X-ray absorption spectroscopy study of Gd³⁺-loaded ultra-short carbon nanotubes

We present an x-ray absorption spectroscopy study of the local structure around the Gd³⁺ion loaded in ultra short (20-100 nm) carbon nanotubes (GNTs). X-ray Gd L₃ absorption near edge structure data shows that the 31.2-μM GNT suspension exhibits a clear characteristic of hydration at the [GdO_n] cluster. Extended x-ray absorption fine structure data show that the Gd³⁺ ion is coordinated by about 9 oxygen ions and that this first coordination shell exhibits an asymmetry similar to that found in triclinic Gd-acetate or Gd[C₂H₃O₂] ₃·4H₂O or GdAc. After correction for the asymmetry using the cumulant of the third order, the Gd-O bond distance is found to be 2.34₅ Å, instead of 2.40₆ Å for a symmetrical (or Gaussian) distribution. It is shorter than that in the Gd-containing MRI contrast agents currently in clinical uses. This may account in part for high proton relaxivity observed for the GNT suspension.