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December 02, 2008

Holographic Precise Mass Control of Nanoscale Particles (50 nanometers to 3 microns) with Re-arrangement in Seconds


Researchers at Purdue University have developed a technique that uses a laser and holograms to precisely position numerous tiny particles within seconds, representing a potential new tool to analyze biological samples or create devices using nanoassembly.

The technique, called rapid electrokinetic patterning, is a potential alternative to existing technologies because the patterns can be more quickly and easily changed, said mechanical engineering doctoral student Stuart J. Williams.

The researchers demonstrated how the method could be used to cause particles to stick permanently to a surface in a single crystalline layer, a structure that could be used in manufacturing. They used their technique to move fluorescent-dyed beads of polystyrene, latex and glass in sizes ranging from 50 nanometers to 3 micrometers.

Future work may involve using a less expensive light source, such as a common laser pointer, which could not be used to create intricate patterns but might be practical for manufacturing.






The technique overcomes limitations inherent in two existing methods for manipulating particles measured on the scale of nanometers, or billionths of a meter. One of those techniques, called optical trapping, uses a highly focused beam of light to capture and precisely position particles. That technique, however, is able to move only a small number of particles at a time.

The other technique, known as dielectrophoresis, uses electric fields generated from metallic circuits to move many particles at a time. Those circuit patterns, however, cannot be changed once they are created.

The new method is able to simultaneously position numerous particles and be changed at a moment's notice simply by changing the shape of the hologram or the position of the light.

"If you want to pattern individual particles on a massive scale using electrokinetic methods as precisely as we are doing it, it could take hours to days, where we are doing it in seconds," Williams said.

The method offers promise for future "lab-on-a-chip" technology, or using electronic chips to analyze biological samples for medical and environmental applications. Researchers are trying to develop such chips that have a "high throughput," or the ability to quickly detect numerous particles or molecules, such as proteins, using the smallest sample possible.

"For example, a single drop of blood contains millions of red blood cells and countless molecules," Williams said. "You always want to have the smallest sample possible so you don't generate waste and you don't have to use as many chemicals for processing the sample. You want to have a very efficient high throughput type of device."

So-called "optical tweezers" use light to position objects such as cells or molecules.

"You can't use mechanical tweezers to move things like molecules because they are too delicate and will be damaged by conventional tweezers," Kumar said. "That is why techniques like optical tweezing and dielectrophoresis are very popular."

The students also have designed an experiment containing one indium tin oxide plate and one gold plate, an important development because gold is often used in biomedical applications.

"It's a technique that you would likely use in sensors, but we also see definite potential ways in which you could use it to manufacture devices with nanoassembly," Wereley said. "But it's really too soon to talk about scaling this up in a manufacturing setting. We're just beginning to develop this technique."


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