School of Chemistry - Theses
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ItemAn AFM study of nanomechanical properties of mammalian nerve and cardiac cells( 2016)The study of nano-mechanical properties of cells (under physiological conditions and/or in presence of pathological or pharmacological agents) by atomic force microscopy (AFM) is of immense interest due to the versatility of the technique. This thesis attempts to understand the nano-mechanical properties of mammalian nerve and cardiac cells under near physiological conditions using AFM. Hertz contact mechanics were employed to obtain elasticity (Young’s modulus) data. Alzheimer’s disease affects different cell types in various regions of the brain. The abnormal extracellular deposition of amyloid peptides is a hallmark of the disease. The effect of the amyloid peptide on two types of nerve cells, transformed (N2A) and primary (cortical) cells, was investigated. Initially preliminary contact mode images followed by 30x30 force grids were obtained for the whole body of N2A cells,. However, due to experimental time limitations after cell removal from the incubator, imaging was compromised and, instead, 10x10 force grids over flat regions of the nerve cells were obtained. This approach was used to obtain force grid data of untreated N2As on Day 5 and 6 from the initial seeding day. Secondly, time-lapse studies were carried out at 12, 36 and 54 hours on both untreated and Aβ1-42 treated N2As and corticals. Force measurements were taken of two regions on each cell type, which showed statistically significant differences in stiffness and adhesion properties for both untreated and Aβ1-42 treated cells. Sub-toxic amounts (5 μM) of Aβ1-42 peptide had a cell specific and time dependent effect. The effects are discussed in terms of Aβ1-42 membrane association, incorporation and internalization and probable affect on the microtubule and actin cytoskeleton network, which is characteristic for each cell type. Various pathological conditions of the heart are attributed to abnormal mechanical properties associated with the left ventricle. Such abnormalities have been imputed to variations in the mechanical properties of sarcomeres (the structural/functional units of cardiomyocytes). Therefore, studies of left ventricular cardiomyocytes were performed. Firstly, live cardiac cells were imaged in near physiological conditions to visualize topographical aspects and specifically the sarcomere regions. T-tubules at the sarcolemmal level were established. Sarcomere features were more prominent at later stages of the experiment. The cardiac cells exhibited what appeared to be highly non-linear topographical properties, that may contribute to non-linear elastic properties. Hence, imaging was followed by force measurements. Simultaneous imaging and force measurements were performed on cardiac cells in the absence and presence of a pharmacological agent, 2,3-butanedionemonomoxime (BDM),, which is an intracellular Ca2+ ion blocking agent. Results were obtained on multiple cells under both untreated and BDM treated conditions. The untreated cells demonstrated higher elasticity and adhesion values compared to BDM treated cells. Finally, in order to ascertain the effect of BDM on the z-band regions (at the sarcolemmal level), a comparison was made between two individual cardiac cells under respective conditions over the same period of 2-12 hours. The mean values of adhesion and elasticity for single cell experiments fell well within the range for that of multiple cells. Overall, the mean Young’s modulus vs. time curves show that stiffness increased with time, for both untreated and BDM treated cells and suggest that the cells continuously alternate between rigor and relax states. BDM appears to produce the relaxation effect and together with the calcium-induced calcium release process has a cumulative effect on the two states over extended time periods. BDM treatment also influences the cell surface adhesion properties.
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ItemEnergy efficiency and advantages of ultrasonic synthesis of nanomaterials( 2015)The physico-chemical effects of ultrasound (US) have been used widely for synthesising various materials. The focus of this project is to evaluate the energy efficiency and advantages of ultrasonic synthetic process. Poly(methyl methacrylate) and poly(methyl methacrylate)-CaCO3 nanocomposites were synthesised by conventional and US-assisted (USK) emulsion polymerization. Although the conversions obtained were similar for both processes, nanocomposites produced by USK were smaller with a narrower particle size distribution. In another study, the photocatalytic activity of CdS nanoparticles synthesized using US were compared with those synthesized using mechanical agitation on the basis of energy input. Samples synthesized using a US horn (USH) and a high shear homogeniser (HSH) showed a lower photocatalytic activity compared to those synthesized in an US bath (USB) and using mechanical stirring (NUS). However, when the power input per unit volume (W/L) is considered, the order of effectiveness of the catalysts is USB>NUS>HSH>USH, suggesting that the mild cavitation conditions generated in the USB process are sufficient to produce an efficient photocatalyst. Overall, US assistance provides improvement in conversions/yields and the dispersive effects help obtain smaller particle sizes and narrower size distributions. However, when the increased energy requirements are taken into account it is obvious that when combining US with conventional material synthesis techniques, it is imperative to choose not only the right amount of energy input but also, the right mode of US input in order to synthesize the most efficacious nanomaterials.
