On the formation of superparamagnetic iron oxide nanoclusters.

Even though clusters consisting of superparamagnetic iron oxide nanoparticles (SPION's) have been widely studied, means for rational tuning of important parameters such as crystallite size and aggregate packing density are rarely discussed. Both the crystallite size and the nanocluster size are important parameters for many applications involving SPION's, and the presented synthesis strategy allows for optimization of the SPION nanoclusters for a given application. Here we show that these parameters can be partially independently tuned through controlled water addition to a synthesis of cationically stabilized, monodisperse nanoclusters of SPION's in the presence of polyethylene imine (PEI). Larger crystallites form with increasing water content, which increases the contribution of attractive van der Waals interactions. The crystallize size within already formed nanoclusters can be further increased through increasing the synthesis time. On the other hand, larger nanoclusters with comparable crystallite sizes form in the absence of PEI. Based on DLS, XRD, TEM and SAXS measurements of the synthesized nanoclusters, a mechanism of formation is proposed that highlight the importance of the crystallite size of the primary particles before cluster formation as a key parameter for achieving well-defined, spherical nanoclusters, and that the critical crystallite size is dependent on the magnitude of electrostatic repulsion for a given system.