It is essential to tune the physicochemical properties of magnetic nanomaterials for their biomedical applications. Herein we report a facile polyol method to synthesize the carboxyl-functionalized magnetite nanocrystal cluster (CMNC) with tunable size and high magnetization. Through introduction of urea as a homogeneous precipitator, the formation of magnetite nanocrystal clusters is facilitated compared to the previous reports. These magnetic clusters, with uniform size and good colloidal stability in water, have a magnetization over 70 emu g−1. The effect of the stabilizer (sodium citrate) on the physicochemical properties of clusters is studied systematically in the polyol process. By simply changing the concentration of sodium citrate in the reaction from 0.001 M to 0.1 M, the diameter of the CMNC can be tuned from approximately 300 nm to 40 nm. The variation of the concentration of sodium citrate also gave rise to a change of the size of the subunits, the surface charge and the magnetization of the clusters due to the adsorption of the citrate molecules on the magnetite nanocrystal clusters. The tuning effect of sodium citrate on the size of CMNCs was attributed to the interaction of different forces in the polyol process: the electrostatic repulsion and the surface tension.
