title: NMR transversal relaxivity of aqueous suspensions of particles of Ln(3+)-based zeolite type materials.
abstract: A series of zeolite-type silicates containing stoichiometric amounts of Ln(3+) ions in the framework (Ln-AV-9 materials), with composition (Na(4)K(2))(Ln(2)Si(16)O(38)).10H(2)O (Ln = Nd, Sm, Eu, Tb, Gd, Dy) has recently been synthesized and characterized. They form paramagnetic microparticles, which as aqueous suspensions have negligible water (1)H longitudinal relaxivities (r(1)) for all Ln(3+) ions studied and quite large transverse relaxivities (r(2)). In this work we further analysed the size distribution of the Ln-AV-9 particles and their r(2)* and r(2) relaxivities. The r(2)* relaxivity effects are explained by the static dephasing regime (SDR) theory. The r(2) relaxivities appear to be strongly dependent on the interval between two consecutive refocusing pulses (tau(CP)) in the train of 180 degrees pulses applied. For long tau(CP) values, the r(2) of the systems saturates at a value, which is always an order of magnitude smaller than r(2)*. These features are explained by a crude model, which takes into account the residual diffusion effect in the static dephasing regime. The large microparticles, although not efficient in T(1) relaxation, are quite effective in enhancing T(2) relaxation, particularly at high magnetic fields. The r(2)* values and the saturation values for r(2) were found to increase linearly with B(0) and mu(0)(2). The largest transversal relaxation rate enhancements were observed for Dy-AV-9 with a saturation value of r(2) of 60 s(-1) mM(-1) and a r(2)* value of 566 s(-1) mM(-1) at 9.4 T and 298 K.
authors.affiliation: Department of Biochemistry, NMR Center and Center of Neurosciences and Cell Biology, Faculty of Science and Technology, University of Coimbra, P. O. Box 3126, 3001-401, Coimbra, Portugal.
authors.forename: Giovannia A
authors.initials: GA
authors.lastname: Pereira
authors.forename: Małgorzata
authors.initials: M
authors.lastname: Norek
authors.forename: Joop A
authors.initials: JA
authors.lastname: Peters
authors.forename: Duarte
authors.initials: D
authors.lastname: Ananias
authors.forename: João
authors.initials: J
authors.lastname: Rocha
authors.forename: Carlos F G C
authors.initials: CF
authors.lastname: Geraldes
mesh_terms: D003287:Contrast Media / Q000737:chemistry; D028581:Lanthanoid Series Elements / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D008855:Microscopy, Electron, Scanning; D013535:Suspensions / Q000737:chemistry; D017641:Zeolites / Q000138:chemical synthesis / Q000737:chemistry*
chemical_list: D003287:Contrast Media; D028581:Lanthanoid Series Elements; D013535:Suspensions; D017641:Zeolites