Advisor:
Shuming Nie

Committee:
Younan Xia
Mostafa El-Sayed
Gang Bao
Adam Marcus (Emory University)

Semiconductor nanocrystalline quantum dots (QDs) have been intensely studied in bioimaging applications due to their exceptional optical properties such as size-tunable narrow emission spectra, broad absorption envelopes, and high resistance to chemical and photodegradation in comparison to conventional fluorophores or fluorescent proteins. An inherent problem that quantum dots have shared with other single emitters is fluorescent intermittency or blinking. The blinking of a single molecule or single QD refers to completely random transitions amid absorption and emission cycles followed by sustained intervals of time without fluorescence regardless of continuous laser excitation. Frequent and prolonged off-times raise difficulties in correspondence between frames when tracking the position of single molecules in cellular experiments due to cells not being homogeneous vessels with a single diffusion coefficient. Even though recently there has been success suppressing the blinking of quantum dots in practice, these methods have been restricted to thick shell particles with diameters nearing 20nm, gradient dots with overlapping multi-peak emission, or inserting the probes in specific solutions of reducing reagents. All of these methods present major difficulties when considering application to live molecular tracking experiments.

The objective of this study is composing a framework by means of physical-chemical theory undertaking the two mechanisms of fluorescent intermittency (Auger recombination and surface carrier trapping) and synthesis measures designed in light of this information, to develop blinking suppressed particles which are better suited for single molecular imaging than currently available QDs then pioneer their use in a biological setting. The central hypothesis is that these imaging probes will provide superior frame correspondence and trajectory reconstruction in tracking studies due to having short infrequent off times with minimal perturbations from size. This hypothesis was constructed from developed theories revealing paths to abate the mechanisms of blinking along with synthetic procedures of QDs which are adaptable and robust while minimizing final sizes. It is understood particles with brief and infrequent off-times offer negligible signal loss, leading to continuous dynamical information at higher acquisition rates. In addition to the enhanced on-times, the emission spectra are expected to be single peaked with thin FWHMs due to synthesis techniques preserving monodispersity, thus enabling multiplexed single molecule tracking of different species. 

The innovation of the study is synthesizing QDs that have greatly diminished fluorescent off-times while preserving small sizes, solution independency, and no multi-peak emission. The proposed work is expected to yield the following outcomes: First, it will establish a framework based on theoretical forethought in which to synthesize smaller blinking suppressed imaging probes. Second, a comparative test of blinking suppressed to conventional core/shell particles elucidating the benefits shown in a single molecule study will be performed.  These studies will present that when adhering to the prescribed framework outlined, one can produce blinking suppressed QDs with sensible sizes for single molecule tracking experiments with minimal signal loss.