket. We expected better binding properties from the -NH bearing compounds due to hydrogen bond formations with amino acid residues inside the allosteric binding pocket of the HIV-1 RT. The success of the second step of the substitution reactions was identified to be dependent on the substituent attached to the triazine ring from the first step. Some reactions were not successful when a stronger nucleophile was used in the first step and a weaker nucleophile was being used as the incoming nucleophile substituting the second Cl atom. Therefore, these reactions were repeated and the order of the reaction rearranged. Temperatures were increased and reaction times were increased at this stage as the reactivity of the triazine ring was reduced and therefore higher kinetic energy was required for successful reactions. In general, the synthesized triazine derivatives bearing two aromatic substituents exhibited the most significant presence of tautomers. The final stage in the synthesis of the trisubstituted triazine derivatives was relatively complex and required much higher temperatures and longer reaction times. The reactions were also performed at smaller scales and difficulties with the purification processes also contributed to the loss of product thereby resulting in lower yields, with one of the compounds giving a yield of 11%. The results obtained from the anti-HIV assay studies from the selected compounds tested, showed that antiviral activity was observed in triazine derivatives with electron withdrawing groups attached to the aromatic substituent as well as -NH and -O linkers at the right and left wing of the triazine core, respectively.Item Use of transaminases for the biosynthesis of enantiopure building blocks of two essential medicines: Ethambutol and Dolutegravir(University of the Witwatersrand, Johannesburg, 2023) Maboya, Josephine; Pienaar, Daniel(S)-2-Amino butan-1-ol and (R)-3–amino butan-1-ol play an important role as intermediates in the synthesis of the anti-tuberculosis drug ethambutol and HIV integrase inhibitor drug dolutegravir respectively. The current industrial preparation of these enantioenriched amino alcohols is quite a challenging process; it typically involves the use of harsh chemicals, results in low yields, and generates hazardous waste materials. Consequently, these methods tend to be expensive, and it has been demonstrated that the cost of these intermediates has a significant impact on the overall costs of the synthesis of the entire drug. Therefore, it is not surprising that the convenient, cost–effective, and environmentally benign production of these optically pure amino alcohols is still the subject of ongoing investigations. The chemo-enzymatic approach holds great potential to replace the conventional routes for the synthesis of enantiopure amines. Transaminase enzymes (ATAs), in particular, have gained much attention over time due to their remarkable capability to transform inexpensive ketone starting materials into valuable enantiopure amino alcohols. Through the utilization of the isopropyl amine donor system, pro-chiral ketone starting materials were effectively transformed into the desired (S)-isopropyl 2-aminobutanoate and (R)-isopropyl 3-aminobutanoate using transaminase biocatalysis. These reactions proceeded well under milder conditions such as ambient temperature and pressure conditions, and impressively under an aqueous environment. Three (S)-enantiomer selective “hit “enzymes were discovered (ATA-189, ATA-194, and ATA-254) for the biotransformation of alpha-keto ester substrate into an enantio-enriched amino ester product, with enantiomeric excess ranging between 95-99% and the yield was 15-73% depending on the enzyme and reaction conditions. However, when it came to dolutegravir intermediate, a different scenario unfolded. In this case, the majority of the ATA enzymes in our enzyme library fortuitously exhibited selectivity for the (R)-enantiomer. In particular, four highly enantioselective enzymes (ATA-254, ATA-261, ATA-262, and ATA-234) were discovered, demonstrating % e.e ranging from 93% to 99.99%, with corresponding yields from 38% to 45%. The successful biotransformation of an inexpensive pro-chiral starting material into highly valuable enantioenriched amino ester intermediates represents a significant achievement. Coupled with an effective reduction method to convert these intermediates into the corresponding amino alcohols, this biotransformation process holds immense potential for enabling the sustainable and cost- effective production of both of the valuable