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.
Advantages and Applications of Carbon Nanotube (CNT) Sensors
Nanotechnology in Sensors: Nanotechnology allows for the production of highly sensitive, lightweight, and cost-effective sensors with minimal power consumption.
Emergence of Carbon Nanotubes (CNTs): Initially made from inorganic semiconductors, sensors now increasingly use CNTs due to their superior properties, such as lightness, strength, high conductivity, and a high specific surface area.
Structure of CNTs: CNTs are cylindrical tubes formed by rolling up layers of graphite. They come in single-walled (SWNTs) and multi-walled (MWNTs) forms.
Advantages of Metallic CNTs: CNTs exhibit ballistic electron transport, allowing high current flow with minimal heating and resistance changes, even at high temperatures (up to 250 °C).
Applications Based on Electrical Property Changes: CNT sensors operate by detecting changes in their electrical properties in response to variations in parameters like temperature or gas adsorption.
Temperature Sensors: CNTs are used as miniaturized temperature sensors that change their electrical characteristics with temperature variations.
Gas Sensors: CNTs detect gas adsorption quickly and sensitively, making them useful for identifying small amounts of chemical vapors, such as in drug or explosive detection. They offer faster response times compared to conventional sensors.
Limitations of CNT Gas Sensors: They can be affected by moisture, temperature, and flow velocity, and struggle with poorly diffusible gases and gas mixtures. Gas ionization sensors can overcome some of these challenges.
Piezoresistive Sensors: These sensors use CNTs to detect strain through changes in their electronic properties under mechanical stress.
Pressure Sensors: CNT-based pressure sensors work by the transition of SWNTs from metal to semiconductor under pressure.
Flow Sensors: These sensors detect flow by measuring changes in electrical signals caused by polar particles in fluids interacting with CNTs.
Biosensors: MWNTs are used in biosensors for monitoring body temperature, pulse, blood glucose, diagnosing illnesses, detecting inflammation, repairing cells, and even as pacemakers, reducing the need for invasive procedures.
Environmental Gas Sensors: CNT sensors can track air pollution sources and assess air quality.
Food Spoilage Sensors: CNTs can be sprayed on plastic surfaces like food wrapping to detect chemicals indicating spoilage, helping to maintain food quality.
Other Types of Sensors: CNTs are also used as optical, stress, strain, and position sensors, with applications in automotive, agriculture (e.g., monitoring greenhouse humidity or chemical applications), and fishing industries (e.g., maintaining pH and water quality in tanks).
Challenges in CNT Sensor Development: Key challenges include producing pure CNTs, controlling their growth mode, and ensuring structural stability (e.g., preventing excessive kinking and buckling). Safety in animal models also needs confirmation for broader adoption.