"Nanotubes are exquisite mass sensors," says Michael Roukes, a professor of physics, applied physics and bioengineering at Caltech, "and this work sets an entirely new bar for detection." However, Roukes, who was not involved in the current research but examined the data in Zettl's published paper, says "they don't quite have the resolution to see individual gold atoms directly yet." During the measurement intervals, Roukes says that "a pile of atoms stick to the device, and statistical analysis of the noise from their random arrivals allows the Berkeley group to deduce the single-atom origin of what they are seeing."

Zettl says that the advantage of using carbon nanotubes for single-atom sensing is that they can operate at room temperature, whereas other systems must be cooled--though not by much, Roukes notes.

Zettl now plans to test his sensors on more complicated molecules, including proteins. In order to be used to analyze complex samples like blood, which contain many different molecules, the nanotube sensors will be organized into arrays. Each sensor would be attached to a binding molecule such as an antibody, which would pick the molecule of interest from the surrounding solution so that it could be weighed. Zettl is currently developing these selective sensors.