School of Chemistry - Theses
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ItemEnergy Transfer Systems for Light Harvesting( 2023-03)Luminescent solar concentrators (LSCs) are solar harvesting devices that employ luminescent materials embedded within a waveguide to collect solar energy. The waveguide is typically made of a low-cost material such as glass or plastic and is designed to capture sunlight and redirect it towards solar cells. The luminescent materials, also known as luminophores, absorb the sunlight and re-emit it at a longer wavelength, allowing it to be trapped within the waveguide and guided towards the edges, where solar cells can convert the light into electricity. However, their performance is associated with several drawbacks, including reabsorption, which refers to the primary emission of the luminophore in the waveguide being reabsorbed by other luminophores in the waveguide. To overcome this, various strategies have been explored such as using different types of luminophores, optimizing the concentration of luminophores, and improving the design of the waveguide. Foster energy transfer (FRET), which reduces luminophore reabsorption, is a crucial aspect for improving energy conversion and device efficiency in LSCs. FRET works on separating the absorption and emission spectra of the luminophores by controlling the intermolecular spacing between donor and emitter molecules. Implementing these strategies can significantly enhance the performance of LSCs and accelerate their practical applications for solar energy harvesting. This thesis aims to advance the understanding of different energy transfer strategies for light harvesting applications. To implement an effective FRET approach, the concentration of luminophores is a crucial factor. The donor luminophore concentration should be higher than that of the emitter to allow energy migration through several donors to reach the emitter. However, increasing the dye concentration often leads to dye aggregation, which can quench the dye's fluorescence properties. To mitigate dye aggregation, molecular insulation in the luminophores can be an effective approach. In this thesis, the molecular insulation strategy is applied to both the donor and emitter luminophores to suppress dye aggregation at high concentrations. Several sterically hindered groups have been employed for both the donor and emitter through the imide position of the luminophore, and their photophysical properties have been observed at various concentrations. The results demonstrate that these sterically hindered groups effectively reduce dye aggregation at high dye concentrations. Furthermore, incorporating luminophores in a polymer backbone enables efficient energy transfer from the donor to the emitter by controlling the distance between the luminophores. The polymer chain acts as a spacer between the luminophores, reducing dye aggregation and suppressing the reabsorption issue. To investigate this approach, a variety of linear polymers with incorporated luminophores were employed in this study, where the luminophores were covalently linked to the polymer chain. Optical properties were analysed in both solid and solution states, and it was determined that this strategy did not adversely affect the luminophore's optical characteristics. Moreover, it was discovered that the more sterically hindered donor efficiently transferred its energy to the emitter, effectively suppressing aggregation-caused quenching (ACQ) in comparison to the less sterically hindered donor molecule. The success of the FRET energy transfer technique in polymer chains has inspired the use of crosslinked luminophore embedded nanoparticles for light harvesting. This approach involves embedding the dye within the polymer particles, resulting in a high dye concentration in a small area and promoting efficient energy transfer. In this study, a series of donor-emitter luminophore embedded crosslinked nanoparticles were synthesized using various luminophore concentrations, and their photophysical properties were studied to examine FRET efficiency. The cross-linked polymer particles effectively reduced luminophore aggregation and reabsorption. These polymer nanoparticles were used to fabricate bulk LSC films, which also demonstrated effective energy transfer. The photophysical observations were subsequently utilized in Monte-Carlo simulations of a large-scale bulk LSC device. The simulation results indicated that the incorporation of cross-linked polymer particles had a significant effect in mitigating the reabsorption process of luminophores in the bulk LSC waveguide.
