- Interfacial Chemistry
- Cross-linked Sheets
- Biological Nanoparticles
- Virus Assemblies
- Cow Pea Mosaic Virus, Horse Spleen Ferritin
- Yellow Turnip Virus, Tobacco Mosaic Virus
Liquid - Liquid Interfaces
The fabrication of functional nanostructured materials for sensing, encapsulation and delivery requires practical approaches to self-assembly on multiple length scales and the synthesis of tough yet permeable structures. Ligand-stabilized nanoparticles assembled into three-dimensional constructs at fluid-fluid interfaces driven by the reduction in interfacial energy were investigated. Studies on the dynamics of the nanoparticles and the self-assembled structures formed at the interface suggest a liquid-like behavior and ordering at the interfaces. Cross-linking of the nanoparticle assembly using functional ligands, affords robust membranes that maintain their integrity even when they are removed from the interface. These composite membranes, nanometers in thickness, are elastic and permeable. Combining other self-assembly processes on a different length scale, i.e. “breath figures”, with the self-assembly of nanoparticles at the oil-water interfaces lead to the formation of hierarchically structured nanoparticle arrays. The assembly of virus and other biological complexes at fluid interfaces was also investigated, where interfacial assembly rendered an easy route to direct and assemble the bioparticles into 2-D and 3-D constructs with hierarchical ordering. These assemblies provide for the potential use of the bioparticles as a natural supramolecular building block to obtain materials with well-defined bio-functionalities.
The organization of inorganic nanostructures within self-assembled organic or biological templates is receiving the attention of scientists interested in developing functional hybrid materials. Recent theoretical arguments have suggested that synergistic interactions between self-organizing particles and a self-assembling matrix material can lead to hierarchically ordered structures. Here we show that mixtures of diblock copolymers and either cadmium selenide- or ferritin-based nanoparticles exhibit cooperative, coupled self-assembly on the nanoscale. In thin films, the copolymers assemble into cylindrical domains, which dictate the spatial distribution of the nanoparticles; segregation of the particles to the interfaces mediates interfacial interactions and orients the copolymer domains normal to the surface, even when one of the blocks is strongly attracted to the substrate. Organization of both the polymeric and particulate entities is thus achieved without the use of external fields, opening a simple and general route for fabrication of nanostructured materials with hierarchical order.
Nanoparticles assemble at the interface between two fluids into disordered, liquid-like arrays, where the nanoparticles can diffuse laterally at the interface. Using nanoparticles dispersed in water and amine end-capped polymers in oil, nanoparticle surfactants are generated in situ at the interface, overcoming the inherent weak forces governing the interfacial adsorption of nanoparticles. When the shape of the liquid domain is deformed by an external field, the surface area increases and more nanoparticles adsorb to the interface. Upon releasing the field, the interfacial area decreases, jamming the nanoparticle surfactants and arresting further shape change. The jammed nanoparticles remain disordered and liquid-like, enabling multiple, consecutive deformation and jamming events. Further stabilization is realized by replacing monofunctional ligands with difunctional versions that cross-link the assemblies. The ability to generate and stabilize liquids with a prescribed shape poses opportunities for reactive liquid systems, packaging, delivery, and storage.
Efficient segregation of water-soluble acid-functionalized single-walled carbon nanotubes (SWCNTs) at the oil/water interface was induced by dissolving low-molecular-weight amine-terminated polystyrene (PS-NH2) in the oil phase. Salt-bridge interactions between carboxylic acid groups of SWCNTs and amine groups of PS drove assembly of SWCNTs at the interface, and were monitored by pendant drop tensiometry and laser scanning confocal microscopy. Impact of PS end-group functionality, PS and SWCNT concentrations, and degree of SWCNT acid modification on interfacial activity were assessed, and a sharp drop in interfacial tension was observed above a critical SWCNT concentration. Interfacial tensions were low enough to support stable oil/water emulsions. Further experiments, including potentiometric titrations and replacement of SWCNTs by other carboxyl-containing species, demonstrated that the interfacial tension drop reflects the loss of SWCNT charge as pH falls near/below the intrinsic carboxyl dissociation constant; species lacking multivalent carboxylic acid groups are inactive. The trapped SWCNTs appear neither ordered nor oriented.
Graphene Oxide Sheets
The interfacial assembly, and its kinetics, of graphene oxide (GO) at the water/oil interface was systematically studied. GO nanosheets were found to segregate to the water/oil interface, interact with quaternized block copolymer chains by the peripheral carboxyl groups on the GO. If the interfacial area is decreased, the GO, assembled at and confined to the interface, jams and then buckles. An analysis of the kinetics of the assembly processes leads to the conclusion that the diffusion of GO to the interface is the rate-determining step. The morphology of the jammed GO film was investigated, and TEM images show that GO sheets are tiled, or form a mosaic at the oil/water interface.