ethambutol and dolutegravir amine intermediatesItem Investigation of rhombohedral 𝑩𝒊𝟐𝑶𝟑 as an oxide conducting electrolyte for solid oxide fuel cell applications(University of the Witwatersrand, Johannesburg, 2023-09) Kerspuy, Tanner Royele Rowan; Billing, Caren; Erasmus, Rudolph M.; Billing, Dave GordonThe synthesis of a bismuth system co-doped with neodymium (Nd3+) and yttrium (Y3+) was at the core of this project. The focus was placed on the synthesis of the rhombohedral phase of bismuth oxide, which has not been observed in pure bismuth oxide. Neodymium was selected as the main dopant (the one used in highest dopant concentration), due to its Shannon ionic radii. Upon doping with Nd3+ as a single dopant, it is observed that a mixture of the rhombohedral and monoclinic phases is obtained, thus noting that the single dopant system using Nd3+ does not stabilise the rhombohedral phase. When using a co-doped system of 15 mol % Nd3+ and 5 mol % Y3+ (15Nd5YSB), it is observed that we are able to obtain a stable phase pure rhombohedral phase, with a total dopant concentration of 20 mol%. The total dopant concentration % ranges selected ranged between 8.5-10 mol %, 20 mol % and 22.5 mol %. The Rietveld refinement of the X-ray diffraction data obtained for both the laboratory and synchrotron-based techniques indicate sample phase purity and phase stability for the samples under investigation. The refinements obtained for the samples indicated that not only one structure model was used to fit the experimental data. The structural models which fit the Rietveld refinements of the experimental data resulted in the observation of pure phase and mixed phase rhombohedral samples being observed. The Nd0.15Y0.05-Bi2O3 (15Nd5YSB)sample resulted in a phase pure rhombohedral structural model. Hereafter all samples will be referred to with the shorthand notation. The thermal analysis techniques are used to indicate the thermal dependence of the samples, this analysis also indicated phase stability across the temperature range of investigation as no phase transitions occurred throughout the heating and cooling cycles, and minimal weight loss is observed. The samples of importance in this study were the 12.5Nd10YSB sample which obtained a conductivity of 2.4511×10-5 S.cm-1 at 500 ℃, and the 15Nd5Y2.5TbSB sample which obtained a conductivity of 2.1725×10-5 S.cm-1 at 500 ℃. The Arrhenius plots obtained indicated stability 3 of these samples across the 200-500 ℃ temperature range with no discontinuities, which suggests no phase transitions, or order-to-disorder transitions. Variable temperature Raman spectroscopy indicated that the behaviour for all the samples analysed using Raman spectroscopy is consistent, however, a deviation was observed for the 15Nd5Y2.5ScSB sample which has a distinctive spot which exhibits different Raman shift behaviour as compared to all other samples. The VT-Raman spectroscopy spectra indicate a distinctive signature Raman peak at ~250 cm-1, which can be concluded to be the Raman peak which is indicative of the rhombohedral 𝐵𝑖2𝑂3, this peak also appears in the low cubic phase % sample after cooling back to room temperature. This assignment of the Raman spectral peak is confirmed through this peak being evident throughout all the spectra obtained and it being consistent throughout all the spectra observed.Item The Synthesis of Pyrido-fused 8-Methoxy Carbazoles by Using a Light-Assisted, Base Mediated Cyclization Reaction(University of the Witwatersrand, Johannesburg, 2023) Magagula, Bongi Florence; Ntsimango, Songeziwe; De Koning, Charles B.Nitrogen-containing compounds such as indoles and carbazoles are significant classes of the N-heterocycles that show great promise as anti-cancer compounds. Indoles such as 2,3-diarylindole, 3-pyranyl indole and carbazoles such as 9-methoxyellipticine are compounds which possess anticancer or antitumor properties. Due to the favourable biological activities of N-heterocyclic compounds, medicinal and synthetic chemists have developed numerous methodologies for their synthesis. In this research project, the broad aim was to synthesize pyrido-fused carbazoles from 5-methoxyindole using methodologies that have been previously used in our laboratories and by other chemists while changing the position of the nitrogen atom on the pyrido-fused carbazoles. The first step in the synthesis of these carbazoles was the treatment of 5-methoxyindole with di-tert-butyl dicarbonate in the presence of 4-dimethylaminopyridine (DMAP) which gave the desired protected indole, tert-butyl 5-methoxy-1H-indole-1-carboxylate