"Nano-Tweezers" Made of Light

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Photo courtesy of David Erickson.

Erickson team's experimental set-up for trapping and releasing nanoparticles.

The promise of nanoscience and nanotechnology lies in our growing ability to observe, characterize, and manipulate matter down to molecular levels. But while our powers to observe and characterize the very small have dramatically expanded—thanks to major advances in instrumentation in recent years—manipulation of matter at the nanoscale continues to pose a challenge. "Handling" tiny nanoscale items is no easy matter, particularly in fluids such as water. There's no physical pair of tweezers capable of holding such objects and moving them around. And yet our ability to fashion new materials for energy and other applications will depend critically on this capability of precisely manipulating micro- and nanoscale objects.

It turns out that light itself provides one effective means of grabbing and holding such tiny objects. In fact, the basic technique of using light in this fashion has been around for nearly three decades. The instruments for holding microscopic particles with light are known as "optical tweezers," and you'll find them in university laboratories across the world. An optical tweezers set-up—basically a desktop or lab bench arrangement—consists of a laser, typically directed by a set of mirrors toward a sample area that is viewable with a powerful optical microscope. The laser beam will actually grab and hold a microscopic sample, provided the sample is dielectric or has a dielectric end or "handle." (Dielectric materials are special insulators that polarize in the presence of an electromagnetic field, with one end of the material becoming positively charged while the other becomes negatively charged. These materials are used in a variety of applications, perhaps most commonly in the capacitors in electric circuits.) The laser beam will grab and hold tiny dielectric particles until the laser beam is shut off, at which point the particle will be released.

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Image courtesy of David Erickson.

Artist's conception of an optical "nanofactory" using light to trap, release, and assemble nanoscale objects.

Optical tweezers have some drawbacks, however. These instruments will only work with dielectric particles larger than about 100 nanometers (nm = 1 billionth of a meter) in diameter. Nanoscience researchers actually need to manipulate much smaller objects than these. In addition, the (relatively) large laser beams typically used in these instruments impart heat to the tiny sample. This can be damaging, even fatal, especially for biological samples. The problem is so common that researchers in this field have coined a special term for it: "opticution." Opticution is the "optical execution" of a sample burned up by a laser beam in an optical tweezers set-up. An additional problem is that the radiating heat tends to push the sample particle away—through a process known as thermophoresis—at the same time that the light is working to hold it.

Now a Cornell University-led research team has devised a new instrument that is not only capable of capturing, holding, and releasing much smaller particles than conventional optical tweezers, but that also manages to do so with a minimal temperature increase to the sample (less than 1 degree Fahrenheit). Their experiment provides a proof-of-concept for a new and highly promising technique for grabbing, holding, and manipulating nanoscale objects—including tiny biological samples—in a liquid environment. The team was led by Cornell University Associate Professor David Erickson, a 2010 winner of a DOE Office of Science (SC) Early Career Research Program award, with support for the research coming from SC's Office of Basic Energy Sciences. The results have been published in the journal Nano Letters.

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Photo courtesy of David Erickson

David Erickson led the team that developed "nano-tweezers."

The key to the Erickson team's ingenious set-up is a specially designed silicon nitride photonic crystal, which acts to concentrate the laser light and confine the light reaching the sample to a particular wavelength. Photonic crystals are novel devices that channel and filter light much as transistors or other semiconductors channel and filter electrons (see "Beyond the Transistor"). In this case, the photonic crystal is employed as a "resonator." The resonator has two important properties. First, it concentrates light at a very bright spot at its center, which pulls in the nanoparticles that are floating by. Second, the resonator operates at a precise wavelength of 1064 nm. The laser light strikes the resonator, and the resonator transfers to the sample just the light confined to that particular wavelength. The key here is that both water and (many) biological molecules (the two typical components of a biological sample) are essentially transparent to light at this wavelength, so they do not absorb heat from the light. The virtual absence of heat, in turn, eliminates the thermophoretic effect, and partly for this reason also the photonic crystal becomes capable of capturing and holding much smaller particles than is possible with conventional optical tweezers.

Using their set-up, Erickson's team was able to trap and release 22 nm polymer particles, 10 nm quantum dots (very tiny particles), and single proteins. The temperature increase to the sample was limited to 0.3 degrees Kelvin, or less than a degree Fahrenheit. The trap and release experiments are captured in videos that are available free of charge online, as part of the article's supporting info (the article itself is accessed either by subscription or a one-time charge).

The Erickson team's achievement amounts to a considerable breakthrough in capabilities for manipulating objects at the nanoscale and could have broad applications across the field.

"The ability to controllably position nanomaterials is a fundamental enabling nanoscience technology," said Erickson, "and so this discovery could eventually enable direct nanoscopic assembly of new types of materials for a wide range of energy technologies."

The technology is being commercialized by a small startup company, Optofluidics, co-founded by Erickson to develop instruments for nanomanipulation. In the long run, the researchers envision the development of optical "nanofactories," which would enable mass production of new types of materials by using techniques based on the "nano-tweezers" concept. Nanofactories may be some years away, but clearly the ability to trap and release particles at this scale—without damaging or destroying them with heat—marks an important step on the road to fashioning entirely new materials through nanotechnology.

—Patrick Glynn, DOE Office of Science, [email protected]

Research Funding

DOE Office of Science, Office of Basic Energy Sciences

Publication

Yih-Fan Chen, Xavier Serey, Rupa Sarkar, Pen Chen, and David Erickson, "Controlled Photonic Manipulation of Proteins and Other Nanomaterials," Nano Letters 12, 1633 (2012).

Related Links

Erickson Lab, Cornell University

Early Career Research Program, DOE Office of Science