Sensors are devices designed to detect or quantify physical-chemical characteristics, such as pressure, temperature, concentration, sound level or light. The process is typically mediated by the conversion of the measured factor into an electric current.
A sensor is assessed by its sensitivity to the signal, the speed of the response, the cost-effectiveness of the device, the capacity for high volume production and its reliability.
Sensors find applications in a vast range of fields, from automotive sensors to biomedical devices. This growing need for sensors has spurred intensive research on novel technologies and materials, such as the nanotube sensor.
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Nanotechnology enables the production of miniaturized sensors that are not just tiny but incredibly sensitive to minute signals. As a result, they add very little weight to the device, use limited power and are cost effective. The initial nanosensors were made from inorganic semiconductors, but the emergence of carbon nanotubes (CNTs) has transformed the world of sensor research.
CNTs are unique among the newer sensors because they are inherently light and small, very strong, with high conductivity for both heat and electrical energy, and with a high specific surface area (ratio of surface area to mass).
CNTs are layers of graphite that have been rolled up to form a cylindrical tube. Both single-walled and multi-walled nanotubes (SWNTs and MWNTs respectively) have been synthesized.
The value of metallic CNTs lies in the ballistic electron transport within them. This means that electrons shoot over long lengths of the tube without scattering. In other words, MWNTs can transport high currents of up to 10 A/cm2 with minimal heating, change in resistance or alteration in form, for long durations, at temperatures as high as 250 oC.
The use of CNT sensors is based upon the changes that occur in their electrical properties in response to any change in other parameters such as temperature or gas adsorption, coupled with their unique characteristics.
They have been used as miniaturized temperature sensors, which respond to changes in temperature by changes in their electric characteristics.
Similarly, gas adsorption causes extremely rapid and sensitive changes in electrical properties of the CNTs, enabling their use as nanoscale sensors for the detection of minute amounts of chemical vapors. The response times are at least one order of magnitude higher when compared to conventional solid-state sensors.
These sensors are very small and can be held at room temperature. They could be useful in drug or explosive detection, for instance, as small cheap sensors that can be mounted throughout sensitive facilities or crowded but security-intensive places like airports.
However, such sensors also experience interference as a result of changes in moisture, temperature, and the velocity of flow. Poorly diffusible gases are not detected well. Also, they cannot accurately differentiate gases from mixtures.
These obstacles are overcome by using gas ionization sensors which break down gases at the tips of the CNTs. These are very sensitive and can be used for rapid chemical sensing based on gas adsorption, at low power.
Piezoresistive sensors have been designed to make use of isotropic CNTs, detecting strain based on changes in the electronic properties of the CNT when under strain, at different locations, and in different directions.
Tunable pressure sensors are based on the transition from metal to semiconductor under pressure in an SWNT.
Flow sensors take advantage of the change in electrical signal in response to the flow of fluid because of the polar particles in the fluid stream cause fluctuations in the coulombic fields, which in turn causes direct scattering of the free carriers.
Biosensors have been developed from MWNTs which can be used to monitor body temperature, pulse, blood glucose levels and to diagnose illnesses, detect inflammation or oncogenes, repair damaged cells, and even to act as a pacemaker.
Such sensors avoid invasive procedures and do not cause inflammation
Environmental Gas Sensors
The use of such sensors could enable tracking air pollution to the source as well as evaluation of air quality.
Food Spoilage Sensors
Sensors to detect food spoilage have also been visualized, by spraying CNTs on a flexible plastic surface such as plastic food wrapping film. These nanotubes then act as sensors to detect the chemicals that indicate rotting. This helps control food quality.
Other Types of Sensor
CNTs are also capable of being used as optical, stress, strain, and position sensors.
Other areas of application include the automotive industry, agriculture (such as monitoring relative humidity in a greenhouse or application of liquid chemicals to plants), and the fishing industry (maintain pH and water quality in the fish tanks).
Many challenges remain to be resolved in the application of nanotube sensors, including the production of CNTs in pure form, perfecting the growth mode and controlling the surface growth. Solutions to these production parameter issues and growth problems, avoiding structural instability such as excessive kinking and buckling, and above all, confirming their safety in animals.
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