We investigate the concentration and size dependent self-assembly of cadmium selenide nanoparticles at an oil/water interface. Using a pendant drop tensiometer, we monitor the assembly kinetics and evaluate the effective diffusion coefficients following changes in the interfacial tension for the early and late stages of nanoparticle adsorption. Comparison with the coefficients for free diffusion reveals the energy barrier for particle segregation to the interface. The formation of a nanoparticle monolayer at the oil/water interface is characterised by transmission electron microscopy.

Bionanoparticles, such as the cowpea mosaic virus, can stabilize oil droplets in aqueous solutions by self‐assembly at liquid interfaces. Subsequent cross‐linking of the bionanoparticles transforms the assemblies into robust membranes that have covalent inter‐bionanoparticle connections. The resulting membranes are nanoscopically thin sheets (see SANS image (SANS=small‐angle neutron scattering)), which were examined by fluorescent labeling.

Molecular and particle-based self-assembly is of importance for studies in the fabrication of uniformly patterned functional surfaces, novel membranes, and delivery vehicles.[1±6] For example, self-assembly of molecular monolayers can be accomplished readily on a variety of flat metal or metal oxide substrates, or at the air/water interface, to give so-called selfassembled monolayers (SAMs).[7±17] When such monolayer assemblies bear functionality, crosslinking chemistries can be used to covalently link the monolayer into a network; [10±13] alternatively, polymerization chemistry can be performed from the surface of appropriately functionalized substrates to impart new properties through such grafting chemistry.[14±16]Relative to flat substrates or interfaces, the spherical surface of a droplet offers four times the surface area, provides greater access to the surrounding environment, and offers a means for encapsulation within a droplet. Nanoparticle assemblies on droplets represent, in many respects, ideal platforms for tuning capsule-environment interactions, as the ligands associated with each nanoparticle provide rich opportunities for tailored interfacial interactions and chemistries.[17±21] In addition, the fluid environment of the assembly provides opportunities for chemical modification of the nanoparticles from either fluid phase. Moreover, the use of nanoparticles rather than micrometer-sized particles for oil/water interfacial assembly leads to a weak dynamic confinement of the particles to the interface, enabling new opportunities to generate complex interfacial patterns.[22]A unique aspect associated with the interfacial assembly of nanoparticles in fluids is the highly mobile nature of the assembly, which gives a rapid diffusion within the interface, and a fast equilibration of the assembly. We have previously shown that tri-n-octylphosphine oxide (TOPO)-covered CdSe nanoparticles segregate to the toluene/water interface to reduce interfacial tension and minimize the Helmholtz free energy of the system. [18] This segregation is strongly size dependent, as larger particles cause a greater reduction in the interfacial energy per particle, and thus exhibit longer residence times at the interface.[23] Our experiments with TOPO-covered CdSe quantum dots revealed the weak nature of this nanoparticle interfacial assembly (a few k b T for particles in the 3±4 nm size range), which becomes even less stable with increasing temperature (e.g., for effecting reactive chemistry in the ligands associated with the nanoparticles). Thus, methods are needed to convert these nanoparticle-based assemblies into nanoparticle-based materials with appreciable mechanical integrity. Our previous efforts towards such structures utilized crosslinking chemistry on functionalized ligands attached to the nanoparticles. For example, on flat oil/water interfacial assemblies, freeradical-induced crosslinking of vinylbenzene-functionalized CdSe nanoparticles was performed successfully to give ultrathin sheets of nanoparticles.[17] ...

The oil/water interfacial assembly of tobacco mosaic virus (TMV) has been studied in situ by tensiometry and small-angle X-ray and neutron scattering (SAXS and SANS). TMV showed different orientations at the perfluorodecalin/water interface, depending on the initial TMV concentration in the aqueous phase. At low TMV concentration, the rods oriented parallel to the interface, mediating the interfacial interactions at the greatest extent per particle. At high TMV concentrations, the rods were oriented normal to the interface, mediating the interfacial interactions and also neutralizing inter-rod electrostatic repulsion. We found that the inter-rod repulsive forces between TMVs dominated the in-plane packing, which was strongly affected by the ionic strength and the bulk solution but not by the pH in the range of pH = 6-8.

Amphiphilic, PEGylated gold nanoparticles, of approximately 2 nm average core diameter, were synthesized by reduction of hydrogen tetrachloroaurate in the presence of the ligand (1-mercaptoundec-11-yl)tetra(ethylene glycol). These PEGylated gold nanoparticles were found to assemble cleanly at the oil-water interface. This self-assembly process gave a microencapsulated oil phase, water as the continuous phase, and a monolayer of gold nanoparticles at the oil-water interface. The capsules could be cross-linked from the organic phase by reaction of the chain-end hydroxyl groups of the PEG ligands with suitable electrophiles such as terephthaloyl chloride.