Correlation between geometrical and structural properties of mixed oxide ultrathin nanotubes and their solar water splitting performance

The objective of this study was to study the effect of Nb alloying with Ti on the photoelectrochemical performance of the resulted oxide upon anodization. In this regard, nanotubes were grown on Ti-Nb alloy via electrochemical anodization and their corresponding photocatalytic behavior was investigated and compared with those grown on an ordinary Ti substrate. After preparing and optimizing the nanotubes dimensions for the required geometrical structure, the as formed tubes were annealed at different temperatures and in air), then characterized with respect to their morphological, structural, and photoelectrochemical properties. From the morphological and structural point of view, optimized and well aligned ultra-thin wall nanotubes were successfully synthesized on the surface of Ti-Nb alloy. To the best of our knowledge, these dimensions have not been reported before. One of the challenges was that the oxide layer formed on the surface of the alloy was not precisely identified in literature, where some authors reported the formation of combination of individual oxides (TiO2 and Nb2O5), whereas, others claimed it was a mixed oxide TiNbOx. Raman and X-ray diffraction test results confirmed the formation of individual anatase and monoclinic Nb2O5 phases. Detailed XRD analysis was performed and the crystallite size as well as microstrain were calculated and found to be minimal indicating negligible effect of lattice induced tension or compression. It is worth mentioning that insignificant structural changes are favorable to maintain good electron mobility. Hence, point defect equations were deduced and it was found that that oxygen vacancies were the prevailing ionic defects rather than electronic Nb compensation. From the aforementioned results, ultrathin wall nanotubes formed on TiNb alloy were achieved, for the first time, with clear representation of the oxide layer composition. Such oxide layer showed better stability upon annealing at high temperatures. Although, UV-Vis test results showed small or negligible enhancement in the absorption, profile the photo-electrochemical measurements showed much higher photocurrent for Ti-Nb oxide alloy than bare TiO2 prepared at the same conditions for the sake of comparison. In conclusion, the Ti-Nb NTs showed enhanced stability over a wide range of temperatures, where the transition from anatase to rutile was shifted to higher temperature in addition to an increase in the photoconversion capability, resulting in a more efficient water splitting process

NASA Laser Light Scattering Advanced Technology Development Workshop, 1988

The major objective of the workshop was to explore the capabilities of existing and prospective laser light scattering hardware and to assess user requirements and needs for a laser light scattering instrument in a reduced gravity environment. The workshop addressed experimental needs and stressed hardware development

Gallium-based Ultraviolet Nanoplasmonics

<p>Nanometer-scale metallic structures have been widely and intensively studied over the last decade because of their remarkable plasmonic properties that can enhance local electromagnetic (EM) fields. However, most plasmonic applications are restricted to the visible and near infrared photon energies due to the limitations of the surface plasmon resonance energies of the most commonly used plasmonic metals: Au and Ag. Plasmonic applications in ultraviolet (UV) are of great interest because Raman scattering sections are larger and do not overlap fluorescence spectra. UV plasmonics also benefit from high spatial resolution and low penetration depth. However, an appropriate UV plasmonic material must be identified.</p><p>We proposed and demonstrated that gallium is a highly-promising and compelling material for UV nanoplasmonics through synthesis of size-controlled nanoparticle arrays, EM modeling of local field enhancement, ellipsometric and spatial characterization of the arrays, and analytical measurement of UV- enhanced Raman and fluorescence spectra. Self-assembled arrays of hemispherical gallium nanoparticles deposited by molecular beam epitaxy on a sapphire support are characterized with spatial and ellipsometric measurements. Spin-casting a thin film of crystal violet upon these nanoparticles permitted the demonstration of surface-enhanced Raman spectra, fluorescence, and molecular photodegradation following excitation by a HeCd laser operating at 325 nm (UV). Measured local Raman enhancement factors exceeding 10<super>7</super> demonstrated the potential of gallium nanoparticle arrays for plasmonically-enhanced ultraviolet detection and remediation.</p>Dissertatio

