Colloidal Silicon Quantum Dots: From Preparation to the Modifications of Self-Assembled Monolayers for Bio-applications

Abstract

Quantum dots (QDs) are semiconductor nanocrystals with unique photophysical properties. Quantum dots have drawn broad research interests in the past three decades, because of their applications in optoelectronic devices, solar cells and fluorescent imaging agents in biomedicine. However, a major issue for the further development of this new class of materials is that many quantum dots are composed of heavy metal elements that are considered unsafe for biological purposes. Therefore, concerns over nanoparticle related toxicity have inspired the design of quantum dots made from materials with biological benign nature, such as crystalline silicon (Si).

The first challenge of working with nanocrystalline silicon quantum dots (SiQDs) is the limited methods available to prepare high quality, surface functionalized nanoparticles. Among the various methods available, colloidal synthesis is of broad interests, for the simple procedures used and solution-based approaches as needed in many applications. In this thesis, chapter three and chapter four describe two new approaches of coping with this challenge, using a one-step method based on thiol-ene chemistry, and a two-step process based on copper catalyzed azide-alkyne cycloaddition (CuAAC) reaction respectively.

The second challenge of applying solution synthesized SiQDs for bio-imaging is their blue photoluminescence that can be affected by biological background signals, as well as the low excitation wavelength that may induce damage to cellular structures. Most responses to this challenge have been focused on material preparation, but limited success has been achieved when solution syntheses are involved. In this thesis, chapter five presents a completely different strategy of resolving this issue by focusing on advanced microscopy. Specifically, fluorescence lifetime imaging microscopy (FLIM) is used to observe SiQDs in intracellular contexts, utilizing their long fluorescence lifetime in the context of one-photon FLIM, two-photon FLIM and energy transfer studies (FLIM-FRET).

Lastly, since surface modified colloidal SiQDs is still in its infancy of development, there are still limited studies showing their applications as biosensors. In chapter six, efforts toward the preparation of the first SiQDs protease sensor is described. This is based on Förster Resonance Energy Transfer (FRET) process involving SiQDs-dye construct, where SiQDs were used as the donor, and conjugated to an organic dye acceptor via an enzyme responsive peptide linker.