Date

2016

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Chemical Engineering

First Adviser

McIntosh, Steven

Other advisers/committee members

Kiely, Christopher J.; Caram, Hugo S.; Korendovych, Ivan V.

Abstract

This work provides a cell-based process using an engineered strain of Stenotrophomonas maltophilia (SMCD1) or cystathionine -lyase (smCSE) enzyme that produces high yields of extracellular, water-soluble semiconductor nanocrystals, also known as quantum dots (QDs) from low-cost precursors in aqueous media. SMCD1 enables controlled growth of cadmium sulfide (CdS) QDs over a period of several hours in culture, allowing precise, extrinsic control of QD size and optical properties with photoluminescence emission spanning the visible spectrum. The as-grown CdS QDs show both zinc-blende and wurtzite type crystal structures with a quantum yield of up to 2.08 %. Cadmium selenide (CdSe) QDs was also successfully synthesized by introducing smCSE enzyme instead of SMCD1 cells. This enzyme was produced and identified from the culture of SMCD1 cells and further overexpressed by engineered Escherichia coli. The optical properties of CdSe QDs were studied and particle size control was also achieved by varying the growth time. Further characterizations confirmed the formation of CdSe nanocrystals with mainly wurtzite type crystal structure. Post-treatments, such as core/shell growth, phase transfer, and photoenhancement were utilized to modify the surface chemistry of these nanocrystals to improve their properties for potential applications. Core/shell structures on both CdS and CdSe cores with different shell structure (ZnS for CdS, CdS for CdSe) were achieved. This significantly improved their optical properties, for example, core/shell CdSe/CdS exhibits a quantum yield up to 12 %. A facile phase transfer protocol was proposed to efficiently transfer the QDs from aqueous phase to organic phase. In addition, photoenhancement via UV illumination was also introduced for CdSe QDs and their photoluminescence was highly improved. Owing to the post-treatments, the qualities of these cell-based QDs are comparable to that from chemical synthesis routes. Our biosynthetic approach to cadmium chalcogenide QDs production provides a viable pathway to realize the promise of green biomanufacturing of these materials for optoelectronic, energy, medicine and other emerging technological applications.

Available for download on Sunday, October 28, 2018

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