Near Infrared-Emitting Quantum Dots: Synthesis, Characterization
\nand Biological Applications.

Quantum dots (QDs) have attracted significant attention in many applications due to \ntheir unique features, such as size-dependent optical absorption and emission, arising \nfrom the quantum confinement effect, which the bulk material does not possess. \nAmong diverse QDs, near infrared (NIR) emitting QDs, such as lead-based QDs, \nwhich can be tuned to emit from below 1000 nanometer to several thousand \nnanometers are particularly interesting. Both their excitation and emission can be \neasily adjusted to lie within the biological windows, highly desirable for some \ndemanding biological applications. Although the synthesis of NIR QDs with a \nuniform and narrow size distribution is well known today, achieving QDs with high \nquantum efficiency and excellent stability is still a big challenge. \nQDs are tiny crystalline particles with typical dimensions in the range of 1-100 nm. \nTheir surface-to-volume ratio is very large, therefore the properties of QDs become \nextremely sensitive to surface characteristics. In order to improve the optical \nproperties of these QDs, great efforts have been made in the past few decades on the \nsurface engineering of QDs. A passivation shell is normally grown over the QDs to \nform a core/shell structure, which in turn improves the optical properties and stability. \nIn the first part of this thesis, the synthesis and optical properties of PbS/CdS \ncore/shell QDs are presented and discussed. We firstly synthesized a series of \ndifferently sized PbS QDs by using the traditional hot injection method, then these \nPbS QDs were coated with a thin CdS shell to form a PbS/CdS core/shell structure by \nthe cation exchange approach, finally the optical properties of these core/shell QDs were studied. \nIn Section I of Part I, we report the development of a reproducible and controllable \nmicrowave-assisted cation exchange approach, for the first time, to quickly synthesize \nhigh-quality, NIR emitting PbS/CdS core/shell QDs. These monodisperse QDs, \nemitting in the range of 1300-1600 nm, show a quantum yield (QY) as high as 57% \nthat is ~1.4 times higher than that of QDs achieved by conventional heating in oil bath. \nMeanwhile, the reaction was successfully scaled up by several times by increasing the \nstarting PbS concentration or by amplifying the reaction volume and the \nas-synthesized core/shell QDs show similarly high QY. \nWe then report anomalous size-dependent photoluminescence (PL) intensity \nvariation of PbS QDs with the formation of a thin CdS shell via the same \nmicrowave-assisted cation exchange approach. We found previously that thin shell \nformation is an effective strategy for increasing the PL intensity of large sized PbS \nQDs. Nonetheless, herein we observed an unusual PL decrease in ultrasmall QDs \nupon shell formation. We attempted to understand this abnormal phenomenon from \nthe perspective of trap density variation and the probability of electrons and holes \nreaching surface defects. To this end, QY and PL lifetime (on the ns-μs time scale) of \npristine PbS QDs and PbS/CdS core/shell QDs were measured and radiative and \nnon-radiative recombination rates were derived and compared. Moreover, transient \nabsorption (TA) analysis (on the fs-ns time scale) was performed to better understand \nexciton dynamics on ultrafast time scales. These experimental results along with \ntheoretical calculations of electron and hole wave functions provide a complete picture of the photophysics governing the core/shell system. Ultimately, a model was \nconstructed to show the energy levels and trap states for various sizes. \nPart II focuses on the photostability and colloidal stability of water dispersible \nPbS/CdS/ZnS core/shell/shell QDs and their potential bio-applications. We report for \nthe first time detailed investigations of the synthesis of NIR, water dispersible, \nstrongly luminescent and highly stable PbS/CdS/ZnS core/shell/shell QDs, their \nproperties in different buffers, their cytotoxicity and further their applications in \ntumor imaging. In particular, we focus on the QDs emitting at 930 and 1220 nm, \nwithin the first and second biological windows, respectively. These QDs were \nsynthesized via our recently developed microwave-assisted approach to grow a ZnS \nshell and to simultaneously exchange initial ligand with mercaptopropyl acid on the \nPbS/CdS core/shell QDs dispersed in an organic phase. These QDs were extremely \nstable in commonly used biological buffers and remarkably, they could keep their \ninitial morphology, dispersion status and PL in phosphate buffered saline buffer (PBS) \nfor as long as 14 months, which was the longest time we investigated with both \ntransmission electron microscopy and PL spectroscopy herein. PL images taken on the \n930 nm emitting PbS/CdS/ZnS core/shell/shell QDs revealed that they could still emit \nstrongly after 30-month storage in PBS. Such long term stability of water dispersible \nQDs is rarely reported in the literature. Their colloidal stability was further \ninvestigated by keeping them in high ionic concentration conditions. Their PL \nintensity did not show any change for at least 3 weeks at high NaCl concentration up \nto 400 mM. The QDs also showed excellent photostability and could keep about 80% of their initial PL intensity after 1 hour of continuous, strong UV illumination. More \ninterestingly, they showed negligible toxicity to cultured cells even at high QDs \nconcentration (50 nM). Given these outstanding properties, the ultrastable and \nbiocompatible QDs were explored for the first time for in vivo tumor imaging in mice. \nWith one order of magnitude lower QD concentration (0.04 mg/mL), significantly \nweaker laser intensity (0.04 W/cm2 vs ~1 W/cm2) and considerably shorter signal \nintegration time (≤ 1 ms vs several hundreds of ms) as compared to the best reported \nrare earth doped nanoparticles, the QDs showed high emission intensity even at \ninjection depth of ~2.5 mm, hard to achieve with visible QDs and other NIR PL \nprobes. \nWe developed a QD-based imaging system based on NIR-emitting PbS/CdS/ZnS \nQDs with minimal noticeable toxicity. Through careful engineering of their emission \nwavelength, we obtained fluorescence imaging nanoprobes with optimal penetration \ndepths in biological tissue. Additionally, this new platform exhibited \nmultifunctionality beyond their use as pure imaging nanoprobes. The system is \ncapable of acting as a biological nanothermometer, based on the reliable \nthermal-dependent behavior of the fluorescence signal. The PbS/CdS/ZnS QDs \nstudied here can easily be exploited to obtain thermal mapping of subskin areas in live \nspecimens, an accomplishment of great relevance for early disease detection and also \nfor real-time therapy monitoring. Moreover, as a result of the intense signal provided \nby the NIR-emitting QDs, we are able to elucidate the real-time biodistribution of the \nQDs by means of in vivo experiments in live mice. We determined the lack of detectable chemical toxicity attributed to the QDs based on cell culture assays, as well \nas the lack of adverse health effects (no significant weight changes or behavior \nabnormalities were found over 4 weeks) for mice injected with a low concentration of \nQDs, coupled with the absence of any fluorescence signal detected in the body organs \nat the end of the experiment. We can therefore ascertain that the ZnS outer shell \nimparts a great deal of bio-compatibility and stability of the PbS/CdS/ZnS QDs \nreported here. Also, the intense fluorescent emission of this material allows for low \ndoses to be used, significantly improving the current state of the art. This greatly \ndiminishes the likelihood of causing adverse health effects on the live specimen when \nused as optical bioimaging probes.

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