  Magnetic field fluctuations arising from fundamental spins are ubiquitous in
nanoscale biology, and are a rich source of information about the processes
that generate them. However, the ability to detect the few spins involved
without averaging over large ensembles has remained elusive. Here we
demonstrate the detection of gadolinium spin labels in an artificial cell
membrane under ambient conditions using a single-spin nanodiamond sensor.
Changes in the spin relaxation time of the sensor located in the lipid bilayer
were optically detected and found to be sensitive to near-individual proximal
gadolinium atomic labels. The detection of such small numbers of spins in a
model biological setting, with projected detection times of one second, opens a
new pathway for in-situ nanoscale detection of dynamical processes in biology.