in excellent yields (90-99%). Exposure of the tert-butyl 5-methoxy-1H-indole-1-carboxylate to lithium 2,2,6,6-tetramethypiperidide followed by quenching with triisopropyl borate and hydrochloric acid gave (1-(tert-butoxycarbonyl)-5-ethoxy-1H-indol-2-yl)boronic acid. Using this and various halogen substituted pyridines, for example 3-bromo-4-methylpyridine in the Suzuki-Miyaura coupling reaction gave tert-butyl 5-ethoxy-2-(4methylpyridin-3-yl)-1H-indole-1-carboxylate (83% yield). This was further reacted with paraformaldehyde and iron (III) chloride or phosphorus oxychloride and DMF. After the removal of tert-butoxycarbonyl protecting group utilizing various methods this produced 5-methoxy-2-(4-methylpyridin-3-yl)-1H-indole-3-carbaldehyde (48% yield). 5-Methoxy-2-(4-methylpyridin-3-yl)-1H-indole-3-carbaldehyde possesses all the carbons of the final compounds and is suitably functionalized to partake in the key photo-induced and ase-mediated cyclization reaction. Previous studies pointed to the necessity of an alkyl protecting group on the indole-N atom. As a result, the indole nitrogen atom was then protected again with a methyl or a benzyl group; where the N-benzyl could be removed at a later stage. For example, reaction of 5-methoxy-2-(4-methylpyridin-3-yl)-1H-indole-3-carbaldehyde dissolved in THF, potassium hexamethyldisilazide and benzyl bromide furnished 1-benzyl-5-methoxy-2-(4-methylpyridin-3-yl)-1H-indole-3-carbaldehyde (83% yield). The key step in this synthesis was the light-assisted, base-mediated cyclization reaction which has been reported by de Koning and co-workers, where a solution of 1-benzyl-5-methoxy-2-(4-methylpyridin-3-yl)-1H-indole-3-carbaldehyde dissolved in dry DMF and potassium tert-butoxide was heated and irradiated with medium mercury lamp yielding the desired pyrido fused carbazole, 11-benzyl-8-methoxy-11H-pyrido[3,4-a]carbazole in a good yield of 70%. Following the outlined synthetic procedure depicted above, we were able to synthesize 5 analogues of 11-benzyl-8-methoxy-1H-pyrido[3,4-a]carbazole.Item Characterization, quantification, and recovery of rare earth elements(rees) in South African coal fly ash samples(University of the Witwatersrand, Johannesburg, 2024) Michael Rampfumedzi, Tshilidzi; Chimuka, LukeRare earth elements (REEs) are naturally distributed throughout the Earth's crust, typically in low concentrations. They are not typically found in isolation but are rather present in various minerals, often in amounts too minute for cost-effective extraction. Fly ash is among the sources that are deemed economically viable for extracting REEs. The objective of this study was to create environmentally sustainable approaches for measuring and reclaiming rare earth elements (REEs) in coal fly ash (CAF) samples. The study involved analyzing fly ash samples collected from various coal power stations using a range of standard and advanced techniques, including X-ray fluorescence (XRF), X-ray diffraction(XRD), scanning electron microscopy (SEM), and inductively coupled plasma mass spectrometry (ICP-MS) and inductively coupled plasma optical emission spectrometry (ICP-OES). The XRF only shows the presence of REEs from all three fly ash samples with a range of 40 to 100 ppm and mineral oxide ranging from 0.1 to 50 %. The XRD results show that fly ash sample is a siliceous-rich sample with abundant minerals such as quartz (SiO2), magnetite (Fe3O4), and mullite (Al4.52Si1.48O9.74). The SEM analysis of the sample confirmed the presence of rare earth minerals, including monazite which is a light atomic mass (LREE), xenotime, a heavy atomic mass (HREE), and perrierite-bearing minerals. The results obtained from the instrumental analysis show that the ICP-MS instrument is the more effective analytical technique for REE analysis in this context as compared to ICP-OES. Using certified reference materials, the results obtained by two acids digestion technique, acids digestion and sodium peroxide fusion in, CGL 111, CGL 124, and AMISO276, were compared to validate whether the methods are reliable. The acid digestion approach demonstrated greater effectiveness in comparison to the sodium peroxide fusion method. The recovery percentage (%) from ICP‒MS showed an excellent percentage yield (80 – 120%) compared to the ICP‒OES instrument (50 –120%). The ICP‒MS data indicate that all fly ash samples have a high concentration of LREEs and a lower concentration of HREEs. Excellent recovery was obtained by ICP‒MS in a developed microwave acid digestion method. The concentration of REEs obtained from ICP - MS and OES in fly ash samples ranged from 50 ppm to 200 ppm for light rare earth elements and 0.5 ppm to 20 ppm for heavy rare earth elements. The total REE ( TREE) concentrations in all fly ash samples range from 400 ppm to 600 ppmItem Geochemical Investigation Into Holocene Palaeoenvironmental Change Along The Southern Cape Coast, South Africa(University of the Witwatersrand, Johannesburg, 2023-06) Dyubele, Viwe; Quick, Lynne; Humphries, MarcClimatic conditions across southern Africa are affected by the complex interaction of different atmospheric and oceanic circulation systems, the understanding of which is important to predicting future climate change. Palaeoenvironmental reconstruction is an essential tool to understand long-term environmental change and the response of ecosystems to such changes. This study examines the geochemical composition of a sediment core (WR1-1) extracted from a freshwater wetland located near Plettenberg Bay on the southern Cape coast. The wetland is located ~4 m above present sea level and positioned ~500 m from the modern coast. Situated within the year-round rainfall zone, the site is influenced by tropical easterly flow and the southern westerlies. Elemental and stable isotope geochemistry are used to reconstruct the palaeoenvironmental change at the site over the last ~8000 cal yr BP. Variations in CaO/Al2O3, Sr/Al2O3 and δ13C indicate that marine conditions dominated from 7300 to 6400 cal yr BP. Marine influence at the site decreased dramatically from ~6300 cal yr BP, as the system transitioned to a freshwater back-barrier wetland. Enrichments in SiO2/Al2O3 and Zr/Al2O3 track changes in depositional energy and suggest that the period 3800 – 3200 cal yr BP was associated with increased aeolian activity. This is interpreted to reflect increased aridity and is consistent with geochemical and pollen records from nearby sites at Eilandvlei and Voëlvlei. This suggests that a shift to more arid conditions during this time was a broad feature of the climate in the year-round rainfall zone of South Africa. The timing of this event corresponds with a marked decrease in Antarctic sea ice and pronounced aridity along the east coast of South Africa, suggesting that mid to late Holocene aridity in the YRZ was likely driven by declines in moisture from both westerly and easterly wind systems.Item Development of a Commercial Manufacturing Process of 9-[(R)-2- (phosphonomethoxy)propyl] adenine (PMPA): A Key Intermediate for the Production of Tenofovir-based HIV Medicines(University of the Witwatersrand, Johannesburg, 2023) Mbutho, Banele; Gohain, Mukut; De Koning, CharlesSouth Africa runs the largest antiretroviral (ARV) program in the world and yet 99% of the active pharmaceutical ingredients (APIs) used to make ARVs are imported from China. Dependence on imported APIs has major cost implications and influences the medication’s security of supply. This project was concerned with making it possible to produce the APIs tenofovir, a precursor for tenofovir disoproxil fumarate and tenofovir alafenamide locally and at a lower cost. A new synthetic route recently introduced by Medicines 4 All (M4ALL) was studied and used in this dissertation. The four-step process that produces an adenine derivative was optimized and scaled into a commercial industrial process producing tenofovir intermediates in repeatable yield and purity. This route was determined to be the most cost-effective since it utilized low cost and commercially available diaminomaleonitrile and triethyl orthoformate as starting materials—contrary to the synthetic routes currently used by the 17 largest tenofovir manufacturers. Key process improvements included a decrease in the number of solvents used and the minimization of by-product formation. Results showed that high yields of tenofovir intermediates were successfully synthesized using this new route. As such, the chemical company we conducted this research in, Chemical Process Technology Pharma will be able to employ this synthetic methodology to affordably produce the APIs used in the manufacturing of ARVs locally improving access to affordable medication.Item Determination of U and Th Radioisotopes in environmental samples by ICP-QMS(University of the Witwatersrand, Johannesburg, 2023-07) Rikhotso, Xikhongelo Valentia; Sehata, James; Chimuka, LukeNot AvailableItem Metal Pincers as Antiviral Agents Targeting SARS-CoV-2 Spike Protein(University of the Witwatersrand, Johannesburg, 2023-08) Bracken, Matthew Lee; Munro, Orde Q.The purpose of this work was to prove the concept that complexes