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 relaxivities are explained by the static dephasing regime (SDR) theory. The corresponding R2 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, τD, 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 R2 ≈ 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 (rp) and the radii of the spheres, within which the refocusing pulses are fully effective (rdiff), were estimated. The values of rp obtained are in agreement with those determined by dynamic light scattering. Because the value of rdiff depends on the external magnetic field B and on the magnetic moment of the lanthanide of interest (μeff2), the R2 relaxivity was found to be proportional to B and to μeff2.