A little over four years ago, researchers at the University of California, Riverside, developed a sensor made from carbon nantoubes for detecting toxic chemicals. So enthusiastic were the researchers with the prospects of their technology that they launched a company, Nano Engineered Applications, that intends to add this sensor to people’s mobile phones.
While the commercial prospects of a smartphone toxin detector are still uncertain, another team of researchers has recently demonstrated a sensing device that also relies on carbon nanotubes (CNTs) to detect different chemicals. Researchers from Japan’s International Center for Materials Nanoarchitectonics and the National Institute for Materials Science, working with collaborators from MIT, combined CNTs with a polymer and discovered that this resulting material offers a powerful sensing platform for toxic chemicals. Their results look to be the first viable demonstration of the power of sensors known as chemresistors.
To demonstrate the effectiveness of their CNT-based material, the researchers integrated it into the electronic circuit of a near-field communication (NFC) tag. Using a smartphone or other device capable of reading the tag, the sensor could be used to detect toxins in seconds and at concentrations as low as 10 parts per million.
The researchers were able to achieve this high speed and sensitivity by wrapping the carbon nanotubes in supramolecular polymers. A supramolecule is actually a number of different molecules that are fused together to act as a single molecule and carry out a particular programmed function. The sensor itself would be considered disposable since only 1 gram of the material is capable of making four million sensors.
In research described in the Journal of the American Chemical Society, the researchers found that when they wrapped the supramolecular polymers around the CNTs, the polymer acted as an insulator that limited the natural high conductivity of the CNTs. When the polymer wrapping comes in contact with a toxic gas, it begins to disassemble, which leads to a big spike in the conductivity of the CNTs—as much as a 3000 times increase in electrical conductivity.
“Think of the wrappers as insulation around CNT wires,” explained Timothy M. Swager, a professor at MIT who worked on the project, in an e-mail interview with IEEE Spectrum. “Reactions with toxic chemicals cause the disassembly and the wires touch and make a circuit. Increased conduction gives detection.”
Sensors like this are known as chemiresistors and have long been thought to be a very powerful tool for detecting small amounts of chemicals, and, according to Swager, this is the first viable demonstration. “You don’t have to move many electrons to measure this and we can inductively power and read the sensor with near-field communication devices, such as a smartphone,” he said.
In future work, the research team is looking to make modifications to the supramolecular polymer to detect different types of toxins more quickly and more sensitively. “We are likely to work with live chemical agents in the future,” Swager added. “I am also using this concept to create sensors with selectivity to different types of toxins.”
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.