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ItemSonosynthesis of Functional Micro/Nano-structures using Biomolecules( 2022)Malnutrition and access to affordable health services are some of the world’s most urgent problems. The development of nutrient and drug delivery systems by using sub-micro particles as carriers has attained much attention for improving the nutritional value of food and the efficacy of diagnostic/therapeutic treatments. Common methods for synthesizing biofunctional particles usually require many reagents and involve multiple steps. In this regard, a novel and advanced approach for material synthesis needs to be developed and investigated to address those limitations. Ultrasonic techniques have emerged as one-pot and eco-friendly methods for the synthesis of organic and inorganic materials. It has been found that acoustic cavitation derived from sound waves can induce emulsification of liquids to form microcapsules and promote chemical modifications of biomolecules. Protein-shelled microcapsules have been synthesized by using low-frequency ultrasound and reducing agents for food-based applications. However, the direct use of food ingredients for the microencapsulation of microcapsules without resorting to additional external agents was never explored and needs to be investigated further for food-based applications. On another note, drug carriers are usually prepared in nanoscale to enhance interaction with cell membranes for achieving efficient therapeutic treatment. Conventional strategies for preparing drug loaded nanoparticles require matrix materials as carriers, resulting in low drug loading capacity and safety issues. Therefore, synthesis of nanodrugs solely made of antibiotic molecules is a better method for development of drug delivery platforms. Many molecules bearing aromatic groups have been successfully sono-assembled into nanoparticles by high-frequency ultrasound, but they are mainly used as drug carriers. Transforming drug molecules into carrier-free nanodrug has not been widely investigated. As such, I intend to expand new research towards other drug molecules with aromatic moieties. In this regard, my Ph.D. project aims to sono-chemically synthesize various micro and nano structures from biomolecules by tuning the frequency/power of ultrasound without the usage of external reagents. The size of the obtained bio-functional structures is controllable, and their compositions are suitable for use in specific applications such as : i) nutrients delivery in food industries; ii) drug delivery for biomedical applications. The fundamental concepts of sono-chemistry for material synthesis, along with biomolecules (proteins, nutrients and antibiotics) based micro/nano structures and their applications are discussed in Chapter 1. Chapter 2 provides an overview of microencapsulation techniques for food industries and fabrication of nanoparticles for antibiotics delivery. In particular, methodology, formulating materials, current challenges, limitations and innovation are discussed. In Chapter 3, the materials, equipment and methodologies involved in the reactions used in this thesis are thoroughly described. Chapter 4 is the first chapter of result and discussion section. Microcapsules made of egg white protein (EWP), as commonly available biopolymers, were first conceptualized. Oil-soluble nutrients (Vitamin A, D and E) were encapsulated into EWP to form nutrients loaded proteinaceous microcapsules by employing 20 kHz ultrasound. This work primarily points out that high availability of free thiol groups in protein solution is crucial in forming stable microcapsules with robust shells, in order to protect micronutrients from degradation against detrimental effects. In Chapter 5, another two plant-based protein isolates extracted from soybean (SPI) and corn (CPI) were also formulated to form microcapsules. This study provided further insights into the structural, chemical and surface properties of proteins for efficient ultrasonic microencapsulation of micronutrients. A double emulsion technique was further developed to co-encapsulate both oil- (vitamin A and D) and water-soluble (vitamin B, C and minerals) micronutrients. In-vitro digestion study showed that the proteinaceous microcapsules enable sustained release of micronutrients, demonstrating their potential in food fortification applications. In Chapter 6, a sono-chemical strategy for transforming antibiotic doxycycline into carrier-free nanodrugs via high-frequency ultrasound (490 kHz) is reported. This study demonstrates that doxycycline undergoes hydroxylation and dimerization processes upon sonication in an aqueous solution to ultimately self-assemble into nanoparticles. The size of obtained particles could be finely controlled by tuning the applied ultrasonic powers. The nanodrugs exhibited antioxidant properties, along with antimicrobial activity against both Gram-positive (S. aureus) and Gram-negative (E. coli) bacterial strains. These results highlight the feasibility of the ultrasound-based approach for engineering carrier-free nanodrug with multiple controlled bio-functionalities. Chapter 7 provides an overall summary of the entirety of my PhD project as well as my conclusion and thoughts on it.
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ItemSynthesis and Modification of ZnSe Nanoplatelets( 2021)Nanoplatelets (NPLs) are a class of nanoparticles which have garnered significant interest in the research community. Unfortunately, much of the focus has been on the usual workhorse material: cadmium selenide. Zinc selenide is a close relative of cadmium selenide, both belonging to the II-VI family of semiconductors, but little research exists on ZnSe NPLs beyond its initial synthesis. In this thesis, ZnSe NPLs are addressed from the bottom up. In Chapter 2, the formation mechanism of ZnSe NPLs and MSCs is investigated. The evolution of nanoparticles in the reaction is monitored while exploring the reaction space. It is demonstrated that the concept of surface reversibility can be used to predict the formation of NPLs and MSCs. Additionally, it is found that MSCs and NPLs compete in the reaction, and selective formation can be induced by varying selenium precursor and the ripening behaviour of the ligand. Along the way, five unreported ZnSe magic-sized clusters (MSCs) are found. Chapter 3 of the thesis is a demonstration of Mn 2+ doping into the ZnSe and ZnS NPLs. Mn 2+dopant incorporation can be confirmed via its photoluminescence and photoluminescence excitation spectra and the photostability is measured. Additionally, the unique Mn 2+ emission is used a probe to investigate the evolution of various ZnSe species. Finally, Chapter 4 is concerned with the post-synthetic shelling of ZnSe nanoplatelets. ZnSe NPLs as synthesized from literature are colloidally and photo-unstable. A common solution to this is to coat the surface of the nanocrystal with a suitable semiconductor material. By modifying the process introduced for CdSe NPLs, the synthesized ZnSe NPLs are shelled successfully via colloidal atomic layer deposition (C-ALD). This allows us to improve its photoluminescence properties and observe unique features associated with Type-II ZnSe/CdS heterostructures.
