Micro-GC nanotube sensor array (above); close up of a nanotube with array (below). Credit: M. STRANO

Imagine looking down at your watch to check the time, while the same watch performs chromatography to determine the chemical composition of the air around you. Although unlikely to be in the shops any time soon, recent advances in carbon nanotube technology and surface chemistry could make very small gas-chromatography devices a future reality.

Michael Strano's group at the Massachusetts Institute of Technology in Cambridge has been developing a highly sensitive, miniaturized gas-chromatography instrument based on carbon nanotubes. Sensitivity was key for this device, Strano wanted to be able to detect analytes in the part-per-trillion range — an amount so small it can be difficult to grasp at first. “This is like opening a small vial in a crowded high-school gymnasium,” explains Strano, “even if you open the vial and shut it quickly, you have actually released hundreds of parts per trillion of this mixture everywhere.”

For the backbone of its new gas-chromatography instrument, Strano's group created single-walled nanotube, chemi-resistor arrays in which the nanotubes were aligned between electrodes.

Changes in the electrical resistance can be analysed as a desired molecule binds the nanotube. To tune the binding of the analyte to the arrays the group also had to develop a new chemistry (C. Y. Lee and M. S. Strano J. Am. Chem. Soc. 130, 1766–1773; 2008). “You can use mean basicity to target whether your molecules will bind very strongly, in between, or not at all,” explains Strano. Then using a microelectromechanical systems device it should be possible to have precise control over which molecules bind to the array.

As a proof-of-principle, Strano's group created a very simple device by etching a chromatography column on a chip. Using a 100-micrometre trench the team showed that mixtures can be separated and, using a fast detector, it is possible to transduce each peak as it comes out.

Strano group's demonstrated parts-per-trillion detection on its device, confirming the potential of this approach to miniaturized, on-chip gas chromatography. And for Strano, seeing an actual physical device working at that level was extremely rewarding. “We are engineers, so we try to make devices that work.”

N.B.