  Electron microscopy (EM) has been instrumental in our understanding of
biological systems ranging from subcellular structures to complex organisms.
Although EM reveals cellular morphology with nanoscale resolution, it does not
provide information on the location of proteins within a cellular context. An
EM-based bioimaging technology capable of localizing individual proteins and
resolving protein-protein interactions with respect to cellular ultrastructure
would provide important insights into the molecular biology of a cell. Here, we
report on the development of luminescent nanoprobes potentially suitable for
labeling biomolecules in a multicolor EM modality. In this approach, the labels
are based on lanthanide-doped nanoparticles that emit light under electron
excitation in a process known as cathodoluminescence (CL). Our results suggest
that the optimization of nanoparticle composition, synthesis protocols and
electron imaging conditions could enable high signal-to-noise localization of
biomolecules with a sub-20-nm resolution, limited only by the nanoparticle
size. In ensemble measurements, these luminescent labels exhibit narrow spectra
of nine distinct colors that are characteristic of the corresponding rare-earth
dopant type.
