Chemical and Biomolecular Engineering - Theses
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ItemEngineering catalytic organic-inorganic materials for sensing applications( 2020)Nanostructured hybrid organic-inorganic materials are a unique class of materials showing distinctive properties that have attracted high interest due to their diverse applications in the fields of energy, environment and medicine. In particular, hybrid materials are promising candidates for sensing applications due to the tunable chemical, structural and functional properties of the organic and inorganic components. Hence, the engineering of novel nanostructured catalytic organic/inorganic materials provides opportunities for the fabrication of advanced nanodevices for biosensing. In this thesis, novel hybrid materials have been prepared and their electrocatalytic, catalytic, and optical properties explored. First, nanostructured electrocatalytic microparticles were synthesized in mild conditions and used with an organic binding agent to prepare carbon electrodes applied in the detection of glucose in biologically relevant media. Second, hierarchically structured hybrid particles displaying enzyme-like catalytic activities were synthesized and used to prepare high-throughput micro-reactors for the detection of bioanalytes via a hybrid organic-inorganic cascade reaction. Finally, a natural occurring polysaccharidic nanoparticle, i.e. glycogen, was engineered to impart adhesive functional properties to a hybrid film and used for the coating of various substrates with different chemical composition. These hybrid coatings embedding metal nanoparticles were employed as catalytic and optically active functional interfaces.
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ItemMicroscopic inter-particle friction and shear rheology of particulate suspensions( 2017)The study of the shear rheology of strongly-flocculated particulate suspensions is important for a range of industrial applications, such as start-up, pipeline flow and slumping. Examples include the improvement of industrial processes, equipment design and product development, such as gels and polymers. It can be achieved by thorough understanding of suspension behaviour and constructing a rheological model to enable an accurate prediction of the behaviour. Above a critical solid concentration, known as the gel point, these suspensions are able to withstand an applied force and show a solid-like characteristic before they yield and flow. Previous studies modelled this behaviour as viscoplastic, which described the sharp transition of no-flow and flow behaviour at a critical point termed the yield stress. Many experimental studies however, contradicted this idea and showed the non-linear elasticity below yielding, time-dependent yield and rate-dependent yielding. Understanding these phenomena requires assimilation of the bulk rheology to the microscopic features of the suspensions. The work presented in this project aims to correlate the micro- and macro- features of suspension behaviour. Constant stress (creep), stepped stress and constant rate experiments were performed on two strongly particulate suspensions, alumina and calcite, at different solids volume fractions using controlled-stress and controlled-strain rheometers with a vane in a large cup technique. In addition, to better understand the stress transfer between particles in the suspension network during shear, a novel experimental Atomic Force Microscope technique for frictional study was developed. This technique allows one to measure friction force between two microsphere particles sliding over each other. The shear rheological data for alumina and calcite were obtained from creep (constant stress), stepped stress and constant rate experiments. The suspensions exhibited non-linear viscoelasticity prior to yielding, non-monotonic shear softening flow, and time- and rate-dependent yielding phenomena. The non-linear viscoelasticity prior to yielding was studied from the creep testing data. The instantaneous and steady-state moduli were extracted. They exhibited softening behaviour with increasing strain and modelled using a modified Cross model. This model indicated that the moduli were constant at very low strain and then eventually softens to a power-law. In addition, the effects of solids concentrations were investigated. Using power-law scaled with the gel point model, the increasing magnitude of the properties from the gel point was predicted. From the stepped stress and constant rate tests, the non-monotonic behaviour of both suspensions was investigated. The steady state stress value from the constant rate test and the data from the stepped stress tests at intermediate shear rate were used to construct the non-monotonic flow curves of both suspensions. The data were modelled using the modified Herschel-Bulkey model. The curves showed shear softening of solids network at lower rates and the increasing of viscous stress with higher shear rates. As the solids concentration increased, the curve shifted up, indicating the increase of solids network and viscous stress of the suspensions. The solids concentration dependency of