<p> Magnetic nanostructures have been subject of intense research as a result of their unique magnetic properties such as superparamagnetism, enhanced magnetic moment, and high magnetic density storage due to shape anisotropy. A highly reproducible and affordable method to fabricate cobalt nanostructures on silicon substrates was devised in this thesis using nanosphere lithography. The surface morphology and magnetic properties of the nanopatterned cobalt thin film were characterized using an optical microscope, scanning electron microscope, atomic force microscope, and a magneto optical Kerr effect (MOKE) magnetometer. Modification to the surface of cobalt thin film was found to extensively alter its magnetic coercivity. Continuous cobalt thin film at 10 nm thick has a coercivity of 102&plusmn;2 Oe, whereas nanostructured cobalt thin films at the same thickness have a coercivity of 167&plusmn;16 Oe. The magnetic coercivity increases by 65&plusmn;18 Oe for an array of close packed cobalt nanostructures using nanosphere templates with diameters ranging from 203 nm to 600 nm. The cobalt nanostructured sample using a 930 nm diameter nanosphere template has a coercivity comparable to a continuous cobalt thin film at 108&plusmn;7 Oe. In addition, these nanostructures exhibit unusual magnetic properties such as multistep behavior and pinching/crossing-over of the magnetization curves with regards to the MOKE signal. These features become more prominent as the diameter of the nanosphere template decreases.</p>

The brightness and photostability of semiconductor quantum dots (QDs) has prompted the exploration of their use in a wide variety of fields. Several examples of QD-based biosensors have been reported but none have actually replaced their preexisting technologies. This work reveals the barriers hindering widespread use of QD based biosensors and examines how QDs can be engineered for improved utility in bioassay designs. The first portion of this project aims to improve Förster Resonance Energy Transfer (FRET) that use QDs as both the donor and acceptor. FRET-based sensors often use fluorescent dyes (FD) or proteins (FPs), but their photo- and chemical instability can be problematic. Contemporary QD-QD FRET systems suffer from unacceptably high background signal due to direct acceptor excitation. Materials engineering is used to create QD donors that are brighter than their QD acceptors to mitigate this effect. First, CdSe/xCdS/xZnS QDs of increasing shell thickness were synthesized and tested in a QD-fluorescent dye system to elucidate the effect of increased donor size on the performance of a FRET sensor. The optimal donors were medium-sized and 8 times brighter than commercially available QDs while retaining ~60% FRET efficiency. When used in a sensor, changes in sensor brightness were visible by eye. Moving towards QD-QD systems, a pH-based aggregation assay was used to test how QD heterostructures comprised of different semiconductor materials perform as FRET donors or acceptors. The fundamental principles uncovered are used to improve contemporary QD-QD FRET sensing and show that sensors can be designed to use color change as a visible, easy-to-decipher readout. Color change-based sensor output is further explored in an allosteric transcription factor-based small-molecule sensor that employs QDs as the sole fluorescent label. A highly modular design is presented that achieves a nanomolar concentration visual limit of detection. The ease of use, and fast, instrument-free readout of the sensor shows promise for its development into a fully integrated point-of-care device, endorsing the value of further developing QD-based in vitro biosensors for clinical or commercial translation. / 2020-06-04T00:00:00Z

<p> The events on September 11^th 2001 and subsequent attacks in America and around the world have brought a renewed interest in the nation&rsquo;s security including the concern over the use of a nuclear or a radiological dispersal device (RDD). Research utilizing NAA and ICP-MS has been done in two separate projects in order to help address some of these concerns. A research assistantship from Savannah River National Laboratory was granted in order to identify the impurities and isotope ratios of ¹⁹²Ir sources (chapters 2-4). An electrochemical dissolution method of the iridium was developed and used for sample preparation for ICP-MS analysis. ICP-MS analysis was then used to identify and quantify impurities and isotope ratios in iridium from various sources. The second research project has developed a series of lanthanide phosphate based nanoparticles for use as tagging and tracking agents (chapters 5-7). The composition of the nanoparticles were varied to provide a unique signature that can be rapidly and precisely measured in the field via neutron activation analysis. The nanoparticles could be used as a real-time in the field method for tracking and identifying materials such as explosives in a post detonation scenario.</p><p>

