14 July 2009

Nano sensors that detect toxic chemicals

There is now a new method to make extremely pure, very small metal-oxide nanoparticles, which may end up detecting toxic industrial chemicals (TICs) and biological war agents.

The challenge is to design a material that reacts quickly and reliably to a variety of chemicals, including TICs, when incorporated into a sensor, said Patricia Morris, associate professor of materials science and engineering at Ohio State University, who is leading a team of researchers developing solid materials that can detect toxic chemicals.

“These are sensors that a soldier could wear on the battlefield, or a first responder could wear to an accident at a chemical plant,” Morris said.

The material under study is nickel oxide, which has unusual electrical properties. Other labs are studying nickel oxide for use in batteries, fuel cells, solar cells, and even coatings that change color.

Morris is more interested in how nickel oxide’s electrical conductance changes when toxic chemicals in the air settle on its surface. Ohio State doctoral student Elvin Beach applies a thin coating of the material onto microelectro-mechanical systems (made in a similar fashion to computer chips), with a goal of identifying known toxic substances.

The design works on the same general principle as another, much more familiar sensor.

“The human nose coordinates signals from hundreds of thousands of sensory neurons to identify chemicals,” Beach said. “Here, we’re using a combination of electrical responses to identify the signature of a toxic chemical.”

The key to making the sensor work is how you make the nickel oxide particles. Beach and Morris devised a new synthesis method that yields very small particles—which give the sensor a large surface area to capture chemical molecules from the air—and very pure particles—which enable the sensor to detect even very small quantities of a substance.

Each particle of nickel oxide measures only about 50 atoms across, which is the equivalent to 5 nanometers (billionths of a meter).

Beach described the synthesis method in very simple terms. “Basically, you mix everything together in a pressure vessel, pop it in the oven, rinse it off, and it’s ready to use,” he said.

Of course, for the process to go smoothly, the researchers have to meet specific conditions of temperature and pressure, and leave the material in the pressure cooker for just the right amount of time. For this study, they set the pressure cooker to around 225°C. They found they can make the particles in as little as 12 hours, but no more than 24 hours.

“Too short a time, and the nickel oxide doesn’t form; too long and it reduces to metallic nickel,” Beach said.

After removing the nickel oxide from the pressure cooker, Beach washes it in a common solvent called methyl ethyl ketone to free up the nanoparticles.

At that point, the material is ready to use. Most other synthesis methods require another additional step, a high-temperature heat treatment.

Starting with a microsensor silicon chip array provided by collaborators, Beach added a layer of particles using a device called a picoliter drop dispenser. A picoliter is a trillionth of a liter.

He describes the dispenser as a kind of inkjet printer that places a droplet of a liquid suspension containing particles onto the chip surface.

Researchers now know the reaction pathway, so they can devise ways to add chemical dopants to the nanoparticles. Dopants would change the function of the sensor, which could speed up the response rate.

A one-gram batch of nickel oxide nanoparticles costs about $5 to make; one chip carries 4 nanograms (billionths of a gram) of material, so each sensor costs only pennies to fabricate.

Other applications could include exhaust or pollution monitoring and air quality monitoring.

For related information, go to www.isa.org/sensors.