Unfolding mechanism of human glutathione transferase M1a-1a

A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg in fulfilment of the requirements for the degree of Doctor of Philosophy May 2018Proteins exist in equilibrium between the native (N) and the denatured (D) states. In order to form the biologically active native state, the amino acid sequence has to fold to form a stable three-dimensional structure. The large scientific community of biochemists and biophysicists has not yet been able to gain a complete understanding of this process. In this study, the unfolding of the homodimeric detoxification enzyme hGST M1a-1a (WT dimer) was investigated. Additionally, an F56S/R81A double-mutant (mutant monomer) was engineered to create a monomeric form of the protein. The mutant monomer was used to gain a better understanding of the unfolding events occurring at the subunit level, in the absence of quaternary interactions. Data from various techniques indicate the mutant monomer to closely resemble the tertiary structure of the subunits in the WT homodimer, making it a suitable model to study the unfolding mechanism of hGST M1a in the absence of quaternary interactions. A four-state equilibrium unfolding mechanism, involving two stable intermediate species, is proposed. HDX-MS studies indicate that disruption of the conserved lock-and-key motif, as well as the structures surrounding the mu loop, results in a destabilisation of domain 1. However, dimer dissociation cannot occur until the mixed charge cluster at the dimer interface has been destabilised. The destabilisation of domain 1 results in destabilisation of α4 and α5 in domain 2, because the domains unfold in a concerted manner. hGST M1a-1a dissociates to form monomeric intermediate (M), with weak interdomain interactions and compromised short-range contacts. The unstable M intermediate self-associates to form an oligomeric intermediate (I). The destabilisation of α6 and α7 in the hydrophobic core of domain 2 drives the formation of the partially structured denatured state. Further investigation will need to be pursued to determine whether hGST M1a-1a unfolds via transient intermediate states; however, the elucidation of the equilibrium unfolding pathway of a complex homodimeric protein is a valuable addition to the ever-growing knowledge base of protein folding.MT 201

Properties of Multivesicular Release from Mouse Rod Photoreceptors Support Transmission of Single-Photon Responses

Vision under starlight requires rod photoreceptors to transduce and transmit single-photon responses to the visual system. Small single-photon voltage changes must therefore cause detectable reductions in glutamate release. We found that rods achieve this by employing mechanisms that enhance release regularity and its sensitivity to small voltage changes. At the resting membrane potential in darkness, mouse rods exhibit coordinated and regularly timed multivesicular release events, each consisting of ~17 vesicles and occurring two to three times more regularly than predicted by Poisson statistics. Hyperpolarizing rods to mimic the voltage change produced by a single photon abruptly reduced the probability of multivesicular release nearly to zero with a rebound increase at stimulus offset. Simulations of these release dynamics indicate that this regularly timed, multivesicular release promotes transmission of single-photon responses to post-synaptic rod-bipolar cells. Furthermore, the mechanism is efficient, requiring lower overall release rates than uniquantal release governed by Poisson statistics

Photodetectors

In this book some recent advances in development of photodetectors and photodetection systems for specific applications are included. In the first section of the book nine different types of photodetectors and their characteristics are presented. Next, some theoretical aspects and simulations are discussed. The last eight chapters are devoted to the development of photodetection systems for imaging, particle size analysis, transfers of time, measurement of vibrations, magnetic field, polarization of light, and particle energy. The book is addressed to students, engineers, and researchers working in the field of photonics and advanced technologies

Image processing on reconfigurable hardware for continuous monitoring of fluorescent biomarkers in cell cultures

