MIT researchers have developed a chemical sensing material whose electrical conductivity dramatically increases when exposed to toxic gases. They were able to integrate the material into the electronic circuit in a near-field communication (NFC) tag, which is embedded in smart cards like those used to get through train ticket gates. They then demonstrated that this technology makes it possible for smartphones to quickly (within five seconds) detect toxic gases at the low concentration of 10ppm.
The chemical sensing material consists of a group of carbon nanotubes (CNTs) individually wrapped with supramolecular polymers – clusters of monomers held together through weak interactions. The material’s electrical conductivity increases by up to 3000 times when it is exposed to electrophilic toxic gases. CNTs alone are highly conductive materials, but when they are wrapped with supramolecular polymers, which serve as insulators, they become poor conductors.
The supramolecular polymers used in this sensor were designed so that weakly-bound sites in the molecules are dissociated when these sites are exposed to toxic gases, causing the wrapping molecules to disassemble. As a result, the original high conductive state of CNTs is restored. The extent of change in conductivity is directly proportional to the concentration of and the duration of exposure to a toxic gas, and the conductivity change can be easily measured by a commercially available resistance meter.
According to the researchers, smartphones can be turned into toxic gas sensors when the chemical sensing material is integrated into the electronic circuit present in a commercially available NFC tag. Users can readily determine the presence or absence of toxic gas by holding an NFC-compatible smartphone over a sensor-embedded NFC tag while making sure that communication between the two devices is intact. The sensor is disposable and 1g of the chemical sensing material makes 4 million sensors, meaning it is feasible to mass-produce the sensor at a low cost.
In future, the researchers plan to develop chemical sensors that are capable of detecting various types of toxic chemical substances more sensitively, quickly and effortlessly by making modifications to the structure of the supramolecular polymer. They also intend to develop a system that contributes to making society safer by selecting the type of wireless communication technology (in terms of communication distance and power consumption) compatible with the application of chemical sensors and considering the integration of chemical sensors with internet cloud technology.