Fabrega, Julia (2009)
Ph.D. thesis, University of Birmingham.
Nanoscale silver particles represent a new generation of cost-effective antibacterial technologies. Due to the increased manufacturing and use of silver nanoparticles (Ag NPs) in consumer goods their release and accumulation into the environment is highly likely but their fate, behaviour and toxicity to organisms is still very much unknown. The present study has investigated the effect that different environmental conditions have on the behaviour and fate of Ag NPs in waters by determining aggregation state, stability and solubility. This work has also determined their interaction and uptake to laboratory grown planktonic and biofilm bacteria, as well as to natural marine biofilms using analytical, electron microscopy and molecular tools. The outcomes of this work describe the effects that environmental factors such as pH, ionic strength and presence of humic substances (HS) have on the stability, behaviour and ultimately fate of Ag NPs in water, with direct implications on bioavailability of NPs to organisms. Higher pH values as well as the presence of organic of matter in the media increased stability and with this the residence time of the particles in suspension. On the other hand, pH values of 6 and also absence of organic matter increased the precipitation of NPs in suspension. Planktonic Pseudomonas fluorescens was highly susceptible to Ag NPs when grown and exposed at pH 9 only. However, toxicity was also mitigated when natural organic matter (HS) was present. Due to the low solubility of the Ag NPs in the media a NP-mediated toxic mechanism is suggested as the mode of toxicity of Ag NPs to planktonic P. fluorescens cells. Environmental parameters were also crucial for the uptake and interaction of Ag NPs to 3-d old Pseudomonas putida biofilms. Ag NPs were uptaken by the cells under all conditions, and decreased biofilm biovolume per surface area when HS were not present. With HS, Ag NPs did not significantly affect biomass; however uptake of Ag NPs doubled under this condition. Ag NPs in suspension had an effect on a natural marine biofilm community. A dosedependent decrease on biomass was recorded, but more importantly Ag NPs stopped biofilm succession and development and/or settling of new taxa on the resident biofilm community. Critical characterisation of Ag NP behaviour under different conditions is crucial for determining which organisms are more likely to interact with Ag NPs in different environmental compartments, and assess the possibility of longer term exposures. This work provides relevant information on the fate and toxicological effects of a short term exposure of Ag NPs to bacterial cells in an aqueous environment, with possible implications on their bioaccumulation and food web transfer.
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