Synthetic routes to the surface functionalization of core-shell nanoparticles through the copper(I)-catalyzed azide-alkyne cycloaddition

The objective of this dissertation was to construct core-shell colloidal particles with a thermoplastic shell containing alkyne units for CuAAC; establish a method to remove the thermoplastic shell for characterization; measure their optical and thermo-dynamic properties; and discover further applications with this particle morphology. Recent trends have shown a re-emergence of research centered around colloidal particles due to their usefulness in biological applications. Colloids around ca. 100 nm show good stability when suspended in aqueous solutions and are not quickly removed by the body. Thus, particles can be modified with fluorescent chromophores to act as biosensors or could be created to contain therapeutic drugs and act as a vehicle for drug delivery. The possibility of colloids being used as biological agents received a large boost with the classification of a series of reactions known as click chemistry; specifically the Copper(I)-catalyzed Azide-Alkyne Cycloaddition (CuAAC). CuAAC allowed for two materials to be covalently attached if one material contained an azide and the other contained a terminal alkyne. The attractiveness of CuAAC for polymer colloids is due to the high-yielding nature of the reation and that it could be carried out at mild conditions in aqueous solutions. This affords an endless variety of functionalities that could incorporated to the surface of polymer colloids with the only drawback being the inability to thoroughly characterize the success of CuAAC reactions on the colloidal surface. In this current project, ca. 130 nm core-shell particles with a thermoplastic shell are surface-functionalized with optically active moieties via CuAAC and used for the following applications: (1) Seeded emulsion polymerization of a soluble shell with a controlled alkyne surface density: A general methodology for producing ca. 100 nm core-shell colloidal particles in which the shell has an elevated alkyne functionality and yet remains thermoplastic is presented. The availability of accessible alkyne groups on the surface of the aqueous-phase particles allows for the in situ surface modification of the particles through a copper(I) catalyzed Huisgen 1,3-dipolar cycloaddition with an azide-terminated surface agent. The core is an extensively crosslinked polymer which can be easily removed by dispersing the particles in a solvent and centrifuging & collecting the cores, leaving the solubilized shells. This allows for the complete characterization of the colloidal surface reactions in the absence of the volumetrically dominant core. The technique is demonstrated with a core-shell colloid composed of a 135 nm crosslinked polystyrene (PS) core coated with a ca. 10 nm thick uncrosslinked poly(methyl acrylate-co-propargyl acrylate) shell. Due to the applicability of this technique for generating particles useful in biomedical imaging or drug delivery applications, the core-shell particles are surface modified with a variety of azide-terminated poly(ethylene glycol)(PEG) derivatives, including a poloxamer which was terminated on either end by an azide and a naphthalimide chromophore. The resulting fluorescent particles had an absorbance at 413 nm and peak emission at 525 nm. The PEG derivatives could be attached to the particles at a grafting density of ca. 0.2-0.3 groups/nm2. (2) Click functionalization of thin films fabricated by roll-to-roll printing of thermoplas-tic/thermoset core-shell colloids: A general methodology for producing ca. 140 nm thermoplas-tic/thermoset, core-shell colloids that are used as an ink in roll-to-roll printing is demonstrated. The printed films are subsequently modified through a dip-click approach using the copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC). The thermoplastic nature of the shell polymers in the par-ticles allows the shell to delaminate when annealed above its glass transition temperature. This results in a printed film that is more robust and functional as it combines the durability of the thermoset core colloids and the flexibility of the thermoplastic shell polymers. The technique was demonstrated using core-shell particles with a crosslinked polystyrene core and co-/terpolymer shell that contained terminal alkynes for click functionalization. The core-shell particles were roll-to-roll printed and then annealed at 100 oC to yield coalesced films. The printed films were dipped in a solution containing an azide-modified fluorescein dye which resulted in the covalent attachment of the dye to the thin films via CuAAC. When the click reaction was allowed to proceed for 24 hours, it was found that ca. 67% of the total functionalization occurred in the first hour. Due to the efficiency of this technique, the potential for large-scale production of printed films where an inline chemical modification via CuAAC could be realized. (3) Morphological dependent excimer emission of carbazole functionalized core-shell particles with a thermoplastic shell: A general methodology to dequench carbazole excimer emission on the surface of core-shell colloids is demonstrated. Carbazole is known to self-quench due to excimer for-mation when carbazole molecules are present in a density wherein intramolecular overlapping is avail-able. The mobility of the carbazole units and the proximity of neighboring carbazole units are key factors to determine whether the low energy transfers will occur. The technique was demonstrated using core-shell particles with a crosslinked poly(methyl methacrylate) core and a thermoplastic, copolymer shell that contained terminal alkynes for click functionalization via copper(I)-catalyzed azide-alkyne cycloaddition. The carbazole units were modified to contain an azide group in order to covalently attach to the surface of the core-shell particles. The functionalized particles displayed excimer formation even at low carbazole concentrations, due to the thermoplastic shell, which al-lowed enough freedom for neighboring carbazoles to overlap. The carbazole excimer emission was dequenched by removing the thermoplastic shell from the particles as this freed the carbazoles from the steric restrictions on the particle surface. These core-shell particles are an ideal match for bio-logical sensors where the dissolution of the polymer shell would be indicated by the dequenching of the chromophore as seen by photluminescence spectroscopy