Superparamagnetism is a phenomenon only seen in nanocrystals. If a micro- or macro-material requires superparamagnetic properties, then the superparamagnetic nanoparticles need to be assembled into complex, hierarchical structures. However, existing methods have problems such as aggregation and clustering of the nanoparticles.

Sanghyuk Wooh, Chung-Ang University, Seoul, Republic of Korea, Héloïse Thérien-Aubin, Max Planck Institute for Polymer Research, Mainz, Germany, and colleagues have developed an evaporation-guided assembly method for producing and shaping superparamagnetic nanoparticles. The shaped superparamagnetic nanoparticles were obtained by drying ferrofluid droplets (a magnetic colloidal suspension) on an amphiphobic surface in the presence of an external magnetic field (pictured). The ferrofluid concentration and the magnetic field can be changed to produce superparamagnetic nanoparticles with different shapes, including barrel-like, cone-like, deflated-ball-like, and twin-tower-like shapes.

The researchers found that the nanoparticles kept their paramagnetic properties in these different shapes. It was found that other colloids can be added into the ferrofluid for the co-assembly of binary, anisotropic particles with superparamagnetic properties. This, in turn, could be used to build macro-structures with superparamagnetic properties.

• Shaping the Assembly of Superparamagnetic Nanoparticles,
  Minghan Hu, Hans-Jürgen Butt, Katharina Landfester, Markus B. Bannwarth, Sanghyuk Wooh, Héloïse Thérien-Aubin,
  ACS Nano 2019.
  https://doi.org/10.1021/acsnano.8b07783