Graduate Thesis Or Dissertation

 

Gold Nanostars: Spectroscopic and Physical Properties Public Deposited

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/3j3337678

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  • Tethering fluorophores on metal nanoparticles for metal enhanced fluorescence (MEF) has attracted a lot of attention from scientists interested in fluorescence based approaches. MEF has many benefits in fluorescence related techniques, because the analytical signal increases and enables potentially lower detection limits or better sensitivity. Many types of nanoparticles have been explored for their abilities to enhance fluorescence. New nanomaterial geometries such as gold nanostars have emerged that draw people’s attention due to the nanostars’ inherent abilities to enhance electric fields and thus the fluorescence. The reason gold nanostars are attractive for MEF is mainly due to the many sharp spikes. Theoretically, the radius of curvature on the nanostars’ tips facilitates strong electric field enhancements that interact with nearby fluorophores and influence the fluorescence intensity. Other physical properties of gold nanostars such as their ability to be dispersed in colloids and the tenability of their plasmons in the nearinfrared region make gold nanostars capable for many applications other than MEF. MEF on gold nanostars is still in the development stage, and the mechanisms of nanostar induced MEF remain unknown. These gaps in knowledge constrain the practical applicability of gold nanostars in fluorescence related fields. Additionally, the stability of gold nanostars in different solution conditions is less well studied. Nanostar stability is critical as it affects the functionalization process of nanostars with different reagents as well as the reproducibility of experiments. Therefore, the lack of literature reports on nanostar stability in various environments restricts and slows down the progress of applying gold nanostars into future applications. Part of my thesis provides experimental evidence of colloidal gold nanostars’ ability to achieve fluorescence enhancement. To the best of my knowledge, this is the first report on MEF by colloidal nanostars under two-photon excitation. In the MEF work, a nanostructure containing nanostars, avidin, and biotinylated DNA linkers was designed to examine the fluorescence signal of a cyanine dye, Cy3, near the nanosurface in nanostar colloids. The observed fluorescence enhancement factors ranged from 1.2 to 3.5, depending on the experimental parameters. The fluorescence enhancement was expected to result from the concentrated electric fields around nanostars sharp spikes. Moreover, I conducted research to better understand the mechanisms of nanostar induced MEF. In these studies, I examined the fluorescence of Cy3 with various fluorophore-tonanostar distances and orientation, and different extent of fluorophore-nanostar spectral overlaps. I found that the spectral overlap between nanostars’ plasmon and the fluorophores’ excitation and emission spectra dominated the fluorescence enhancement mechanism. From these results, I have preliminary data that the near-field and pre-filter effect mechanisms caused the observed trend in fluorescence enhancement. To contribute knowledge to future research involving gold nanostars, I performed experiments to determine the stability of gold nanostars over time and in colloids with different ionic strengths and pH values. The results showed that gold nanostars’ plasmon blue shifted over time, and became unstable in colloids with the salt concentration over 35 mM or pH lower than 2, where the plasmon shifts were greater than 30 nm. Moreover, I modified the gold nanostars with different surfactants {mercaptobenzoic acid (MBA) and sodium dodecyl sulfate(SDS)} to gain insight on how to improve the nanostars’ stability. Generally speaking, the MBA coating kept the nanostars’ plasmon steady over time; SDS helped keep the zeta potential stable over time and strengthened the nanostars’ stability in colloids with different salt concentrations and pH values. Overall, the fundamental research about the MEF and stability of gold nanostars addressed in the thesis is to inform and guide prospective experimental designs relevant to nanostars. These studies are significant steps towards applying gold nanostars into future applicable fields.
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  • Ongoing Research
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  • 2019-06-11 to 2020-07-11

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