A team of MIT researchers have created economical chemical sensors using chemically altered carbon nanotubes, that facilitate smartphones or other wireless devices to sense trace quantities of poisonous gases. With the aid of these sensors, the researchers aim to build lightweight, low-cost radio-frequency identification (RFID) badges that can be applied for individual security and safety.

MIT researchers have developed low-cost chemical sensors, made from chemically altered carbon nanotubes, that enable a smartphone or other wireless devices to detect trace amounts of toxic gases. (Illustration: Christine Daniloff/MIT)

These types of badges could be worn by people working with dangerous chemicals prone to leaks, and by military personnel on the battlefield to quickly identify the presence of chemical weapons, such as choking agents or nerve gas.

“Soldiers have all this extra equipment that ends up weighing way too much and they can’t sustain it,” says Timothy Swager, the John D. MacArthur lead author on a paper describing the sensors that was published in the Journal of the American Chemical Society and Professor of Chemistry. “We have something that would weigh less than a credit card. And [soldiers] already have wireless technologies with them, so it’s something that can be readily integrated into a soldier’s uniform that can give them a protective capacity.”

The sensor is basically a circuit filled with carbon nanotubes, which are generally extremely conductive but have been enclosed in an insulating material that preserves them in a very resistive state.

Exposure to particular toxic gases causes the insulating material to break apart, and the nanotubes turn out to be highly more conductive. This transmits a signal that can be read by a smartphone integrated with near-field communication (NFC) technology, which allows devices to broadcast data over minimal distances.

The sensors are adequately sensitive to sense less than 10 parts per million of specific lethal gases within five seconds.

We are matching what you could do with benchtop laboratory equipment, such as gas chromatographs and spectrometers, that is far more expensive and requires skilled operators to use. Swager

Furthermore, each sensor costs approximately a nickel to manufacture; around 4 million can be manufactured from about 1g of the carbon nanotube materials.

You really can’t make anything cheaper. That’s a way of getting distributed sensing into many people’s hands. Swager

The other co-authors of the paper from Swager’s lab include Shinsuke Ishihara, a postdoc who is also a member of the International Center for Materials Nanoarchitectonics at the National Institute for Materials Science, in Japan; and PhD students Joseph Azzarelli and Markrete Krikorian.

Wrapping nanotubes

In the last few years, Swager’s lab has created other low-cost, wireless sensors, called chemiresistors, which have identified ripeness of fruit, spoiled meat, and many other things. The sensors are all designed identically with carbon nanotubes that are chemically altered, so their ability to convey an electric current alters when exposed to a specific chemical. For this project, the team built sensors that are extremely sensitive to “electrophilic” substances (simply put - electron-loving, chemical substances), which are frequently lethal and used for chemical weapons.

To achieve this, they developed a novel type of metallo-supramolecular polymer, a material comprising metals bound to polymer chains. The polymer serves as insulation, enclosing each of the sensor’s numerous single-walled carbon nanotubes, dividing them and maintaining them in an extremely resistant state to electricity. But electrophilic substances cause the polymer to disassemble, thereby allowing the divided carbon nanotubes to join together, which facilitates optimized conductivity.

In their study, the team drop-cast the nanotube/polymer material onto gold electrodes. The electrodes were then exposed to diethyl chlorophosphate, a reactive simulant of nerve gas and a skin irritant. With the aid of a device capable of measuring electric current, they noticed a 2,000% spike in electrical conductivity following five seconds of exposure. Likewise conductivity increases were noticed for trace quantities of many other electrophilic substances, such as thionyl chloride (SOCl 2 ), a reactive simulant in choking agents.

Conductivity was considerably lower in response to basic volatile organic compounds, and exposure to a majority of non-target chemicals in fact increased resistivity.

Swager states that developing the polymer was a fragile balancing act but significant to the design. The polymer material should be able to hold the carbon nanotubes apart. But as it disassembles, each monomer has to interact more feebly, allowing the nanotubes to regroup.

We hit this sweet spot where it only works when it’s all hooked together. Swager

Resistance is readable

The researchers developed an NFC tag to construct their wireless system. This tag is capable of turning on when its electrical resistance falls down to below a specific threshold.

Smartphones deliver short electromagnetic field pulses that vibrate with an NFC tag at radio frequency, thereby instigating an electric current, which transmits information to the phone. However, it is not possible for smartphones to vibrate with tags whose resistance is greater than 1 ohm.

The nanotube/polymer material was then applied to the antenna of the NFC tag. The resistance of the material reduced to a point that allowed the smartphone to ping the tag when the material was displayed to 10 parts per million of SOCl 2 for five seconds. Swager states that this is basically an ON/OFF indicator that helps determining the presence of lethal gas.

Researchers feel that a wireless system like this could be used to identify leaks in lithium thionyl chloride batteries used in military systems, fire alarms and medical instruments.

Alexander Star, a professor of chemistry and bioengineering and clinical and translational science at the University of Pittsburgh, highlights that the design developed by the researchers for a wireless sensor or dosimeter for electrophilic substances could enhance the safety of soldiers.

The authors were able to synthesize a [carbon nanotube] composite sensitive to … a class of chemicals of high interest for sensing. This type of device architecture is important for real-life application, due to the fact that a chemical weapon dosimeter worn by military and security personnel requires rapid reading. Star

Swager says that the next step is to examine the sensors on live chemical agents that are difficult to find and more dispersed particularly at trace levels. This test he says will take place outside the lab. He also points out the possibility of developing a mobile app capable of carrying out more refined measurements of an NFC tag’s signal strength. Variations in the signal denote lower or higher concentrations of a toxic gas. “But creating new cell phone apps is a little beyond us right now,” Swager says. “We’re chemists.”

The research was supported by the Japan Society for the Promotion of Science and the National Science Foundation.