Small Sensors Show Big Potential

Signal, Dec 2006 by Kenyon, Henry S

Arrays of sensitive microscopic detectors offer new ways to identify harmful gases and liquids quickly.

Researchers have developed nanoscale sensors capable of detecting trace amounts of chemical and biological agents. The tiny devices can be placed on microchips, creating the potential for highly accurate networked sensors embedded in a variety of equipment and systems.

Conducted by Arizona State University (ASU), Tempe, Arizona, and Motorola's Embedded Systems Research Laboratories, Tempe, the work focuses on using single-walled carbon nanotubes (SWNTs) to detect chemical agents. According to Dr. Nongjian Tao, a professor in ASU's department of electrical engineering who is heading the research, SWNTs are known for their electronic properties, which makes them good material for microchips and transistors.

Tao explains that chemical and biological sensors typically operate by converting chemical reactions into electrical signals. These reactions occur when molecules bind to the surface of the sensor. The engineering challenge lies in converting these molecular-level events into a useful electronic signal.

An SWNT consists of a sheet of carbon atoms rolled into a tube. Every atom in this structure is exposed to the environment, which allows the tube to detect any changes such as chemicals binding to its surface. This high sensitivity makes carbon nanotubes a popular choice for sensors, Tao states. When used as a sensor, the inside of the SWNT is not important because the chemical interactions take place on the outer surface of the tube. The tube's small size is useful for embedding high-density arrays on small platforms such as microchips. SWNT sensitivity provides an advantage because it allows for rapid signal response.

Developing the small SWNT structures into sensors is difficult. Modifying SWNTs to attract specific chemicals strongly will change the tubes' electronic properties, ruining their ability to work as sensors. Tao's solution attaches peptides to the surface of the tubes because peptides do not significantly alter the tubes' electronic properties and are chemically stable.

Peptides are molecules consisting of roughly 20 amino acids that form the building blocks of proteins, natural biological systems that can recognize a variety of substances. By changing the sequencing of the amino acids, scientists can make different types of peptides. "That's the reason antibodies can recognize antigens. The human body can make antibodies by tuning the sequence of the amino acids," Tao says.

Researchers use this building block approach to create different peptides that are attached to an array of SWNTs embedded on a microchip. The various peptides are modified to detect specific chemicals and biological agents. Tao explains that this process allows each nanotube on the chip to distinguish a particular substance. "You can tune the functionality of the device by tuning the sequence and the length of the peptides," he observes.

ASU researchers demonstrated the proof of concept by setting the tubes to detect several simple chemicals and heavy metal ions. Tao notes that the heavy metal ions were chosen because the U.S. Food and Drug Administration and the U.S. Environmental Protection Agency had provided funding to develop methods of detecting these ions in drinking water. He adds that SWNTs can be modulated to detect a range of substances from toxic gases in the air to chemicals in water.

In addition to carbon nanotubes, Tao's team also studies the use of conducting polymers as detectors. He explains that the concept can be used in a manner similar to SWNTs. Because these polymers are naturally conductive, the electric current flowing through a polymer-equipped device can be measured to detect any binding events taking place on the device's surface. Polymers are flexible and can be synthesized for specific purposes; however, conductive polymers do not have the electronic properties of carbon nanotubes.

Polymers used as chemical detectors are referred to as conducting polymer nanojunctions, which consist of two semiconducting metal electrodes with a small gap in between. This space is bridged with the conducting polymer to create a nanoscale junction. Nanoscale sensors have several advantages. Tao explains that the small size reduces the surface-to-volume ratio of a device, making it more sensitive. The smaller volume also decreases response time to a reaction.

Tao notes that similar methods are used to convert conducting polymers and SWNTs into sensors. The polymers, when activated with probe molecules, can recognize different chemicals and biological agents. Peptides also can be attached to polymer nanojunctions. He shares that this type of chemistry is somewhat easier to accomplish with polymers than with nanotubes. "When you're trying to modify or attach something to carbon nanotubes [with peptides], sometimes you can actually change the properties of the carbon nanotubes. Conductive polymers give you more freedom to choose the right way to do it," he says.