Hierarchical materials synthesis at soft all-aqueous interfaces

Fluid–fluid interfaces are known to function as templates for nanomaterial precipitation and subsequent assembly, both in biological and technological contexts. Here we present a method for hierarchical magnetic (iron oxide) materials synthesis at soft, all-aqueous liquid interfaces that act as templates for nanoparticle precipitation and macromolecule-assisted assembly. The method simply involves fusion of two reactant-loaded aqueous drops, with one drop containing a macromolecular polyelectrolyte. We elucidate the role played by macromolecule–metal complexes in presenting a soft interfacial ‘scaffold’ for controlled hierarchical materials synthesis. We also demonstrate the continuous fabrication of such hierarchically structured particles in microfluidic devices, and discuss the general applicability of this method.