Self-assembled peptide amphiphile nanostructures containing Gd-based magnetic moieties. (A) AFM image of nanofibers. (B) Atomic force microscopy (AFM) image of nanospheres. (C) Molecular design of 1, 2, 3. (D) MR images of phantom gels formed from (1) the control PA of 1, (2) a mixture of 1 and 2, (3) a mixture of 1 and 3, and (4) Gd-DTPA. The mixture of 1 and 3 demonstrated the highest contrast due to the increase in τr of GdIII chelates. (Reproduced with permission from 43 and 39. Copyright 2005 American Chemical Society)