Fluorescence microscopy is a widespread tool in biological research. It is the primary modality for bioimaging and empowers the study and analysis of multitudes of biological processes. It can be applied to fixed biosamples, that is samples with frozen biological features by mean of chemical linkers, or live biosamples providing useful insights on the spatio-temporal behavior of fluorescently stained biomarkers. Current fluorescent microscopy techniques use digital image sensors which are used to leverage quantitative studies instead qualitative outcomes. However, state-of-the-art techniques are not suitable for integration in small, contained and (semi-)autonomous systems. They remain costly, bulky and rather quantitatively inefficient methods for monitoring fluorescent biomarkers, which is not on par with the design constraints found in modern Lab-on-a-Chip or Point-of-Use systems requiring the use of miniaturized and integrated fluroscence microscopy. In this thesis, I summarize my research and engineering efforts in bringing an embedded image processing system capable of monitoring fluorescent biomarkers in cell cultures in a continuous and real-time manner. Three main areas related to the problem at hand were explored in the course of this work: simulation, segmentation algorithms and embedded image processing. n the area of simulation, a novel approach for generating synthetic fluorescent 2D images of cell cultures is presented. This approach is dichotomized in a first part focusing on the modeling and generation of synthetic populations of cells (i.e. cell cultures) at the level of single fluorescent biomarkers and in a second part simulating the imaging process occurring in a traditional digital fluorescent microscope to produce realistic images of the synthetic cell cultures. The objective of the proposed approach aims at providing synthetic data at will in order to test and validate image processing systems and algorithms. Various image segmentation algorithms are considered and compared for the purpose of segmenting fluorescent spots in microscopic images. The study presented in this thesis includes a novel image thresholding technique for spot extraction along with three well-known spot segmentation techniques. The comparison is undertaken on two aspects. The segmentation masks provided by the methods are used to extract further metrics related to the fluorescent signals in order to (i) evaluate how well the segmentation masks can provide data for classifying real fluorescent biological samples from negative control samples and (ii) quantitatively compare the segmentations masks based on simulated data from the previously stated simulation tool. Finally, the design of an embedded image processing system based on FPGA technologies is showcased. A semi-autonomous smart camera is conceived for the continuous monitoring of fluorescent biomarkers based on one of the segmentation methods incorporated in the previously stated comparison. Keeping the focus on the need for integration in fluorescence microscopy, the image processing core at the heart of the smart camera results from the use of a novel image processing suite; a suite of IP cores developed under the constraints dictated by the bioimaging needs of fluorescence microscopy for use in FPGA and SoC technologies. As a proof of concept, the smart camera is applied to the monitoring of the kinetics of the uptake of fluorescent silica nano-particles in cell cultures

High accessibility and chemical homogeneity in mesoporous materials: silicas, gold-containing composites and phosphates

El trabajo recopilado en esta tesis está basado en la formación de materiales mesoporosos y sus posibles usos como catalizadores. Este trabajo se ha estructurado en cuatro partes diferenciadas. En primer lugar se presenta una breve introducción a los materiales mesoporosos revisando su evolución en la historia, los diferentes métodos de síntesis, su versatilidad en cuanto a funcionalización y aplicaciones y acabando con un repaso al trabajo realizado en el grupo de investigación. El segundo capítulo está centrado en el mejor conocimiento de la sílice UVM-7, un material mesoporoso de carácter bimodal desarrollado por el grupo. En primer lugar se estudia en detalle la estructura del material para confirmar la interconexión en tres dimensiones de los mesoporos. Esto se consigue a través de medidas de tomografía electrónica, combinadas con la formación de réplicas de carbono y la preparación de composites con oro y el posterior estudio microscópico de su dispersión en el material. Una vez conocida al detalle su estructura, se trabaja en la expansión de los mesoporos utilizando alcanos como agentes expansores, suponiendo que, dado su carácter hidrofóbico, estos se alojarán en el interior de las micelas de surfactante. En este trabajo se estudia la cantidad real de alcano alojada en las micelas a través de medidas espectrofotométricas y se trata de explicar los resultados obtenidos en base a estudios coloidales. Finalmente, para poner en valor la expansión obtenida se inmovilizan enzimas e ibuprofeno en los materiales con mayor expansión. El tercer capítulo recoge dos trabajos en los que se ha colaborado con diferentes grupos de catálisis. Se forman composites de sílice con diferentes dominios oxídicos utilizando dos estrategias diferentes. En el primer caso, se forman composites tipo UVM-7 con Ni/Sn/Ce utilizando una síntesis en “one-pot”, siguiendo del método de los atranos. En el segundo caso, se impregna el material UVM-7 para formar óxidos cristalinos de TiO2, ZrO2 y CeO2 en su superficie. Posteriormente se depositan partículas de Au sobre los materiales y se prueban en la catálisis de la condensación selectiva de benzilamina a N-benzilidenbenzilamina y en la oxidación de CO a CO2 respectivamente. El trabajo realizado por nuestra parte recoge la síntesis y caracterización de los soportes, así como la comprensión de los resultados catalíticos en base a la estructura de los distintos materiales. Por último se incluye un capítulo sobre materiales mesoporos no silíceos en el que se estudia la formación de un fosfato de hierro mesoporoso así como de fosfonatos de hierro. Para ello se utiliza un surfactante como agente director estructural, igual que en el caso de la UVM-7. Se caracterizan los materiales con diferentes técnicas y se propone una estructura.The work collected in this PhD Thesis is based on the formation of mesoporous materials and their use as catalysts. This work is divided in four differentiated pars. The first one includes a short introduction to mesoporous materials and their evolution, the possible synthetic methods, their versatility among functionalization and applications. This chapter ends with a revision to the previous work of the research group. The second chapter is focused on UVM-7 silica, a bimodal mesoporous material developed in the group. The structure of this material is studied in detail to confirm the tridimensional interconnexion of the mesopores. This is achieved with the use of electron tomography, combined with the formation of carbon replicas and the preparation of gold-composites, followed by a microscopic study of gold dispersion in the material. Once known the nature of the structure, mesopore expansion is explored using alkanes as swelling agents. Given the hydrophobic nature of these molecules they are supposed to be located inside the surfactant micelles. In this work the real amount of alkane incorporated is studied, using spectrophotometric techniques and the results of these measures are explained based in colloidal studies. Finally, different enzymes and ibuprofen have been immobilized in the material, as molecular probes. The third chapter includes two collaborations with catalytic groups. Silica composites with different oxidic domain have been formed, following two different strategies. In the first one, UVM-7 like composites containing Ni/Sn/Ce have been formed with a “one-pot” synthesis, using the atrane method. In the second one, UVM-7 silica has been impregnated to form crystalline TiO2, ZrO2 and CeO2 on the surface. Gold nanoparticles have been deposited on these materials and they have been used for catalysis of selective aerobic oxidative condensation of benzylamine to N-benzylidenebenzylamine and Co to CO2 oxidation, respectively. The work done by our group contains synthesis and characterization of supports, as well as understanding of catalytic results based in the structure of the materials. Finally, non-silica mesoporous materials have been studied. In the final chapter we have studied the formation of mesoporous iron phosphate and phosphonates. As for UVM-7, surfactant is used as structural directing agent. The obtained materials have been characterized using different techniques and a structure is proposed

