Analytical Chemistry Analytical Chemistry
The reaction between polyanaline and formaldehyde is shown above. The polymer is normally planar (A) but
contorts on bonding (B) which changes its conductivity. An array of similar polymer-based sensors comprises
the electronic nose (bottom).
By Tyler Irving
Posted November 2011
The human nose contains millions of cells coated with more than 1,000 types of olfactory receptors. Now, researchers at the National Research Council (NRC) have found a way to mimic this process, creating a prototype ‘e-nose’ that can detect a range of substances, from toxic pollutants to dangerous goods.
Three years ago, a group led by Gerardo Diaz-Quijada at the NRC’s Steacie Institute for Molecular Sciences was asked to develop a more effective chemical sensor for formaldehyde, a toxic compound that often leaches out of building materials. They did this using polyaniline (PANI), a polymer that consists of phenyl rings connected by nitrogen groups. The unique structure of PANI contains many de- localized bonds, which allow electrons to flow along the chain and give it a mild conductivity. At the same time, the nitrogen groups are reactive and can temporarily bond to molecules like formaldehyde. This bonding contorts the polymer and changes the electrical conductivity. By detecting this change, the sensor can identify the presence of formaldehyde in less than a second, even at the parts per billion level.
Even more promisingly, PANI’s structure — and therefore its reactivity — can be easily altered by the addition of various side chains. Diaz- Quijada imagines creating a family of related polymer sensors, each one of which would bond to a given molecule in a slightly different way. “This is exactly how we sense smells with our noses,” Diaz-Quijada says. “You smell something not because we have highly selective receptors, but because the molecule will partially bind to many different receptors and your brain is able to decode this as a pattern.” This pattern or ‘fingerprint’ model explains how humans are able to detect about 10,000 different odours, despite having far fewer unique receptors.
The formaldehyde detector is currently on track to be commercialized within a year, although the e-nose
will take longer to develop. The team is currently shopping the prototype to companies interested in sensing
applications from environmental pollutants to explosives. Diaz- Quijada estimates that the e-nose might be
sniffing around airports or factories in about five years. 
Photo Credit: Gerardo Diaz-Quijada
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