Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

A sensor is described which utilizes the phenomenon of surface plasmon resonance to detect changes in refractive index of chemical or biochemical samples applied to a surface modified optical fiber. The sensor is constructed by polishing a short section of the lateral surface of an optical fiber to its evanescent field surrounding the fiber core. One or more thin films are applied to the polished section of the fiber to produce the sensing element. One of the films is the metal silver, which acts as the support for the surface plasmon. Under the proper conditions, TM polarized energy propagating in the fiber can be coupled to a surface plasmon electromagnetic mode on the metal film. This coupling depends on the wavelength, the nature of the fiber, the refractive index and thickness of the thin films applied to the fiber, and the refractive index of a chemical sample in contact with the modified surface. The fiber to plasmon coupling is seen as a large attenuation of the light reaching the distal terminus of the fiber. / Ph. D.

The detection and active monitoring of hydrogen gas levels is essential in many areas of industry including manufacturing, storage, and transport of this gas. Previously, methods used to detect hydrogen gas have relied upon monolithic technologies to create resistive-based sensors which are impractical for many applications requiring electrical isolation or operation in the presence of strong electromagnetic fields. Materials can be sputter deposited along cylindrical optical fiber-based sensors to create novel fiber-optic chemical sensors. Herein, the mechanisms allowing the detection of hydrogen gas using a modified Extrinsic Fabry-Perot Interferometric (EFPI) sensor are presented in this work. A new custom-designed, custom-built radio frequency (RF) magnetron sputtering system has been used to deposit thin films of palladium metal along with cylindrical substrates. The surface morphology of the deposited films are investigated using a variety of analytical tools, including Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (AES), and Auger Electron Spectroscopy (AES). The system is used to deposit thin films of palladium along a cylindrical EFPI fiber-optic sensor to produce a new fiber-optic hydrogen gas sensor. Experimental results obtained in a controlled hydrogen atmosphere are used to demonstrate the high resolution and fast response time associated with these new hybrid sensors. / Master of Science

Instrument miniaturization is led by the desire to perform rapid diagnosis in remote areas with high throughput and low cost. In addition, miniaturized instruments hold the promise of consuming small sample volumes and are thus less prone to cross-contamination. Gas chromatography (GC) is the leading analytical instrument for the analysis of volatile organic compounds (VOCs). Due to its wide-ranging applications, it has received great attention both from industrial sectors and scientific communities. Recently, numerous research efforts have benefited from the advancements in micro-electromechanical system (MEMS) and nanotechnology based solutions to miniaturize the key components of GC instrument (pre-concentrator/injector, separation column, valves, pumps, and the detector). The purpose of this dissertation is to address the critical need of developing a micro GC system for various field- applications. The uniqueness of this work is to emphasize on the importance of integrating the basic components of μGC (including sampling/injection, separation and detection) on a single platform. This integration leads to overall improved performance as well as reducing the manufacturing cost of this technology. In this regard, the implementation of micro helium discharge photoionization detector (μDPID) in silicon-glass architecture served as a major accomplishment enabling its monolithic integration with the micro separation column (μSC). For the first time, the operation of a monolithic integrated module under temperature and flow programming conditions has been demonstrated to achieve rapid chromatographic analysis of a complex sample. Furthermore, an innovative sample injection mechanism has been incorporated in the integrated module to present the idea of a chip-scale μGC system. The possibility of using μGC technology in practical applications such as breath analysis and water monitoring is also demonstrated. Moreover, a nanotechnology based scheme for enhancing the adsorption capacity of the microfabricated pre-concentrator is also described. / Ph. D.

