Plasmonic enhancement of fluorescence for biomedical diagnostics
Stranik, Ondrej (2007) Plasmonic enhancement of fluorescence for biomedical diagnostics. PhD thesis, Dublin City University.
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The enhancement of fluorescence that can result from the proximity of fluorophores to metallic nanoparticles (NP’s) is investigated. This plasmonic enhancement, which is a result of the localized surface plasmon resonance (LSPR) at the metal surface, can be exploited in order to improve the signal obtained from optical biochips and thereby lower the limits of detection. The scale of the enhancement depends on many parameters such as NP size and shape, metal type and NP-fluorophore separation. Throughout the work, theoretical calculations were carried out, and, where relevant, theoretical predictions were compared with experimental measurements. Characterisation techniques used include TEM, AFM as well as optical fluorescence and absorption. The first section deals with the production of ordered arrays of nanostructures, of varying size and composition, on glass substrates using a nanosphere lithography technique. The ability to tune the peak wavelength of LSPR was demonstrated. Fluorescent dyes were then pin-printed onto the NP layer and the fluorescence enhancement was measured. The second body of work involved characterising the enhanced fluorescence from dyes attached to free NPs in solution. NPs of sizes ranging from 5 to 50nm radius and with different gold/silver alloy compositions were prepared by wet chemistry. The NPs were coated with silica shells to control the dye-NP separation and to minimise quenching. The dependence of the enhancement on NP size was found to agree well with theoretical calculations based on the Mie theory. The final body of work focused on the development of strategies applicable to polymer biochips. This included the development of techniques for immobilising NPs on plastic substrates. A range of dyes and a range of NP shapes were investigated. Dye-NP separation was controlled to nanometer precision by layer-by-layer deposition of polyelectrolytes. In this configuration, both dye quenching and enhancement effects were observed and characterised. The key result to emerge from this work was that it is possible to design an optical biochip enhancement platform where the NP shape, size and composition are optimised for the selected dye label and where the average dye-NP separation is designed to achieve maximum enhancement.
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