A facile solvothermal approach was used to synthesize ultrasmall monodisperse magnetite (Fe3O4) nanoparticles with precise size control of one nanometre, in which iron acetylacetonate [Fe(acac)3] acted as the iron source, n-octylamine as the reductant, and n-octanol as the solvent. By varying the volume ratios of n-octylamine to n-octanol, 4 nm, 5 nm and 6 nm Fe3O4 nanoparticles were obtained. Compared with the popular thermal decomposition methods to Fe3O4 nanoparticles, this solvothermal process, without the need for a N2 atmosphere and reflux conditions, was more convenient. The intrinsic crystallography of the obtained magnetite nanoparticles was characterized by XRD and HRTEM, the particle size, shape, superlattice and self-assembly were observed by TEM, and the surface of nanoparticles was studied by FT-IR and XPS. The magnetic properties of the different particle sizes were measured at room temperature. Furthermore, the relaxation efficiency of three samples as high-performance magnetic resonance imaging contrast agents depending on the particle size was investigated in cetyltrimethyl ammonium bromide (CTAB) aqueous solution.
