Molecules in motion

Amanda D. Francoeur, amanda.francoeur@laurin.com

Moving and then trapping freely dispersed molecules can be a challenging process, especially if they are suspended in liquid. To lessen the difficulty, a novel method has been developed using infrared light to thermally drive and control particles.

The technique functions like an optical conveyor belt, helping bring together biological elements on a micrometer scale. The method may improve diffusion-limited surface reactions, cell signaling control and observation of biomolecules over time.

Optical divide

Dieter Braun and Franz Weinert from the physics department at Ludwig Maximilians University used a technique employing thermophoresis, the repositioning of particles as a result of temperature gradients. The molecules, 1-μm fluorescent single-stranded DNA, were placed in 2-μm-thick bulk water between a glass surface and a metal-coated base. An infrared laser operating at 1455 nm and 5 W was directed at the base, heating the liquid locally to a diameter of 70 μm. A thermal gradient developed with cooler liquid at the top of the container, pushing the DNA to the top.

The laser was then moved in a radial pattern, causing the solution to change in viscosity and the fluid on either side of the laser beam’s moving spot to contract and expand and thus flow away from the center. The liquid at the top moved in the opposite direction of the “belt,” conserving mass and creating a steep gradient in the Z-direction. With this motion, the molecules within the container were pulled toward the center, where they accumulated. Overall, it took approximately three seconds for the entire process to take place.

The researchers established that the conveyor belt method can assemble molecules from a few nanometers to hundreds of micrometers in length. For smaller molecules that would be less affected by thermophoresis, researchers could increase the radius of the trap to increase the concentration level in the center.

Unlike conventional methods such as using gravity to drive fluid, the conveyor belt technique implements flow by thermoviscous expansion. However, temperature gradients are not a factor in the conveyor’s efficiency; instead, its effectiveness is determined by the speed of the flow.

Advantages to the system include not having to use microfluidics, electrodes or surface modifications. Because of its optical operation, the trap can be repositioned, reducing the time needed to collect all of the particles in comparison to diffusion into an immobile trap. The study was published online Oct. 6, 2009, in Nano Letters.