  Spin-labeling of molecules with paramagnetic ions is an important approach
for determining molecular structure, however current ensemble techniques lack
the sensitivity to detect few isolated spins. In this Letter, we demonstrate
two-dimensional nanoscale imaging of paramagnetic gadolinium compounds using
scanning relaxometry of a single nitrogen vacancy (NV) center in diamond.
Gadopentetate dimeglumine attached to an atomic force microscope tip is
controllably interacted with and detected by the NV center, by virtue of the
fact that the NV exhibits fast relaxation in the fluctuating magnetic field
generated by electron spin flips in the gadolinium. Using this technique, we
demonstrate a reduction in the $T_1$ relaxation time of the NV center by over
two orders of magnitude, probed with a spatial resolution of 20 nm. Our result
exhibits the viability of the technique for imaging individual spins attached
to complex nanostructures or biomolecules, along with studying the magnetic
dynamics of isolated spins.
