Discovery of Enhanced Interaction Between Carbon Nanotubes and Ammonia

This content is excerpted from Sensor Technology Alert and Newsletter, a sensor intelligence service published by the Technical Insights unit of Frost & Sullivan.


In work funded by the National Science Foundation, researchers at Temple University have discovered that the presence of oxygen groups on single-walled carbon nanotubes (SWNTs) can enhance the interaction between such nanotubes and ammonia. Researchers from the University of Pittsburgh and Emory University contributed to the study.

Eric Borguet of the department of chemistry, at Temple University, indicated that scientists have previously shown that, in using carbon nanotubes for sensors, their conductivity can be changed by the presence of oxygen.

"Theorists have tried for a long time to explain this interaction, and their calculations have typically shown that the interaction between the carbon nanotubes and ammonia is very weak, and in fact, very few ammonia molecules would stick to the nanotubes at room temperature," stated Borguet.

However, he noted that such theorists are studying pure nanotubes, often referred to as "perfect" nanotubes without oxygen.

Using infrared spectroscopy, Borguet and his collaborators revealed that the SWNT purification process--which introduces oxygen to the nanotubes in order to remove impurities introduced in the nanotube fabrication process--changes the interaction between the nanotube and a chemical species, such as ammonia.

"It is no longer pure carbon; there are oxygen-containing groups on the purified nanotubes," stated Borguet. "And, we believe that it is the presence of those groups that enhances the interaction between the nanotubes and the ammonia molecules at any temperature."

"We take the nanotubes and heat them up to 500 Kelvin in vacuum and then cool them down to 94 Kelvin, and we see that ammonia can stick. But, as we treat the nanotubes to higher and higher temperatures, the ammonia signal at 94 Kelvin goes down," stated Borguet.

"One of the things that is happening as we heat to higher and higher temperatures is that we are driving off the oxygen-containing functionality," noted Borguet. "Once that oxygen-containing functionality is gone, 'poof,' the ability of the ammonia to stick is gone. But, if we re-expose the SWNTs to room temperature and ambient air, the ability to interact comes back."

The oxygen-generating functionality may possibly, therefore, be rekindled as a result of exposing the SWNTs to room temperature and air after heating. However, the researchers were not able to detect the oxygen after exposure to air; therefore, the nanotubes may be reoxidizing at a very small level.

Borguet noted that, although the researchers are unable to detect the ammonia sticking to the SWNTs at higher temperatures, the lack of detection may result from using the infrared spectroscopy technique.

"There may be another technique with a higher sensitivity that can detect the presence of ammonia," Borguet stated. "We can't say there is no ammonia, but if there is, it is below our group's detection capability."

Dr. Borguet told Sensor Technology that an improved understanding of how oxygen functionality on SWNTs impacts the interaction of ammonia with SWNTs will be important in gaining a deeper understanding of the sensitivity and specificity characteristics of such nanotubes with respect to ammonia. Moreover, considerably more research is required to understand the interaction between oxygen-containing SWNTs and other types of gases.

In addition, the discovery of oxygen impacting the interaction of ammonia with SWNTs could possibly, eventually, help facilitate the development of small sensors for homeland security applications. For example, with sufficient understanding of how oxygen functionality on a nanotube interacts with different types of gases, one might be able to create a chemical nose comprised of carbon nanotubes that could identify and detect chemicals associated with different hazards.

Dr. Borguet noted that the aforementioned findings represent an initial key step in the journey toward realizing such a chemical nose. Moreover, he has stated that the aforementioned work "could be an important discovery because theorists have all been calculating using 'perfect' nanotubes, but the experiments are not being carried out on 'perfect' nanotubes. The theorists can no longer ignore that there is going to be oxygen-containing functionality when looking at the effects of these nanotubes in the future."

According to Frost & Sullivan's research service on An Assessment on the Future of Carbon Nanotubes--Strategic Analysis of Market & Potential (published June 2004), global revenues for single-walled carbon nanotubes are projected to reach about $199.8 million in 2007. At that time, the distribution of the overall revenues of SWNTs by application is expected to be: field emission devices--35.0%; electronics (including semiconductor components, memories, sensors, and probes)--37.8%; composites--5.1%; fuel cells--1.3%; batteries--1.0%; and research institutions--19.8%.

The Nanotechnology-Applications and Markets in North America research service, from the Technical Insights Division of Frost & Sullivan (published September 2004) reviews and assesses advancements in nanotechnology in the North American region.

One of the drivers of opportunities for nanotechnology in electronics arena identified in the study is the need for small sensing systems. In this vein, the study notes that, 'The current need for small inexpensive sensing systems is driving the development of nanotechnology in this area. Sensors that can collect data from battlefields and from hazardous areas, such as radioactive zones, or sensors that can collect data from living organisms are being developed rigorously. Since biosensors integrate electronics and biology, nanotechnology plays a dual role in terms of shrinking the electronics and interfacing it with the biological world.'