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Laser, holograms could expedite sample analysis

BioPhotonics
Feb 2009
Caren B. Les, caren.les@laurin.com

WEST LAFAYETTE, Ind. – Researchers have developed a method that uses a near-infrared laser and holograms to rapidly position nanoscale particles. The noninvasive technique, known as rapid electrokinetic patterning, holds promise for the development of lab-on-a-chip technology for the analysis of biological samples in the medical and environmental fields. The tool could enable the design of sensor technologies that move particles to a specific region on an electronic chip for rapid detection and analysis.

SNhologramBioScan.jpg

These images were taken from a video illustrating a technique that uses a laser and holograms to precisely position clusters of numerous tiny particles within seconds, representing a potential new tool to analyze biological samples or create devices. Here, an accumulation of 1-μm particles (red dots) are translated to the center of the illumination location (marked with a yellow circle). The translation and accumulation occur in 0.5 s. The full video is available online at ecommons.cornell.edu/handle/1813/11399. Courtesy of Birck Nanotechnology Center, Purdue University.

The technique – which evolved through the curiosity of two Purdue University doctoral students, Stuart J. Williams and Aloke Kumar, and their work with Steven T. Wereley, associate professor of mechanical engineering – has advantages over two existing techniques for manipulating nanoparticles: optical trapping, which uses a highly focused beam of light to capture and position particles, and dielectrophoresis, which uses electric fields generated from metallic circuits to move many particles at a time. The former can move only a small number of particles at a time, while the latter produces circuit patterns that cannot be changed once they are created. And patterning individual particles on a massive scale using electrokinetic methods could take hours or days, in comparison with the new technique, which takes only seconds, Williams said.

Williams was working on AC electrokinetic methods to manipulate fluids and particles, including dielectrophoresis, and Kumar was working on optical trapping techniques, specifically a new holographic optical trapping unit that can program highly focused light patterns to dynamically trap particles and precisely translate them, when they decided to team up. Their investigation into the simultaneous application of electrokinetic and optical trapping forces resulted in the development of an optically induced electrokinetic technique for microparticle manipulation and assembly.

Williams said that their method involves the introduction of a liquid sample between two parallel electrode plates separated by 50 μm. The electrodes are biased with an AC signal at frequencies typically less than 150 kHz. The researchers used the Bioryx 200, a highly focused near-infrared Nd:YAG laser-based system from Arryx Inc. of Chicago, to apply the holographic illumination to the surface of either plate. The holographic optical landscapes were implemented by a computer-controlled spatial light modulator.

The illumination from the laser directly heats the liquid sample and creates thermal gradients, Williams said. Changes in temperature produce gradients in the dielectric properties of the fluid, and the electric field acts upon these changes, resulting in fluid motion. A microfluidic vortex is created, with the center of the recirculation located at the laser focal point. The vortex carries the particles toward the center of the optical illumination, where they group together in a patterned crystallinelike monolayer, taking the general shape of the illuminated regions. The particles have been patterned on indium tin oxide and gold electrodes, the latter of which often are used in biomedical applications.

The technique’s versatility and dynamic capabilities are significant, Williams said. The fluid motion is created by illumination-induced thermal gradients, and its speed is controlled by the AC signal. The motion by itself can be used to mix or pump fluids within a microchannel, he added.

He also noted that the particles can be concentrated, patterned and translated with the illumination, which, because it is computer-controlled, can be precisely reconfigured and repeated. The particle accumulation is AC-frequency-dependent, so the particles are captured selectively, leading to characterization or separation of samples.

Any of these characteristics can be used independently or simultaneously to support a variety of lab-on-a-chip applications, he said. The technique can be applied toward the assembly of particles or biological cells to create artificial architectures, including photonic crystals or bioengineered tissues, he added.

Because the technique simultaneously induces fluid motion to concentrate a sample, it can be used to improve the efficiency of lab-on-a-chip sensors and processes, including bead-based assays, Williams said.

The students won an award for their work during the 12th International Conference for Miniaturized Systems for Chemistry and Life Sciences in San Diego in October. They plan to characterize the electrokinetic mechanisms responsible for the technique, a process that is expected to produce more efficient and controlled results in future investigations. They also aim to directly apply the technique to the relevant technological areas.


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