Fe₃O₄-LiMo₃Se₃ Nanoparticle Clusters as\nSuperparamagnetic Nanocompasses

Abstract

A scaleable chemical approach to functional nanoscale analogues of the magnetic compasses in\nmagnetotactic bacteria is described. LiMo₃Se₃-Fe₃O₄ nanowire−nanoparticle composites were synthesized\nby a reaction of 3-iodopropionic acid treated LiMo₃Se₃ nanowire bundles with oleic acid-stabilized Fe₃O₄\nnanoparticles of 2.8, 5.3, and 12.5 nm size in tetrahydrofuran. Transmission electron micrographs show\nthat the composite consists of Fe₃O₄ nanoparticles attached to the surfaces of the 4−6 nm thick nanowire\nbundles. UV/vis spectra reveal absorptions from the nanowire (506 nm) and magnetite components\n(280−450 nm), and IR spectra show characteristic bands for the propionic acid linkers and for the residual\noleic acid ligands on the magnetite particles. In the presence of excess oleic acid, the nanocomposites\nundergo rapid disassembly, suggesting that Fe₃O₄ nanoparticles are bonded to nanowires via carboxylate\ngroups from the linkers. Ultrasonication of a dispersion of the composite in THF produces individual\nLiMo₃Se₃-Fe₃O₄ clusters, which are 340 ± 107 nm long and 20 ± 5 nm thick, depending on the sonication\ntime and Fe₃O₄ nanoparticle size. Field cooled and zero-field cooled magnetization measurements reveal\nthat the blocking temperature (<i>T</i>_B = 100 K) of the clusters with 5.3 nm Fe₃O₄ is increased as compared\nto the free nanoparticles (<i>T</i>_B = 30 K). Directional dipolar interactions in the clusters lead to magnetic\nanisotropy, which makes it possible to align the clusters in a magnetic field (900 Oe).