Sensor design gets systematic

PORTLAND, Ore. — Sensor manufacturers have continued to improve the sensitivity of their designs through engineering innovations derived from trial-and-error experimentation. Electrical engineers have been guided by "black art" principles, which sensor-gurus claim in abundance. Unfortunately, no overarching framework has been available that incorporates these principles into a methodology for new sensor designs.

Now EEs at Purdue University (West Lafayette, Ind.) claim to have invented that missing framework, providing a new method of modeling sensor designs that is already solving long-standing puzzles.

"Other groups have come up with a whole array of conflicting principles regarding how to make better sensors," said Ashraf Alam, an EE and professor of electrical and computer engineering at Purdue. "But we have unified those principles in a systematic way, so that now there is a consistent framework regarding how to make sensors better." He performed the research with his student, an EE doctoral candidate, Pradeep Nair.

To test their sensor design principles, the researchers addressed the issue of which nanoscale sensor designs are optimal for sensor applications where target molecules stick to the sensing element. EEs have long known that when sensing individual molecules—from smoke detectors to biological and chemical sensors—the smaller the sensing element, the better. The reason smaller is better, however, has been only anecdotally related to how diffusion of the target molecule limits the speed at which a sensor can act.

Alam and Nair, however, now claim to have provided a theoretical solution to this puzzle that they subsequently verified by experiment. First, the researchers compared a traditional flat planar sensor element with a cylindrical single-nanotube sensor element. The results showed that the smaller cylindrical sensor was at least 100 times more sensitive—more than could be accounted for by conventional smaller-is-better theories.

"We were surprised to see that the cylindrical sensor is orders of magnitude more sensitive than planar sensors," said Alam.

The reason why was even more surprising. EEs have often speculated that nanoscale sensors were better because the sensing element was closer in size to the molecules being detected—that the so-called "swamping effect" of a large planar sensor could be mitigated by making the sensing element smaller. But the Purdue EEs said that is not the reason.

The reason that nanoscale cylindrical sensors are better than flat planar sensors, they said, is that the target molecules can diffuse onto the surface of the planar sensor only from the "front" side, whereas the cylindrical nanotube has no "front," thus canceling out the slowing effect of diffusion.

"What happens when you have a small cylindrical sensor like a nanotube or nanowire is that the molecules you are trying to sense can come from any direction—which per unit area makes it more likely to sense a molecule than a traditional planar sensor," said Alam.

Because cylindrical nanoscale sensors were known to be more sensitive, but were hard to make, some sensor designers have resorted to nanocomposite (also called nanonet) sensing elements that utilize multiple cylindrical nanotubes or nanowires grown in a bushy jumble-of-nanowires that resembles the opening move in the game of "pick-up sticks."

"Several manufacturers are using these pick-up-stick-like sensors—some claiming they are even better than single cylindrical sensors," said Alam. "But we found that while these sensors were better than planar sensors, they were not as good as the single nanowire sensor."