Thesis (M.Tech.: Food Technology)-Dept of Biotechnology, Durban University of Technology, 2007 xvi, 106 leaves / Alicyclobacillus acidoterrestris is a spore-forming spoilage micro-organism found in fruit juices whose spores are not destroyed by typical pasteurisation. Once its spores outgrow and multiply in finished juice products, they produce two volatile taint compounds namely guaiacol and 2,6-dibromophenol. In the food industry margins for errors are small and monitoring of products to avert such errors is crucial. Conventional microbiological monitoring is one such technique for spoilage micro-organisms another being automated systems which can detect taints. Both these categories were evaluated in this study with the electronic nose and gas chromatograph being the specific automated systems being assessed. Sensory evaluation was also assessed as a diagnostic tool in the detection of taints. Isolation and identification of what was thought to be A. acidoterrestris was a laborious and expensive exercise which eventually proved inconclusive. A pure culture was purchased and juices were then inoculated with two levels of A. acidoterrestris spores and incubated. Juices from each level of inoculation were evaluated at different time intervals via the above-mentioned monitoring techniques. Of the three media assessed in the microbiological method, Bacillus acidoterrestris medium (BAM) was found to be the most effective for enumerating A. acidoterrestris followed by K-medium (KM) then Orange Serum Agar (OSA). While BAM was still indicating the presence of A. acidoterrestris KM and OSA were not (counts of <10cfu/g). This illustrated that this micro-organism could be easily overlooked if KM or OSA were being used to enumerate them. Considering that many workers actually do use KM and OSA as their media of choice in enumerating A. acidoterrestris (perhaps because BAM is very tedious to prepare) the cause for concern is a real one. Assessment of the resultant taints via sensory evaluation after inoculation and incubation reveals the inability of many panellists to detect taints at levels (as assessed by GC) far above their documented threshold values. While GC is an extremely useful and powerful tool, the level of expertise and skill required to use such an instrument cannot be overlooked nor can the expense involved. With regard to the electronic nose assessment for the presence of the volatile taint compounds, an important finding was that the electronic nose indicated significant differences between test and control samples when panellists performing sensory evaluation did not. This also correlated to an interval when enumeration on OSA illustrated no A. acidoterrestris after several days of inoculation and incubation and BAM and KM did. Without implying that the electronic nose has no drawbacks, it has proved, in this instance to be a simple and easy piece of equipment to use. It can be used to detect taints produced under simulated spoilage conditions at reduced analysis times, levels of expertise, cost and energy.

Currently, researchers at Sandia National Laboratories are creating software that is designed to determine the source of a toxic release given sensor readings of the toxin concentration at fixed locations in the building. One of the most important concerns in solving such problems is computation time since even a crude approximation to the source, if found in a timely manner, will give emergency personnel the chance to take appropriate actions to contain the substance. The manner in which the toxin spreads depends on the air flow within the building. Due to the turbulence in the air flow, it is necessary to calculate the flow field on a fine mesh. Unfortunately, using a fine mesh for every calculation in this problem may result in prohibitively long computation times when other features are incorporated into the model. The goal of this thesis is to reduce the computation time required by the software mentioned above by applying two different mesh coarsening strategies after the flow field is computed. The first of these strategies is to use a uniformly coarse mesh and the second is to use our knowledge of the air flow in the building to construct an adaptive mesh. The objective of the latter strategy is to use a fine mesh only in areas where it is absolutely necessary, i.e., in areas where there is a great change in the flow field.

The main topic of this thesis is the performance improvement of microresonators as mass-sensitive biochemical sensors in a liquid environment. Resonant microstructures fabricated on silicon substrates with CMOS-compatible micromachining techniques are mainly investigated. Two particular approaches have been chosen to improve the resolution of resonant chemical/biochemical sensors. The first approach is based on designing a microresonator with high Q-factor in air and in liquid, thus, improving its frequency resolution. The second approach is based on minimizing the frequency drift of microresonators by compensating for temperature-induced frequency variations. A disk-shape resonant microstructure vibrating in a rotational in-plane mode has been designed, fabricated and extensively characterized both in air and in water. The designed resonators have typical resonance frequencies between 300 and 1,000kHz and feature on-chip electrothermal excitation elements and a piezoresistive Wheatstone-bridge for vibration detection. By shearing the surrounding fluid instead of compressing it, damping is reduced and quality factors up to 5800 in air and 94 in water have been achieved. Short-term frequency stabilities obtained from Allan-variance measurements with 1-sec gate time are as low as 1.1 10-8 in air and 2.3 10-6 in water. The performance of the designed resonator as a biological sensor in liquid environment has been demonstrated experimentally using the specific binding of anti-beta-galactosidase antibody to beta-galactosidase enzyme covalently immobilized on the resonator surface. An analytical model of the disk resonator, represented by a simple harmonic oscillator, has been derived and compared with experimental results. The resonance frequency and the Q-factor of the disk resonator are determined from analytical expressions for the rotational spring constant, rotational moment of inertia, and energy loss by viscous damping. The developed analytical models show a good agreement with FEM simulation and experimental results and facilitate the geometrical optimization of the disk-type resonators. Finally, a new strategy to compensate for temperature-induced frequency drifts of resonant microstructures has been developed based on a controlled stiffness modulation by an electronic feedback loop. The developed method is experimentally verified by compensating for temperature-induced frequency fluctuations of a microresonator. In principle, the proposed method is applicable to all resonant microstructures featuring excitation and detection elements.

