Cobalt magnetic nanoparticles as theranostics: conceivable or forgettable?

Superparamagnetic nanoparticles, exposed to an external variable magnetic field, undergo rapid excitation/relaxation. So-called soft magnets, typically iron-based, rapidly and completely relax when the magnetic field returns to zero. Instead, cobalt-based (CoB) hard magnets retain residual magnetization, a characteristic related with the procedure for nanoparticles (NPs) production. Many researchers are still attracted by the potential of CoB NPs for theranostics as multifaced signal probes for imaging, microrobots, enhanced thermo/radiation therapy, and drug release. Since iron oxide NPs are the only magnetic NPs approved for human use, they are of reference for analyzing the potential of the disregarded CoB NPs. In vitro observed toxicity of CoB NPs, largely attributable to cobalt ions and other chemical species released by dissolution, excluded them from further investigations in humans. Nevertheless, experimental evidences documenting the in vivo toxicity of engineered CoB NPs remain very few. The surface functionalization adds newer properties and could improve the biocompatibility of NPs, critical for the clinical exploitation. In our opinion, it would be worth to further exploit the potential of finely tunable properties of CoB NPs in in vivo systems in order to establish a systematic database of properties and effects suitable for human application.