  Diamond nanoparticles (nanodiamonds) can transport active drugs in cultured
cells as well as in vivo. However, in the latter case, methods allowing to
determine their bioavailability accurately are still lacking. Nanodiamond can
be made fluorescent with a perfectly stable emission and a lifetime ten times
longer than the one of tissue autofluorescence. Taking advantage of these
properties, we present an automated quantification method of fluorescent
nanodiamonds (FND) in histological sections of mouse organs and tumor, after
systemic injection. We use a home-made time-delayed fluorescence microscope
comprising a custom pulsed laser source synchronized on the master clock of a
gated intensified array detector. This setup allows to obtain
ultra-high-resolution images 120 Mpixels of whole mouse organs sections, with
subcellular resolution and single-particle sensitivity. As a proof-of-principle
experiment, we quantified the biodistribution and aggregation state of new
cationic FNDs able to transport small interfering RNA inhibiting the oncogene
responsible for Ewing sarcoma. Image analysis showed a low yield of
nanodiamonds in the tumor after intravenous injection. Thus, for the in vivo
efficacy assay we injected the nanomedicine into the tumor. We achieved a
28-fold inhibition of the oncogene. This method can readily be applied to other
nanoemitters with $\approx$100 ns lifetime.
