NMR relaxivity of Ln3+-based zeolite-type materials
Résumé
A series of zeolite-type silicates containing framework lanthanides and termed Ln-AV-9 have been synthesized. Fundamentally different from conventional zeolites, these materials have the composition (Na4K2)(Ln2Si16O38)·10H2O (Ln = Nd, Sm, Eu, Tb, Gd, Dy). 29Si and 23Na MAS NMR spectra of Sm-AV-9 indicate the presence of six and two distinct sites, respectively, in agreement with the crystal structure. Gd-AV9 exhibits a simple Curie paramagnetic behaviour with an effective magnetic moment of 7.97 µB, while Eu-AV-9 has a much lower magnetic susceptibility, with a very shallow temperature dependence. The 1H longitudinal relaxivities of water (r1), obtained for aqueous suspensions of several Ln-AV-9 materials with different Ln3+ ions, are very small (0.04–0.09 s−1 mM−1) and almost independent of the Ln3+ ion, while their transverse relaxivities (r2) are much larger (16–60 s−1 mM−1) and proportional to μ4eff, where μeff is the magnetic moment of the lanthanide ion present. While r1 of Gd-AV-9 suspensions is very small, the r2 is substantial and proportional to B20. The dependence of r2 on μ4eff and B20 has been fitted using the model of outer-sphere diffusion of water around the particles in the long-echo limit. The results indicate that aqueous suspensions of Ln-AV-9, although inefficient in longitudinal relaxation, are very effective in enhancing transverse relaxation, particularly at high magnetic fields. They are, therefore, attractive as T2 MRI contrast agents.