This thesis describes spectroscopic studies of three different systems: silver nanoparticles, the dye molecule rhodamine 6G adsorbed on graphene, and the type IV pili and c-type cytochromes produced by the dissimilatory metal-reducing bacterium Geobacter sulfurreducens. Although these systems are quite different in some ways, they can all be considered examples of nanomaterials. A nanomaterial is generally defined as having at least one dimension below 100 nm in size. Silver nanoparticles, with sub-100 nm size in all dimensions, are examples of zero-dimensional nanomaterials. Graphene, a single atomic layer of carbon atoms, is the paradigmatic two-dimensional nanomaterial. And although bacterial cells are on the order of 1 µm in size, the type IV pili and multiheme c-type cytochromes produced by G. sulfurreducens can be considered to be one- and zero-dimensional nanomaterials respectively. A further connection between these systems is their strong interaction with visible light, allowing us to study them using similar spectroscopic tools. The first chapter of this thesis describes research on the plasmon-mediated photochemistry of silver nanoparticles. Silver nanoparticles support coherent electron oscillations, known as localized surface plasmons, at resonance frequencies that depend on the particle size and shape and the local dielectric environment. Nanoparticle absorption and scattering cross-sections are maximized at surface plasmon resonance frequencies, and the electromagnetic field is amplified near the particle surface. Plasmonic effects can enhance the photochemistry of silver particles alone or in conjunction with semiconductors according to several mechanisms. We study the photooxidation of citrate by silver nanoparticles in a photoelectrochemical cell, focusing on the wavelength-dependence of the reaction rate and the role of the semiconductor substrate. We find that the citrate photooxidation rate does not track the plasmon resonance of the silver nanoparticles but instead rises monotonically with photon energy. These results are discussed in terms of plasmonic enhancement mechanisms and a theoretical model describing hot carrier photochemistry. The second chapter explores the electronic absorption and resonance Raman scattering of the dye molecule rhodamine 6G (R6G) adsorbed on graphene. Graphene has been shown to quench the fluorescence of adsorbed molecules and quantum dots, and some previous studies have reported that the Raman scattering from molecules adsorbed on graphene is enhanced. We show that reflective contrast spectroscopy can be used to obtain the electronic absorption spectrum of R6G adsorbed on graphene, allowing us to estimate the surface concentration of the dye molecule. From these results we are able to calculate the absolute Raman scattering cross-section for R6G adsorbed on bilayer graphene. We find that there is no evidence of enhancement but instead that the cross-section is reduced by more than three-fold from its value in solution. We further show that a model incorporating electromagnetic interference effects can reproduce the observed dependence of the R6G Raman intensity on the number of graphene layers. The third and final chapter describes the preliminary results from studies of the dissimilatory metal-reducing bacterium Geobacter sulfurreducens. This anaerobic bacterium couples the oxidation of organic carbon sources to the reduction of iron oxides and other extracellular electron acceptors, a type of anaerobic respiration that necessitates an electron transport chain that can move electrons from the interior of the cell to the extracellular environment. The electron transport chain in G. sulfurreducens has not been completely characterized and two competing mechanisms for the charge transport have been proposed. The first holds that G. sulfurreducens produces type IV pili, protein filaments several nanometers in width, with intrinsic metallic-like conductivity. According to this mechanism, the conductive pili mediate electron transport to extracellular acceptors. The second proposed mechanism is that charge transport proceeds by electron hopping between the heme groups in the many c-type cytochromes produced by G. sulfurreducens. In this picture, the observed conductivity of the pili is due to hopping through associated cytochrome proteins. Our aim is to explore these alternative mechanisms for electron transport in G. sulfurreducens through electrical and optical studies. We report the work we have done thus far to culture and characterize G. sulfurreducens, and we show that preliminary micro-Raman studies of G. sulfurreducens cells confirm that we can detect the spectroscopic signature of c-type cytochrome proteins. Future directions for this ongoing work are briefly discussed.