the extracted properties was also studied. The data showed an exponential increase with higher solids concentrations. The time- and rate-dependent yielding phenomena were studied using the creep testing and constant rate data. From the creep test, the breakage time of suspension at different stress values can be extracted. The decreasing trend with higher stresses in breakage time suggests the time-dependent yielding of particulate suspensions. The rate-dependent yielding was observed from the peak stresses at various rates from the constant rate experiment. The peak stress data, modelled using the modified Herschel-Bulkey model, showed decreasing trend at lower rates and eventually increased with higher rates. The extracted properties were also found to be solids concentration dependence and could be predicted with an exponential function. The study of particle-particle interactions in this work focused on the friction force between two spherical micrometer-sized particles. A novel experimental setup for friction force measurements using Atomic Force Microscope (AFM) was developed. A particle probe attached on the cantilever was pushed against an immobilized particle on a microscope slide. The topography of the scanned particle was obtained while measuring the normal and lateral deflections of the cantilever. Given calibration constants, these deflections were converted to forces and analysed. The results show a non-linear friction behaviour and the breakdown of Amontons’ law, which suggests a linear correlation between normal and friction forces. The friction results implied that friction between two particles are more complicated than a constant sliding friction. This work extends our fundamental understanding of the relationship between particle-particle forces and shear rheology of particulate suspensions. The results from AFM study suggest a more complicated mechanism of friction between two microsphere particles. The non-linear behaviour of friction between two particles was one of the underlying causes of the shear rheological phenomena in particulate suspension.
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ItemNanoparticles and macromolecules in flow( 2016)The optimised fabrication of nanomaterials requires understanding of their behaviors in flow. Here we report on investigations into the flow induced structured changes in colloidal suspensions and polymer solutions. Fabrication of nano-devices requires understanding of the fundamental aspects of flow induced changes in suspensions or polymers. Understanding the behaviors of the dynamics of particles in flow gives insights into fluid mechanics at these length scales. In this work, rheo-optics are combined with numerical simulations to study flow induced phenomena in the colloidal systems. The rheo-optical methods are used to measure the real time optical changes of the sheared nanomaterial suspensions. The simulations are developed to derive the optical changes from the microstructure or motion changes of the suspending nanomaterials in flow. The thesis reports on three main areas of study. The first two studies are on the flow induced alignment of prolate nanoparticles in the red form poly-4BCMU solutions and gold nanorods aqueous sucrose solutions. The absorption spectra have been measured over a range of shear rates using polarized and unpolarized incident light, and the reversible optical changes indicate that the nanoparticles do not undergo aggregation during measurement. The measured absorbance anisotropy is attributed to the flow induced particle alignment which reach the limitation at high Peclet numbers. The spectral changes are consistent with the Jeffery's orbits (cooperating with the Brownian rotation) for large nanoparticles. While, for the nanorods with the long axis <100nm, the spectral shifts are no longer consistent with the modified Jeffery's orbits, but with the rods flipping between extreme orientations of the Jeffery's orbits. This indicates that the effect of the Brownian motion and hydrodynamic forces on the nanoscale rods needs being reconsidered. The viscoelastic effect on the flow-induced aggregation is studied in the dilute colloidal polystyrene nanoparticles suspensions. The real-time aggregation processes have been recorded via measuring optical absorption/scattering in flow. The observed absorbance decreases over time are attributed to the flow-induced coagulation. The aggregation processes still follow the Smoluchowski coagulation equation in a revised version. Suspensions in a series of media are studied to evaluate the effect of the media rheological properties on the particle aggregation. The data shows that elasticity reduces the aggregation while the solution viscosity increases the aggregation rate. In conclusion, the flow induced alignment and aggregation in the nanomaterials suspensions were studied using the rheo-optical method. The classical hydrodynamic theory describes the rotation of larger prolate nanoparticles in flow, but is no longer efficient for the rod like particles with the long axis 100nm. The solution viscosity accelerates the aggregation of nanoparticles while the elasticity has the opposite effect.
