NMR Transversal Relaxivity of Suspensions of Lanthanide Oxide Nanoparticles

Abstract

Aqueous suspensions of paramagnetic lanthanide oxide nanoparticles have been studied by NMR relaxometry. The observed R * 2 relaxivities are explained by the static dephasing regime (SDR) theory. The corresponding R 2 relaxivities are considerably smaller and are strongly dependent on the interval between the two refocusing pulses. The experimental data are rationalized by assuming the value of the diffusion correlation time, τ D , to be very long in a layer with adsorbed xanthan on the particle's surface. In this layer, the refocusing pulses are fully effective and R 2 ≈ 0. Outside this layer, the diffusion model for weakly magnetized particles was applied. From the fit of the experimental relaxation data with this model, both the particle radii (r p ) and the radii of the spheres, within which the refocusing pulses are fully effective (r diff ), were estimated. The values of r p obtained are in agreement with those determined by dynamic light scattering. Because the value of r diff depends on the external magnetic field B and on the magnetic moment of the lanthanide of interest (μ eff 2 ), the R 2 relaxivity was found to be proportional to B and to μ eff 2 .