  There is a growing need for biolabels that can be used in both optical and
electron microscopies, are non-cytotoxic, and do not photobleach. Such
biolabels could enable targeted nanoscale imaging of sub-cellular structures,
and help to establish correlations between conjugation-delivered biomolecules
and function. Here we demonstrate a subcellular multi-modal imaging methodology
that enables localization of inert particulate probes, consisting of
nanodiamonds having fluorescent nitrogen-vacancy centers. These are
functionalized to target specific structures, and are observable by both
optical and electron microscopies. Nanodiamonds targeted to the nuclear pore
complex are rapidly localized in electron-microscopy diffraction mode to enable
"zooming-in" to regions of interest for detailed structural investigations.
Optical microscopies reveal nanodiamonds for in-vitro tracking or
uptake-confirmation. The approach is general, works down to the single
nanodiamond level, and can leverage the unique capabilities of nanodiamonds,
such as biocompatibility, sensitive magnetometry, and gene and drug delivery.
