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(2020)ThesisThe propagation polymer chains in RDRP can be isolated in the dormant form and be re-activated at chain ends to generate propagating radicals, which initiate subsequent chain extensions. Since the effect of remote substituents on kinetics decreases rapidly with their distance from the reaction site (chain ends), propagating radicals could be assumed to be only determined by the characteristics of terminal and penultimate units. Therefore, the chemical structure (substitution and stereochemistry) of dormant chain ends is crucial for the investigation of kinetics of RDRP. The investigation of stereochemical effect of dormant chain ends is challenging due to limited models which are highly accessible and able to represent long polymer chains. In this thesis, two ideal models (diastereomeric macro-RAFT agents with different chemical structures and two inherent diastereomers for each) were designed and prepared by single unit monomer insertion (SUMI) technology to examine their stereochemical effect on kinetics of RDRP. In the first work, diastereomeric CPDTC-Ane was synthesized by inserting the monomer anethole (Ane) into a trithiocarbonate, 2-Cyano- 2-propyl dodecyl trithiocarbonate (CPDTC) via SUMI. Two isolated diastereomeric oligomers, as the models for dormant polymer chains, were employed as initiators (chain transfer agents) for PET-RAFT (photoinduced electron/energy transfer reversible addition-fragmentation chain transfer) polymerization, respectively. Discriminatory photo-activation behavior was observed in separated diastereomer system with the consumption of Dia2 much faster than that of Dia1 under identical reaction conditions. However, such huge difference can be neutralized by chain transfer in mixed diastereomer system. The mechanistic insight into such interesting behavior was suggested with the aid of quantum chemical calculations. A bit more complicated system, diastereomeric CPDTC-St-PMI obtained by the insertion of two monomers (Styrene (St) and N-phenylmaleimide (PMI)) into CPDTC in proper sequence, was explored in the second work. Stereochemical effect on both initiation step and monomer addition step can be explored in the system of diastereomeric CPDTC-St-PMI due to the generation of diastereomeric radicals after photo-activation. Discriminatory kinetic behaviors were observed in both mixed and separated diastereomer systems, implying stereochemical effect still significantly affects kinetics in RAFT polymerization.
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(2020)ThesisThe rapid development of portable and wearable devices requires new generation energy storage systems possessing high energy density, high safety warranty, and low-cost. Despite the outstanding performance of lithium-ion batteries, their further application is severely limited by their safety risks. Those safety issues are derived from the heavy reliance on toxic, flammable, and volatile organic electrolytes. In contrast, aqueous zinc-based batteries have the advantages of non-flammability, low-cost, and eco-friendliness, which are ideally suitable for portable applications. However, the energy density and reversibility of most reported aqueous Zn-based batteries are far from satisfactory. First of all, there are not many choices for cathode materials with high discharge plateaus. Next, side reactions and corrosion occur simultaneously on zinc anode during the cycling process. As a commercial cathode material, lithium cobalt oxide (LCO) attracts increasing attention due to its high discharge plateaus, high volumetric capacity, and excellent reversibility. However, the cycling stability of LCO in aqueous solutions is still a challenging problem due to the severe side-reactions. In this thesis, a high-voltage zinc-based hybrid battery with LCO cathode and the mild-alkaline electrolyte is developed. In the first chapter, the stability of the LCO cathode was improved by investigating the effects of pH and salt concentrations on battery cycling performance. In the following chapter, the effects of different electrolyte anion on the corrosion and stripping/plating behaviours were studied. It was demonstrated that the reversibility of LCO could be improved significantly with increasing electrolyte pH. Additionally, the mild-alkaline acetate electrolyte exhibits a wide electrochemical window to 2.15V, and improved reversibility of zinc anode compare with other electrolytes (based on Cl-, SO42-, and NO3- ions). The hybrid Zn/LCO battery with this mild-alkaline acetate-based electrolyte provides a flat and high discharge voltage at 1.94 V, resulting in a high energy density of 162 Wh/kg. The battery remained 72 % capacity after 120 cycles with 96 % coulombic efficiency even at 0.2 C.
