| Posted: September 25, 2009 |
Cheap, sensitive carbon nanotube sensors could detect explosives, toxins in water |
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(Nanowerk News) A sensitive new Stanford-developed disposable chip detects low concentrations of the explosive trinitrotoluene (TNT) and a close chemical cousin of the dreaded toxic nerve agent sarin in water samples. The research appears online this week in the journal ACS Nano ("Sorted and Aligned Single-Walled Carbon Nanotube Networks for Transistor-Based Aqueous Chemical Sensors").
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Traces of TNT can leach into streams near munitions-making and testing sites, and then be detected downstream.
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Terrorists could try to mix sarin into a reservoir or water mains. An electronic sensor that can instantly detect very low concentrations in water would be a desirable technology for staying ahead of potential attacks, said chemical engineering Associate Professor Zhenan Bao, who leads the group that developed the chip.
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| A diagram of a nanotube transistor on a flexible chip for detecting toxins or explosives in a water sample.
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Although many researchers around the world have devised a wide variety of chemical sensors, Bao said, the Stanford chip offers a rare combination of low-cost materials, low power usage, robust and repeatable performance in water, instant response and physical flexibility. To date, lower detection limits have only been achieved with complex, expensive, non-portable optical systems.
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Extreme sensitivity
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"We have used semiconducting carbon nanotube network transistors to make extremely sensitive sensors that are capable of operating stably under water," Bao said. "We showed sensitivity in the range of a few parts per billion for detection of explosive compounds such as TNT."
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A nanotube is a rolled-up sheet of carbon atoms that is only one atom thick. Every atom is therefore on the tube's surface. This makes single-walled nanotubes very sensitive to nearby molecules that would drift by in a water sample, says postdoctoral researcher and article co-author Melburne LeMieux.
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The Stanford sensors are more sensitive than other waterborne, nanotube-based sensors because they are built using processes developed in Bao's lab that ensure a high-density of well-aligned nanotubes that are almost purely semiconducting. Errantly placed or jumbled nanotubes would reduce the sensor's sensitivity. Semiconducting nanotubes – those that switch electrical current on and off rather than always conducting it like a wire – can detect a wider range of molecular interactions with greater sensitivity than purely conducting nanotubes.
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The chip itself is made of an inexpensive, flexible plastic substrate, rather than the more expensive, rigid silicon that underlies most computer chips. The researchers also use a thin polymer gate electrical insulator layer, which allows the device to operate on less than 1 volt of electricity.
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"By combining our nanotube self-sorting deposition process with our ultrathin cross-linked polymer dielectric [insulator] formulation, we've enabled underwater, nanotube-based chemical sensors," said paper first author Mark Roberts, a former graduate student who is now a postdoctoral researcher at Sandia National Laboratories.
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Unmistakable results
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In lab tests, LeMieux and Roberts found that when the chip was exposed to water with just 2 parts per billion of either TNT or dimethyl methylphosphonate (a sarin cousin), the chips instantly reported unmistakable changes in electrical current.
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"It's very rewarding when you add in the chemical and you see this very rapid response," LeMieux says. "I don't know if it's must-see TV, but it's definitely worth watching."
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To make real contributions to security, the authors acknowledge, the chips will have to be packaged into a field-worthy device with a power supply and a wireless transmitter. They also will require more sophisticated nanotube circuitry, or microfluidics that can precisely sort through all the chemicals that would likely be present in a real-world water supply.
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Those are both next steps for the group.
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Support for the research came from fellowships from NASA, the Sloan Research Foundation and the Intelligence Community Postdoctoral Fellowship Program, as well as from the National Science Foundation.
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