Relaxometric properties of gadolinium-grafted mesoporous SBA-15 silica materials with varying pore size

Gadolinium loaded mesoporous silicas are interesting for MRI contrast agent application due to high r(1) relaxivities. Here, gadolinium grafted mesoporous SBA-15 materials with varying pore diameters are investigated by field dependence profiling, temperature studies and high field r(1) (11.7 T, 500 MHz) measurements. SBA-15 hybrid materials with varying pore diameters are prepared via pseudo surface organometallic chemistry (pseudo-SOMC) of Gd[N(SiHMe2)(2)](3)(THF)(2) and their Nuclear Magnetic Resonance Dispersion (NMRD) profiles (range 4-80 MHz, pore diameters 5.4, 6.5 and 7.7 nm) are presented. The profiles have the shape of a slowly tumbling system, with a maximum at relatively high field strength (30-50 MHz) and are characterized by increasing r(1) with increasing pore size and decreasing temperature. The same trends are observed at 500 MHz, where r(1) increases from 6.5 to 14.1 mM(-1) s(-1) when the pore diameter increases from 5.4 to 9.1 nm. Modeling of the NMRD profiles proposes that the Gd-modified SBA-15s have as much as five inner-sphere water molecules (q = 5), a relatively slow global (tau(g) = 100 mu s) and fast local rotation (tau(R) = 0.08 ns) and an efficient water accessibility to the metal center (tau(m) = 1.15 ns). A model originally developed for zeolites is used to describe the water dynamics in the SBA-15 silicas, and implies that the observed trend with pore size is due to a combination of an increasing number of free water molecules per Gd3+ (x) and a decreasing residence lifetime of water inside the pores (tau(p)). (C) 2013 Elsevier Inc. All rights reserved.