Investigating the fate and transport behavior of cadmium selenide quantum dots and transition metal oxide nanoparticles in aquatic and soil environments
Navarro, Divina Angela G.
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The advancements in nanomaterials research have indeed brought about promising and exciting technologies. These new materials are bound to enter the aquatic and soil environments with commercialization and widespread use. Though CdSe and CdSe/ZnS quantum dots (QDs), and HfO 2 and ZrO 2 metal oxide nanoparticles (MO NPs) have yet to enter the mainstream market, an understanding of their behavior in the environment is important in assessing their potential risks and in making informed regulatory policy. Hence, the study of the environmental fate and transport of these NPs represents a timely and worthwhile endeavor. The work presented in this dissertation focuses on understanding the interactions and processes that will influence the stability and solubility of QDs and MO NPs in aquatic and soil systems. The role of natural organic matter (NOM) in the stabilization of colloidal dispersions of hydrophobically-coated CdSe QDs and HfO 2 , ZrO 2 , and solid-solution Hf 0.37 Zr 0.63 O 2 NPs was investigated using simple phase transfer experiments. QDs coated with tri- n -octylphosphine oxide, tetradecylphosphonic acid, and oleic acid and MO NPs coated with tri- n -octylphosphine oxide, which were otherwise insoluble in water, phase transferred into aqueous solutions of humic substances (HS) within 24 hours of mixing. Phase-transferred QDs and MO NPs were intact and temporarily stabilized by HS. Data suggest two synergistic mechanisms that facilitate phase transfer of QD/MO-HS agglomerates: (1) an overcoating mechanism involving dispersion interactions between nonpolar moieties of HS and hydrocarbon chains of the organic capping ligands, and (2) a coordinative mechanism involving displacement of the organic capping ligands by Lewis basic functionalities of HS. The structure of the organic ligands, as well as the nature of the NP crystal surface influenced the kinetics of phase transfer and extent of stabilization of QDs and MO NPs. This HS-mediated phase transfer was also demonstrated in two natural surface water samples from Buffalo and Tonawanda Creeks in Erie County, NY. The potential of QDs to leach into the groundwater or be taken up by plants were also investigated. Differences in the partitioning and mobility of water-dispersible CdSe QDs coated with mercaptopropionic acid and CdSe/ZnS coated with a polymer bearing carboxylic acid pendant groups were examined using small-scale soil columns. Potential root uptake of water dispersible CdSe/ZnS QDs by the model plant species, Arabidopsis thaliana. Results suggested that though both QDs exhibited limited soil mobility, the presence of chelating acids in soil can enhance the leaching potential of intact QDs. Longer incubation (15 days) of QDs in soil also indicated some degradation of CdSe and CdSe/ZnS QDs. Indeed, retention of intact QDs in soil could be a concern for terrestrial plants. Plant uptake experiments revealed that Arabidopsis exposed to QDs that are dispersed in Hoagland’s solution for 1-7 days did not internalize of intact QDs. QDs remained adsorbed onto the root surfaces with or without humic acid addition. Despite no evidence of nanoparticle internalization, the ratio of reduced glutathione relative to the oxidized glutathione in plants decreased when plants were exposed to QD dispersions containing humic acids, suggesting that QDs caused oxidative stress on the plant at this condition. The observed oxidative stress was most likely induced by the plant’s exposure to leached ions in solution, Cd 2+ and SeO 3 2- , and not from intact QDs.