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Analytical Chemistry

Electronic Tweezers Grab Nanoparticles

Nanoscience: An electron beam can both trap and snap images of nanoparticles in solution

by Katherine Bourzac
October 19, 2012

Beam Me Up
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Credit: Nano Lett.
The graph shows the trajectory of a gold nanoparticle as an electron beam grabs and pulls it along in a chamber of water. The circles indicate position of the beam, and colored line shows where the particle is at a given time. The inset micrograph shows a single gold nanoparticle (red arrow) trapped in the beam (white halo).
20121019lnj1-nptweezer.jpg
Credit: Nano Lett.
The graph shows the trajectory of a gold nanoparticle as an electron beam grabs and pulls it along in a chamber of water. The circles indicate position of the beam, and colored line shows where the particle is at a given time. The inset micrograph shows a single gold nanoparticle (red arrow) trapped in the beam (white halo).

By using a modified transmission electron microscope, researchers demonstrate that a beam of electrons can pick up and carry nanoparticles, as well as produce pictures of them (Nano Lett., DOI: 10.1021/nl302788g). The so-called electronic tweezers could help scientists in diverse tasks, such as building up new materials nanoparticle by nanoparticle, and performing precise biophysics experiments on single cells, the researchers say.

In the past, scientists have manipulated atoms and microsized particles, including single cells, using a beam of laser light called optical tweezers. But grappling with nanoparticles is more difficult, says Haimei Zheng, a materials scientist at Lawrence Berkeley National Laboratory. The force required to trap a particle with optical tweezers increases as the particle gets smaller. Optical tweezers can move particles as small as 10 nm, but such devices require a lot of energy, powerful lasers, and complex equipment.

Making it easier to manipulate nanoparticles would have “a huge impact in nanoscience,” Zheng says. A new set of tweezers for nanoparticles could assemble carefully patterned arrays of nanoparticles with novel electronic or optical properties. Similar to biophysical experiments run with optical tweezers, such a device could help researchers study the forces between nanoparticles or between nanoparticles and cells.

Zheng and her collaborators were not the first to think of using an electron beam to grab onto nanoparticles. Other researchers had previously used electron beams to move particles in molten aluminum or in a vacuum. These experiments showed that electron beams could move nanoparticles, but were of limited use for the applications Zheng envisioned.

Zheng developed the electronic tweezers with Paul Matsudaira, an engineer at the National University of Singapore, by modifying a transmission electron microscope, which produces images by passing a stream of electrons through a sample. To test the beam’s nanoparticle moving abilities, they added a droplet of water containing 10-nm diameter gold nanospheres to a small chamber. When they focused the electron beam into the chamber and moved the beam, they found that the particles traveled to where the electron beam was most intense.

The researchers demonstrated that the electron beam on the microscope not only trapped the particles, but also produced snapshots of the gold nanospheres. Optical tweezers can’t perform this double duty, Zhang says. The researchers also used the beam to assemble nanoparticles into a clump on a surface, suggesting future applications in assembling new kinds of materials, she says.

The setups for optical and electronic tweezers are quite different, Zheng says, but the optical devices require about 10,000 times as much power to move nanoparticles as the electron beam needed.

Zheng says she doesn’t know how the beam traps the nanoparticles, but she suspects that when the electron beam ionizes the water around the particles it creates negative pressure that pushes on them. Her team is now investigating whether the beam can be used to trap nanoparticles of different shapes and compositions.

Gregory Timp, an electrical and biological engineer at the University of Notre Dame, says the forces created by the electronic tweezers are comparable to those produced in complicated optical tweezers systems. He also points out that, unlike previous electron-beam traps, this system works with particles in water, an environment compatible with experiments at the interface of biology and nanotechnology. “This is a big stepping stone,” Timp says.

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