A microscale technique known as optical trapping uses beams of light as tweezers to hold and manipulate tiny particles. Stanford researchers have found a new way to trap particles smaller than 10 nanometers - and potentially down to just a few atoms in size – which until now have escaped light’s grasp.
To grasp and move microscopic objects, such as bacteria and the components of living cells, scientists can harness the power of concentrated light to manipulate them without ever physically touching them.
The process of optical trapping – or optical tweezing, as it is often known – 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.
Unfortunately, existing optical tweezers are not adept at handling these tiny building blocks.The problem is inherent to the light beam itself. Optical trapping typically uses light in the visible spectrum (with wavelengths between 400 and 700 nanometers) so that scientist can actually see the specimen as they manipulate it.
Due to a physical constraint called the diffraction limit of light, 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 nanometers – half the shortest visible wavelength of 400 nanometers.
Thus, if the specimen in question is only 2 nanometers wide – the size of a typical protein – trapping it in a space of 200 nanometers allows only very loose control at best. Scale-wise, it is akin to guiding a minnow with 20-meter-wide fishing net.
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. A strong conductor like silver or gold holds its electrons weakly, giving them freedom to move around near the metal’s surface.
The researchers applied plasmonic principles to design a new aperture that focuses light more effectively. The aperture is structured much like the coaxial cables that transmit television signals, the researcher 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 aperture and emerge on the other end as a powerful, concentrated beam of light.
As nanoscale tools go, this new optical trap would be quite a versatile gadget. While the researchers first envisioned it in the context of materials science, its potential applications span many other fields including biology, pharmacology, and genomics.
This story is reprinted from material from Stanford University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.