Nanosensors are chemical or mechanical sensors that can be used to detect and convey information about the presence of chemical species and nanoparticles, as well as monitoring physical parameters such as temperature. Nano refers to objects measured in billionths of a meter known as nanometers. The materials that incorporate the nanosensors include structures such as nanowires, nanotubes, and nanopores.
At a nano-scale, the surface area has a larger effect on material behavior than it does for bigger objects. As a result, properties such as conductivity, magnetism and reflectivity change compared to larger bodies. Nanosensors have increased specificity over regular sensors due to the fact they can operate at similar scales to biological processes, which allows for functionalization with chemical/biological molecules and recognition events that cause detectable physical changes.
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Types of Nanosensor and How They Work
The different type of nano-sized sensors can be split into chemical, mechanical, biological and optical sensors. Nanosensors work by measuring electrical changes in the sensor materials.
Electrochemical nanosensors detect resistance changes in nanomaterials upon the binding of an analyte. The changes happen due to changes in scattering, or the depletion or accumulation of charge carriers. An example of a chemical nanosensor is carbon nanotubes, which are used to sense ionization of gaseous molecules, as well as to detect atmospheric concentrations of gases at the molecular level
Mechanical nanosensors work similarly to electrical sensors, as they also measure electrical changes. Microelectromechanical systems (MEMS) used in car airbags are a type of mechanical nanosensor, and they monitor changes in capacitance.
Phototonic nanosensors are used to quantify concentrations of clinical samples. An example would be the chemical modulation of a hydrogel film volume. As the hydrogel swells or shrinks upon chemical stimulation, the Bragg grating changes color and light is diffracted at different wavelengths, which is then correlated with the concentration of a target analyte.
Biological nanosensors are analytical devices that incorporate a biological sensing element integrated within a physiochemical transducer. They produce a chemical signal that is proportional to the concentration of the target analyte. An example of a biological nanosensor includes lipid bilayers in biosensors to reconstitute natural membranes in vitro.
Other types on nanosensors include electromagnetic, plasmonic, spectroscopic, magnetoelectronic and spintronic nanosensors.
How They are Made
Nanosized sensors can be made with three different methods. These methods include:
- Top-down lithography – The most common type of nanosensor synthesis. It involves starting out with a large block of material and carving out the desired form.
- Bottom-up assembly - Involves assembling sensors out of minuscule atoms or molecules. The process is mainly used for building starter molecules for self-assembling sensors.
- Molecular self-assembly - Uses an already complete set of components to automatically assemble themselves into a finished product.
Nanosensors can be used in areas such as:
- Monitoring the environment
- Medical diagnostics
- Monitoring physical parameters
- The food industry
Nanosensors are used in the environmental field to sense biological and chemical agents present in air and water. Mercury can be detected in both air and water using dandelion-like Au/polyaniline (PANI) nanoparticles with surface-enhanced Raman spectroscopy. Air can be sampled to measure pollutants, as well as using nanosensors measure solar irradiance, aerosol-cloud interactions, climate forcing, and biogeochemical cycles. Water quality and distribution can also be monitored using nanosensors
Medical diagnostics use blood borne nanosensors or in lab-on-a-chip type devices. Viruses and bacteria can also be detected using nanosensors. These sensors make use of the variations in electrical conductivity, such as that of carbon nanotubes with bonded antibodies. If there is a matching virus or bacteria attached to the antibody, a change can be measured in conductivity.
Temperatures of both living cells and nanofluids can be measured using nanosensors. For cells, nanothermometers are inserted into separate cells, and semiconductor crystals change color in response to temperature changes. Fine gage thermocouples can also be used to measure temperatures in ex vivo tissues. Measuring the temperature of nanofluids is required due to heat management being an issue in electronics. Nanofluids with superior thermal conductivity characteristics are needed and sensors are required to measure nano effects.
Recent developments in nanoscience and nanotechnology are also being applied to the food industry. Nanostructured materials have applications in areas such as new packaging materials and encapsulated food components. Nanostructured systems in food are also being researched, including polymeric nanoparticles, liposomes, nanoemulsions, and microemulsions. The materials allow for an enhanced solubility, can improve bioavailability, facilitate controlled release, and protect bioactive components during manufacture and storage.