School of Chemistry - Theses
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ItemExcitation energy transfer in nanocrystal systems( 2014)The development of sensing platforms capable of accurately and sensitively detecting toxins, small molecules and DNA, has been an area of intense interest by researchers in defence, medicine and public safety. The photophysical phenomenon of Förster Resonance Energy Transfer (FRET) provides a route to achieving a high quality transduction signal for analyte detection, and the creation of next-generation sensing platforms. However, whether this model is valid to describe energy transfer in nanoparticle-dye systems has yet to be satisfactorily proven. A detailed background to FRET in general, and its application to nanoparticle fluorophores specifically, will be provided in Chapter 1. A detailed description of experimental protocols for both the synthesis of semi-conducting nanocrystal fluorophores, so called ‘Quantum Dots’, and a number of rigid, dye labelled polyproline peptides will be detailed in Chapter 2. Back- ground information about specific experimental techniques, including peptide synthesis and single particle spectroscopy, is also provided. In Chapter 3 energy transfer between Quantum Dots (QDs) and molecular dye molecules, is detailed. Using rigid polyproline spacers to controllably vary the separation it is found that, in contrast to simple dye-dye systems, the ratio of dye adsorbed to each QD must be explicitly accounted for by the Poisson distribution. Moreover, it is found that dye fluorescence quantum efficiency is also distance dependent, which obscures enhancements to the dye fluorophore due to energy transfer. However, accounting for both of these effects, it is found that the efficiency of energy transfer from QDs to adsorbed dye molecules obeys an R−n dependence with n = 6, as predicted from Förster Resonance Energy Transfer (FRET). In Chapter 4 energy transfer between ZnO nanoparticles to adsorbed dye molecules, is detailed. Remarkably, energy transfer in this system is found to occur from an intraband electronic state. It is found that the entire broad emission from radiative decay of this state, is uniformly reduced upon dye adsorption with a concomitant enhancement in the dye emission. These observations demonstrate that energy transfer from ZnO involves a single electronic state coupled to the phonon modes of the crystal lattice. It is also found that this energy transfer to the adsorbed dye molecules is very efficient, regardless of the orientation of the dipole moment of the dye molecule and the distance to the defect state. Finally in Chapter 5, a novel parameter for quantifying fluorescence intermittency, ‘blinking’, in QDs is presented. As energy transfer at the single particle level is often confounded by this phenomenon, additional insight that leads to its suppression is desirable. The parameter is essentially a measure of the total fractional correlation of single QD time-trajectories, which is found is related to the ensemble photoluminescence quantum yield and effectively independent of the manner in which the time-trajectory is binned.