A series of magnetic iron oxide nanoparticle clusters and porous nanoparticles with high magnetization are synthesized using the polyols with different reductive abilities. To investigate the morphology evolution in a polyol process, HRTEM is used to study the products. When ethylene glycol is used as the solvent, iron oxide nanoparticles form and then self-assemble into compact nanoparticle clusters (ca. 300 nm) through oriented attachment growth with a few misarranged nanoparticles in the clusters. At last these clusters develop into flower-like clusters with single crystal-like diffraction. When 1,2-propylene glycol is employed as the solvent, iron oxide nanoparticles quickly self-assemble into nanoparticle clusters (ca. 50 nm) with many misarrangements. As the reaction time extends, these clusters collapse and split into porous nanoparticles. According to the morphology evolution of iron oxide nanoparticles in different polyols, we propose our reasonable supposition on the whole process. These findings may help to understand the role of the polyol as solvent and obtain the novel secondary structures of nanoparticles in a polyol process.

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.