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Mayilo, Sergiy (2009): Exploiting Energy Transfer in Hybrid Metal and Semiconductor Nanoparticle Systems for Biosensing and Energy Harvesting. Dissertation, LMU München: Faculty of Physics
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Abstract

In this work, gold and semiconductor nanoparticles are used as building blocks for nanostructures, in which energy transfer is investigated. Nanoparticles have size-dependent controllable optical properties. Therefore, they are interesting objects to study different aspects and applications of energy transfer. Fluorescence quenching by gold nanoparticles is investigated and used to develop novel immunoassays for medically relevant molecules. The range of fluorescence quenching by gold nanoparticles is effective over longer distances than for dye molecules. The reason for this is the large absorption cross-section of gold nanoparticles and the radiative rate suppression of dyes caused by gold nanoparticles. The influence of gold nanoparticles on radiative and non-radiative rates of Cy3 and Cy3B dyes is studied here. A competitive, homogeneous immunoassay for digoxigenin and digoxin, a drug used to cure heart diseases, is developed. Dye-labeled digoxigenin is bound to the gold nanoparticles functionalized with anti-digoxigenin antibodies, quenching the dye fluorescence. Unlabeled digoxigenin partially replaces the dye-labeled digoxigenin leading to an increase of fluorescence. The assay has a limit of detection of 0.5 nM in buffer and 50 nM in serum. Time resolved spectroscopy reveals that the quenching is due to energy transfer with an efficiency of 70%. A homogeneous sandwich immunoassay for cardiac troponin T, an indicator of damage to the heart muscle, is developed. Gold nanoparticles and fluorophores are functionalized with anti-troponin T antibodies. In the presence of troponin T the nanoparticles and fluorophores form a sandwich structure, in which the dye fluorescence is quenched by a gold nanoparticle. The limit of detection of the immunoassay in buffer is 0.02 nM and 0.11 nM in serum. Energy transfer, with up to 95% efficiency, is responsible for the fluorescence quenching, as found through time resolved spectroscopy. Energy transfer is demonstrated in clusters of CdTe nanocrystals assembled using three methods. In the first method, clusters of differently-sized water soluble CdTe nanocrystals capped by negatively charged mercaptoacid stabilizers are produced through electrostatic interactions with positively charged Ca(II) cations. The two other methods employ covalent binding through dithiols and thiolated DNA as linkers between nanocrystals. Energy transfer from smaller nanocrystals to larger nanocrystals in aggregates is demonstrated by means of steady-state and time-resolved photoluminescence spectroscopy, paving the way for nanocrystal-based light harvesting structures in solution. Multi-shell onion-like CdSe/ZnS/CdSe/ZnS nanocrystals are presented. In these structures the CdSe core and the CdSe shell produce two emission peaks upon UV light excitation. When the emission peaks are well matched, the resulting emission appears as pure white light. The shade of the white light can be controlled by annealing the particles. Evidence for intra-nanocrystal energy transfer is presented.