Assembly, cross-linking and encapsulation using functionalized nanoparticles at liquid interfaces
The assembly of nanoparticles at the interface of immiscible fluids holds promise for the preparation of new materials that benefit from both the physical properties of the nanoparticles and the chemistry associated with the ligands. Shaking nanoparticle solutions in organic solvents with water, results in the formation of nanoparticle-coated droplets that range in size from 10 μm to 200 μm. A strategy to control the size of these emulsions is described, by passing the droplets through commercial track-etch membranes with known pore sizes. Extrusion reduces the droplet size by breaking the droplets while passing theough the membrane pores, and reforming in the presence of excess nanoparticles in solution to form droplets as small as 1-5 μm.
Crosslinking of nanoparticles at a liquid interface lends greater stability to the interfacial assembly, leading to ultrathin nanoparticle-based capsules, which possess mechanical integrity even after removal of the interface. Two approaches towards crosslinking are used in this thesis. Norbornene-functionalized CdSe/ZnS are used to afford facile capsule visualization by fluorescence confocal microscopy, as well as ease of crosslinking in mild conditions by means of ring-opening metathesis polymerization (ROMP). The crosslinked capsules can be used to encapsulate materials, and display size-selective retention capability, governed by the interstitial spaces between the nanoparticles. In a second approach to making hybrid capsules and sheets, horse spleen ferritin bionanoparticles and aldehyde-functionalized CdSe quantum dots are co-assembled at an oil-water interface. The cross-linked materials formed by reaction of the aldehyde functionality on the quantum dots with the surface-available amines on the ferritin bionanoparticles can be disrupted by addition of acid, thus leading to pH-degradable capsules and sheets.
The driving force for assembly of nanoparticles at liquid interfaces is the reduction of the interfacial energy between the two liquids. The factors governing the amount of interfacial stabilization provided by the nanoparticles, namely the size and ligand coverage of the nanoparticles, are examined using the example of mixed assemblies of two different types of nanoparticles. Assemblies of 10 nm cobalt nanoparticles are disrupted upon the addition of 2.5 nm CdSe nanoparticles. The studies in this thesis demonstrate that the lower density of ligand coverage on CdSe quantum dots can overcome the large difference in size between the two nanoparticles, thus displacing the cobalt nanoparticles from the interface.
Finally, preliminary results using amphiphilic graft copolymers instead of nanoparticles for interfacial stabilization of liquids are discussed. The resulting capsules are used for encapsulation and release of nanoparticles. In a technique termed repair-and-go, these nanoparticle-filled capsules are used for repairing cracked surfaces by passing the capsules over hydrophilic substrates containing hydrophobic cracks.
0794: Materials science
0795: Materials science