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 [Formula: see text] nor transverse relaxation rate [Formula: see text] changes monotonously with the particle size [Formula: see text]. Therefore, size can affect the magnetism of magnetic nanoparticles, but it is not the only factor. Then, we defined the relaxation rates [Formula: see text] (i = 1 or 2) using the induced magnetization of magnetic nanoparticles, and found that the correlation relationship between [Formula: see text] relaxation rate and [Formula: see text] relaxation rate is slightly worse, with a correlation coefficient of [Formula: see text] = 0.8939, while the correlation relationship between [Formula: see text] relaxation rate and [Formula: see text] relaxation rate is very obvious, with a correlation coefficient of [Formula: see text] = 0.9983. The main reason is that [Formula: see text] relaxation rate is related to the magnetic field inhomogeneity, produced by magnetic nanoparticles; however [Formula: see text] 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 [Formula: see text] relaxation time, so it is not directly related to magnetic field inhomogeneity.