Structure and Functionality of Novel Nanocomposite Granules for a Pressure-Sensitive Ink with Applications in Touchscreen Technologies

Tactile sensors are now ubiquitous within human-computer interactions, where mouse and keyboard functionality can be replaced with a trackpad or touchscreen sensor. In most technologies the sensor can detect the touch location only, with no information given on the force of the touch. In this thesis, functional components of a novel nanocomposite ink are developed, which when printed, form a pressure-sensitive interface which can detect both touch location and touch force. The physical basis of the force-sensitive response is investigated for the touchscreen sensor as a whole, as well as the intrinsic force-sensitivity of the ink components. In an earlier form the nanocomposite ink, that was the starting point of this study, contained agglomerates of conductive nanoparticles which were formed during blending of the ink, and provided the electrical functionality of the sensor. Here, novel nanocomposite granules were pre-fabricated prior to inclusion in the ink. The granules were designed such that they exhibited well-defined size, structure and strength. Control of these parameters was achieved through selection of the granule constituents, as well as the energy and duration of the granulation process. When incorporated into the ink and screen-printed to form a pressure-sensitive layer in a touchscreen test device, the functional performance could be assessed. Sensors containing pre-formed granules showed improved optical transmission, compared to sensors containing the same mass loading of nanoparticles forming spontaneous agglomerates. Agglomerates tend to create a larger number of small scattering centres which scatter light to larger angles. The spatial variation in the force-resistance response, as well as the sensitivity of this response, was also linked to the distribution of the granules within the pressure-sensitive layer. The physical basis of the force-resistance response is two-fold. Firstly, mathematical simulations showed that deflection of the upper electrode increased the number of granules contacted with increasing applied force and therefore decreased the resistance through the sensor. Secondly, a force-sensitive resistance of the granules themselves was also observed at high forces. Analysis of the non-linear current-voltage characteristics suggested the presence of non-linear conduction pathways within the granules. Using a random resistor network model, the non-linear current contribution decreased after approximately 0.7 N force. To understand this effect, a model based on the physical basis of quantum tunnelling mechanisms was also applied, however this provided a poor fit to the data and no further understanding could be gained