Robert C. Haddon on New Advancements on Single-Walled Carbon Nanotubes

Fast Moving Fronts Commentary, March 2011

Robert C. Haddon

Article: Chemically functionalized single-walled carbon nanotubes as ammonia sensors

Authors: Bekyarova, E;Davis, M;Burch, T;Itkis, ME;Zhao, B;Sunshine, S;Haddon, RC
Journal: J PHYS CHEM B, 108 (51): 19717-19720, DEC 23 2004
Addresses: Univ Calif Riverside, Ctr Nanoscale Sci & Engn, Riverside, CA 92521 USA.
Univ Calif Riverside, Ctr Nanoscale Sci & Engn, Riverside, CA 92521 USA.
Carbon Solut Inc, Riverside, CA USA.
(Addresses have been truncated)

Robert C. Haddon talks with and answers a few questions about this month's Fast Moving Fronts paper in the field of Chemistry.

SW: Why do you think your paper is highly cited?

The paper demonstrated the importance of chemistry in improving the response of single-walled carbon nanotube (SWNT) devices to gas molecules. At that time it was already well established that SWNTs hold great potential as a sensor material, because their delocalized electronic structure is very sensitive to dopants.

Researchers were looking for ways to improve the SWNT-sensor performance and initial experiments in the literature made use of non-covalent functionalization schemes to achieve this goal; the disadvantage of the non-covalent approach is the lack of control of the chemical composition and electronic structure, which leads to devices with irreproducible characteristics.

In this paper we showed that SWNTs, which are functionalized with a conducting polymer, poly(m-aminobenzene sulfonic acid)—PABS, exhibit enhanced sensitivity to ammonia as compared to purified nanotubes. We also proposed a mechanism of gas detection, which involved modulation of the electron density on the SWNTs mediated by the covalently attached polymer chain.

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

The study describes a new approach to the development of carbon nanotube-based sensors, which relies on covalent modification of the nanotubes. The chemistry in this work plays a multiple role—it introduces functional groups, which can be tailored for specific interaction with a range of gas molecules, based on their chemical nature and at the same time it modifies the electronic structure of the nanotubes while rendering the material water soluble.

"The development of a new sensor technology will further our capability for reliable detection of toxic compounds and compounds of diagnostic importance, which has social and political implications."

It is also a synthesis of knowledge about the mechanism of gas detection with carbon nanotubes, which involves a response of the semiconducting SWNTs to electron donating and accepting molecules.

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

SWNTs have a one-dimensional structure with every atom exposed on the surface, which makes them the ideal molecular wire. When SWNTs are exposed to toxic gases, such as nitrogen dioxide (NO2) or ammonia (NH3), the resistance of the SWNTs changes dramatically and it can either decrease—as in the case of NO2—or increase—for example upon interaction with ammonia. The high sensitivity of the SWNTs, taken together with their small size and highly robust chemical structure, make them an attractive candidate for sensor materials that provide the components for large-scale integrated sensor devices.

Tailored design of the attached functional groups to the nanotubes allows the fabrication of an array of sensors, which can detect a range of toxic gases. Importantly, because the nanotubes are functionalized they can be dispersed in solvents and then deposited as thin films on pre-patterned devices. Because of its simplicity the proposed concept can be easily deployed in existing sensor platforms.

SW: 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?

I became interested in carbon nanotubes soon after the announcement of their discovery in 1991, but I did not become involved until 1996 when Smalley reported in Science the synthesis of high-quality carbon nanotubes. I was convinced that chemistry would open new horizons for this fascinating material and began active research on the chemical modification and applications of carbon nanotubes.

We prepared and characterized the first soluble nanotubes in 1998 ("Solution Properties of Single-Walled Carbon Nanotubes," Science 282: 95, 1998) and this opened a route to tailor the nanotube properties for a variety of applications, provided a means for dissolution, and greatly facilitated their characterization. We demonstrated the covalent and ionic functionalization of single-walled carbon nanotubes and showed that this chemistry brought about modifications of the carbon nanotube interband electronic transitions that were observable by near-infrared spectroscopy.

SW: Where do you see your research leading in the future?

The chemistry of carbon materials is a fascinating field, which continues to evolve and offer tremendous opportunities. My recent research efforts are focused on the application of chemistry to engineer a band gap into graphene; covalent carbon-carbon bond formation reactions can be used to change the hybridization of the graphitic atoms from sp2 to sp3 in order to modify the conjugation length of the delocalized carbon lattice and we have already demonstrated a number of promising functionalization reactions.

Graphene is now envisioned as one of the most promising candidates on the roadmap of next-generation materials for the electronics industry, and chemistry offers the means for the realization of the potential of graphene in this regard.

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

The development of a new sensor technology will further our capability for reliable detection of toxic compounds and compounds of diagnostic importance, which has social and political implications. It will help to reduce the risk of accidents and terrorist attacks. Functionalized carbon nanotube sensors have tremendous potential in biomedical, food, and environmental monitoring, with the potential to improve our quality of life.End

Robert C. Haddon, Distinguished Professor
Director, Center For Nanoscale Science And Engineering
University Of California, Riverside
Riverside, Ca, USA




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