Non-interacting magnetic nanometal particles with diameters of ≤5 nm have been predicted to exhibit superparamagnetism, but they are very difficult to synthesize because such small nanometals are unstable under ambient conditions and are rapidly oxidized into respective oxide phases. The challenge of synthesizing air-stable nanometals such as iron, cobalt, and nickel has been recently addressed in our study published in Nature Communications; they were stabilized by trapping between the nanosheets of thermally reduced graphene oxide (TRGO). Such zero-valent nanoparticles with a particle sizes from 1 to 6 nm exhibit peculiar morphologies which can be controlled via metal concentration. In particular, the work shows that hybrids of iron nanoparticles and TRGO with 10 wt.% of iron are superparamagnetic at room temperature and retain superparamagnetism even at 5 K. The corrected saturation magnetization of 185 Am2 kg–1 is among the highest values reported for iron-based superparamagnets. A PEG-functionalized Fe/TRGO hybrid exhibited enhanced colloidal stability and in vitro biocompatibility, and proved to be a promising powerful T2 negative contrast agent for MRI, with contrast properties comparable to or better than most commercially used MRI agents once its in vivo bio-related features are assessed in details. It is believed that TRGO trapping is a generally applicable method for the synthesis of nanometals and alloys (e.g., FePt nanoparticles) with particle sizes and morphologies that can be controlled by careful selection of metal precursors that are used and their concentrations. This could then facilitate development of new nanotechnologies that exploit entrapped superparamagnetic nanometals for MRI and targeted drug delivery or to create energy materials (spin batteries) and sensors. Moreover, this method provides a space to expand the diverse applications of TRGO by enabling the preparation of TRGO derivatives that have clean surfaces for further functionalization while still enabling magnetic separation and reusability.
Tucek, J.; Sofer, Z.; Bousa, D.; Pumera, M.; Hola, K.; Mala, A.; Polakova, K.; Havrdova, M.; Cepe, K.; Tomanec, O.; Zboril, R. Air-Stable Superparamagnetic Metal Nanoparticles Entrapped in Graphene Oxide Matrix. Nat. Commun. 2016, 7, 12879. DOI: 10.1038/ncomms12879