of bioavailable metal ions may be designed to target specific solvent-exposed amino acid residues on therapeutic protein targets. The complexes synthesized and studied were novel Zn(II) and Cu(II) NNN amide pincers. The chelates were designed by in silico methods to target solvent-exposed tyrosine residues on the receptor binding domain of SARS-CoV-2. These tyrosine residues are crucial for binding host cell receptors and by targeting these groups, the metal pincers may potentially act as antiviral fusion inhibitors for the treatment of COVID-19. Biophysical studies were carried out to determine the binding affinity between the chelate and phenolic residues. These studies identified the most likely binding site for the metal complex on the SARS-CoV-2 spike protein epitope. The novel chelates were crystalized and found to adopt hexameric metallocycle architecture.Item The microwave assisted synthesis of doped carbon dot/carbon nano-onion composites: A novel all-carbon counter electrode for dye-sensitized solar cell(University of the Witwatersrand, Johannesburg, 2023) Masemola, Khanyisile; Moloto, Nosipho; Maubane-Nkadimeng, Manoko S.; Coville, Neil J.Human society's development is heavily reliant on stable energy supply, and fossil energy sources have long been a very reliable energy source for this objective. However, being a non-renewable energy source, fossil fuel depletion is unavoidable and impending in this or future generations. To solve this issue, renewable energy, particularly solar energy, has received a lot of attention since it directly turns solar energy into electrical power with no environmental consequences. Various photovoltaic technologies based on organic, inorganic, and hybrid solar cells have been successfully manufactured to date. However, much study has been concentrated on organic solar cells for household and other commercial uses due to its inherent cheap module cost and ease of production. But dye-sensitized solar cells (DSSCs) have been reported to be the most efficient and simplest applied organic solar cell technology. In this study, carbon dot: onion-like carbon nanomaterial composites (Cdots: OLCNs) were synthesized for possible future application in electronic devices with particular attention to dye-sensitized solar cells. The nitrogen-doped carbon dots (NCdots) and functionalized onion-like carbon nanomaterials (OLCNs) were synthesized using a one-step hydrothermal microwave assisted irradiation method and flame pyrolysis method using liquid fuels, respectively. The as-synthesized OLCNs where purified and washed using an organic solvent n-pentane to obtain pristine OLCNs (p-OLCNs) which were further functionalized with N2 gas to obtained nitrogen-doped CNOs (N- OLCNs) and H2O2 to give oxygenated OLCNs (ox-OLCNs). For the synthesis of NCdots, various precursors (ethylenediamine, urea and fumaronitrile) were used to evaluate the effect of different nitrogen sources on the properties of these materials. Photoluminescence spectroscopy showed that the resulting NCdots exhibited the conventional excitation-dependency behavior. The NCdots which presented with the highest fluorescence quantum yield (made from ethylenediamine) were used to make the subsequent NCdot: OLCNs composites. The as-prepared p-/ox-/N-OLCNs all showed similar morphologies typical of chain-like carbon nanostructures, according to transmission and scanning electron microscopy studies, but with varying particle sizes of 42 nm, 125nm and 85 nm, respectively. The corresponding nanocomposites were used as counter electrode materials in DSSCs. The application of all the nanocomposites in the DSSCs resulted in cell efficiencies, current densities, open circuit voltages and fill factors that were lower than that of a conventional platinum (Pt) electrode. All nanocomposites tested presented with cell efficiencies <1%. Furthermore, the cells displayed some photovoltaic effect of minimal activity in the absence of light, under dark field conditions implying it is still a photovoltaic material. This photocurrent generated by the cell in the dark is suggested to be a dominant contributor to the low performance of the cells. However, what was remarkable was that this photovoltaic effect, primarily due to the thermal activity from the long lasting glow of the NCdots specifically, was found to be stable and efficient in response as infrared radiation even without being illuminated with light for 5 minutes. This suggests that the NCdots: OLCNs composites have potential application, possibly as efficient diodes rather than for use in DSSCs.