Binary assembly of colloidal gold nanorods and octahedral nickel ferrite nanocrystals driven by depletion force

Binary assembly of nanoparticles with different shapes, sizes, and materials yields novel structures with geometric diversity. Through meticulous control of the assembly structure, it is possible to engineer specific types of inter-building block interactions, such as plasmonic, electronic, and magnetic coupling, which are crucial for tailoring the material's overall functionality. However, achieving control over the structural symmetry of these assemblies and exploring the potential regulatory mechanisms, including the orientational alignment of non-spherical nanoparticles, remains a challenge. Here, gold nanorods (GNRs, length ≈ 62 nm, diameter ≈ 17 nm) and octahedral nickel ferrite nanocrystals (∼17 nm) were used as building blocks, by regulating the weak interactions between the nanoparticles, the regulatory mechanism of the binary assemblies was explored. High concentration nickel ferrites modified with polyethylene glycol (PEG) tended to contact substrate in a plane-to-substrate manner to ensure structural stability, and further formed planar stripe-shaped structures. Droplet evaporation assembly experiments showed that with PEG serving as the depletant, the combined effects of depletion and entropy patchiness led to end-to-end assembly of GNRs at the periphery of the stripes, instead of a more stable side-by-side arrangement. Furthermore, through the interfacial assembly method, the anchoring effects of GNRs at the edge of nickel ferrites were further mediated. Comparative experiments on GNR assembly with and without depletants showed that incorporating oleic acid and dodecanethiol enhanced the depletion and van der Waals attraction between oil-soluble GNRs, resulting in side-by-side assembly at the periphery of nickel ferrite superlattices. This work has demonstrated a strategy for effectively controlling multi-component assembly, promoting the development of complex nanoscale superstructures with potential applications in optical/plasmonic devices, bimetallic heterojunctions, and heterogeneous catalysts.