Peidong Yang and colleagues made their device by attaching a tin oxide nanowire waveguide to the tapered end of an optical fibre. Light travelling along the fibre can be effectively coupled into the nanowire. This light is re-emitted when it reaches the tip and so can be used to provide high-resolution optical images of the inside of a single living cell when the tip is inserted into the cell. The nanowire is flexible and can be repeatedly bent and buckled, which means it can be re-used many times.

Optical light has long been used to study biological cells, because they are transparent to light at these frequencies. However, the technique is limited by the so-called diffraction limit of light, so structures smaller than half the wavelength of the incident light cannot be resolved. Recent advances in nanophotonics have now made it possible to overcome the diffraction limit, and thus image sub-cellular components, but the machines used are complex, expensive to build and bulky.

The new nanowire endoscope overcomes all these problems, say Yang and co-workers. The team tested its device on HeLa cells and found that light emitted from the endoscope was closely confined to the nanowire tip. "This means that the device can provide highly directional and localized illumination," explained Yang.

The endoscope is also non-invasive because it can be safely inserted into a cell cytoplasm without causing any damage to the cell. The blue light emitted by the nanoprobe is not dangerous either because the volume illuminated is very small – on the order of picolitres. This is in contrast to conventional fluorescence sensing techniques based on sub-micron tapered optical fibres that are relatively big and cone-shaped. The illumination volume here remains large and, what is more, the fibres can easily rupture cell membranes.

Delivering payloads

Although single-cell delivery systems based on carbon and boron nitride nanotubes already exist, such devices have relatively long delivery times of up to 30 minutes. The new nanowire probes are much quicker: Yang's team attached quantum dots to their nanowire tip – using photo-activated linkers that can be cleaved with low-power UV radiation – and observed that the tip released its quantum dot cargo into the target cells in just one minute. The researchers confirmed their result with confocal microscopy scanning of the cells.

"In the future, this nanowire endoscope should be able to do point-delivery of genes, proteins, therapeutic drugs or other cargo with high spatial resolution," Yang told nanotechweb.org. "In addition, such a device may also be used to electrically or optically stimulate a living cell with a simultaneous high-solution optical signal being output from the same nanowire probe."

The work is detailed in Nature Nanotechnology.