IntroductionCarbon NanotubesDevelopment of Carbon Nanotube-based SensorsReferencesFuture research
Cylindrical allotopes of carbon are grouped together as nanotube structures. The importance of carbon nanotubes is starting to become more apparent in the sensor industry. A recent paper by Dr. Katherine A. Mirica et al (2012) describes the structural and functional principle to carbon nanotubes for their application in the detection of harmful gases.
Structurally, carbon nanotubes are long, thin sheets of carbon atoms that can then be shaped into cylindrical forms. The key functional capacity to these carbon nanotubes is their ability to conduct electricity and transmit this wave of energy across their structure, making these nanotubes ideal for the development of highly-sensitive sensor components. The tensile strength and elastic modulus to a carbon nanotube structure allows for a wider range of application for this material in multiple industries.
The electrical property to carbon nanostructures is possible due to the symmetry and electronic configuration to this material. The chiral vector to the single-walled carbon nanotube determines the sensitivity of the nanotubes. By having a cylindrical form, carbon nanotubes maintain their level of conductance.
This also means that the electrical transport that takes place along the nanotube structure is enhanced. During electrical conductance, the single free electron to the carbon atom will travel along the length of the nanostructure in a unidirectional manner.
During exposure to a noxious gas, the gas particle will land on the nanotube structure and change the shape of the nanotube by creating a kink in the shape of this structure, which alters the unidirectional electrical conductance that travels along this material, and this is the main principle to carbon nanotube sensors.
Development of Carbon Nanotube-based Sensors
Chemists at Massachusetts Institute of Technology (MIT) have developed carbon nanotube powder and compressed this material into a new form of pencil lead that can inscribe sensors onto a sheet of paper. The following video focuses on making gas sensors out of carbon nanotubes, which opens application fields for this technology in the food industry, healthcare and homeland security.
The current sensor by MIT researchers is designed to detect ammonia gas, which is known to be an industrial toxic chemical. This research opens an exciting new path for customising carbon nanotube-based sensors for a range of volatile gases. A key process in the fabrication of these gas sensors involves dissolving these nanotubes in dichlorobenzene, a solvent used to purify the nanotubes. However, with dichlorobenzene being a chemical carcinogen, this process becomes hazardous and not as accurate.
The research by Katherine A. Mirica et al (2012) tested the pencil containing the carbon nanotube powder by inscribing a line onto a sheet of paper made out of gold particles (the gold sheet delivered the electrode elements to help the electrical conductance move through the inscribed sensors). Application of an electrical current to the carbon nanotube material, a voltage runs through this structure in the presence of gas particles, and as mentioned earlier in this article, if the electrical current is altered it indicates that a gas particle has attached to the carbon structure and changed the flow of electrical energy.
Carbon nanotubes are sensitive to the surrounding environment because they are structurally capable of absorbing gas particles. For this reason, further research should focus on the thermopower, resistance, and the density of the nanotube complex to provide a deeper understanding of how these parameters can affect the electrical impedance of this material and, furthermore, how ambient conditions could also impact the sensitivity to such carbon-based sensors.
Due to the inherent form and structure of carbon nanotubes, this material has made some impressive progress to say it has a history of less than 20 years in its application. There are various methods to integrating nanotubes with different sensors via processes such as direct-growing, drop-deposition, printing, etc.
As mentioned, the purification method for nanotubes is hazardous and can be unreliable, which makes the production of such sensors on a large scale a costly operation. Future challenges will aim to find reliable and efficient methods to purify carbon nanotube material without being hazardous to the human and nanostructure.