Nanophotonic endoscope looks inside single living cells

Ashley N. Paddock, ashley.paddock@photonics.com

A new nanophotonic endoscope can take high-resolution images of the inside of a single living cell and deliver therapeutic drugs and other cargo without injuring or damaging the cell.

Lawrence Berkeley National Laboratory and University of California researchers attached a tin oxide nanowire waveguide to the tapered end of an optical fiber to create the imaging system. Light traveling along the optical fiber 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 fiber optic fluorescence imaging with nanowire waveguides has enabled researchers to manipulate light at the nanoscale inside living cells so that they can study the biological processes within those cells. In addition, the researchers discovered that the nanowire-based endoscope can detect optical signals from subcellular regions.

This schematic depicts the subcellular imaging of quantum dots in a living cell using a nanowire endoscope. Images courtesy of Lawrence Berkeley National Laboratory.

Because cells are optically transparent, they can be noninvasively imaged with visible light in three dimensions, and visible light allows 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 bulky. The scientists miniaturized their endoscope system by incorporating their nanophotonic component into a benchtop fiber optic setup, which was low in cost.

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.

To test its nanowire endoscope as a local light source for subcellular imaging, the team optically coupled it to an excitation laser, then waveguided blue light across the membrane and into the interiors of individual HeLa cells.

“The optical output from the endoscope emission was closely confined to the nanowire tip and thereby offered highly directional and localized illumination,” said Peidong Yang, a chemist with Berkeley Lab’s Materials Sciences Div. “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 picoliter scale.”

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.

Having demonstrated the biocompatibility of their nanowire endoscope, Yang then tested its capabilities for delivering payloads to specific sites inside a cell. While carbon and boron nitride nanotube-based single-cell delivery systems were reported, they suffered from delivery times that range from 20 to 30 minutes, plus a lack of temporal control over the delivery process.

To surmount these limitations, the team members attached quantum dots to the tin oxide nanowire tip of their endoscope using photoactivated linkers that can be cleaved by low-power ultraviolet radiation. Within one minute, their functionalized device was able to release its quantum dot cargo into the targeted intra-cellular sites.

The report was published online in Nature Nanotechnology (doi: 10.1038/ nnano.2011.226).

In the future, Yang said the endoscope could be used not only for optical imaging and cargo delivery, but also for stimulating living cells optically or electrically.