Carbon nanotubes that act like miniature radio receivers can detect and weigh individual gold atoms, according to a new study from the University of California, Berkeley. Researchers say that the sensors could one day be used to detect individual biomolecules, such as proteins, which might be employed to monitor the air for small traces of bioterror agents, or for rapid bedside diagnostics on unfiltered blood samples.
These mass sensors, which function as nanomechanical cantilevers, identify individual atoms and proteins by weighing them. They work on the same principle as a diving board, says Alex Zettl, a professor of physics at Berkeley who developed the nanotube sensors. When a swimmer is on the edge of the board bending her knees in preparation for a dive, the board moves up and down relatively slowly. After she jumps, the board continues to vibrate but at a higher frequency. Similarly, when an atom or a molecule sticks to a carbon nanotube with one end fixed like a diving board, increasing its mass, the resonant frequency decreases. The trick is detecting these changes in frequency without using complicated equipment. “You need electronics that read out that it’s moving and at what frequency,” says Zettl.
Zettl’s mass sensor is a single carbon nanotube anchored to a negative electrode at one tip, with the other tip facing a positive electrode. Electrons flow from the negative electrode through the nanotube and jump to the positive electrode, where the current is read. As the nanotube wiggles back and forth, the current flowing to the positive electrode varies. At the peak of its oscillation, fewer electrons make the jump.
When no sample is present, the nanotubes’ rate of oscillation is held steady by syncing it with a radio wave of constant frequency–the magnetic component of the electromagnetic radio wave pulls on the nanotube. When a sample is introduced, the oscillations slow measurably, allowing Zettl to determine the weight of the sample. “It’s like we’re playing the nanoradio and throwing atoms at it,” he says. “As we play it, we can hear them.”
In a paper published today in Nature Nanotechnology, Zettl and graduate student Kenneth Jensen describe using the nanoradio to sense and weigh individual gold atoms at room temperature. Zettl says that he chose gold atoms as proof of principle and will now try to detect complex molecules such as proteins with the system.
“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.