Magnetic nano- and micro-particles based on Gd-substituted magnetite with improved colloidal stability

A series of Fe3-xGdxO4 (x=0, 0.1, 0.2) nanoparticles with an average diameter of around 8 nm were prepared by the coprecipitation method and coated by citric acid (CA). The nanoparticles show superparamagnetic behavior at room temperature and transition to a blocked state, at a temperature from similar to 89 K to similar to 213 K, depending on Gd concentration. The saturation magnetization of Fe3-xGdxO4 tended to drop for samples with a higher content of Gd. High colloidal stability is mandatory in medical applications of magnetic nanoparticles, and here we demonstrate a new procedure for its improvement. A colloidal sample of Fe3O4@CA was mechanically milled, after which dynamic light scattering and zeta potential measurements were used to monitor the hydrodynamic size and colloidal stability of the acquired suspensions. After 90 min of milling, the average hydrodynamic diameter decreased by 40%, and size distribution changed from polymodal to monomodal, while the negative zeta potential increased from -30.5 mV to -52.8 mV. Additionally, Fe2.80Gd0.20O4@CA nanoparticles were embedded in human serum albumin to produce magnetic microspheres (MMS), which could be used as a drug delivery platform. FE-SEM images showed that magnetic nanoparticles form clusters within MMS.