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ItemSynthesis, structure and reactivity of ligand stabilized coinage metal nanoclusters( 2013)The coinage metal nanoclusters (CMNCs), defined as copper, silver or gold, constitute an intermediate state of matter that exist between molecules and bulk material. The properties of CMNCs differs to that of molecules and bulk material due to quantum confinement effects. These nanostructured materials have attracted significant attention owing to their fundamentally interesting architectures, and unique properties with applications in areas such as catalysis, optical materials, medical imaging, models for hydrogen storage. Tailoring the properties of such promising materials has proven challenging and requires a fundamental understanding of their assembly, structure and reactivity. The aim of this thesis is: (i) the primary application of mass spectrometric techniques to monitor the formation of CMNCs which result from the addition of sodium borohydride to a solution consisting of a coinage metal salt and the bidentate ligand, bis(diphenylphsphino)methane (dppm) under various synthetic conditions; (ii) to apply this information in developing synthetic approaches to optimize clusters of interest and apply a mass spectrometry (MS) directed synthesis leading to the isolation of crystalline material suitable for structural characterization by X-ray crystallography (iii) apply MS based analysis methods to provide information on the reactivity of CMNCs in solution and the reactivity and structure of mass selected CMNCs in the gas phase. Electrospray ionization mass spectrometry (ESI-MS) and UV-Vis spectroscopy were used to monitor the formation of gold nanocluster cations in the condensed phase via the sodium borohydride (NaBH4) reduction of methanolic solutions containing AuClPPh3 and dppm. ESI-MS highlights the formation of complexes prior to the addition of NaBH4 as [Au2(dppm)2]2+, [Au(PPh3)2]+, [Au2(dppm)3]2+, [Au(dppm)2]+,[Au2Cl(dppm)2]+. The cationic complex product distribution can be monitored over a range of metal to ligand ratios to minimize the colloid precursor [Au(PPh3)2]+. The addition of NaBH4 where the optimized metal to ligand ratio was determined as AuClPPh3:dppm is 1:2 results in the formation of the following types of gold nanoclusters [Au9(dppm)4]3+, [Au9(dppm)5]3+, [Au5(dppm)3(dppm-H+)]2+, [Au10(dppm)4]2+, [Au11(dppm)5]3+, [Au11(dppm)6]3+, [Au13(dppm)6]3+ and [Au14(dppm)6(Ph2PCHPPh2)]3+. The gas phase unimolecular chemistry of these cations was examined by (i) collision induced dissociation (CID) and electron capture dissociation resulting in the gas phase synthesis of the novel clusters [Aux(dppm)y]z+ (x = 2,3 , 6–13; y = 1–6 and z = 1–3) and [Aux(dppm)y(dppm-H+)]z+ (x = 5,14; y= 2,5; z = 2,3) via ligand loss and core fission fragmentation channels. (ii) electron capture dissociation (ECD) of mass selected multiply charged gold cluster cations where an additional fragmentation channel arises due to C-P bond activation. ESI-MS was also applied to study the reactivity that results from silver salts in the presence of dppm, that are treated with sodium borohydride. It was observed by ESI-MS that no all metallic silver clusters had formed. Instead there existed abundant and relatively monodisperse trinuclear silver(I) hydride clusters. The synthesis could be refined by careful MS based analysis to result in the isolation of crystalline material of (i) [Ag3(μ3-H)(μ3-Cl)(dppm)3]BF4, and (ii) [Ag3(μ3-H)(dppm)3](BF4)2. These clusters could be mass selected to generate novel gas phase clusters in the gas phase. The multiply charged cation [Ag3(μ3-H)(dppm)3]2+ was also investigated by ECD and EID. The silver hydride cluster cation [Ag10H8(dppm)6]2+ was observed during the synthesis of trinuclear silver clusters. This cluster has yet to be isolated.
