NMR relaxivity of Ln3+-based zeolite-type materials

Abstract

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 µ_B, 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 μ⁴_eff, where μ_eff 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 μ⁴_eff 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.