Magnetic properties, responsiveness, and stability of paramagnetic dumbbell and ellipsoid colloids.

A layer-by-layer method is used to synthesize paramagnetic metal/polymer multi-layered colloidal particles. By taking advantage of recent advances in the synthesis of polymer colloids with various shapes and layer-by-layer assembly, superparamagnetic dumbbell colloids are fabricated with different aspect ratios and asymmetries between lobes. We characterize the surface coverage of magnetic nanoparticles on the anisotropic host particles and the magnetic response of the particles by magnetometry. The synthesized particles are colloidally stable, but in a magnetic field, form persistent microstructures through induced interactions. These column-like microstructures are reversible and readily disperse by Brownian motion. The magnetic nanoparticle concentration during synthesis provides a means to control the magnetic response of the composite nanoparticle. The method is extended to prolate ellipsoid nanoparticles with aspect ratios between 3.4 and 4.2.