Theses and Dissertations from UMD
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Item COMBINED EFFECTS OF SUNLIGHT AND TITANIUM DIOXIDE NANOPARTICLES ON DIETARY ANTIOXIDANTS AND FOOD COLORS(2014) Li, Meng; Lo, Yangming M; Food Science; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The breakthroughs in material science have enabled industrialized fabrication and production of nanomaterials. To date, nanoscale materials have been shown to exhibit improved functionalities, providing numerous novel applications. Titanium dioxide (TiO2) nanomaterials have been widely utilized in the food industry due to their unique properties under light. Upon light irradiation, TiO2 nanoparticles (NPs) generate highly active reactive oxygen species (ROS) therefore can be potentially used as light tunable antibacterial packaging materials. Moreover, it has also been reported that a considerable amounts of TiO2 NPs is found as an ingredient in food, cosmetics, personal care, and pharmaceutical products. With improved photoactivity, nano TiO2 generates higher amounts of ROS upon light irradiation that can result in oxidative damage. The present study investigates the combined effect of sunlight irradiation and TiO2 nanoparticles (NPs) on sensitive antioxidants and food colors. Upon simulated sunlight irradiation, TiO2 NPs weakened the radical scavenging ability of antioxidants by photocatalytic decomposition or surface adsorption. The decomposition of a widely used food azo dye FD&C Yellow No. 5 (tartrazine) by sunlight activated TiO2 NPs was also investigated. The mechanism is pH dependent, involving the depletion of two main ROS species, hydroxyl radical and singlet oxygen. Compared with the photocatalyst TiO2 sample P25, food-grade TiO2 NPs also showed strong ROS promoting ability and resulted in the degradation of selected synthetic dyes, including tartrazine, allura red and sunset yellow, as well as the semi-synthetic food color chlorophyllin sodium copper salt. Thus, TiO2 NPs should be used with caution when added to or used in contact with food ingredients that depend solely on the existing antioxidants and colors in the system. The ability of TiO2 to generate ROS was found to be phase-dependent. The rutile phase TiO2 generated the least amount of ROS when compared to anatase phase and did not lead to noticeable color degradation in the studied light irradiation period. Thus, when possible, rutile phase TiO2 should be used in food as it provides a more stable system compared to anatase and mixed phases. The results in this study provide clear insights on setting up proper protocols for evaluating and administrating nanosized TiO2 in food uses.Item The Novel Use of Nitroxide Antioxidants as Free Radical Scavengers in Ultra-High Molecular Weight Polyethylene (UHMWPE) for Total Joint Replacements(2010) Chumakov, Marina Konstantinovna; Al-Sheikhly, Mohamad; Bioengineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Ultra-high molecular weight polyethylene (UHMWPE) has been the standard load-bearing material used in total joint replacements since the 1960s. However, oxidative degradation can lead to premature aging and wear of UHMWPE, requiring implant revision. The novel use of nitroxide antioxidants to prevent oxidation in UHMWPE was proposed and the resulting structure and property changes were evaluated in this work. Standard sterilization and crosslinking methods of Co-60 gamma or high energy electron beam radiation produce alkyl free radicals on the polymer chain. Alkyl radicals react to form bimolecular crosslinks and long-lived allyl radicals at high dose rates; at low dose rates they tend to react with oxygen to form peroxyl radicals. The peroxyl radicals further interact with the polymer chain producing hydroperoxides and more free radicals, leading to oxidative degradation. As an alternative to post-irradiation remelting, which allows radical recombination but reduces fatigue strength, antioxidants can be introduced into UHMWPE to scavenge residual radicals. Nitroxides are stable organic compounds that have a strong paramagnetic signal and are very efficient in preventing lipid peroxidation and in providing radioprotection in biological tissues. The nitroxides used are 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and 4-hydroxy-TEMPO (Tempol). Through radical-radical interactions, radiolytically-produced alkyl radicals in UHMWPE are scavenged by the nitroxide radical. This is demonstrated through Electron Paramagnetic Resonance (EPR) spectroscopy where the paramagnetic nitroxide signal decays as it interacts with carbon-centered radicals in UHMWPE. Pulse radiolysis kinetics studies also show that alkyl radicals in UHMWPE preferentially react with nitroxides in the absence of oxygen. Controlled infiltration of UHMWPE with nitroxides is also observed using EPR. The resulting crosslink densities were investigated using Thermomechanical Analysis. It was observed that the addition of nitroxides after irradiation does not alter the crosslink density. Tensile testing of crosslinked and nitroxide-doped UHMWPE demonstrates increased ultimate tensile strength and toughness and the material exhibits an increase in crystallinity. Additionally, accelerated aging of specimens containing trace levels of nitroxide show very low oxidation levels when placed in an aggressive oxygen environment. Consequently, low concentrations of nitroxides diffused into UHMWPE after crosslinking produce an oxidation-resistant and highly crosslinked material for improved implant performance.