It is well known that the dispersibility of ultrasmall iron oxide nanoparticles is significant for the application in biomedicine. The cost of the synthesis using a thermal decomposition method is high and the synthesized product has high cytotoxicity and poor water dispersibility. Up to now, to the best of our knowledge, although the synthesis of ultrasmall iron oxide nanoparticles by a hydrothermal method using vitamin C as a reducing agent has been widely studied, the water-phase synthesis at room temperature is still not reported. The researchers from the Ningbo Institute of Materials Technology & Engineering (NIMTE) led by Prof. Aiguo Wu and Dr. Leyong Zeng have explored the water-phase synthesis of ultrasmall monodisperse metal-iron oxide nanoparticles as T1-weighted MRI contrast agents.

Using an improved hydrolysis method of inorganic salts assisted with water-bath incubation, ultrasmall water-soluble metal-iron oxide nanoparticles (including Fe₃O₄, ZnFe₂O₄ and NiFe₂O₄ nanoparticles) were synthesized in aqueous solutions, which can be used as T₁-weighted contrast agents for magnetic resonance imaging (MRI). The morphology, structure, MRI relaxation properties and cytotoxicity of the metal-iron oxide nanoparticles were studied. The results show that the average sizes of nanoparticles are about 4 nm, 4 nm and 5 nm for Fe₃O₄, ZnFe₂O₄ and NiFe₂O₄ nanoparticles, respectively.

Moreover, the nanoparticles have good water dispersibility and low cytotoxicity. The MRI test show the strong T₁-weighted, but the weak T₂-weighted MRI performance of metal-iron oxide nanoparticles. The high T₁-weighted MRI performance can be attributed to the ultrasmall size of metal-iron oxide nanoparticles. Therefore, the metal-iron oxide nanoparticles with good water dispersibility and ultrasmall size can have potential applications as T₁-weighted contrast agent materials for MRI.

Fig. 1 TEM images of ZnFe₂O₄ (a) and NiFe₂O₄ (b) nanoparticles. Both ZnFe₂O₄ and NiFe₂O₄ nanoparticles are well dispersed, and the size is also uniform. The size of ZnFe2O4 nanoparticles is about 3-5 nm (the average size was 4 nm), and the size of NiFe2O4 nanoparticles is about 4-7 nm (the average size was 5 nm).

Fig. 2 Plots of 1/T₁ (a) and 1/T₂ (b) over concentrations of MFe₂O₄ (M = Fe, Zn, Ni) nanoparticles. Based on the T₁, T₂ values and the corresponding concentrations of MFe₂O₄ (M = Fe, Zn, Ni) nanoparticles, the longitudinal and transverse relaxivities (r₁ and r₂) of MFe₂O₄ (M=Fe, Zn, Ni) nanoparticles could be calculated, respectively. It can be found that the r₁ values of MFe₂O₄ (M = Fe, Zn, Ni) nanoparticles were 5.991, 7.928, and 6.850 mM^-1 s^-1, and the r₂ values were 15.534, 14.642, and 12.921 mM^-1 s^-1, respectively. The r₂/r₁ ratios of MFe₂O₄ (M = Fe, Zn, Ni) nanoparticles were 2.59, 1.85, and 1.89, respectively. The low r₂/r₁ ratio values indicated that the MFe₂O₄ (M = Fe, Zn, Ni) nanoparticles had strong T₁-weighted MRI relaxation properties, and could be good candidates as T₁-weighted MRI contrast agents.

Above research work has been published in Phys Chem Chem Phys 2012; 14: 2631-2636.

Group Leader Prof. Aiguo Wu (aiguo@nimte.ac.cn)
（Images by NIMTE）