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

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Ren, Fuqiang (2016). Near Infrared-Emitting Quantum Dots: Synthesis, Characterization and Biological Applications. Thèse. Québec, Université du Québec, Institut national de la recherche scientifique, Doctorat en sciences de l'énergie et des matériaux, 144 p.

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Résumé

Quantum dots (QDs) have attracted significant attention in many applications due to their unique features, such as size-dependent optical absorption and emission, arising from the quantum confinement effect, which the bulk material does not possess. Among diverse QDs, near infrared (NIR) emitting QDs, such as lead-based QDs, which can be tuned to emit from below 1000 nanometer to several thousand nanometers are particularly interesting. Both their excitation and emission can be easily adjusted to lie within the biological windows, highly desirable for some demanding biological applications. Although the synthesis of NIR QDs with a uniform and narrow size distribution is well known today, achieving QDs with high quantum efficiency and excellent stability is still a big challenge. QDs are tiny crystalline particles with typical dimensions in the range of 1-100 nm. Their surface-to-volume ratio is very large, therefore the properties of QDs become extremely sensitive to surface characteristics. In order to improve the optical properties of these QDs, great efforts have been made in the past few decades on the surface engineering of QDs. A passivation shell is normally grown over the QDs to form a core/shell structure, which in turn improves the optical properties and stability. In the first part of this thesis, the synthesis and optical properties of PbS/CdS core/shell QDs are presented and discussed. We firstly synthesized a series of differently sized PbS QDs by using the traditional hot injection method, then these PbS QDs were coated with a thin CdS shell to form a PbS/CdS core/shell structure by the cation exchange approach, finally the optical properties of these core/shell QDs were studied. In Section I of Part I, we report the development of a reproducible and controllable microwave-assisted cation exchange approach, for the first time, to quickly synthesize high-quality, NIR emitting PbS/CdS core/shell QDs. These monodisperse QDs, emitting in the range of 1300-1600 nm, show a quantum yield (QY) as high as 57% that is ~1.4 times higher than that of QDs achieved by conventional heating in oil bath. Meanwhile, the reaction was successfully scaled up by several times by increasing the starting PbS concentration or by amplifying the reaction volume and the as-synthesized core/shell QDs show similarly high QY. We then report anomalous size-dependent photoluminescence (PL) intensity variation of PbS QDs with the formation of a thin CdS shell via the same microwave-assisted cation exchange approach. We found previously that thin shell formation is an effective strategy for increasing the PL intensity of large sized PbS QDs. Nonetheless, herein we observed an unusual PL decrease in ultrasmall QDs upon shell formation. We attempted to understand this abnormal phenomenon from the perspective of trap density variation and the probability of electrons and holes reaching surface defects. To this end, QY and PL lifetime (on the ns-μs time scale) of pristine PbS QDs and PbS/CdS core/shell QDs were measured and radiative and non-radiative recombination rates were derived and compared. Moreover, transient absorption (TA) analysis (on the fs-ns time scale) was performed to better understand exciton dynamics on ultrafast time scales. These experimental results along with theoretical calculations of electron and hole wave functions provide a complete picture of the photophysics governing the core/shell system. Ultimately, a model was constructed to show the energy levels and trap states for various sizes. Part II focuses on the photostability and colloidal stability of water dispersible PbS/CdS/ZnS core/shell/shell QDs and their potential bio-applications. We report for the first time detailed investigations of the synthesis of NIR, water dispersible, strongly luminescent and highly stable PbS/CdS/ZnS core/shell/shell QDs, their properties in different buffers, their cytotoxicity and further their applications in tumor imaging. In particular, we focus on the QDs emitting at 930 and 1220 nm, within the first and second biological windows, respectively. These QDs were synthesized via our recently developed microwave-assisted approach to grow a ZnS shell and to simultaneously exchange initial ligand with mercaptopropyl acid on the PbS/CdS core/shell QDs dispersed in an organic phase. These QDs were extremely stable in commonly used biological buffers and remarkably, they could keep their initial morphology, dispersion status and PL in phosphate buffered saline buffer (PBS) for as long as 14 months, which was the longest time we investigated with both transmission electron microscopy and PL spectroscopy herein. PL images taken on the 930 nm emitting PbS/CdS/ZnS core/shell/shell QDs revealed that they could still emit strongly after 30-month storage in PBS. Such long term stability of water dispersible QDs is rarely reported in the literature. Their colloidal stability was further investigated by keeping them in high ionic concentration conditions. Their PL intensity did not show any change for at least 3 weeks at high NaCl concentration up to 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 interestingly, they showed negligible toxicity to cultured cells even at high QDs concentration (50 nM). Given these outstanding properties, the ultrastable and biocompatible QDs were explored for the first time for in vivo tumor imaging in mice. With one order of magnitude lower QD concentration (0.04 mg/mL), significantly weaker laser intensity (0.04 W/cm2 vs ~1 W/cm2) and considerably shorter signal integration time (≤ 1 ms vs several hundreds of ms) as compared to the best reported rare earth doped nanoparticles, the QDs showed high emission intensity even at injection depth of ~2.5 mm, hard to achieve with visible QDs and other NIR PL probes. We developed a QD-based imaging system based on NIR-emitting PbS/CdS/ZnS QDs with minimal noticeable toxicity. Through careful engineering of their emission wavelength, we obtained fluorescence imaging nanoprobes with optimal penetration depths in biological tissue. Additionally, this new platform exhibited multifunctionality beyond their use as pure imaging nanoprobes. The system is capable of acting as a biological nanothermometer, based on the reliable thermal-dependent behavior of the fluorescence signal. The PbS/CdS/ZnS QDs studied here can easily be exploited to obtain thermal mapping of subskin areas in live specimens, an accomplishment of great relevance for early disease detection and also for real-time therapy monitoring. Moreover, as a result of the intense signal provided by the NIR-emitting QDs, we are able to elucidate the real-time biodistribution of the QDs 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 as the lack of adverse health effects (no significant weight changes or behavior abnormalities were found over 4 weeks) for mice injected with a low concentration of QDs, coupled with the absence of any fluorescence signal detected in the body organs at the end of the experiment. We can therefore ascertain that the ZnS outer shell imparts a great deal of bio-compatibility and stability of the PbS/CdS/ZnS QDs reported here. Also, the intense fluorescent emission of this material allows for low doses to be used, significantly improving the current state of the art. This greatly diminishes the likelihood of causing adverse health effects on the live specimen when used as optical bioimaging probes.

Type de document: Thèse Thèse
Directeur de mémoire/thèse: Ma, Dongling
Co-directeurs de mémoire/thèse: Vetrone, Fiorenza
Mots-clés libres: énergie matériaux
Centre: Centre Énergie Matériaux Télécommunications
Date de dépôt: 09 avr. 2019 21:12
Dernière modification: 09 avr. 2019 21:12
URI: http://espace.inrs.ca/id/eprint/8024

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