Biosensors are instrumental in the detection of analytes in a wide range of areas including enzyme kinetics and disease diagnosis. A proof-of-principle upconversion nanophosphor (UCNP) based biosensor based on luminescence energy transfer between UCNPs, acting as the energy transfer donor, and enzymes and biologically relevant proteins, the energy transfer acceptor is reported here. Analyte detection has been performed by ratiometric sensing by monitoring the change in the multiple emission bands of the UCNPs. Chapter 1 is an introduction into the emerging field of UCNPs as biosensing agents. These nanoparticles offer numerous advantages over current biosensing agents (namely organic dyes and quantum dots) including resistance to photobleaching and photoblinking, long emissive lifetimes, a large anti-Stokes' shift and near infrared (nIR) excitation to eliminate autofluoresence, and multiple characteristic emission bands for sensing multiple analytes. Chapter 2 describes the synthesis and characterisation of Yb3+/Tm3+ and Yb3+/Er3+ co-doped UCNPs via a range of different preparative methods; thermal decomposition, microwave irradiation and a one-step solvothermal process to produce hydrophilic UNCPs. In addition, commercial UCNPs, kindly donated by Phosphor Technology, were also characterised and used as a benchmark for characterisation of the newly synthesised UCNPs. Chapter 3 describes the detection of the enzyme pentaerythritol tetranitrate reductase (PETNR), through energy transfer between the commercial Yb3+/Tm3+ doped UCNPs and the enzyme using ratiometric sensing. These proof-of-principle results were published in Dalton Transactions. In addition, ratiometric change of the UCNP emission bands was able to monitor the enzyme-substrate turnover in a two electron redox reaction. Chapter 4 describes techniques for increasing the scope and sensitivity of the proof-of-principle UCNP-enzyme biosensing system. Small, hydrophilic Yb3+/Tm3+ and Yb3+/Er3+ doped UCNPs, synthesised in chapter 2, were able to detect glucose oxidase and cytochrome c, in addition to PETNR. Covalent attachment of PETNR to Yb3+/Tm3+ doped UCNPs was additionally achieved. Chapter 5 describes the incorporation of UCNPs into optical ring resonators (ORRs) in order to develop a lost cost, label-free, rapid response biosensor. Drop casting and inkjet printing methods for the deposition of UCNPs onto these devices were investigated and emission of UCNPs was achieved, for the first time, by ORR excitation.

abstract: Rapid development of new technology has significantly disrupted the way radiotherapy is planned and delivered. These processes involve delivering high radiation doses to the target tumor while minimizing dose to the surrounding healthy tissue. However, with rapid implementation of these new technologies, there is a need for the detection of prescribed ionizing radiation for radioprotection of the patient and quality assurance of the technique employed. Most available clinical sensors are subjected to various limitations including requirement of extensive training, loss of readout with sequential measurements, sensitivity to light and post-irradiation wait time prior to analysis. Considering these disadvantages, there is still a need for a sensor that can be fabricated with ease and still operate effectively in predicting the delivered radiation dose. The dissertation discusses the development of a sensor that changes color upon exposure to therapeutic levels of ionizing radiation used during routine radiotherapy. The underlying principle behind the sensor is based on the formation of gold nanoparticles from its colorless precursor salt solution upon exposure to ionizing radiation. Exposure to ionizing radiation generates free radicals which reduce ionic gold to its zerovalent gold form which further nucleate and mature into nanoparticles. The generation of these nanoparticles render a change in color from colorless to a maroon/pink depending on the intensity of incident ionizing radiation. The shade and the intensity of the color developed is used to quantitatively and qualitatively predict the prescribed radiation dose. The dissertation further describes the applicability of sensor to detect a wide range of ionizing radiation including high energy photons, protons, electrons and emissions from radioactive isotopes while remaining insensitive to non-ionizing radiation. The sensor was further augmented with a capability to differentiate regions that are irradiated and non-irradiated in two dimensions. The dissertation further describes the ability of the sensor to predict dose deposition in all three dimensions. The efficacy of the sensor to predict the prescribed dose delivered to canine patients undergoing radiotherapy was also demonstrated. All these taken together demonstrate the potential of this technology to be translatable to the clinic to ensure patient safety during routine radiotherapy. / Dissertation/Thesis / Doctoral Dissertation Chemical Engineering 2019

