  Sensing of a few unpaired electron spins, such as metal ions and radicals, is
a useful but difficult task in nanoscale physics, biology, and chemistry.
Single nitrogen-vacancy (NV) centers in diamond offer high sensitivity and
spatial resolution in the optical detection of weak magnetic fields produced by
a spin bath, but often require long acquisition times, of the order of seconds.
Here we use a novel approach, based on coupled spin and charge dynamics in
dense NV ensembles in strongly fluorescent nanodiamonds (NDs) to sense external
magnetic dipoles. We apply this approach to various paramagnetic species,
including gadolinium complexes, magnetite nanoparticles and hemoglobin in whole
blood. Taking advantage of the high NV density, we demonstrate detection of
subnanomolar amounts of Gd spins with ultrashort acquisition time approaching
10 ms. Strong luminescence, high sensitivity and short acquisition time make
dense NV- ensembles in NDs a powerful tool for biosensing and bioimaging
applications.
