Electronic nose sniffs sweet, sour, spicy aromas

An electronic-nose technology was demonstrated this week at the Composites at Lake Louise Conference in Alberta, Canada. The electronic nose, invented by Harry Tuller, an electrical engineer from Massachusetts Institute of Technology, mimics biology through a low-cost thin-film fabrication technique that combines the best aspects of organic and inorganic materials.

According to Tuller, the nose does not have individual detectors for each smell, but an array of sensor channels that categorize aromas generally, such as sweet, sour and spicy. The relative amounts of each aroma characterize a smell's signature. Tuller's research team copied this architecture using a low-cost direct-writing technique for mimicking the biology of smells. The resultant electronic nose harnesses an experimental Hewlett-Packard programmable inkjet head to print arrays of detectors side-by-side in thin films using maskless lithography. By directly writing sensor films to a quartz substrate, the programmable printhead can create arrays of smell sensors that work like a nose, but which can be calibrated to sense the aroma of noxious gases including those wafting off toxins and explosives.

"So far, we have demonstrated how to use HP's programmable inkjet cartridge to create templates of thin films using our ink formulas, and shown that it works for nitrogen oxide [NO] emissions from diesel exhaust," said Tuller, a professor in MIT's materials science department. "To create our electronic nose, we are now working on integrating [onto the same chip] arrays of thin-film sensors that are each sensitive to a different type of gas."

Key to success
The key to the project's success was a technique to maximize the surface area of his electronic nose's sensor pads, according to Tuller, who also credits his collaborators—postdoctoral fellow Kathy Sahner and graduate student Woo Chul Jung. By texturing the sensor arrays, the researchers have increased sensitivity to the point that the sensors are sensitive to even a single layer of the molecules.

"It turns out sensitivity is proportional to surface area, which we increased tenfold by first putting down a layer of hollow, very thin-walled spheres made out of an organic polymer, and then on top of that we deposit our inorganic ink," said Tuller. "Then we heat the whole thing up so that the organic material burns away, leaving a highly textured inorganic thin film."

For their first test example, the researchers printed the ceramic material barium carbonate on top of a quartz crystal, then applied an alternating voltage to resonate it at about 10MHz. As NO gas molecules were adsorbed by the sensing layer atop the quartz crystal, the frequency of resonance dropped.

"Even a monolayer can change its mass and thus its resonant frequency," said Tuller.

The initial sensor coating was barium carbonate, which is sensitive to NO, but the researchers have also tested the technique with other coatings to sense other chemicals. Next, the researchers are building arrays of the sensors on the same chip, so that similarly to the electronic nose, chemicals can be identified as distinctive signatures of responses from a standard set of sensors.

"We are currently formulating a set of inks that we can put down in a pattern on top of an array of resonators, each resonator covered with a different ink," said Tuller.

Ink experiments
The researchers are also experimenting with inks that change their resistance when gases are adsorbed on them, so that they can pattern an array of general-purpose sensor pads first, then functionalize them for detecting different combinations of chemicals by putting down a second layer of coatings on top—a different one atop each sensor pad.

Tuller's lab is funded by the National Science Foundation.

- R. Colin Johnson
EE Times