No abstract

Surface self-assembly of spherical nanoparticles of sizes below 10 nm into hierarchical heterostructures is under arising development despite the inherent difficulties of obtaining complex ordering patterns on a larger scale. Due to template-mediated interactions between oil-dispersible superparamagnetic nanoparticles (MNPs) and polyethylenimine-stabilized gold nanoparticles (Au(PEI)NPs) at the water−oil interface of microemulsions, complex nanostructured films can be formed. Characterization of the reverse microemulsion phase by UV−vis absorption revealed the formation of heteroclusters from Winsor type II phases (WPII) using Aerosol-OT (AOT) as the surfactant. SAXS measurements verify the mechanism of initial nanoparticle clustering in defined dimensions. XPS suggested an influence of AOT at the MNP surface. Further, cryo-SEM and TEM visualization demonstrated the elongation of the reverse microemulsions into cylindrical, wormlike structures, which subsequently build up larger nanoparticle superstructure arrangements. Such WPII phases are thus proven to be a new form of soft template, mediating the self-assembly of different nanoparticles in hierarchical network-like filaments over a substrate during solvent evaporation.

Nanoparticle interactions can mediate assemblies and self-organization into colloidal superstructures, if then control of the size and stabilization is highly required towards the fabrication of functional nanomaterials. In the proposed approach, different types of water-dispersible and oil-dispersible nanoparticles were incorporated in water-in oil (w/o) microemulsions stabilized by Aerosol-OT (AOT) as surfactant. Phase separation at higher surfactant and water contents resulted in the formation of a heterocoagulate, which can be separated with an external magnet. This magnetic heterocoagulate, which can contain 3 different types of nano-particles, can be dispersed in longer-chain carboxylic acids. An additional phase-transfer into oil-in water (o/w) dispersions was mediated by an auxiliary surfactant in waterethanol. The resulting nanostructures from heterocoagulates formed at higher temperatures consisted on inner domains of water-dispersible nanoparticles embedded in a matrix-shell of oleyl-capped nanoparticles. Thus, a new encapsulation strategy is achieved under relative mild conditions, combining three types of nanoparticles and providing compartmentalization and magnetic-responsiveness.