The gadonanotubes: structural origin of their high-performance MRI contrast agent behavior

We present an X-ray absorption spectroscopy study of the local structure around the Gd³⁺ ions loaded within ultra-short carbon nanotubes (US-tubes). For this material, called gadonanotubes (GNTs), detailed modeling and analyses of the X-ray absorption near edge structure (XANES) and extended fine structure (EXAFS) data have helped decipher the structural origin of their exceedingly high-performance MRI contrast agent behavior. Analysis revealed that the Gd³⁺ ions are surrounded by ∼9 oxygen ions in the first coordination sphere with ∼3 O^2- ions and ∼6 O^2- ions, respectively, centered around 2.24 Å and 2.41 Å and that the Gd³⁺-ion sites in the GNTs are small. These sites are proton decorated once in solution through formation of the -(OH) and/or -(OH₂) ligands. A large hydration number due to small Gd³⁺-ion sites and short Gd-O bonds, and thus short Gd-H distances, are the essential structural details pertinent to the high proton relaxivity observed for GNTs suspended in water.