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Article: Nanoelectronic carbon dioxide sensors Authors: Star, A;Han, TR;Joshi, V;Gabriel, JCP;Gruner, G Journal: ADVAN MATER, 16 (22): 2049-+, NOV, 18 2004 Addresses: Nanomix Inc, Emeryville, CA 94608 USA. Nanomix Inc, Emeryville, CA 94608 USA. Univ Calif Los Angeles, Dept Phys, Los Angeles, CA 90095 USA.

Alexander Star talks with ScienceWatch.com and answers a few questions about this month's Fast Moving Fronts paper in the field of Materials Science.

Why do you think your paper is highly cited?

Carbon nanotubes, and specifically carbon nanotube-based chemical and biological sensors, continue to be an exciting area of research. Carbon nanotubes, with diameters comparable to the size of individual molecules, offer the advantage of high sensitivity. It is also possible to integrate numerous carbon nanotube-based sensors on a single silicon chip, which allows for the simultaneous detection of numerous analytes in real time with low power consumption. Our article demonstrated an application of carbon nanotube-based sensor technology for the detection of carbon dioxide (CO₂) gas—a very important chemical analyte.

Does it describe a new discovery, methodology, or synthesis of knowledge?

This paper describes the first account of a carbon nanotube-based field-effect transistor (NTFET) for the detection of CO₂ gas at concentrations as low as 500 parts per million (ppm). While CO₂ detection has been achieved using other technologies, primarily nondispersive infrared (NDIR) sensors, these techniques suffer due to power consumption and size. NTFETs are very small in size, can be produced at low cost, and have a low power consumption, thus rendering them attractive for disposable medical sensing as well as for wireless sensing in industrial and environmental applications.

Would you summarize the significance of your paper in layman's terms?

(A) Conceptual drawing of a carbon nanotube network device coated with polymer layer for CO₂ detection. (B) Atomic Force Microscopy (AFM) image of network of carbon nanotubes functionalized with a polymer layer for CO₂ sensing.

The significance of this paper relates to the development of a nanosensor that is capable of detecting CO₂ gas, in real time, and under normal environmental conditions. These devices represent a new generation of low-cost, low-power CO₂ gas sensors that can be embedded into disposable airway adapters, masks, and resuscitation bags to monitor human respiration in medical settings.

The significance of our research results was immediately recognized by the scientific community. Our paper was highlighted in Nature journal in the article "Nanotubes keep tabs on breathing," published online November 12, 2004. The National Science Foundation (NSF), which funded our research through their Small Business Innovation Research (SBIR) program, has also issued their press release "Monitoring life, one breath at a time" (NSF PR 04-148; November 10, 2004).

How did you become involved in this research, and how would you describe the particular challenges, setbacks, and successes that you've encountered along the way?

This research was conducted at Nanomix Inc., a startup nanotech company, which develops and commercializes nanoelectronic sensors for healthcare providers. The measurement of CO₂ levels in respiration is a standard of care according to the American Heart Association, the American Association of Respiratory Care, and the American Society of Anesthesiologists. So, we at Nanomix were extremely interested in applying carbon nanotube-based sensors for detection of CO₂.

The particular challenge in this case was inherent insensitivity of carbon nanotubes to CO₂. At that time, NTFETs have already demonstrated detection of electron donating (NH₃) and electron withdrawing (NO₂) molecules. While bare NTFET devices are sensitive to the presence of strong electron donors and acceptors, they are not sensitive to weak Lewis acids or bases such as H₂, CO₂, and CH₄. Therefore, special recognition chemistry is required to induce chemical processes that can modify the NTFET device characteristics.

After a long path of trial and error, we have adapted a mixture of poly(ethyleneimine) (PEI) and starch polymers as the CO₂ selective recognition layer. Upon exposure to CO₂ gas, the chemical reactions in the hydrated polymer layer lower the local pH and alter the charge transfer to the semiconducting carbon nanotube channel, resulting in the change of the NTFET electronic characteristics.

Where do you see your research leading in the future?

The measurement of carbon dioxide gas levels in human respiration has great medical diagnostics value. Our CO₂ sensor technology can be potentially used in many medical applications, including respiratory monitoring for endotracheal tube verification, intra/inter hospital transfer, adequacy of CPR, sleep apnea screening, procedural sedation monitoring, metabolic testing, and many others. 

Do you foresee any social or political implications for your research?

In addition to the described medical applications, our CO₂ nanosensors can impact the society in more than one way. CO₂ is one of the man-made greenhouse gases which are widely blamed for global warming. All technological solutions to curb CO₂ emission to the atmosphere will benefit enormously from this low-cost nanosensor technology.

For example, reduction of carbon dioxide emission from coal-fired power plants can be facilitated by CO₂ capture and storage in deep geological formations. Monitoring and verification of CO₂ storage is an essential part of this approach, as it is very important to directly monitor the ground surface to detect if CO₂ is seeping back into the atmosphere.

Although CO₂ monitoring using non-dispersive infrared technology is readily available, it is rather expensive for expanded sensor network applications. Development of more efficient and cost-effective technologies for CO₂ detection is important for the advancement of low-carbon energy technologies.

Another potential application of CO₂ nanosensors hits closer to home. Carbon dioxide concentration is a dynamic measure of indoor air quality. The cost of heating and air conditioning buildings can be reduced significantly by active control schemes that use carbon dioxide measurements to regulate the makeup of airflow. In the future, nanosensors can replace the costly infrared sensors used in today's demand-controlled ventilation applications.

Alexander Star, PhD
Assistant Professor
Department of Chemistry
University of Pittsburgh
Pittsburgh, PA, USA

KEYWORDS: NANOELECTRIC CARBON DIOXIDE SENSORS, FIELD-EFFECT TRANSISTORS, NANOTUBE TRANSISTORS, SINGLE, FUNCTIONALIZATION, DEVICES, MEMORY, FET, PERFORMANCE, ADSORPTION, BIOSENSORS.

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• March 2011 - Fast Moving Fronts
• Alexander Star on Nanoelectronic Carbon Dioxide Sensors