Colloidal Dumbbells Form Chiral Chains

Silica nanoparticles joined together by iron oxide line up in a magnetic field to make chiral colloidal helices, reports a group led by Jérôme Bibette at the Paris-based Industrial Physics & Chemistry Higher Educational Institution (Nature 2008, 455, 380). The researchers used iron oxide to cement together pairs of 1-μm-diameter silica nanoparticles to make dumbbell shapes. When symmetric dumbbells were prepared from silica particles of similar size, they assembled in a magnetic field to form a chain in which the dumbbells aligned perpendicular to each other, with the iron oxide running down the center of the chain. When silica particles of different sizes were used to make asymmetric dumbbells, the aligned dumbbells formed a helical structure. Using the spheres as building blocks for macromolecules could provide a way to study steric interactions without confounding factors such as hydrogen bonding or electrostatic forces, the researchers say. The helices could also interact with circularly polarized light to rotate, potentially functioning as a motor to turn electromagnetic radiation into mechanical work.