New devices are inexpensive, powerful - Detection - gas detectors

A new type of sensor based on porous silicon and a unique metallization process could offer enhanced sensitivity, reduced power demands, and lower cost compared to existing technologies for detecting gaseous compounds important in environmental, food, and biomedical applications. Because they are based on silicon wafers, manufactured using integrated circuit production techniques, and operate at room temperature employing relatively low voltages, the new sensors could be integrated into electronic equipment and used to build sensing arrays.

"The sensors show a rapid reversible response to low concentrations of these gases at room temperature," says James L. Gole, professor of physics, Georgia Institute of Technology, Atlanta. "They operate on a voltage much less than that of a watch battery and would be small enough to be taken into the field with a troop contingent or any other group concerned about the presence of harmful gases. The sensors are so simple that they could ultimately be mass-produced for pennies apiece."

Sensors based on porous silicon have been built before, but their practicality has been limited because of high resistance in the electrodes connected to the porous silicon and the power requirements of as much as five volts. Utilizing a unique metallization process, however, Gole and his collaborators dramatically reduced the resistance of the electrodes built into the silicon, allowing their sensors to operate at between one and 10 millivolts--a dramatic improvement over earlier sensors.

The new devices can detect ammonia, hydrochloric acid, and nitrogen oxides at concentrations of between 10 and 100 parts-per-million--if not lower--compared to 100 to 1,000 parts-per-million for the higher-voltage sensors. Because the chemical reaction they use to detect the gases can be rapidly reversed, the new devices are reusable. Moreover, after long-term use, they can be regenerated with a simple chemical treatment.

The introduction of gases onto sensitive porous surfaces causes dramatic changes in their conductance. Simple and inexpensive electronic equipment can be used to measure these changes. That could allow the sensors to be integrated onto a microelectronic chip and utilized as part of an "artificial nose" to detect a range of potentially toxic compounds.

The combination of low cost, low power consumption, room temperature operation, and simple production for the devices opens up new possibilities for sensing applications. "We believe these sensors can be inexpensively built in arrays, which opens up interesting opportunities for mixture analysis in water quality, environmental sensing, food toxin detection, and agricultural uses," maintains Peter Hesketh, a professor in the School of Mechanical Engineering. "We also see the possibilities for biomedical analysis useful in blood analysis, pathogen testing, and analyzing allergic reactions."

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