May 3, 2006
Atomic force microscope sorts molecules out
Researchers at IBM's Almaden Research Center, US, have used an atomic force microscope (AFM) to separate out molecules of different types. Applying a voltage to the AFM tip caused the molecules to move along the tip at different rates, in a small-scale form of the electrophoresis separation technique.
"This very clever use of an atomic force microscope has the potential to provide ultra-fast and ultra-sensitive molecular separations," said David Garfin, president of the American Electrophoresis Society. "Today, electrophoresis separations in gels or capillary tubes typically take minutes to hours. It would be a tremendous improvement if this early-stage technology can be developed to deliver over a wide range of molecular sizes the precise sub-second separations that the IBM scientists have demonstrated."
The IBM team demonstrated the technique by separating fragments of DNA. Molecules with five base pairs took 5 ms to travel down an 11.2 µm long AFM tip, while molecules with 16 base pairs took 15 ms.
"We controlled the passage of as few as 10 molecules, which indicates that this approach should be very useful for analysing very small biological samples and in writing extremely small features," said H. Kumar Wickramasinghe of IBM.
To perform the method, the team first stored molecules in a reservoir around the base of the AFM tip by applying a voltage that moved the molecules up the tip from a drop of aqueous solution at its apex. Then the scientists reversed the polarity of the voltage so that molecules moved back down the tip at a rate depending on the type of the molecule. Pulsing the voltage and moving the tip enabled controlled deposition of different types of molecule at selected locations on a substrate.
The team also used the technique to draw the IBM logo by depositing 59–79 nm-wide lines of 5-base-pair fragments of DNA. Used in this way, the technique could have applications in fields such as nanoelectronic circuit manufacturing and molecular electronics.
"Our new method acts more like an inkjet printer than a fountain pen," said Wickramasinghe. "For example, we write only when the electric field is applied, not continuously while in contact with the surface. We can also control the deposition rate by varying the electric field strength and our new technique is much less sensitive to the chemical properties of the molecules or the writing surface."
The researchers reported their work in Applied Physics Letters.
About the author
Liz Kalaugher is editor of nanotechweb.org.