Preparation Of Magnetic Fe₃O₄ Nanoparticles And The Study Of Their Relaxation Properties

Magnetic Fe₃O₄ nanoparticles have been widely used as magnetofection, protein purification, magnetic resonance imaging (MRI) and other biomedical researches owing to their unique physical and chemical properties. Among these applications, MRI, using magnetic Fe₃O₄ nanoparticles as contrast agent, arouses increasing attention. The efficiency of magnetic Fe₃O₄ nanoparticles as contrast agent is quantified by the relaxation rate of the suspension, which is closely related to the particle size. It is of great significance, therefore, to have a controlled preparation of nanoparticles on the application of magnetic resonance imaging.Magnetic Fe₃O₄ nanoparticles were synthesized by coprecipitation, modified by citric acid, and then suspended stably in water. Based on a careful characterization, we investigated the influence of citric acid amount added in the reaction on the particle size. It was demonstrated that the size and saturation magnetization of Fe₃O₄ nanoparticles decreased with the increase of citric acid, which led to an increase of the surface coating of citric acid.We assumed a nanoparticle volume distribution with the log-normal volume distribution, and nanoparticle was a core-shell model. Theμ0M-μ0H curve was fitted based on Langevin function. The size of nanoparticles was calculated according to the parameters. The result suggested that the assumed log-normal volume distribution correctly reflected the reality and nano-particles was a core-shell structure.The influence of iron concentration and measurement waiting time on the relaxation properties of Fe₃O₄ had been studied. It had been found that both longitudinal relaxation time and transverse relaxation time were reduced with the increase of the iron concentration and did not have a dependence on waiting time. As the size of nanoparticles decreased, longitudinal relaxation rate and transverse relaxation rate were reduced, which showed that citric acid was an effective modifying small molecule for iron oxide nanoparticles.Finally, the relaxation properties of four magnetic nanoparticles were measured. The impact of particle size, surface modification layer thickness and material structure on relaxation properties had been studied. The result showed that when the surface modification were the same, the relaxation time decreased with the increase of the particle size; when the particle size were the same, the relaxation rate decreases as surface modification layer turned thicker; relaxation rate of mesoporous nanoparticles are bigger when surface modification layer thickness and nanoparticle size were under the same conditions.