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ItemAssembly of polymer matrices enveloping cubic lyotropic liquid crystalline nanoparticles for drug delivery applications( 2012)Cubic lyotropic liquid crystalline nanoparticles (cubosomes™) exhibit great potential as drug delivery vehicles due to their nanoscale size, biocompatible constituents, and high loading potential for hydrophobic, hydrophilic, and amphiphilic agents. However, they also suffer from some limitations which have restricted their clinical effectiveness. For example, they release their cargo in a rapid, uncontrolled manner— a phenomenon known as burst release. In addition, the lipids which form reverse cubic phase typically do not contain surface functional groups for the immobilisation of targeting or stealth providing moieties. Polymeric capsules, in particular those made with the layer-by-layer technique, are able to modify the release properties of a loaded drug according to the number and nature of polymer layers. Many of the polymers employed also contain available functional groups for additional chemistry. However polymeric capsules can be difficult to efficiently load with therapeutic agents, particularly when the drugs are lipid soluble. Additionally, the removal of the capsule core template often requires conditions that can cause instability. This thesis examined the use of polymers to modulate the properties of cubosomes with the intention to aid stability, limit burst release, add potential functionality, and increase the payload. Different methods used to prepare stable, well dispersed amphiphilic cubosomes (high pressure homogenisation, extrusion, and ultrasonication) were analysed and compared. The effect of an additive to the aqueous environment (such as sodium chloride or phosphate buffered saline (PBS)) was also investigated. Certain additives to the amphiphile matrix such as the charged lipids cetyl trimethylammonium bromide (CTAB), dioctadecyl-dimethylammonium bromide (DODAB) or sodium dodecyl sulphate (SDS) were found to cause structural changes to both bulk and dispersed cubic phase but could be tolerated up to a certain quantity before complete destabilisation occurred. Integrating cubic nanoparticles and polymer matrices was first accomplished by coating silica microparticles. This resulted in a multilayered polymer coating representing an embedded layer of cubosomes surrounded by poly(allylamine hydrochloride) (PAH) and poly(styrene sulfonate) (PSS) polyelectrolytes. Upon removal of the silica core, stable polymer microcapsules containing embedded cubic nanoparticles were obtained. A diversity of molecular encapsulation matrices is offered through the capsule core, polyelectrolyte layers, and the embedded cubosomes of these sub-compartmentalised, nanostructured microcapsules. Individual cubic nanoparticles surrounded by polyelectrolyte multilayers were prepared next. The polymers were able to interact with the non-charged cubic lipid nanoparticles by utilising a polyelectrolyte modified with hydrophobic side chains (poly(methacrylic acid-co-oleyl methacrylate), PMAO) as an initial layer. Three bi-layers of poly(L-lysine) (PLL) and poly(methacrylic acid) (PMA) were then sequentially added. In order to separate accrued polymer aggregates from the coated lipid nanoparticles, a simple technique was developed whereby centrifugation separated the less dense cubosomes for collection. Modulation of the drug release properties and attenuation of the burst release from coated cubosome particles was demonstrated using two model drugs (fluorescein and perylene). The modified polymer PMAO was then utilised as an alternative stabiliser for lyotropic liquid crystalline nanoparticles. The charge-stabilised particles were tested against the most commonly utilised steric stabiliser Pluronic F127 for stability and drug release characteristics. Although PMAO-stabilised nanoparticles still exhibited burst release, improved particle stability was observed over time and over a range of temperatures, including storage under refrigeration. A lesser amount of PMAO stabiliser and less energy input were also required to disperse the bulk lipid into discrete, uniform nanoparticles compared to Pluronic F127. These studies demonstrate the viability of combining layer-by-layer polymer matrix technology with cubic lyotropic liquid crystalline nanoparticles to enhance the future of drug delivery.