Research Stories

by Linley Erin Hall

Humans have a set of built-in chemical-detection devices. For example, the nose identifies chemicals by their smells. The tongue identifies chemicals by their tastes. Using these senses and others, people can interact with the chemical world that surrounds us.

Computers, on the other hand, do not have this skill.

"In terms of interacting with the chemical world, we still have limited technologies," says NJ Tao, a professor at Arizona State University's Ira Fulton School of Engineering.

Tao is an electrical engineer. He says that voice activation features allow computers to interact with the human voice. The mouse allows for interaction with a keyboard. But computers still cannot interact with the chemical world.

"A computer does not recognize the world in terms of chemistry or molecules," he says.

Tao and his colleagues are trying to change this. They are creating a set of ultra small, nano-sized sensors that can detect single molecules of different substances. Such sensors have applications in healthcare, homeland security, pollution monitoring, and many other areas.

Tao's ASU group aims to incorporate these sensors into handheld instruments that can communicate with computers via wired or wireless technologies. This will produce fast, extremely accurate results.

The work requires expertise in chemistry, electrical engineering, computer science, and communications engineering. "Our work is a very interdisciplinary project," says Erica Forzani, assistant professor of research in electrical engineering.

To date, the ASU researchers have developed several different kinds of nanosensors. All of them rely on a sensor's ability to detect a target molecule. The target molecule goes through a chemical reaction with part of the sensor so that the two attach to one another. Scientists call this a "binding event."

The sensors then convert this binding event into a signal that can be measured electronically. The difference is in the type of signal that each sensor produces.

One sensor is based on tiny tuning forks. These are shaped like the tool used by piano tuners. The tiny versions are made of the mineral quartz rather than metal. When a molecule binds to a fork, it changes the fork's vibration. This yields a mechanical signal that can be measured.

The researchers have also created a sensor in which a molecule binds to a nanowire between two nanoelectrodes. This generates an electrical signal.

Forzani says that working at the nano scale provides many advantages. "When you scale down the size of a sensing device, basically the surface to volume ratio is huge. It changes drastically," she says. "This fact gives you a big advantage in terms of improving the sensitivity."

These sensors are incredibly sensitive. The ASU researchers can actually see the effects of individual molecules binding to a nanowire or tuning fork.

Tao and his colleagues tweak the basic design of these sensors so that they work well for different applications. And the applications currently under development are wide-ranging.

For example, one nanowire-based sensor will detect ammonia in human breath. Ammonia is an indicator of kidney malfunctions and stomach ulcers. Healthcare professionals currently determine ammonia levels in the body through a blood test that must be sent to a laboratory. That takes time. A nanosensor-based breath test would be much easier for patients, and give near-immediate results.

Nanowire sensors have also been used to test for metals such as copper and nickel ions in drinking water. These sensors are portable. That is a big advantage.

"We can run this with batteries and have it on a handheld device. Someone can go to the site and test the water," says Alvaro Diaz Aguilar, a doctoral student in Tao's group.

Data gathered on location can then be sent back to the lab wirelessly. In contrast, the method currently used by the EPA to monitor water is much more time consuming. Samples must be taken to the lab because the equipment used is too big and bulky to move.

The ASU researchers have developed tuning fork nanosensors that detect various molecules. They have also found some surprising applications.

For example, tuning fork sensors could be made to be sensitive to infrared radiation, also known as IR, or heat. Better IR sensors would help soldiers to see in the dark.

Most IR sensors in use today require cooling. That makes them bulky and more difficult for soldiers to carry around. On the other hand, the tuning fork sensor is small and works regardless of temperature. The tuning fork sensor is also very inexpensive. Tao says that each tuning fork costs less than a dime, and they are relatively easy to modify.

The ASU group is different from most others working on sensors. They are focused on creating devices that will work in the real world on real problems. They are thinking about how to integrate these nanosensors with electronics to read the signals, display the data, and even send the data through wireless networks. As such, the team combines the expertise of electrical engineers with chemists. They also work with researchers at Motorola on aspects of the devices such as wireless communication.

Tao says that nanosensors have a lot of promise. But the ASU researchers are still faced with many challenges. In particular, a sensor that works in the lab doesn't necessarily work as well (or at all) in the real world.

Laboratories are clean. They have relatively constant temperature and humidity. Not so the real world.

Testing is different as well. Researchers initially test their sensors on samples that contain just the few molecules they want to analyze. But many applications require sensors that can simultaneously detect lots of different molecules while ignoring others. For example, water in a polluted stream might contain many different harmful substances, including bacteria, algae, metals, and other chemicals. The ASU group is currently developing ways to increase the sensors' capabilities.

If one type of sensor is not adequate for a particular application, then an array of sensors can be used. Francis Tsow is a graduate student in Tao's lab. He compares a tuning fork array to the human nose.

"Our nose sensors are not very specific in certain situations. But we have many different kinds of them. So we basically form a map for each individual chemical," he says.

The individual sensors may all bind to the same group of chemicals, but their responses to each one will differ. This allows researchers to detect a particular chemical based on its pattern of responses on the array.

Despite these challenges, nanosensor research at ASU is moving full steam ahead.

"We're trying to take an integrated approach," Tao says. "If you're trying to optimize a device on the device level, it may not be good enough. You need to think about the system level. You have to think about sample collection and delivery, data processing and transmission. Everything needs to be considered, not just a single sensing element."

Nanosensor research at ASU is supported by the Environmental Protection Agency, the National Institutes of Health, Motorola, and the Defense Intelligence Agency. For more information, contact NJ Tao, Ph.D., Ira A. Fulton School of Engineering, 480.965.4456. Send e-mail to njtao@asu.edu. Visit at http://fulton.asu.edu