Characterization and Relaxation Properties of a Series of Monodispersed Magnetic Nanoparticles

Magnetic iron oxide nanoparticles are relatively advanced nanomaterials, and are widely used in biology, physics and medicine, especially as contrast agents for magnetic resonance imaging. Characterization of the properties of magnetic nanoparticles plays an important role in the application of magnetic particles. As a contrast agent, the relaxation rate directly affects image enhancement. We characterized a series of monodispersed magnetic nanoparticles using different methods and measured their relaxation rates using a 0.47 T low-field Nuclear Magnetic Resonance instrument. Generally speaking, the properties of magnetic nanoparticles are closely related to their particle sizes; however, neither longitudinal relaxation rate r(1) nor transverse relaxation rate r(2) changes monotonously with the particle size d. Therefore, size can affect the magnetism of magnetic nanoparticles, but it is not the only factor. Then, we defined the relaxation rates r'(i) (i = 1 or 2) using the induced magnetization of magnetic nanoparticles, and found that the correlation relationship between r'(1) relaxation rate and r1 relaxation rate is slightly worse, with a correlation coefficient of R-2 = 0.8939, while the correlation relationship between r'(2) relaxation rate and r(2) relaxation rate is very obvious, with a correlation coefficient of R-2 = 0.9983. The main reason is that r(2) relaxation rate is related to the magnetic field inhomogeneity, produced by magnetic nanoparticles; however r(1) relaxation rate is mainly a result of the direct interaction of hydrogen nucleus in water molecules and the metal ions in magnetic nanoparticles to shorten the T-1 relaxation time, so it is not directly related to magnetic field inhomogeneity.