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April 28, 2009

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Researchers use light beams to grab molecules

Jan. 1, 2009
Courtesy National Science Foundation
and World Science staff

Us­ing a beam of light shunted through a ti­ny sil­i­con chan­nel, re­search­ers say they’ve cre­at­ed a nano­scale, or mo­le­cu­lar-scale, trap that can cap­ture DNA mo­le­cules.

The work is part of re­search in­to sys­tems de­signed to ma­ni­pu­late nano­scale ob­jects so that they can be moved to de­sired places for anal­y­sis or for build­ing ti­ny struc­tures, such as machines.

When DNA molecules sus­pended in a ti­ny stream of wa­ter flow through a nano­scale chan­nel, they can be cap­tured by a field of light if that light is con­fined in a de­vice called a slot wave­guide. The pres­sure from the light can then pro­pel the DNA along the wave­guide chan­nel to br­ing the molecules to new lo­ca­tions. Such ma­nip­u­la­tion could prove val­u­a­ble for as­sem­bling nano­scale struc­tures, driv­ing pow­er­ful sen­sors and de­vel­op­ing a range of oth­er tech­nolo­gies. (Cred­it: Ni­colle Ra­ger Full­er, Na­tion­al Sci­ence Founda­tion)

Re­search­ers from Cor­nell Uni­ver­s­ity in New York re­ported the find­ings in the Jan. 1 is­sue of the re­search jour­nal Na­ture.

En­gi­neers need to “ma­ni­pu­late mat­ter at the scale of mo­le­cules and atoms,” said Wil­liam Schultz, a pro­gram of­fic­er at the U.S. Na­tion­al Sci­ence Foun­d­a­t­ion who over­saw the fund­ing for the re­search. “The Cor­nell re­search­ers have made an im­por­t­ant step in real­iz­ing the full po­ten­tial of these de­vices.”

Light had been used to ma­ni­pu­late nano­scale ob­jects be­fore, but the new tech­nique al­lows re­search­ers to ma­ni­pu­late them more pre­cisely and over long­er dis­tances, de­vel­op­ers said.

Phys­i­cists found dec­ades ago that light can be thought of ei­ther as a form of waves, or as streams of weight­less par­t­i­cles. Al­though the two pic­tures are hard to rec­on­cile, re­search­ers of­ten find they must use both de­scrip­tions of light in­ter­changeably as they de­scribe phe­nom­e­na re­lat­ed to light.

With this in mind, one way to think about the new re­search is that it pro­vides a way to con­dense streams of light par­t­i­cles “to a very small area,” said Cor­nell en­gi­neer Da­vid Er­ick­son, one of the co-authors of the stu­dy. This is done by stream­ing the par­t­i­cles, called pho­tons, “a­long a spe­cial type of wave­guide,” a de­vice that con­strains their path.

When molecular-sized bits of mat­ter such as DNA float near these pho­tons, they’re “sucked in and pushed along with the flow,” he added. “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 tar­get ob­jects, “get­ting them to stay in our de­vice and keep them mov­ing along it.”

Er­ick­son and col­leagues crafted a wave­guide that would shunt light in­to a nar­row beam con­tain­ing ti­ny chan­nels 60 to 120 bil­lionths of a me­ter wide, de­signed to keep light waves fo­cused. These con­dense a wave’s en­er­gy to scales as small as the tar­get mo­le­cules, overcoming pri­or lim­ita­t­ions caused by light dif­frac­tion, or spread­ing out of the waves.

The re­search­ers used wa­ter so­lu­tions con­tain­ing ei­ther DNA or ti­ny na­no­par­t­i­cles, or nano­scale ob­jects, wash­ing the flu­ids over the “mi­cro­chan­nels.”

“What we’re hop­ing to do now is bet­ter un­der­stand some of the un­der­ly­ing phys­ics to see what else might be pos­si­ble with this ap­proach,” said Er­ick­son. “Hope­fully in the fu­ture we can shut­tle around in­di­vid­ual strands of DNA the same way we now shut­tle around light,” as is done with op­ti­cal fibers.

In fu­ture ver­sions of the sys­tem, he added, the light will both cap­ture the par­t­i­cles and trans­port them. 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.

* * *

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Using a beam of light shunted through a tiny silicon channel, researchers say they’ve created a nanoscale, or molecular-scale, trap that can capture individual DNA molecules. The work is part of research into systems designed to manipulate nanoscale objects so that they can be moved to desired places for analysis or for building tiny structures, such as machines. Researchers from Cornell University in New York reported the findings in the Jan. 1 issue of the research journal Nature. Engineers need to “manipulate matter at the scale of molecules and atoms,” said William Schultz, a program officer at the U.S. National Science Foundation who oversaw the funding for the research. “The Cornell researchers have made an important step in realizing the full potential of these devices.” Light had 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, developers said. Physicists found decades ago that light can be thought of either as a form of waves, or as streams of weightless particles. Although the two pictures are hard to reconcile, researchers often find they must use both descriptions of light interchangeably as they describe phenomena related to light. With this in mind, one way to think about the new research is that it provides a way to condense streams of light particles “to a very small area,” said Cornell engineer David Erickson, one of the co-authors of the study. This is done by streaming the particles, called photons, “along a special type of waveguide,” a device that constrains their path. When molecular-sized bits of matter such as DNA float near these photons, they’re “sucked in and pushed along with the flow,” he added. “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 target objects, “getting them to stay in our device and keep them moving along it.” Erickson and colleagues crafted a waveguide that would shunt light into a narrow beam containing tiny channels 60 to 120 billionths of a meter wide, designed to keep light waves focused. These condense a wave’s energy to scales as small as the target molecules, overcoming prior limitations caused by light diffraction, or spreading out of the waves. The researchers used water solutions containing either DNA or tiny nanoparticles, or nano-scale objects, washing the fluids over the “microchannels.” “What we’re hoping to do now is better understand some of the underlying physics to see what else might be possible with this approach,” said Erickson. “Hopefully in the future we can shuttle around individual strands of DNA the same way we now shuttle around light,” as is done with optical fibers. In future versions of the system, he added, the light will both capture the particles and transport them. 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.