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(2021)ThesisActivated sludge is widely used in wastewater treatment plants (WWTPs) to treat municipal and industrial wastewater. The formation of activated sludge flocculates underpins the separation of biomass from nutrient depleted wastewater through gravity. Although research has been conducted to understand and improve activated sludge treatment, there remains a lack of knowledge on underlying mechanisms in activated sludge floc formation. Consequently, the knowledge base for preventing activated sludge malfunctions such as pin flocs or bulking is lacking. This project is based on the hypothesis that activated sludge flocs are nucleated by particulate organic matter (POM) that passes through primary screening and sedimentation and enters secondary biological treatment units. Colonization of particulate organic matter by activated sludge bacteria was compared with colonization of glass beads. The types of organic matter tested includes chitin, keratin, lignocellulose, lignin and cellulose. Bacterial community profiles were generated from DNA extracted from these biopolymers after incubation in aerated activated sludge over two weeks in the presence and absence of regular nutrient amendment regimes (reduced carbon and nitrogen). Scanning electron microscopy images of colonized surfaces were generated. Keratin showed poor attachment ability for bacterial colonization on its surface in activated sludge. Bacterial colonization and biofilm formation processed fastest on chitin among tested POM and chitin-biofilms have higher diversity bacterial communities which could potentially withstand environmental disturbance. The addition of reduced C, N resources in activated sludge promote the dominance of Proteobacteria in activated sludge and bacteria capable of decomposing POM tend to occupy more on corresponding POM over time. The abundance of key bacteria involved in nitrification and denitrification on specific POM was explored and provided useful information for adjusting engineered biofilms to a high efficiency for nitrogen pollutants removal. Furthermore, this study successfully isolated five strains performing ammonium, nitrite, nitrate transformation and identified them as: Pseudomonas guangdongensis, Acinetobacter tandoii, Pseudomonas pseudoalcaligenes, Pseudomonas stutzeri, Rhizobium borbori. N-pollutant degradation was observed with chitin colonized by N processing bacteria in artificial wastewater with 50 mg/L NH4+, NO2- and NO3-. Potential exists for constructing synthetic activated sludge flocs through using chitin as a surface substratum to be deployed in WWTPs for N-pollutant degradation. Taken together, this research proved the feasibility of constructed biofilm deployed in the wastewater treatment in future and gave some credence to the notion of a development strategy in WWTPs.
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(2021)ThesisMicro/nanoscaled particles represent a group of intelligent materials that can precisely and rapidly respond to biological microenvironments and improve therapeutic outcomes. In order to maximize biomedical application potentials, developing nanoscale particles that are able to catalyse in-situ substrates to address the biological problems is highly desired but still remains a critical challenge. Herein, a 2D nanosheet-based catalytic nanoparticles with enzyme mimicking behaviour is developed for enhanced drug delivery toward the tumour microenvironment as well as the antibacterial treatment. The nanoparticles are constructed via a facile one-pot method and exhibit ultrathin monolayer nanosheet morphology and could be further attaching drugs as cargo to fulfil the drug delivery task. The 2D structure of the LDHs allows high catalytic activity, leading to a responsive, sustained, and relatively high amount of substrate transformation. Herein, in this thesis, H2O2 was used as the model substrate to demonstrate the nanoparticles catalyzing ability, as well as the potential applications. The Ferrous ion-containing LDHs was tested for different medicinal purposes, tumour targeting drug deliveries and antibacterial treatment
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(2020)ThesisCurrently, Nafion – a proton exchange membrane based on polymer fluorosulfonic acid ionomer has become dominant in the market owing to its high ionic conductivity, physical and chemical stability at moderate temperatures. However, it is strongly dependent on water content to maintain high proton conductivity. Therefore, it cannot be used in several applications such as metal hydride-air batteries in which water is detrimental. In this thesis, proton exchange membranes consisting of heteropoly acid and polyvinyl alcohol were investigated to achieve high proton conductive polymer electrolytes working at room temperature and low humidity conditions. The encapsulation of heteropoly acid into metal-organic framework MIL-101 was also studied to enhance proton conductivity. First, heterophony acids as proton donors were shaped into a membrane by incorporating with polyvinyl alcohol which functions as structural support. At room temperature and 25 % relative humidity, the proton conductivity of SiWA/PVA membranes was 1.2 x 10-2 S.cm-1 which is close to the value of Nafion membrane at fully hydrated condition. However, the proton conductivity of the SiWA/PVA membrane was reduced to 0.5 x 10-3 Scm-1 after being heated to 80 °C which suggests some leaching of heteropoly acid and water molecules during operation. To solve this problem, silicotungstic acid was encapsulated into mesoporous chromium (III) terephthalate metal-organic framework (MIL-101) in an attempt to prepare high proton conductivity membranes (MIL-101/SiWA/PVA), which can operate at room temperature and low humidity conditions. This novel method was proven successful as the proton conductivity increased to 2.4 x 10-2 S.cm-1 at 25 °C and 25 % Relative humidity. This result is two times higher than the proton conductivity of SiWA/PVA membrane. Under low level of hydration, when the MIL- 101/SiWA/PVA membranes were dried at 80 °C, the proton conductivity experienced a reduction from 2.4 x 10-2 to 1.9 x 10-2 S.cm-1 which is also higher than that of SiWA/PVA membranes at the same conditions. This means that by immobilising heteropoly acid (i.e. SiWA) in MIL-101, the proton conductivity can be improved. This is due to continuous channels provided by MIL-101 facile proton transport even at low level of hydration.
