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.
Optical imaging of cells has taught us much of what we know in biology, but physical limitations prevent resolutions that would let us see many subcellular features. Now researchers at Lawrence Berkeley National Lab and University of California, Berkeley are reporting the development of a nano-scale endoscope that can both image intracellular objects and deliver payloads precisely to the inside of living cells without any observed damage to them.
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.
The device consists of an optical fiber that delivers laser light to a tin oxide nanowire waveguide attached at the tip. The nanowire’s narrow profile allows it to be inserted into the cell without damaging the membrane. Once positioned, light delivered via the optical fiber is passed to the nanowire that re-emits it within a very narrow field of view.
Using the new technique, the researchers were able to deliver quantum dots and visualize intracellular objects that a traditional optical microscope cannot.
To 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.
“The 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.”
Having 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.
Press release: A Single Cell Endoscope
Abstract in Nature Nanotechnology: Nanowire-based single-cell endoscopy
Flashback: How The Diffraction Barrier Was Broken to Achieve Sub 80 nm Optical Microscopy
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