Carbon Nanotube Sensors for the Detection of Noxious Gases

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By detecting the adsorption and desorption times of an airborne toxin, it is possible to locate the source of the toxin and the amount of time it would take for this exposure to intoxicate life forms in neighboring locations.

Nanotechnology is making a fundamental contribution to the design and development of sensor technology for its application in ‘electronic nose’ devices capable of isolating trace amounts of toxic airborne particles.

One such ‘electronic nose’ prototype has been developed by Nosang Myung, a Professor at the University of California. The electronic sensor device is made of carbon nanotubes that detect airborne toxins at concentrations of parts per billion (ppb). The structural components to this sensor device include humidity and temperature sensors, a USB port and a chip.

Carbon Nanotubes – Functional Principle

The electronic band to a nanotube includes single carbon atoms that form a hexagonal lattice. The carbon atom lattice is bound together via covalent bonds. A total of three carbon atoms via sp2 molecular orbits form the structure of a nanotube. The even number of electron arrangement in a nanotube structure allows for this material to become a semiconducting element.

Nanotubes display a level of electrical conductance and so can be considered as semiconductors. The chirality of a nanotube will determine how a nanotube structure will perform. For example, if a nanotube structure is rolled up evenly, this organized display allows for effective conductance of electricity across the structure.

With each carbon atom covalently bound to three surrounding atoms, the remaining electron moving freely in the outer shell of each carbon atom gives this structure an electrochemical property.

In the detection of toxic airborne particles, the binding of a toxic molecule at a level of parts per billion, the contact between the toxin molecule and the carbon nanotube will create a change in the shape of the nanotube structure and this will affect the level and speed of electrical conductance across the nanotube structure. The adsorption of specific molecules on their surface brings about a change in the V-I curves.

The basic principle here is that, in the event of a defect (e.g., via contamination with a noxious gas molecule), the defect in the tube lattice will result in part of the nanotube conducting electricity and the other part of the tube behaving as a semi-conductor.

The adsorption of the noxious chemical onto the nanotubes will also cause a charge transfer and narrows the energy gap between the carbon atoms thus increasing the conductance of each tube.

The following video is an example of carbon nanotube deformation and offers a basic insight into how the nanotube would behave in the event of interaction with a foreign particle.

The fabrication process involved in the development of carbon nanotube sensors is achieved using the dielectrophoresis method. During this fabrication process, electrokinetic motion of polarized material takes place in electrical fields which allows for the separation and positioning of carbon nanotubes.

Dielectrophoresis ensures that a strong electrical potential can be isolated between carbon nanotubes and the electrodes this material is exposed to. During the initial stages in this process, the carbon nanotubes are submerged in ethanol and taken through a process of ultrasonication (generating low- and high-pressure waves in a liquid).

Applications

The prototype to this sensor device will be integrated into three technological platforms: a smartphone, wearable device, and a handheld device. The prototype device developed by Professor Nosang Myung has been tested in industrial sites to isolate combustion emission and gas leaks.

The device has also helped measure pesticide levels in agriculture, which makes this device applicable to the farming industry and also useful for monitoring water contamination. One exciting application for this sensor device is the possibility of using this device for testing chemical warfare agents in the military.

Gas sensing technology will continue to play an important part in the detection of environmental pollution. Conventional gas sensors that are applied to monitor airborne contaminants have poor sensitivity to and feedback on the detection of gases, such as oxide thin-film sensors.

The introduction of nanotube-based sensors will offer a better solution to a more sensitive sensor device with detection levels down to 1 ppb and less. The only issue with the nano-based sensors is that the fabrication process is complex and could be difficult to expand on a large-scale.

References and Further Reading

• Bandaru P.R. Electrical properties and applications of carbon nanotube structures. Journal of Nanoscience and Nanotechnology 2007; 7: 1–29.
• Li Q, et al. Structure-dependent electrical properties of carbon nanotube fibers. Advanced Materials 2007; 19:3358–3363.
• Rosso M.A. (2001). Origins, properties, and applications of carbon nanotubes and fullerenes. Submitted in partial fulfillment of the requirements for the graduate course IT 283 Advanced Materials and Processes. California State University Fresno.
• Wang Y, et al. A review of carbon nanotubes-based gas sensors. Journal of sensors 2009; volume 2009: 1–24.
• Wongwiriyapan W, et al. Single-walled carbon nanotube thin-film sensor for ultrasensitive gas detection. Japanese Journal of Applied Physics 2005; 44(16):L482–L484.
• Irina V.Zaporotskova, et al. Carbon nanotubes: Sensor properties. A review. Modern Electronic Materials, 2016, 2, 4: 95-105.

This article was updated on the 1^st August, 2019.