Annealing of magnetic nanoparticles for their encapsulation into microcarriers guided by vascular magnetic resonance navigation

Iron, cobalt and iron–cobalt nanoparticle properties, such as diameter, saturation magnetization (Ms), crystal structure, surface composition and stability in physiological solutions, were investigated according to the annealing temperature used prior to their encapsulation into poly(d, l-lactic-co-glycolic acid) (PLGA) microcarriers. These new 60-μm microparticles should exhibit an Ms around 70 emu g−1 to be guided in real time from their intravascular injection site to a tumor with a magnetic resonance imaging scanner. The challenge in the preparation of the nanoparticles consisted in limiting Ms loss by oxidation and the release of metallic ions. It was found that when the annealing temperature reached 650 °C, Fe nanoparticles coalesced, the mean diameter reached (O) 361 ± 138 nm and Ms increased to 171 emu g−1. These nanoparticles exhibited a core of α-Fe and a shell of Fe3O4. On the opposite, Co nanoparticle properties were not affected by the annealing temperature: O and Ms were around 120 nm and 140 emu g−1, respectively. FeCo (60:40, atomic percent) nanoparticles coalesced at an annealing temperature >550 °C, O and Ms reached 217 nm and 213 emu g−1, respectively. Co and FeCo nanoparticles with a Co atomic proportion >15 % were coated with a graphite shell when the temperature was set to 550 °C. In physiological solution, Fe and Co nanoparticles significantly released more ions than FeCo nanoparticles. After the preparation steps prior to their encapsulation, the Ms of Fe and FeCo nanoparticles decreased by 25 and 3 %, respectively. FeCo–PLGA microparticles possessed a relatively high Ms (73 emu g−1) while that of Fe–PLGA microparticle (20 emu g−1) was too low for efficient targeting. The graphite shell was efficient to preserve Ms during the encapsulation.