3.2. Synthesis of the SPIONs by Co-Precipitation

SPIONs were synthesized by co-precipitation, which is the most widely applied synthesis technique [30]. Among all the parameter that can influence the properties of the particles, the ratio of iron salts has been shown to vary their sizes [49]. With this purpose, a 100 mL solution with a molar ratio of iron salts Fe^II/Fe^III of 0.5 were prepared, containing 0.01 M of HCl to avoid further oxidation of the Fe^II. Then, 30 mL of ammonia solution (30% wt) was added and a black precipitated instantly appeared. The solution was left to rest for 2 h and then washed with distilled water several times assisted by a permanent magnet.