MIT chemical engineers used carbon nanotubes to build an ultra-sensitive electronic detector for deadly gases such as the nerve agent sarin, mustard gas, ammonia, and VX nerve agents. The technology shows potential for environmental or security applications. For example, it can be designed into a low-cost, low-energy device that’s carried in a pocket or deployed inside a building to monitor hazardous chemicals.
The sensor has exhibited record sensitivity to molecules mimicking organophosphate nerve toxins such as sarin. More specifically, it can detect minute quantities as low as 1 femtomole, roughly equivalent to a concentration of 25 parts per trillion.
Sarin can kill at very low concentrations in as little as 10 minutes, so highly sensitive detection is imperative to save lives. The new detector is much more sensitive than needed to detect lethal doses.
Another bonus is that the nanotube sensors require only about 0.0003 W. Basically, one sensor could run forever on a regular battery.
The research team used an array of carbon nanotubes aligned across microelectrodes to build their super-sensitive detector. Each tube consists of a single-layer lattice of carbon atoms, rolled into a long cylinder that acts as a molecular wire. The “wire” has a diameter of about 1/50,000 of the width of a human hair. When a particular gas molecule binds to the carbon nanotube, the tube's electrical conductivity changes. Each gas affects conductivity differently, so gases can be identified by measuring the conductivity change after binding.
The researchers achieved new levels of sensitivity by coupling the nanotubes with a miniature gas-chromatography column that’s etched onto a silicon chip smaller than a penny. The column rapidly separates different gases before feeding them into the nanotubes.
An imporatnt feature is that the sensor is passively reversible at this level of sensitivity. To achieve this, the team needed to decrease how strongly the nanotube sensor binds different gas molecules on its surface, allowing the sensor to detect a series of gas exposures in rapid succession.
This can be done by coating the nanotubes with amine-type molecules, which donate an extra pair of electrons to the nanotubes. The coating lets gas molecules bind to nanotubes but detach a few milliseconds later, allowing another molecule from the column to move in. With a network of these reversible sensors, a gas can be tracked as it spreads through a large area.