PLASMON ENHANCED SINGLE MOLECULE FLUORESCENCE IN ZERO MODE WAVEGUIDES (ZMWS)

Plasmonic nanostructures are an extensive research focus due to their ability to modify the photophysical properties of nearby fluorophores. Surface plasmons (SP), defined as the collective oscillation of delocalized electrons, are the fundamental characteristic primarily responsible for altering those photophysical properties. Studying fluorophores at the single-molecule level has received significant attention since more specific information can be extracted from single molecule-based studies, which otherwise could be obscured in ensemble studies. However, single-molecule studies are inherently challenging because the signal from a single molecule is usually dim, making it difficult to detect. The situation is even worse in a crowded environment due to higher background noise, such as cellular autofluorescence, in the case of cell-based studies. Thus, one possible way out of this single-molecule detection problem is to couple the fluorophore with a plasmonic nanostructure which can potentially enhance the fluorescence intensity leading to an improvement in the signal-to-noise ratio. Throughout the projects presented here, I studied the fluorescence characteristics of single fluorophore molecules coupled in a plasmonic nano-aperture termed Zero Mode Waveguides (ZMWs). I utilized single fluorophores of different origins, such as organic dyes and quantum dots (QDs), in ZMWs of different metallic compositions. By probing ZMWs made from the mixture of aluminum and gold, with a range of ATTO dyes emitting across the visible wavelength, we found that the surface plasmon resonance of ZMWs is tunable by optimizing the metal ratio. Apart from the ATTO dyes, I investigated the photoluminescence (PL) behavior of single QDs in ZMWs and observed a significant enhancement in PL intensity and a substantial improvement in the blinking characteristics of the QDs, which are beneficial for the utility of QDs as a bio-imaging agent or a single-photon source. Single QDs in ZMWs exhibited a significant enhancement in biexciton quantum yield, which is crucial for their potential application in lasing, where materials with a high optical gain are desired. I also examined the fluorescence properties of the single fluorophores in gold ZMWs in the presence of a gold nanoparticle (AuNP). I observed a more significant enhancement in fluorescence intensity in the gap between AuZMW and AuNP compared to the case of only AuZMWs or only AuNPs. The experimental designs and the resulting findings throughout the three projects presented here should be a valuable resource for the future development of plasmon-mediated single-molecule studies