Synthesis and Biological Applications of Heavy-Metal-Free Semiconductor Nanocrystals

Semiconductor nanocrystals, also called quantum dots (QDs), are an interesting class of materials exhibiting size-tunable optical properties. QDs are attractive for a variety of applications, such as biological sensing and imaging, high color definition display technologies, and photovoltaics. The most widely studied QDs are compound semiconductors of the type CdX and PbX (with X= S, Se, and Te). The absorption and fluorescence emission wavelengths of these QDs span the visible and near infrared (NIR) regions of the electromagnetic spectrum. However, the highly toxic heavy metals Cd and Pb contained in these materials are problematic for their widespread use in commercial applications. Substitution of the heavy metals with Zn and Sn can yield QDs that are less toxic and more environmentally friendly. ZnSe QDs are highly luminescent in the UV-blue region of the spectrum and can be engineered to emit at longer wavelengths by doping them with transition metals, such as Mn or Cu. Encapsulation of ZnSe QDs with an inorganic shell, such as ZnS, has been shown to increase their stability and fluorescence intensity (quantum yield). However, the thermodynamic stability of such core/shell particles must be studied to understand the feasibility and long-term stability of atomically-abrupt interfaces between the core and the shell. The thermodynamic stability of ZnSe/ZnTe and ZnTe/ZnSe core/shell QDs was explored in this study. It was found that ZnSe/ZnTe core/shell QDs are thermodynamically more stable than ZnTe/ZnSe core/shell QDs. Functionalization of the surface of the QDs with biomolecules enables their use in biological sensing and imaging applications. A ZnSe-based QD-DNA biosensor was fabricated and characterized using a novel portable time-domain LED fluorimeter that enables nanosecond fluorescence lifetime measurements. SnSe QDs that absorb near-infrared (NIR) radiation are attractive nano-materials for applications in photovoltaics, photodetectors and photothermal therapy. A new synthesis method for small-size (\u3c 4nm) SnSe QDs was developed that employs an air-stable tin(II) chloride-oleylamine complex and selenium powder dissolved in trioctylphosphine as the precursors. The growth rate and morphology of the nanocrystals were studied as functions of the processing conditions. Optimal synthesis conditions that allow precise control over the final particle size and prevent particle aggregation were identified. The SnSe QDs were coated with a ZnSe shell, capped with 11-mercaptoundecanoic acid and dispersed in aqueous solution to enable bioconjugation with amino-modified biomolecules for biological applications. To meet the increasing demand for QDs, the development of new, highly-efficient processes for their synthesis is required. A continuous flow reactor was developed that enables efficient synthesis of ZnSe QDs using microemulsions as templates for controlling both the size and size distribution of the particles. The operating conditions of the reactor were optimized to maximize particle quality and conversion of precursors