Fine-tuning carbon nanotubes

Since their discovery in the 1990s, carbon nanotubes have ensnared the imagination of chemists. Among them are researchers at Pacific Northwest National Laboratory who are putting these fine filaments--ten-thousand times smaller than a hair--to work as biosensors and improving the way carbon nanotubes can be chemically customized to form the basis for a wide variety of devices.

Yuehe Lin, a PNNL staff scientist, recently reported at the national meeting of the American Chemical Society the first successful lab test of a disposable organophosphate (OP) sensor he fashioned from carbon nanotubes.

Besides posing a serious environmental hazard, OP compounds are raw material for nerve agents. Crews responding to a terrorist's nerve-agent attack might not know what hit them until too late. Lin chemically fused carbon nanotubes to enzymes that act as catalysts in neurotransmitters, the impulses that enable nerve cells to communicate. He peppered a 2- by 4-millimeter sensor surface with carbon-tube bits and their bound enzymes. In the presence of OP, enzyme activity is dampened. The nanotubes, acting as electrodes, sensed the inhibition as a muted signal and passed that information to an off-the-shelf electrochemical detector. The detector was plugged into a notebook computer for an instant reading of OP, at traces as little as 5 parts per billion.

Testing another sensor configuration, Lin showed carbon nanotubes could be used in broad biomedical applications--in this case, in measuring blood sugar. In a paper published earlier this year in the journal Nano Letters, Lin described how he stood carbon nanotubes about 50 nanometers wide on end. They were treated with the enzyme glucose oxidase and anchored to epoxy-covered material that acted as an electrode contact; their tips protruded through a polished sensor surface, enabling them to come into contact with blood, which started a catalytic reaction whose energy was conveyed via the carbon-nanotube electrodes. The stronger the signal, the higher the blood sugar. Lin credits the sensor's utility to the reactivity of the enzyme, the excellent conductivity of carbon nanotubes and the enormous nanoelectrode array--about a million carbon nanotubes integrated on a surface the size of a pencil lead.

Meanwhile, Leonard Fifield, Chris Aardahl and their PNNL colleagues, along with Larry Dalton of the University of Washington, have developed a promising new way to attach molecules to the surfaces of carbon nanotubes, which they reported in the Journal of Physical Chemistry B.

To be modified for specific applications, nanotubes typically have to be induced to become reactive by subjecting them to strong acidity or other harsh conditions. "These processes can mute the strength and conductivity that make pristine nanotubes interesting for use in active devices such as biosensors and regenerable sponges for atmospheric carbon dioxide," Fifield said.

The new PNNL technique employs a "supercritical fluid," in this case carbon dioxide, to load specially designed molecules onto the nanotubes without compromising the tubes' strength and sensitivity.

"The use of supercritical carbon dioxide, which has both gaslike and liquidlike properties, eliminates the need for harmful organic solvents," Fifield said. "It also enables us to control how much of a nanotube surface is coated with molecules and to control the thickness of the coating."