Fabrication of magnetic clusters and rods using electrostatic co-assembly

Using a novel protocol for mixing oppositely charged colloids and macromolecules, magnetic clusters and rods are fabricated using 10 nm-iron oxide nanoparticles and polymers. Here, we show that as the dispersions undergo the so-called desalting transition, spherical clusters in the range 100 nm - 1 {\mu}m form spontaneously upon dialysis or dilution. With a magnetic field applied during the dialysis, a one-dimensional growth of the aggregates is initiated, resulting in the formation of 1 - 100 {\mu}m rods of average diameter 200 nm. In this paper, we demonstrate that the nanostructured rods have inherited the properties of the iron oxide particles, namely to be superparamagnetic. We also discuss the dependence of the magnetic properties as a function of the nanoparticle diameter.