Characterization and Relaxation Properties of a Series of Monodispersed Magnetic Nanoparticles

Abstract

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 r 1 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.

Keywords: Langevin model; contrast agent; magnetic field inhomogeneity; magnetic nanoparticles; relaxation; relaxation rate.

Figure 1

TEM images of SHP series magnetic nanoparticles samples. (a) SHP-05; (b) SHP-10; (c) SHP-15; (d) SHP-20; (e) SHP-25; (f) SHP-30.

Figure 2

Hydrodynamic size distribution of SHP series magnetic nanoparticles. The discrete points are the measured hydrodynamic size distributions, and the solid lines are the fitting curve obtained using lognormal distribution.

Figure 3

Waiting time dependence of $T_2$ relaxation time. It can be seen that the $T_2$ relaxation time of magnetic nanoparticle samples with different particle sizes hardly varies with the waiting time $t_w^{\prime }$ under the current test conditions.

Figure 4

Relaxation rate of SHP series magnetic nanoparticle sample. (a) Inverse of longitudinal relaxation time $1/T_1$ and (b) inverse of transverse relaxation time $1/T_2$ with respect to Fe ion concentration $c_{\mathrm{Fe}}$.

Figure 5

Relaxation rate of SHP series magnetic nanoparticle samples. (a) $r_1$ relaxation rate; (b) $r_2$ relaxation rate.

Figure 6

The ratio $r_2/r_1$ of SHP series magnetic nanoparticles.

Figure 7

Relaxation rate of SHP series magnetic nanoparticle sample. (a) Inverse of longitudinal relaxation time $1/T_1$ and (b) inverse of transverse relaxation time $1/T_2$ with respect to the induced magnetization $M$.

Figure 8

Relaxation rate of SHP series magnetic nanoparticle samples. (a) $r_1^{\prime }$ relaxation rate; (b) $r_2^{\prime }$ relaxation rate.

Figure 9

Linear regression of (a) $r_1$ relaxation rate with $r_1^{\prime }$ relaxation rate and (b) $r_2$ relaxation rate with $r_2^{\prime }$ relaxation rate.