Tiny tweezers made from a bowtie of light

Researchers have built the world's smallest tweezers, capable of picking up a single virus or molecule.

The device could be a boon for scientists who want to manipulate biological specimens or build tiny structures from nanocrystals, says physicist Mathieu Juan from Sydney's Macquarie University.

"To my knowledge these are the smallest tweezers ever built," he says. "They will allow people to manipulate, scan and move around very small objects such as viruses."

Unlike everyday tweezers, the new device uses a highly focused beam of light to grip and manipulate objects.

Juan and his co-authors, led by Professor Romain Quidant from the Institute for Photonic Sciences in Barcelona, Spain, describe the technology this week in the journal Nature Nanotechnology.

The researchers focused a beam of laser light through a metal-coated optical fibre. At the tip of the fibre they created an opening shaped like a bow-tie, made of two overlapping triangles.

It's the shape of this opening that allows the beam of light to be controlled with such "exquisite precision," says Juan.

The device is based on a mechanism known as "self-induced back action", he explains. In essence, this means that optical tweezers are designed to shape themselves to the presence of the object they are picking up.

"In other words the trapped specimen plays an active role in the trapping mechanism," the authors write.

Where the two triangles of the bow-tie shape meet, a very gentle force is generated, which does not result in any temperature increase that might damage a biological molecule, Juan says.

The researchers report that they used the device to pick up and move around a plastic sphere just 50 nanometres across - a thousandth the width of a human hair.

Over the course of several minutes, they were able to move the trapped sphere over large distances.

"This was a proof of concept," Juan notes. "Most likely we would be able to push the limit further down to even smaller objects such as biological molecules."

Scientists have been hunting for ways to manipulate smaller and smaller objects, he says, particularly in the biological sciences where fragile structures are easily destroyed by heat or physical pressure.

The new device, the authors say, promises to answer their needs.

"This non-invasive approach is foreseen to open new horizons in nanosciences by offering an unprecedented level of control of nanosized objects, including heat-sensitive biospecimens."