These ideal light emitting probes represent a significant step in scrutinizing
the behaviors of proteins and other components in complex systems such as a
Labeling a given cellular component and tracking it through a typical biological
environment is fraught with issues: the probe can randomly turn on and off,
competes with light emitting from the cell, and often requires such intense
laser excitation, it eventually destroys the probe, muddling anything you'd
be interested in seeing.
"The nanoparticles we've designed can be used to study biomolecules one
at a time," said Bruce Cohen, a staff scientist in the Biological Nanostructures
Facility at Berkeley Lab's nanoscience research center, the Molecular Foundry.
"These single-molecule probes will allow us to track proteins in a cell
or around its surface, and to look for changes in activity when we add drugs
or other bioactive compounds."
Molecular Foundry post-doctoral researchers Shiwei Wu and Gang Han, led by
Cohen, Imaging and Manipulation of Nanostructures staff scientist Jim Schuck
and Inorganic Nanostructures Facility Director Delia Milliron, worked to develop
nanocrystals containing rare earth elements that absorb low-energy infrared
light and transform it into visible light through a series of energy transfers
when they are struck by a continuous wave, near-infrared laser. Biological tissues
are more transparent to near-infrared light, making these nanocrystals well
suited for imaging living systems with minimal damage or light scatter.
"Rare earths have been known to show phosphorescent behavior, like how
the old-style television screen glows green after you shut it off. These nanocrystals
draw on this property, and are a million times more efficient than traditional
dyes," said Schuck. "No probe with ideal single-molecule imaging properties
had been identified to date—our results show a single nanocrystal is stable
and bright enough that you can go out to lunch, come back, and the intensity
To study how these probes might behave in a real biological system, the Molecular
Foundry team incubated the nanocrystals with embryonic mouse fibroblasts, cells
crucial to the development of connective tissue, allowing the nanocrystals to
be taken up into the interior of the cell. Live-cell imaging using the same
near-infrared laser showed similarly strong luminescence from the nanocrystals
within the mouse cell, without any measurable background signal.
"While these types of particles have existed in one form or another for
some time, our discovery of the unprecedented 'single-molecule' properties these
individual nanocrystals possess opens a wide range of applications that were
previously inaccessible," Schuck adds.