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Posted: March 18, 2007
A nanotechnology nose for sommeliers
(Nanowerk News) Researchers in the Organic Electronics Group at the University of California, Berkeley are working on printed electronic nose vapor sensors that, for instance, could monitor if wine has gone bad or the contents of a bottle of medicine is spoiled.
Most organic semiconductors are sensitive to their environment. Unfortunately, they typically show sensitivity to multiple vapors and are therefore nonspecific. Fortunately, specificity may be obtained by arraying thin-film transistors with different channel materials and subsequently pattern matching.
In comparison to an all-silicon nose, such a system offers similar sensitivity (ppm level detection) and specificity (single functional group differentiation) at lower cost. Silicon noses typically require one additional mask per sensor element, and often need many elements for high specificity. The use of printed sensors with high functional specificity eliminates these needs, and should enable the realization of integrated noses with a cost of less than 5 cents. That is over 1000 times cheaper than current electronic noses. In the longer term, it should be possible to integrate signal processing within the organic platform itself.
To separate the sensor shift due to analyte exposure from the sensor shift due to bias stressing, differential architectures are used, with two identical transistors wired into a bridge. One transistor is protected with an overlying polymer layer. Both transistors are biased at identical voltages, ensuring that the bridge
accounts for the bias stress. Upon analyte exposure, only the unprotected transistor shows an additional shift, which may be measured.
Nanotechnology plays a part in constructing these sensor devices. Gold is used to construct the transistor gates. Gold in its bulk state melts at 1,000° C. Nanoparticles of gold, however, melt at 100° C, ensuring that the gold can be melted without destroying the underlying polymer sheet. The reactive thin-films in the transistor are around 20 nanometers thick.
To demonstrate their electronic nose, the researchers studied the spoilage of wine. Calibration experiments performed with pure ethanol and acetic acid show that differential signals are expected as wine spoils due to the accumulation of acetic acid. When fresh wine is slowly converted to stale wine, all the device
parameters, including mobility, threshold voltage, and drive current, drift upward. The researchers say that wine spoilage detection should therefore be possible, basically by having such a sensor inside the wine bottle. This technology could be extended to a wide range of consumer product monitoring.
Further reading: "Towards the nanoscopic electronic nose" is a Nanowerk Spotlight where we introduced resarch that showed that metal oxide nanowires can be used as a platform for the fabrication of the next generation ultra small e-noses and probably substitute in the future the current devices based on polymer or oxide thin-films.