Ultrafast sorting of molecules

Need to separate a dozen or so molecules of one kind from a dozen of another kind? IBM scientists now have a way to do that-in just milliseconds. H. Kumar Wickramasinghe, Kerem Unal, and Jane Frommer at IBM's Almaden Research Center in San Jose, Calif., utilize a highly miniaturized version of electrophoresis. Instead of using a gel or capillary tube as the sorting medium, however, they use the 11.2-µm-long conical tip of an atomic force microscope (Appl. Phys. Lett. 2006, 88, 183105). An aqueous mixture of two DNA oligonucleotides is moved into a nanoreservoir at the base of the tip. Voltage pulses are then used to drive the molecules down toward the tip. Because their mobility depends on their size, the molecules travel at different rates, "eluting" off the tip and onto the underlying substrate at different times. By moving the tip over the substrate, the scientists can deposit a dozen molecules of A here, a dozen molecules of B there. The sorting process is more than 10,000 times faster than conventional electrophoresis and has the potential to improve medical lab tests and other technologies.