Understanding the microstructure of single walled nanotubes (SWNTs) in solution is an essential step in the development of fluid processing techniques for the creation of multifunctional macroscopic SWNT materials and is also useful in the more fundamental study of rigid rod solutions. In this thesis, the microstructure of SWNT solutions in ClHSO 3 and in mixtures of ClHSO 3 and 102% H 2 SO 4 is studied by investigation of their viscoelastic properties. These results are compared to previous investigations of SWNTs in 102% H 2 SO 4 and in ClHSO 3 at higher concentrations in order to study the effects of concentration and inter-SWNT attractive potential. Attractive interactions between the SWNTs are found to have a strong effect on percolation threshold concentration. A percolation transition is also observed in solutions at a fixed concentration as the solvent strength is decreased. Measurements obtained below the percolation transition are compared to the predictions of existing rigid rod solution models.

Thesis (Ph. D.)--University of Hong Kong, 2009. / Includes bibliographical references (leaves 119-127). Also available in print.

<p> Carbon nanotubes are a relatively new area of research which has gained significant attention in published literature. One reason for this interest is their use in multi-phase composites, specifically where they can enhance traditional polymer matrices. Many authors have attempted to adapt conventional micromechanical analyses reserved for microfibers to the nano scale. A review of these works is presented. In depth analysis is provided on one of these two phase (nanotube and matrix) models, the Anumandla-Gibson model, originally published in 2006. A discussion of its strengths and sensitivities is given, with numerical data to support the conclusions. It is extended to three-phase composites through the use of classical laminated plate theory. A literature survey is conducted to gather published two and three-phase experimental results for comparison. Two phase experimental results agree well with the present model, whereas three phase data was limited, but initial comparisons were promising.</p>

<p> This thesis investigates the use of EEG to track the spatial locus of covert, visual attention. Three experiments are described that were to detect the position of visual attention as it was deployed towards targets as they appeared. The first experiment uses flickering fields placed in the periphery of the visual field to induce SSVEPs, to be used to track the position of attention which varies horizontally between them. The flickers failed to produce significant SSVEP activity. However attention locus could still able to be tracked by endogenous lateralizations of 12Hz and 18Hz activity. A second experiment was then designed to track attention locus as it varied either horizontally or vertically using only endogenous EEG activity in the alpha (10Hz), low-beta (18Hz), high-beta (24Hz) and gamma (36Hz) bands. Tracking proved successful in all but a small number of subjects. Horizontally varying attention was associated with lateralizations of the alpha band and low-beta band, while vertically varying attention was associated with varying alpha band and low-beta band activity in the occipito-parietal junction over the central sulcus. A third experiment was then performed to track attention locus as it varied in two dimensions. Using a combination of the features found to be informative in the second experiment, tracking proved successful in up to nine bins of two-dimensional visual space. Tracking in either the horizontal or vertical dimension was also successful when attention varied in two dimensions. The success of this method shows that EEG can be used to passively detect the spatial position of attention, at varying degrees of position, as a person attends to objects they see.</p>