Researchers at the US Lawrence Berkeley National Laboratory (Berkeley, CA, USA) and the University of California (UC Berkeley; USA) created the novel endoscope by attaching a tin oxide nanowire waveguide to the tapered end of an optical fiber. Light travelling along the optical fiber was effectively coupled into the nanowire, where it was reemitted into free space upon reaching the tip. The nanowire tip is extremely flexible, due to its small size and high aspect ratio, yet it can endure repeated bending and buckling so that it can be used multiple times. Another possible application of the system is biosensing and single-cell electrophysiology.
To test the nanowire endoscope as a local light source for subcellular imaging, the researchers optically coupled it to an excitation laser and guided blue light across the membrane and into the interiors of individual HeLa cells, the most commonly used immortalized human cell line for scientific research. Illuminating the intracellular environment of the cells with blue light using the nanoprobe did not harm the cells, since the illumination volume was at the pico-liter scale. Neither did the contact with the cell cytoplasm induce cell death, apoptosis, significant cellular stress, or membrane rupture. The study was published on December 18, 2011, in Nature Nanotechnology.
“By combining the advantages of nanowire waveguides and fiber-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,” said lead author chemist Peidong Yang, PhD, of the Berkeley Lab's Materials Sciences Division. “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 specificity.”
Once the biocompatibility of the nanowire endoscope was demonstrated, the researchers tested the device’s capabilities for delivering payloads to specific sites inside a cell. To do so, they attached quantum dots to the tin oxide nanowire tip of the endoscope using photo-activated linkers that can be cleaved by low-power ultraviolet (UV) radiation. Within one minute, the functionalized nanowire endoscope was able to release its quantum dot cargo into targeted intracellular sites.
The directional blue laser light was then 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 photo activation to release the dots had no significant effect on cell viability.
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