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ItemThe environmental degradation of pyrotechnic magnesium powder( 2002)Magnesium powder is one of the most common and important metal fuels used in military pyrotechnic systems. However, the major problem with magnesium based pyrotechnic systems is the susceptibility of the magnesium powder to react with moisture in the environment, particularly at elevated temperatures and relative humidities, typical of military storage conditions found in Northern Australia. This study of the bulk and surface chemical and physical changes associated with the environmental degradation of commercial pyrotechnic magnesium powder at elevated temperature and elevated relative humidity results in the formation of magnesium hydroxide (Mg(OH)2, magnesite or hydrated magnesium carbonate (MgCO3.xH2O), magnesium oxide (MgO) and hydromagnesite or basic magnesium carbonate (Mg5(C03)4(OH)2.4H2O). This was confirmed by auger and x-ray photoelectron and infrared spectroscopy and x-ray diffraction. All of these products form as a result of the reaction of the magnesium with moisture and air but they each form in different amounts and at different rates depending on the grade (particle size/surface area) and the type (particle shape) of the magnesium powder. For all the powders, the major degradation product is magnesium hydroxide, which was confirmed by all the analysis techniques. The temporal ageing of magnesium and the subsequent formation of bulk magnesium hydroxide follow a three stage sigmoidal relationship. The first stage represents surface nucleation; the second or main stage represents the progression of the reaction from the surface to the particle interior and exhibits extensive surface and sub-surface cracking; and the third stage represents the exhaustion of the available magnesium. The equation for the second (main) stage of the reaction for the formation of Mg(OH)2 is given by the pseudo first order rate equation where the rate of the reaction is independent of relative humidity and shows Arrhenius dependence. The formation of Mg(OH)2 proceeds from the outside of the particle inwards but is not diffusion controlled; the reaction is controlled by the rate at which the magnesium/reaction products interface moves into the particle. The surface morphology of the particles shows the formation of hexagonal and trigonal plates and trigonal pyramids and prisms. Crystallite formation is also observed in the interior of the particles. The formation of Mg(OH)2 is a complex heterogeneous process involving many separate steps which follows a Rideal type mechanism; the water diffuses to the particle surface, is chemisorbed and then reacts directly with the metal surface to form Mg(OH)2.
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ItemThe interaction kinetics of a melittin derivative with a phospholipid membrane( 2010)A deeper understanding about the lipid-peptide interactions contributes significantly to the development of drug delivery systems. The utilization of a model to scrutinize the lipid-peptide interactions helps to overcome the resistance of anti-microbial agents and the unselectiveness of the anti-cancer agents. Cytolytic peptides, the peptides that able to lyse various bacteria or mammal cells, become one of the anti-microbial agent and anti-cancer agent candidates to overcome those problems. A fluorescently labeled melittin derivative and 1,2 dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) small unilamellar vesicles (SUVs) were used as a model to study the lipid-peptide interaction. One of the cytolytic peptide, melittin, is an α-helical peptide which has 6 positive charge in physiological condition. Melittin is labeled with Alexa 430. Using the steady-state fluorescence spectroscopy and Fluorescence Lifetime Imaging Microscopy (FLIM), the information on the melittin microenvironment changes through the spectral characteristic and the lifetime of Alexa 430, can be monitored. DPPC SUVs behavior was observed through the Rayleigh light scattering intensity change. The data shows that the interaction between melittin with DPPC SUVs is dependent upon the lipid-peptide ratio. At lipid-peptide ratio of 100:1, or high lipid-peptide ratio, melittin is associated with the vesicle without vesicles size change. This is indicated by the increase of Alexa 430 lifetime and quantum yield with negligible light scattering change. At lipid-peptide ratios of 50:1, 40:1 and 30:1; or the medium lipid-peptide ratio, Alexa 430 lifetime and quantum yield raise significantly followed by the increase of light scattering that be sign of further melittin insertion accompanied by the vesicles fusion. The data implies the pore formation without involving melittin insertion to the hydrocarbon chain structure. At lipid-peptide ratios of 20:1 and 10:1, or the low lipid-peptide ratio, light scattering data shows a decrease in vesicle size, which is attributed to vesicles micellization by melittin. Furthermore, the different time scale of the kinetic progress parameters; the fluorescence intensity, the lifetime and the Rayleigh light scattering intensity; signify the multi steps process which takes place during the interaction. In high lipid-peptide ratio, melittin associate with the vesicles rapidly without further vesicles size changes. In medium lipid-peptide ratio, the data implies the formation of Toroidal-pore followed by slow vesicles fusion. Meanwhile, at low lipid-peptide ratio, membrane micellization occurs in a very short time indicating the existence of the Carpet model. However, the limitation of the observation time frame gives uncertainty whether Toroidal-pore and Carpet model is the most suitable model to illustrate the lipid-peptide interaction. Hence, this research proves that melittin disrupt the zwitterionic membrane through a complex mechanism depends on the lipid-peptide ratio which need further investigation.