Browsing by Subject "nanomaterials"

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    Effect of carbon nanotubes on transepithelial resistance in barrier epithelial cells
    (Office of the Vice Chancellor for Research, 2012-04-13) Lewis, Shanta; Blazer-Yost, Bonnie L.; Petrache, Horia I.; Roark, Torri
    The burgeoning of the nanotechnology industry has revolutionized engineering, medicine and the fashion industry amongst many other technologies. The arrays of products that are synthesized from nanomaterials include high definition TV and computer screens, artificial organs, as well as antibacterial food containers. With these novel applications there is a paralleled concern of the negative implications of nano-material contamination in our environment and food chains. We are interested in carbon nanotubes because they are the most abundantly occurring nanoparticles that are found in the workplace. We have recently conducted studies in barrier epithelial cells to show that long single-wall and multi-wall carbon nanotubes (CNTs) caused a decrease in the transepithelial electrical resistance, a measure of the barrier function ,of renal principal cells at very low concentrations (0.4 ng/cm2- 4 g/cm2). These results suggested that nanoparticles may also cause an effect on other barrier epithelial cells such as those lining the human digestive and respiratory tracks. After 48 hours of CNT exposure, to airway and colon cell lines, Calu-3 and t84 respectively, the calculated resistances were approximately half of the control monolayers’, indicating that the barrier function of the tissue had been compromised, while the cellular monolayer remained intact. We sought to determine the mechanism of action of the nanoparticles, by investigating the interaction of CNTs with model lipid membranes using a bilayer clamp amplifier. Measurements showed that the presence of nanoparticles caused transient disruptions in lipid membranes made of phosphatidylcholine lipids. Nanotubes also caused transient interruptions in the current allowed by the ion channel reporter (gramicidin A). These results began to elucidate the mode of action of the particles and indicated that it is important to develop a complete understanding of how nanoparticles interact with cells if we are to safeguard against changes that these materials will cause in vivo.
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    Role of surface ligand chemistry on shape evolution and optoelecronic properties of direct band gap semiconductors
    (2017) Teunis McLeod, Meghan; Sardar, Rajesh
    The expansion of the applications of direct band gap semiconductor nanocrystals (NCs) has been a result of the control colloidal synthetic methods offer on the optoelectronic properties. These properties are readily controlled by the surface chemistry and even a small change in the surface passivating ligand can show profound effects. Furthermore, the choice of surface passivating ligand also impacts the NC shape evolution, which in turn influence the surface area, quantum yield, and charge transport properties that are critical to optimize device fabrication. In this dissertation, the unique aspects of surface chemistry that control both NC shape evolution and optoelectronic properties are investigated. We began by investigating how surface chemistry controls the shape evolution of methyl ammonium lead bromide (CH3NH3PbBr3) perovskite NCs. In addition to the surface passivating ligand, the reaction temperature and solvent system were also examined. Through a series of control experiments, the critical parameter for the formation of quantum wires (QWs) was found to be the presence of a long chain acid, while the quantum platelets (QPLs) required a long chain amine and chlorinated solvent, and quantum cube (QC) formation was kinetically driven. The higher ordered stacking of the QPLs and bundling of the QWs was also found to be controlled by surface ligand chemistry. Next we further examined how surface chemistry impacts shape evolution, but in the system of metal chalcogenide NCs. We developed a versatile, low temperature, and gram scale synthesis of QWs, QPLs, and quantum rods (QRs) using both cadmium and zinc as metal precursors and sulfur and selenium as chalcogenide precursors. Through systematic investigation of both the surface chemistry and reaction progression, the growth and formation mechanism was also determined. The 1D QW growth required a long chain amine while the QPLs required the presence of both a long and short chain amine to drive 2D growth. Finally, the QRs would found to be a kinetically-controlled process. Ultrasmall semiconductor NCs are known to possess high surface to volume ratios and therefore even a minute change in surface chemistry will have a significant impact on the optoelectronic properties. Our investigation focused on (CdSe)34 NCs, and how exchanging native amine ligands with various chalcogenol based ligands influences these properties. These NCs lie in the strong confinement regime and therefore have a higher probability of undergoing exciton delocalization, resulting in red shifts of the first excitonic peak and reduction of the optical band gap. Additionally, we examined different characteristics of the ligand (level of conjugation, electron withdrawing or donating nature of para-substitution, binding mode and head group) to examine how these parameters impact exciton delocalization. We observed the highest shift in the optical band gap (of 650 meV) after exchanging the native amine ligands with pyrene dithiocarbamate. Through this investigation it was determined that ligand characteristics (specifically conjugation and binding mode) have significant influence in the proposed hole delocalization. Finally, we continued the investigation of how surface chemistry controls optoelectronic properties of ultrasmall NCs, but expand our work to those of methyl ammonium lead halide. We developed a low temperature and colloidal synthesis of white-light emitting NCs with a diameter of 1.5 nm. Through precise manipulation of the surface halide ions, it was possible to tailor the emission to match that of nearly pure white light.