No description available.

Thesis (M.Tech.: Food Technology)-Dept of Biotechnology, Durban University of Technology, 2007 xvi, 106 leaves / Alicyclobacillus acidoterrestris is a spore-forming spoilage micro-organism found in fruit juices whose spores are not destroyed by typical pasteurisation. Once its spores outgrow and multiply in finished juice products, they produce two volatile taint compounds namely guaiacol and 2,6-dibromophenol. In the food industry margins for errors are small and monitoring of products to avert such errors is crucial. Conventional microbiological monitoring is one such technique for spoilage micro-organisms another being automated systems which can detect taints. Both these categories were evaluated in this study with the electronic nose and gas chromatograph being the specific automated systems being assessed. Sensory evaluation was also assessed as a diagnostic tool in the detection of taints. Isolation and identification of what was thought to be A. acidoterrestris was a laborious and expensive exercise which eventually proved inconclusive. A pure culture was purchased and juices were then inoculated with two levels of A. acidoterrestris spores and incubated. Juices from each level of inoculation were evaluated at different time intervals via the above-mentioned monitoring techniques. Of the three media assessed in the microbiological method, Bacillus acidoterrestris medium (BAM) was found to be the most effective for enumerating A. acidoterrestris followed by K-medium (KM) then Orange Serum Agar (OSA). While BAM was still indicating the presence of A. acidoterrestris KM and OSA were not (counts of <10cfu/g). This illustrated that this micro-organism could be easily overlooked if KM or OSA were being used to enumerate them. Considering that many workers actually do use KM and OSA as their media of choice in enumerating A. acidoterrestris (perhaps because BAM is very tedious to prepare) the cause for concern is a real one. Assessment of the resultant taints via sensory evaluation after inoculation and incubation reveals the inability of many panellists to detect taints at levels (as assessed by GC) far above their documented threshold values. While GC is an extremely useful and powerful tool, the level of expertise and skill required to use such an instrument cannot be overlooked nor can the expense involved. With regard to the electronic nose assessment for the presence of the volatile taint compounds, an important finding was that the electronic nose indicated significant differences between test and control samples when panellists performing sensory evaluation did not. This also correlated to an interval when enumeration on OSA illustrated no A. acidoterrestris after several days of inoculation and incubation and BAM and KM did. Without implying that the electronic nose has no drawbacks, it has proved, in this instance to be a simple and easy piece of equipment to use. It can be used to detect taints produced under simulated spoilage conditions at reduced analysis times, levels of expertise, cost and energy.

Piezoresistive microcantilevers can be used for the detection of biological and chemical substances by measuring the change in surface stress. Design parameters for the cantilever and piezoresistor dimensions are investigated analytically and through finite element modelling. Based on these results, six optimized cantilever types are designed and fabricated with microfabrication methods. The electrical and mechanical properties of these devices as well as their deflection and surface stress sensitivities are characterized and compared to the models. A second generation of cantilevers that incorporates heater areas to trigger or enhance chemical reactions is designed and fabricated. In addition to the measurements done for the first generation devices, the thermal properties for both steady-state and transient operation of these microcantilevers are characterized.