Plasmonic Optical Tweezers Could Trap Tiny Proteins

An innovative aperture design based on plasmonics could focus light so effectively that tiny beams could trap and manipulate particles as small as a few atoms.

Optical trapping — also called optical tweezing — involves sculpting a beam of light into a narrow point that produces a strong electromagnetic field. The beam attracts tiny objects and traps them in place, just like a pair of tweezers. However, there are natural limits to this technique. The process breaks down for objects significantly smaller than the wavelength of light; optical tweezers cannot grasp supersmall particles such as individual proteins, which are only a couple of nanometers in diameter.

Now, scientists at the Stanford School of Engineering have shown theoretically that light passed through their novel aperture would stably trap objects as small as 2 nm. A prototype of the microscopic device is now being built.

Illustration showing the new aperture design (left) with two layers of silver separated by another of silicon dioxide. The structure focuses light in a novel way to trap smaller particles than ever before. The focused beams are shown in the illustration on the right. Courtesy of Amr Saleh, Stanford School of Engineering.

“Optical tweezers seemed like a really cool way of assembling new materials,” said materials scientist Jennifer Dionne, who imagined an optical tool that would help her precisely move molecular building blocks into new configurations.

Current optical tweezers, however, are not adept at handling these tiny building blocks. The problem is inherent in the light beam itself. Optical trapping typically uses visible light (with wavelengths between 400 to 700 nm) so that scientists can see the specimen as they manipulate it.

Because of the diffraction limit, the smallest space in which optical tweezing can trap a particle is approximately half the wavelength of the light beam. In the visible spectrum, this would be about 200 nm.

If a specimen under investigation is only 2 nm wide — the typical size of a protein — trapping it in a 200-nm space allows only very loose control at best. In addition, the optical force that light exerts on an object diminishes as the object gets smaller.

Assistant Professor Jennifer Dionne. Courtesy of Joel Simon.

"If you want to trap something very small, you need a tremendous amount of power, which will burn your specimen before you can trap it," said doctoral candidate Amr Saleh.

To eliminate this problem, some researchers attach the specimen to a much larger object that can be dragged around with light. Dionne noted, however, that important molecules like insulin or glucose might behave quite differently when attached to giant anchors from how they would on their own. To isolate and move a tiny object without frying it, the researchers needed a way around the limitations of conventional optical trapping.

The most promising method of moving tiny particles with light relies on plasmonics, a technology that takes advantage of the optical and electronic properties of metals, Dionne said. A strong conductor like silver or gold holds its electrons weakly, giving them freedom to move around near the metal's surface.

When lightwaves interact with these mobile electrons, they move in what Dionne describes as "a very well defined, intricate dance," scattering and sculpting the light into electromagnetic waves called plasmon-polaritons. The very short wavelengths of these oscillations enable them to trap small specimens more tightly.

Plasmonic principles were used to design a new aperture that focuses light more effectively. The aperture is structured much like the coaxial cables that transmit television signals, Saleh said. A nanoscale tube of silver is coated in a thin layer of silicon dioxide, and those two layers are wrapped in a second outer layer of silver. When light shines through the silicon dioxide ring, it creates plasmons at the interface where the silver and silicon dioxide meet. The plasmons travel along the aperture and emerge on the other end as a powerful, concentrated beam of light.

Doctoral candidate Amr Saleh. Courtesy of Joel Simon.

Although it is not the first plasmonic trap, the Stanford device promises to trap the smallest specimens recorded to date. Saleh and Dionne have shown theoretically that their design can trap particles as small as 2 nm. With further improvements, their design could be used to optically trap molecules even smaller.

The researchers first envisioned the nanotool in the context of materials science, but it could be applied to fields including biology, pharmacology and genomics.

Dionne said that she would first like to trap a single protein, and try to unravel its twisted structure using visible light alone. The beam of light could also be used to exert a strong pulling force on stem cells, which has been shown to change how these important building blocks differentiate into various kinds of cells.

Saleh, on the other hand, is interested in moving and stacking tiny particles to explore their attractive forces and to create new, "bottom up" materials and devices.

Details of the study appear in Nano Letters (doi: 10.1021/nl302627c).

For more information, visit: www.stanford.edu