Fluorescence confocal image of a single living HeLa cell shows that via nanoendoscopy a quantum dot cluster (red dot) has been delivered to the cytoplasm within the membrane (green) of the cell. (Courtesy of Berkeley Lab)
endoscope that can provide high-resolution optical images of the
interior of a single living cell, or precisely deliver genes, proteins,
therapeutic drugs or other cargo without injuring or damaging the cell,
has been developed by researchers with the U.S. Department of Energy
(DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab). This
highly versatile and mechanically robust nanowire-based optical probe
can also be applied to biosensing and single-cell electrophysiology.
team of researchers from Berkeley Lab and the University of California
(UC) Berkeley attached a tin oxide nanowire waveguide to the tapered end
of an optical fibre to create a novel endoscope system. Light
travelling along the optical fibre can be effectively coupled into the
nanowire where it is re-emitted into free space when it reaches the tip.
The nanowire tip is extremely flexible due to its small size and high
aspect ratio, yet can endure repeated bending and buckling so that it
can be used multiple times.
combining the advantages of nanowire waveguides and fibre-optic
fluorescence imaging, we can manipulate light at the nanoscale inside
living cells for studying biological processes within single living
cells with high spatial and temporal resolution,” says Peidong Yang, a
chemist with Berkeley Lab’s Materials Sciences Division, who led this
research. “We’ve shown that our nanowire-based endoscope can also detect
optical signals from subcellular regions and, through light-activated
mechanisms, can deliver payloads into cells with spatial and temporal
who also holds appointments with the University of California
Berkeley’s Chemistry Department and Department of Materials Science and
Engineering, is the corresponding author of a paper in the journal Nature Nanotechnology describing
this work titled “Nanowire-based single-cell endoscopy.” Co-authoring
the paper were Ruoxue Yan, Ji-Ho Park, Yeonho Choi, Chul-Joon Heo,
Seung-Man Yang and Luke Lee.
Images of a nanowire endoscope in close contact with a quantum dot cluster in a HeLa cell (left), and separated vertically from the cluster by 2 mm (middle) and horizontally by 6 mm (right). Colored circles and arrows mark the position of the cluster and movement of the endoscope.
significant advancements in electron and scanning probe microscopy,
visible light microscopy remains the workhorse for the study of
biological cells. Because cells are optically transparent, they can be
noninvasively imaged with visible light in three-dimensions. Also,
visible light allows the fluorescent tagging and detection of cellular
constituents, such as proteins, nucleic acids and lipids. The one
drawback to visible light imaging in biology has been the diffraction
barrier, which prevents visible light from resolving structures smaller
than half the wavelength of the incident light. Recent breakthroughs in
nanophotonics have made it possible to overcome this barrier and bring
subcellular components into view with optical imaging systems. However,
such systems are complex, expensive and, oddly enough, bulky in size.
we had shown that subwavelength dielectric nanowire waveguides can
efficiently shuttle ultraviolet and visible light in air and fluidic
media,” Yang says. “By incorporating one of our nanophotonic components
into a simple, low-cost, bench-top fibre-optical set-up, we were able to
miniaturize our endoscopic system.”
test their nanowire endoscope as a local light source for subcellular
imaging, Yang and his co-authors optically coupled it to an excitation
laser then waveguided blue light across the membrane and into the
interiors of individual HeLa cells, the most commonly used immortalized
human cell line for scientific research.
optical output from the endoscope emission was closely confined to the
nanowire tip and thereby offered highly directional and localized
illumination,” Yang says. “The insertion of our tin oxide nanowire into
the cell cytoplasm did not induce cell death, apoptosis, significant
cellular stress, or membrane rupture. Moreover, illuminating the
intracellular environment of HeLa cells with blue light using the
nanoprobe did not harm the cells because the illumination volume was so
small, down to the picolitre-scale.”
This schematic depicts the subcellular imaging of quantum dots in a living cell using a nanowire endoscope. (Courtesy of Berkeley Lab)
demonstrated the biocompatibility of their nanowire endoscope, Yang and
his co-authors next tested its capabilities for delivering payloads to
specific sites inside a cell. While carbon and boron nitride
nanotube-based single-cell delivery systems have been reported, these
systems suffer from delivery times that range from 20-to-30 minutes,
plus a lack of temporal control over the delivery process. To overcome
these limitations, Yang and his co-authors attached quantum dots to the
tin oxide nanowire tip of their endoscope using photo-activated linkers
that can be cleaved by low-power ultraviolet radiation. Within one
minute, their functionalized nanowire endoscope was able to release its
quantum dot cargo into the targeted intracellular sites.
microscopy scanning of the cell confirmed that the quantum dots were
successfully delivered past the fluorescently labeled membrane and into
the cytoplasm,” Yang says. “Photoactivation to release the dots had no
significant effect on cell viability.”
highly directional blue laser light was used to excite one of two
quantum dot clusters that were located only two micrometers apart. With
the tight illumination area and small separation between the light
source and the dots, low background fluorescence and high imaging
contrast were ensured.
the future, in addition to optical imaging and cargo delivery, we could
also use this nanowire endoscope to electrically or optically stimulate
a living cell,” Yang says.
nanowires used in these experiments were originally developed to study
size-dependent novel electronic and optical properties for energy
This research was supported by the DOE Office of Science and a grant from the National Institutes of Health.
Nanowire-based single-cell endoscopy