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(2020)ThesisVolatile organic compounds (VOCs) are organic chemicals that could cause health effects on short-term and long-term exposure. Therefore, the detection of VOCs is important for early monitoring of hazards. However, current technologies for VOC detection have drawbacks such as relying on expensive instruments and skilled operators, and are time-consuming. To overcome these challenges, this thesis aims to develop an easy-to-use VOC sensor that could potentially be used on-sites by employing polydiacetylenes (PDAs), a class of conjugated polymers which are excellent materials for colorimetric sensors due to their chromatic properties visible by the naked eye. There have been many developments in PDA-based sensors, however, shortcomings still exist. On one hand, the conventional method to fabricate PDA hinders its scalability for large-scale PDA production. On the other hand, although PDA-based VOC sensors recently developed lead to excellent detection performance, these approaches can only be realized by tedious monomer synthesis procedures. This thesis focuses on overcoming these two problems. In this thesis, the first application of the solvent injection method for large-scale PDA synthesis was presented. The formation of PDA particles similar to those formed by the conventional film hydration method was confirmed by a study on particle size and morphology using dynamic light scattering and electron microscopy. The functionality of the PDA was confirmed via ammonia detection. Large-scale production of PDA vesicles (0.1 L and 0.25 L) was achieved. The results indicate that the solvent injection method is a viable alternative to the conventional method for large-scale PDA production. Next, a simple approach to fabricate a paper-based PDA/polymer VOC sensing array using a drop-casting technique was developed. Compared with pure PDAs, the incorporation of polymers led to an enhanced sensitivity towards acetone vapor. The rationale of polymer incorporation was based on the solubility and affinity of polymers to different VOCs, which creates varying molecular interactions between polymers and VOCs. Length of diacetylene monomers, polymer molecular weight, and the ratios of PDA/polymer were investigated to tune the system. In the last section, the colorimetric behavior of the PDA sensor was investigated in response to vapors of five VOCs. The work presented in this thesis highlights new opportunities for the development of large-scale PDA materials and PDA-based VOC sensors.
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(2021)ThesisThe molecular weight distribution (MWD) has a significant impact on the properties of polymeric materials, however, the characterisation of polymer MWDs has been limited to statistical parameters such as the number-average molecular weight (Mn) and dispersity (Đ). These parameters do not fully express the features of polymer MWDs, thus limiting the ability to rationally design complex polymeric materials with tailored MWDs. Herein, a platform for the design and synthesis of arbitrary polymer MWDs is developed and experimentally validated. The platform is based on the description of polymer MWDs as a mathematical function, rather than individual statistical parameters. As such, the complete shape of arbitrary polymer MWDs can be designed using a developed software. The software requires only a calibration using model monomodal MWDs directly obtained from GPC to design theoretical MWD. Using this platform in conjunction with a flow-mediated polymerisation approach, a range of arbitrarily shaped polymer MWDs were successfully designed and prepared. Finally, complex triblock copolymer mixtures with tailored compositions and overall MWD were fabricated via a one-pass flow-mediated polymerisation using this computer-guided approach.
