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

Abstract

Gadolinium loaded mesoporous silicas are interesting for MRI contrast agent application due to high r1 relaxivities. Here, gadolinium grafted mesoporous SBA-15 materials with varying pore diameters are investigated by field dependence profiling, temperature studies and high field r1 (11.7T, 500MHz) 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 4–80MHz, pore diameters 5.4, 6.5 and 7.7nm) are presented. The profiles have the shape of a slowly tumbling system, with a maximum at relatively high field strength (30–50MHz) and are characterized by increasing r1 with increasing pore size and decreasing temperature. The same trends are observed at 500MHz, where r1 increases from 6.5 to 14.1mM−1s−1 when the pore diameter increases from 5.4 to 9.1nm. 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 (τg=100μs) and fast local rotation (τR=0.08ns) and an efficient water accessibility to the metal center (τm=1.15ns). 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 (τp).