Nanoparticle Conjugates and Materials through Manipulating Surface Chemistry

Chapter 1: Functionalization of nanocrystals is essential for their practical application, but synthesis on nanocrystal surfaces is limited by the available chemistries to introduce new functionality and create conjugated materials. Applications such as the development of imaging probes and hybrid materials require new methods for conjugating and interfacing nanoparticles with biological systems. The introductory chapter will present relevant background on the types of nanocrystals used in this work as well as the state and drawbacks of current nanocrystal functionalization techniques.Chapter 2: As a basis for surface modification, the development of polymer encapsulation techniques was investigated to enable phase transfer to aqueous dispersions and to display reactive functional groups at the interface. Use of this encapsulation procedure allows for reproducible and tunable surface composition for a variety of chemistries and further modifications. Modifications can be used to display different functional groups on the nanoparticle surface for conjugation or to modulate physical properties.Chapter 3: One surface conjugation strategy is the use of copper-catalyzed azide-alkyne cycloaddition (CuAAC). It is among the most popular methods for ligating molecules to surfaces, but as Cu(I) ions quickly and irreversibly quench semiconductor quantum dot (QD) fluorescence, development of this chemistry has been largely useless for QDs. A combinatorial fluorescence assay was developed to screen for non-quenching synthetic conditions for CuAAC on QD surfaces, and we identified conditions for complete coupling without significant quenching. Using these findings, I synthesized unquenched QD-peptidyl toxin conjugates and imaged their specific and voltage-dependent affinity for potassium channels in live cells.Chapter 4: The use of the split domain SpyTag/SpyCatcher system forms the basis for another facet of conjugate design that can incorporate biomolecules through a small engineered peptide tag. This peptide-protein pair is a genetically-encodable tool to incorporate specific, covalent interactions between the two components which can be included in separate materials. The reaction is traceless and agnostic to a wide range of reaction conditions, integrating a degree of modularity into our systems. With this linkage as a design rule, we generate stable biomolecule conjugates as probes for imaging and as the basis for higher order structures.Chapter 5: Self-assembly offers a scalable and reproducible bottom-up approach to fabricate patterned nanomaterials, but they have been limited in their ability to combine high conjugation yields with programmability. We employed protein engineering to modify the bacterial S-layer proteins SbsB and RsaA to create biomolecular scaffolds for the controlled deposition of multiple types of nanoparticles. Using the isopeptide bond-forming SpyCatcher conjugation, we have enabled dense coverage of a wide range of nanoparticles on the free-floating SbsB lattice including gold nanoparticles (Au NPs), quantum dots (QDs) and upconverting nanoparticles (UCNPs). Using orthogonal conjugation strategies, we created arrays with Au NP-QD pairings that conferred plasmonic enhancement of QD radiative decay. In addition to purified sheets, we show that SpyCatcher ligation of QDs to cell surface RsaA proteins produces crosslinking into an extended 3D cellular network. Confocal and atomic force microscopies demonstrate a dense and ordered layer of QDs on the cell surface, while mechanical analysis of the supracellular material demonstrates a >30-fold enhancement of storage modulus. The modularity inherent to the design tolerates changes to the nanocrystal composition and permits regeneration of the material after damage.Chapter 6: While the applications of nanoparticle conjugates are varied, they all stem from the central need for methods that extend the reach of synthetic chemistry on the nanocrystal surface. Development of these techniques allows the creation of hybrid materials of one or more nanocrystal types interacting with proteinaceous and cellular biomaterials. I recapitulate these findings to describe the current state of the art and the future of the nanocrystal chemistry field