  We present a new technique that combines off-axis Digital Holography and Dark
Field Microscopy to track 100nm gold particles diffusing in water. We show that
a single hologram is sufficient to localize several particles in a thick sample
with a localization accuracy independent of the particle position. From our
measurements we reconstruct the trajectories of the particles and derive their
3D diffusion coefficient. Our results pave the way for quantitative studies of
the motion of single nanoparticle in complex media.
