High-strength magnetically switchable plasmonic nanorods assembled from a binary nanocrystal mixture

Next-generation 'smart' nanoparticle systems should be precisely engineered in size, shape and composition to introduce multiple functionalities, unattainable from a single material(1-3). Bottom-up chemical methods are prized for the synthesis of crystalline nanoparticles, that is, nanocrystals, with size-and shape-dependent physical properties(4-6), but they are less successful in achieving multifunctionality(7-9). Top-down lithographic methods can produce multifunctional nanoparticles with precise size and shape control(2,3,10,11), yet this becomes increasingly difficult at sizes of similar to 10 nm. Here, we report the fabrication of multifunctional, smart nanoparticle systems by combining top-down fabrication and bottom-up self-assembly methods. Particularly, we template nanorods from a mixture of superparamagnetic Zn0.2Fe2.8O4 and plasmonic Au nanocrystals. The superpara-magnetism of Zn0.2Fe2.8O4 prevents these nanorods from spontaneous magnetic-dipole-induced aggregation, while their magnetic anisotropy makes them responsive to an external field. Ligand exchange drives Au nanocrystal fusion and forms a porous network, imparting the nanorods with high mechanical strength and polarization-dependent infrared surface plasmon resonances. The combined superparamagnetic and plasmonic functions enable switching of the infrared transmission of a hybrid nanorod suspension using an external magnetic field.