Simple magnet manipulates CO nanoparticles

Centimeter-long chains of surfactant-coated cobalt nanoparticles have been assembled and disassembled by using a weak magnet, the first reported method for reversible manipulation of magnetic nanoparticles in solution (Langmuir, published online Oct. 12, dx.doi.org/10.1021/la0506473). The ability to reliably control magnetic nanoparticle assemblies is potentially important for information storage devices and medical magnetic resonance imaging, according to Angela R. Hight Walker and coworkers at NIST, who carried out the work. Nanoparticles in colloidal suspensions typically assemble into ordered structures when deposited on a substrate and when the solvent is evaporated. The NIST team used a magnetic field of 0.05 tesla—the same strength as a refrigerator magnet—to assemble nanoparticle chains while still in solution. The 15-nm-diameter nanoparticles act like tiny bar magnets and individually align with the magnetic field, which incites dipole interactions to couple the particles together into chains. When the magnetic field is removed, the chains slump into 3-D coils that settle out of solution (shown). When the solution is agitated, the chains fall apart to re-form a colloidal suspension.