Corralling Brownian motion

From Baltimore, at a meeting of the American Physical Society

If you think making a little kid sit still for a camera is hard, try it with a protein in a water droplet. Such tiny objects jitter constantly from collisions with molecules of the water around them, and that activity quickly drives a protein molecule out of a typical microscope’s view.

A new microscope system can compensate for those jitters, known as Brownian motion. The system has held minuscule fluorescent objects in its view for seconds at a time. The newfound stability promises scientists a means to study important biological agents in their natural environment, report the gadget’s inventors. “We can trap smaller objects than can be trapped by any other means—all the way down to individual proteins,” claims Adam E. Cohen of Stanford University.

He and William E. Moerner, also of Stanford, built their jitter-correction system around a particle trap made of two glass slides. The researchers etched one slide with shallow, intersecting channels, each attached to an electrode. Fluids move through these channels in and out of a hair-width trapping region at the intersection. The scientists then mounted this transparent trap above the lens of an inverted optical microscope equipped with a laser to excite the trap’s contents.

When a fluorescent particle moves within the trap, the system immediately applies electric signals to fluid in the channels. The signals propel the fluid and drive the particle toward the center of the microscope’s view.

Among the items that the team has trapped are viruses and nanocrystals. To confine yet-smaller protein molecules, however, the researchers had to add glycerol to the water to make it more viscous. A new, faster-feedback version of the trap won’t need the glycerol, Cohen says.

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