Innovative hybrid composite nanomaterials

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Bilkent University
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Digital lighting and bio-imaging are two emerging crucial research fields. Nanotechnology stands in the center of these applications by providing nano-scale particles possessing large surface-to-volume ratios, high effciency, and low toxicity while allowing for functionalization, effcient quality lighting and improved biocompatible bio-imaging. Some of the frequently employed nanoparticles in optoelectronics and imaging are colloidal semiconductor quantum dots, colloidal conjugated polymer nanoparticles, and colloidal iron oxide nanoparticles, all of which we have studied using colloidal approaches to make hybrid composites for lighting and imaging in this thesis. Fluorescent inorganic nanoparticles of colloidal quantum dots (QDs) attract significant interest for many optoelectronic and biomedical applications. Although they possess numerous advantages including broad absorption band, high quantum yield, and narrow emission spectrum, there are serious concerns on their recycling due to their cadmium-based composition. Alternatively, relatively low toxic organic uorescent polymer nanoparticles or oligomer nanoparticles have stepped forward. However, their reduced emission effciency and stability in solid state is an important limitation for their use in wide-spread solid-state lighting applications. To address these problems, in the first part of this thesis, we proposed and demonstrated the design of new hybrid composite material systems of oligomer nanoparticles to be used in solid-state lighting. We first showed that the emission effciency and stability of the oligomer nanoparticles in solid state are significantly improved based on our proposed crystallization technique. Here, using this simple and low-cost approach, oligomer nanoparticle monoliths were obtained from the powders of these crystals. Despite the disadvantages of using QDs, their high quantum effciency and narrow-band emission still make them a valuable asset for solid-state lighting. However, the decrease in solid-film effciencies is still an important issue to be addressed. With this perspective, in this thesis we utilized the incorporation of QDs into crystalline matrices allowing for the nonradiative energy transfer (NRET) to improve the emission capability of the nano-emitters. Since it is an interesting crystalline semiconductor organic molecule, we employed anthracene as the host donor medium and incorporated the quantum dots being exciton acceptors. Here, we systematically investigated the NRET from each anthracene emission peak to QDs and demonstrated the use of this composite system on LEDs as color converters and the polarization ratio change of quantum dots within this crystal system. Magnetic resonance imaging (MRI), for which we also developed colloidal contrast agents using nanoparticles (NPs) as the second part of this thesis, is a powerful diagnostic tool providing good soft tissue contrast and high spatial resolution. It produces T1- and T2-weighted images, in which the region of interest is observed as brighter and darker contrast, respectively. Superparamagnetic iron oxide (IO) NPs are an important member of T2-weighted contrast agents possessing low toxicity. However, they suer from poor anatomic details due to their darker contrast. Therefore, combining T1- and T2-weighted features in a single IO NP (dual-modal contrast) is a major step for improving MRI contrast. In order to meet the requirement for dual-modal contrast agents, which possess both T1- and T2-weighted imaging capability, in this thesis we synthesized highly monodisperse superparamagnetic cubic IO NPs. Magnetic characterizations along with in vivo MRI experiments demonstrated that these nanoparticles hold great promise for dual-modal imaging. This increased dual-modal eect without paramagnetic material doping or decreasing the size of nanoparticles smaller than 5 nm directed us to understand the relation of the T1 and T2 relaxations depending on the IO NP size and shape. Here, we showed the presence of intrinsic paramagnetic phase in magnetite IO NPs. Moreover, we demonstrated that this contribution is higher in IO NPs possessing cubic shape compared to the spherical counterparts, which explains the increased dual-modal effect in the monodisperse superparamagnetic nanocubes.

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Oligomer nanoparticles, Quantum dots, Light-emitting diodes (LEDs), Nonradiative energy transfer, Superparamagnetic iron oxide nanoparticles (SPIONs), Magnetic resonance imaging (MRI)
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