Browsing by Subject "zinc oxide"
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Item Application of Luminescent Materials to Optical Sensing(2014-09-05) Ritter, Sarah CDevelopment of sensors for detection of various chemical and biological species is an important and ever-growing field. In particular, optical-based sensors enable a remote, rapid method for continuous or on-demand monitoring. Monitoring humidity is important across many applications, such as humidity control within moisture-sensitive environments and in medical, semiconductor, and food science fields. Following a study of photobleaching, defect-related emission of zinc oxide nanoparticles was monitored as a function of relative humidity. An important next step is its application to monitoring toxic gases, as air pollution has been identified as a major health concern. Of importance for the biomedical field is monitoring key blood analytes for human health. Monitoring blood pH is critical for specific patient groups, such as those suffering from diabetic ketoacidosis and congenital lactic acidosis. A pH-sensitive fluorophore was loaded within red blood cells for use as a continuous blood analyte monitor. Future work will focus on glucose, as current estimates show that one out of every three children born in 2000 will develop diabetes in his or her lifetime ? thus, the global impact of this disease is immense. Results from ZnO studies indicate that photobleaching is related to the surface area to volume ratio. ZnO nanoparticles display a linear response to humidity with a sensitivity of 0.008417 RH^-1 and 0.01898 RH^-1 for nitrogen and air environments, respectively. Owing to reversibility and high sensitivity, ZnO nanoparticles have great potential as optical-based environmental sensors. Results from dye-loaded ghost studies indicate that fluorescence intensity of intracellular dyes report on extracellular pH. Resealed ghosts loaded with a fluorescein isothiocyanate-glycylglycine conjugate reversibly track pH with a resolution down to 0.014 pH unit. For use in vivo, the development of an NIR pH-sensitive dye was paramount. Unfortunately, all NIR dyes tested exhibited poor pH sensitivity while displaying sensitivity to external factors (e.g., temperature, concentration, proteins). However, circulation kinetics of resealed ghosts were easily monitored once injected in vivo with an optical fiber-based system. Although the cells were rapidly removed from circulation, the loaded ghosts resulted in higher signal than would be expected for free dye alone. Once optimized, the resealed ghosts could serve as a long-term, continuous, circulating biosensor for the management of diseases.Item Microwave-Assisted Synthesis of II-VI Semiconductor Micro- and Nanoparticles towards Sensor Applications(2013-01-15) Majithia, RavishEngineering particles at the nanoscale demands a high degree of control over process parameters during synthesis. For nanocrystal synthesis, solution-based techniques typically include application of external convective heat. This process often leads to slow heating and allows decomposition of reagents or products over time. Microwave-assisted heating provides faster, localized heating at the molecular level with near instantaneous control over reaction parameters. In this work, microwave-assisted heating has been applied for the synthesis of II-VI semiconductor nanocrystals namely, ZnO nanopods and CdX (X = Se, Te) quantum dots (QDs). Based on factors such as size, surface functionality and charge, optical properties of such nanomaterials can be tuned for application as sensors. ZnO is a direct bandgap semiconductor (3.37 eV) with a large exciton binding energy (60 meV) leading to photoluminescence (PL) at room temperature. A microwave-assisted hydrothermal approach allows the use of sub-5 nm ZnO zero-dimensional nanoparticles as seeds for generation of multi-legged quasi one-dimensional nanopods via heterogeneous nucleation. ZnO nanopods, having individual leg diameters of 13-15 nm and growing along the [0001] direction, can be synthesized in as little as 20 minutes. ZnO nanopods exhibit a broad defect-related PL spanning the visible range with a peak at ~615 nm. Optical sensing based on changes in intensity of the defect PL in response to external environment (e.g., humidity) is demonstrated in this work. Microwave-assisted synthesis was also used for organometallic synthesis of CdX(ZnS) (X = Se, Te) core(shell) QDs. Optical emission of these QDs can be altered ased on their size and can be tailored to specific wavelengths. Further, QDs were incorporated in Enhanced Green-Fluorescent Protein ? Ultrabithorax (EGFP-Ubx) fusion protein for the generation of macroscale composite protein fibers via hierarchal self-assembly. Variations in EGFP- Ubx?QD composite fiber surface morphology and internal QD distribution were studied with respect to (i) time of QD addition (i.e., pre or post protein self-assembly) and (ii) QD surface charge ? negatively charged QDs with dihydrolipoic acid functionalization and positively charged QDs with polyethyleneimine coating. Elucidating design motifs and understanding factors that impact the protein-nanoparticle interaction enables manipulation of the structure and mechanical properties of composite materials.Item Vapor transport techniques for growing macroscopically uniform zinc oxide nanowires(2009-08) Baker, Chad Allan; Hall, Matthew John; Shi, Li, Ph. D.ZnO nanowires were grown using carbothermal reduction and convective vapor phase transport in a tube furnace. Si <100> substrates that were 20 mm x 76.2 mm were sputter coated with 2 nm to 50 nm gold which formed nanoparticles on the order of 50 nm in diameter through a process of Ostwald ripening upon being heated. Growth temperatures were varied from 800ºC to 1000ºC, flow rates were varied from 24 sccm to 3300 sccm, and growth durations were varied from 8 minutes to 5 hours. Vapor phase Zn, CO, and CO2, produced by carbothermal reduction and suspended in an Ar atmosphere, were flowed over the Si substrates. The Au nanoparticles formed an eutectic alloy with Zn, causing them to become liquid nanodroplets which catalyzed vapor-liquid-solid nanowire growth. The nanowires were also synthesized by self-catalyzing vapor-solid growth in some cases. Using the tube furnace never resulted in more than 50% of the substrate being covered by nanowires. It was found that a bench-top furnace could achieve nearly 100% nanowire coverage by placing the 20 mm x 76.2 mm sample face down in a quartz boat less than 2 mm above the source powder. This was because minimizing the distance between the sample and the source powder was critical to achieve macroscopically uniform growth consistently.