Unraveling the synthesis and assembly of gold-iron oxide hybrid nanoparticles

Assembling magnetic particles into an ordered superlattice is an attractive and cost-effective method to create magnetic responsive materials. Introducing noble metals into the magnetic iron-based nanoparticles (NPs) can further tailor the material properties. Such heterogeneous assemblies exhibit distinctive and tunable optical and magnetic properties. However, the growth and assembly of iron-based hybrid nanomaterials involve intricate trade-offs among various complex forces during wet chemical reactions. Consequently, their dynamic assembly mechanism remains unclear, making it challenging to achieve controllable heterogeneous assembly and self-assembly induced by intrinsic magnetic interactions. Here, we observed the heterogeneous assembly between gold (Au) and iron oxide (FeOx) domains, as well as the self-assembly of the Au-FeOx hetero-NPs into superlattices. The heterogeneous assembly between the FeOx component and Au seed NPs enables the control of hetero-NPs, encompassing core-shell, flower-shaped, to dimeric morphologies. Moreover, pure FeOx NPs and Au-FeOx hetero-NPs showed spontaneous self-assembly into hexagonal close-packed superstructures in the SAXS pattern, occurring after reaching 7.5 and 9.1 nm, respectively. These self-assembly processes are dominated by the magnetic dipolar interactions related to the critical size of the FeOx domains. This work sheds light on developing the next generation of magnetic-functional materials with hierarchical assembly.