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«By Yusuf Nur A thesis submitted to The University of Birmingham for the Degree of DOCTOR OF PHILOSOPHY School of Geography, Earth and Environmental ...»

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found out that NPs with different coating agents of similar final concentrations manifested different effects on the bacteria exposed. PVP coated 10 nm core sizes gold NPs with final concentration of 10 ppm have reduced the growth of bacteria compared to the Minimum Davis Media blank. This adverse effect was manifested by the bacterial cells in the form of outer membrane damage as confirmed by the TEM images of the cells. Blebbings and tubular formation on the surface of the bacterial outer membrane and wide holes on the membrane were signs of membrane damages after NPs exposure. Complete bursting of effected bacterial cells was also visualised. In contrast, citrate - capped NPs of similar concentration and core size as the aforementioned PVP coated NPs have slightly increased the growth of the bacteria This finding supports the idea that surface chemistry of the NPs is a very important factor in terms of the effect of the NPs on the bacterial cells. Any interaction between the NPs and the bacterial cells starts with the contact of the coating agent of the NPs on the outer cell membrane. Consequently, the bacteria was grown in both PVP and citrate solution without any other carbon source. It was found out that bacteria can utilise citrate as carbon source but not the PVP. This finding has explained why bacteria growth was slightly increased in the citrate - capped NPs solution. Finally, gold ions of similar concentration as the above mentioned NPs solutions have completely inhibited the bacteria growth and caused the immediate death of the bacterial cells.

What makes this research unique and different from many ecotoxicology studies is the four step process employed here:

 Synthesising well - constrained NPs.

 Fully characterising them and studying their stability for the relevant period of time.

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ensure any change of the physicochemical properties of the NPs in the media since any property change may complicate the data interpretation after exposure.

 Post exposure characterisation of both NPs and bacterial cells.

The aforementioned four steps are necessary to carry out in order to relate NPs property to the cell response. Although it is possible that some researches have characterised the NPs after synthesis, studies that sought to fully characterise the NPs both in the growth media and after exposure are rare or non – existent.

Notwithstanding the fact that the effect of the coating agents on the outer cell membrane is visualised in the form of blebbing and wide wholes on the surface of the membrane, the nature and the mechanisms of the interaction between NPs and cells need further research. The bacterial population in the environment can exist either as unicellular planktonic or in more sessile communities in the form of biofilm.

Therefore, both the transport of the NPs in the biofilm and their effects on the structure of the biofilm will be an interesting area for further investigation and will contribute to our understanding of this relatively new technology on the natural environment.

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