Optical trapping of femtoliter droplets

To develop effective drugs, researchers need to identify which molecules interact and what conformations they adopt when they do so. Isolating molecules simplifies their study but requires chemically attaching them to a surface. Now a group has used an optical trap to control a molecule contained in a femtoliter droplet.

In the July 3 issue of Applied Physics Letters, scientists from the Center for Advanced Research in Biotechnology in Rockville, Md., and from the National Institutes of Standards and Technology in Gaithersburg, Md., report on their technique. They encapsulated molecules in water by mixing them ultrasonically with fluorocarbons. The water naturally associated with the molecules and separated from the fluorocarbons, creating the droplets. Because the refractive index of water is higher than that of fluorocarbons, the scientists could use an optical trap formed by an infrared laser to corral and stabilize the molecules in a location convenient for their study. In addition, the method did not require a purification step.

To establish that others could use their technique to study fluorescently labeled molecules, the researchers excited free fluorescent dye molecules and fluorescently labeled DNA with a 532-nm laser. An avalanche photodiode detected the fluorescence.

Using this method, the researchers successfully measured the fluorescence of single sulforhodamine B, tetramethylrhodamine-labeled DNA, free Cy3 and Cy3-labeled DNA. They tested the stability of red fluorescent protein, a more fragile fluorophore, and found that at least 50 percent of the molecules survived ultrasonication.

In addition, they used the technique to make FRET measurements on single-stranded DNA molecules with Cy3 and Cy5 attached.

The scientists also mixed individual water droplets by using the optical trap, which could allow the study of molecular interactions.