Design and synthesis of biocompatible fluorescent semi-conductor nanocrystals for in-vivo and in-vitro imaging/sensing applications

Abstract

Quantum dots (QD) are unique materials in which their optical properties are decoupled from their solution properties via the tunability of surface ligands. The primary focus of this thesis is the design and synthesis of new ligand coatings to render QDs water soluble, pushing the boundaries of QD applications in biology both in-vivo and in-vitro. On the in-vivo front, ultrasmall QDs (-5 nm hydrodynamic diameter) were synthesized via the use of Cysteine as a zwitterionic ligand coating to generate the smallest biocompatible QDs known to date, allowing for the first time collection of quantitative in-vivo renal clearance data of inorganic nanoparticles in a mouse as a model for design of future clearable nanoparticle in-vivo probes and drug delivery vehicles. On the in-vitro front, a suite of multifunctional ligands were synthesized to produce QDs that exhibit low non-specific binding to cells, small hydrodynamic diameter (HD), tunable surface charge, high quantum yield, and good solution stability across a wide pH range. These ligands feature dihydrolipoic acid for tight binding to the QD surface, a short poly(ethylene glycol) (PEG) spacer for water solubility and biocompatibility, and an amine or carboxylate terminus for covalent derivatization. We successfully demonstrated covalent attachment of energy acceptor dyes to enable sensing applications via Forster Resonance Energy Transfer (FRET), and attachment of proteins to enable high-affinity cell labeling and single particle tracking. In addition, QDs solubilized with these ligands could be derivatized via metal-affinity driven conjugation chemistry with polyhistidine-tagged proteins, which facilitated the purification of monovalent QDs for the first time via gel electrophoresis. Further improvement on ligand stability focused on addressing the problem of thiol oxidation, and a new class of multifunctional polymer ligands were developed featuring multiple imidazole moieties for multidentate interactions with the QD surface. The polymers are synthesized via reversible addition-fragmentation chain transfer (RAFT)-mediated polymerization to produce molecular weight controlled monodisperse random copolymers from three types of monomers that feature imidazole groups for QD binding, polyethylene glycol (PEG) groups for water solubilization, and either primary amines or biotin groups for derivatization.