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ItemUltrasound driven synthesis of bio-functional nanostructures( 2019)Polyphenolic-, amino acids- and doxorubicin-based nanostructures are of great interest due to their multifarious applications in biomedical field as antibiotics, antioxidants, antimicrobial and anti-cancer agents. Most of the research on the polyphenolic and amino acids based nanostructures adhere to the formation of coordination complexes with metals, self-assembly techniques or chemical functionalization and crosslinking reactions. To improve the efficacy of therapeutic agents, a variety of nanoparticles have been developed for the controlled and targeted delivery of doxorubicin. These include biopolymers-based nano/microcapsules, carbon-based nanoparticles, polymer-drug conjugate, dendrimers, liposomes, micelles, inorganic nanoparticles and nucleic acids nanostructures. The development of simple, one pot and effective synthesis routes to fabricate bio-nanomaterials is in high demand. In particular, the use of polyphenols, doxorubicin and single amino acids as building blocks to fabricate nanostructured materials is still unexplored. In this PhD work, I have used ultrasound-based technologies to synthesize phenolic, amino acid and doxorubicin based micro and nanoparticles for different biomedical applications. Chapter 1 provides an overview on the structural and bio-functional properties of nanoparticles and methods to synthesize nanoparticles for biomedical use, including the ultrasonic techniques have been discussed. Furthermore, fundamentals of ultrasound are also provided. In the literature review (Chapter 2), several studies dealing with the polyphenol, doxorubicin and amino acid molecules have been summarised. In the last section of this chapter, numerous investigations on synthesis of diverse types of nanostructures using ultrasound are reviewed. In Chapter 3 materials and methods, equipment and all other experimental details used are described. Chapter 4 provides a fundamental understanding on the ultrasonic coupling of simple phenolic molecules, where acoustic bubble acts as a catalytic binding site for the generation of bioactive oligomers without the need for utilizing any enzymes, catalysts (organic or inorganic) and other toxic reagents. It has been observed that the extent of oligomerization and nanoparticles formation depends on the ultrasonic frequency, concentration and physiochemical properties of the phenolic building blocks. Chapter 5 demonstrates that cavitation bubbles are simple micro-reactors with reactive surfaces to perform one-pot multiple reactions on complex polyphenolic molecules to convert tannic acid to ellagic acid, namely (i) hydrolysis of an ester linkage, (ii) C–C coupling reactions, (iii) condensation reactions and (iv) crystallization of the product into regularly shaped particles. The size and shape of the crystals can be controlled by ultrasonic frequency, power and time. The synthesized particles exhibit fluorescence properties, anticancer and antioxidant activity and could be further used for drug loading and delivery. In Chapters 6 and 7, the role of the acoustic field in the formation of supramolecular nanoaggregates using tryptophan and phenylalanine as building blocks was investigated. It has been demonstrated that the acoustic bubbles driven at high frequency standing wave, in addition to provide a reactive surface for the dimerization of biomolecules, can also provide an energy source to fuel and refuel the dissipative out of equilibrium assembly of these molecules below the critical aggregation concentration. Furthermore, the unique optical and bio-functional properties of nanoparticles for bioimaging and probing the intracellular trafficking of a drug have also been studied. In Chapter 8 the sound-driven self-assembly of the anticancer drug doxorubicin was investigated to generate nanoparticles solely made of drug molecules. The newly developed nanoparticles were tested on different types of cancer cells and the drug was found to be active in drug resistant cell lines. In addition, the mechanism of action of drug nanoparticles was investigated. Chapter 9 provides a summary of the entire PhD work.