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(2022)ThesisChitosan is a promising material for making edible, active and biodegradable packaging films for foods; however, pure chitosan films have poor mechanical and barrier properties. This Master of Philosophy study was conducted with the aim to improve the physicochemical and biological properties of chitosan films by incorporating epoxy activated agarose (EAA) and three flavonoids, namely catechin, quercetin and luteolin into the film. Chitosan films were prepared with chitosan of three molecular weights (low, medium and high) and by drying at 21 °C, 40 °C and 50 °C. EAA and the flavonoids were incorporated into chitosan, both at 1-10%. With increased MW of chitosan, the film thickness, tensile strength (TS), elongation at break (EAB), and swelling ability increased while the moisture content, solubility, water vapor permeability (WVP) and the melting temperature declined. Higher drying temperatures led to greater TS and higher melting temperature for the films. Incorporation of the EAA significantly improved the moisture related properties and flexibility of the chitosan films. Moreover, with higher amounts of EAA, the film thickness and opacity increased while the TS and thermal stability declined. Incorporation of flavonoids had significant (type and concentration dependent) impact on the physicochemical and biological properties of chitosan films. Addition of flavonoids up to 5% resulted in films with greater TS, EAB and thermal stability, whereas at concentrations of up to 3%, the films produced had improved WVP. All the chitosan-flavonoid composite films exhibited antimicrobial activity against Listeria monocytogenes, Salmonella typhimurium, Escherichia coli and Staphylococcus aureus. Beef samples wrapped with pure chitosan or chitosan-flavonoid composite films had significantly lower microbial counts and a more reddish color after two weeks of storage at 4 °C than those packaged with cling wrap. Storage of the chitosan films at 21 °C and 4 °C for six weeks resulted in significant reductions in the TPC, TFC, antioxidant activity and the flexibility of the films, which occurred at a faster rate at 21 °C. Overall, this study demonstrated that incorporation of EAA and flavonoids at appropriate levels can significantly improve some of the physicochemical and biological properties of chitosan films.
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(2022)ThesisCentral nervous system diseases, including neurodegenerative diseases and brain malignancies, remain a critical medical challenge. Nanoparticle-enabled therapy and diagnosis have demonstrated promises to address brain diseases. However, one of the physical barriers is the blood-brain barrier (BBB), which largely restricts nanoparticle transport to the brain. To address this issue, many strategies have been developed. One of the important strategies is binding nanoparticles to a targeting ligand (e.g., transporter proteins) that deliver nanoparticles to brain tissues via transcytosis. However, this approach is limited by the poor specificity of transporter proteins, and none of these proteins are unique to the brain, so this can lead to undesirable efficiency in crossing the blood-brain barrier. In this thesis, I use brain microvascular endothelial cell membranes that are homologous to those in the BBB to coat iron oxide nanoparticles, preserving the complex antigenic information on the cell membranes and achieving biofunctionalization of nanomaterials. I demonstrated that the cell membrane-coated iron oxide nanoparticles have a core-shell structure, good biocompatibility, and stability. By using in vitro models, the cell membrane-coated iron oxide nanoparticle exhibited cell-specific targeting and BBB crossing efficiency.
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Antimicrobial Polymers as Adjuvants/Synergists of Antibiotics to Combat Multidrug-Resistant Bacteria(2022)ThesisThe emergence of multidrug-resistant (MDR) bacteria due to the overuse and misuse of antibiotics in the medical and agricultural sectors has now become a critical global healthcare issue. Antimicrobial peptides (AMPs) and synthetic mimics thereof have shown promise in combating MDR bacteria effectively, mainly because of their mechanism of action that disrupts bacteria cell membrane, consequently hindering resistance development in bacteria. However, these antimicrobials also exhibit toxicity to healthy mammalian cells at high dosage. To overcome this toxicity issue, the application of combination therapy alongside traditional antibiotics could enable the administration of these membrane-disrupting antimicrobials at lower dosage. Herein, this thesis investigates the synergetic effects of new tri-systems for combination therapy against Gram-negative bacteria which contain: i) an AMP (colistin methanesulfonate); ii) an antimicrobial polymer as AMP mimic and; iii) commercial antibiotics. It was found that colistin and the antimicrobial polymer could combine synergistically with any of the three antibiotics, doxycycline, rifampicin, and azithromycin, against wild type and MDR strains of Pseudomonas aeruginosa. Crucially, given the lower dosage of antimicrobial polymer used in these combination systems, the therapeutic index (also known as selectivity index), which is an indicator of an antimicrobial system to preferentially target bacteria over mammalian cells, is higher than the standalone agents. Furthermore, in this thesis, other selected antimicrobial polymers that are active toward mycobacteria instead of Gram-negative were also investigated as potential adjuvants or synergists to potentiate the antimicrobial activity of antibiotics against Mycobacterium smegmatis via a two-component system. Among the different families of antibiotics screened, it was found that these polymers only act as adjuvants of aminoglycosides. Overall, this thesis yields valuable new insights on combination therapy that will be useful toward combating MDR bacteria in clinical settings.