Optical tweezers have long been used to transport particles in the size range of micrometers to a few hundred nanometers. Now, in a significant step toward providing nanotech with useful ‘lab on a chip’ devices, researchers have used slot waveguides to condense light energy to scales as small as 60 nm allowing them to trap 75-nm polystyrene nanoparticles and DNA molecules and transport them optically. From “Nano ‘Tractor Beam’ Traps DNA“:
Using a beam of light shunted through a tiny silicon channel, researchers have created a nanoscale trap that can stop free floating DNA molecules and nanoparticles in their tracks. By holding the nanoscale material steady while the fluid around it flows freely, the trap may allow researchers to boost the accuracy of biological sensors and create a range of new ‘lab on a chip’ diagnostic tools.
…”For this research to emerge in the marketplace in a device such as a ‘lab on a chip’, it is essential for engineers to be able to manipulate matter at the scale of molecules and atoms, particularly while the matter is contained within a fluid stream only slightly larger than the particles themselves,” says William Schultz, the National Science Foundation (NSF) program officer who oversaw the researchers’ grant. “NSF and other funding agencies have made nano-science and -technology a high priority. The Cornell researchers have made an important step in realizing the full potential of these devices.”
Light has been used to manipulate cells and even nanoscale objects before, but the new technique allows researchers to manipulate the particles more precisely and over longer distances.
“At the nanoscale, we can think of light like a series of massless particles called photons,” says Cornell engineer David Erickson, one of the co-authors of the study. “We’ve demonstrated a way to condense these photons down to a very small area and stream them along a special type of waveguide, a device that acts like a nanoscale optical fiber. When pieces of matter, like DNA or nanoparticles, float near these streaming photons, they are sucked in and pushed along with the flow. The effect is sort of like moving a truck by throwing baseballs at it. The trick is that we found a way to have a large number of highly efficient “collisions” between the photons and the nanoparticles, getting them to stay in our device and keep them moving along it.”
…The breakthrough is the use of the slot waveguide, which condenses a light wave’s energy to scales as small as the target molecules, overcoming prior limitations caused by light diffraction. Because the waveguide is also a “nanochannel” it can both trap and transport objects using light.
…”Ultimately we imagine being able to take all the ultrafast and highly efficient optical devices that have been developed for communications and other applications over the last 20 years and apply them to the manipulation of matter in different types of nanosystems. Hopefully in the future we can shuttle around individual strands of DNA the same way we now shuttle around light.”
In future iterations of the system, the light will both capture the particles and transport them, so the DNA would arrive at the trap and then be directed to another location, such as a sensor or a staging ground for the assembly of a structure.
The research is published in Nature (abstract). The authors demonstrated that the trapping they achieved was substantially stiffer than achieved by earlier methods…0.20 vs 0.013 pN per nm. They also note that the sub-wavelength slot waveguides used can be integrated into lab-on-a-chip platforms using existing manufacturing techniques. They conclude that the approaches they demonstrated “allow discrete optical manipulation and transport of nanoscopic objects with greater precision than is available with existing approaches. The fusion of nanofluidics and optical manipulation in this manner could lead to new methods of bioanalysis and directed assembly.” The “directed assembly” part sounds like it might be useful for assembling devices and systems from DNA and other molecules.
—Jim
