Classification
Nanosensors are classified as Physical nanosensors and Chemical nanosensors.
Physical nanosensors
Physical nanosensors are used to measure magnitudes such as mass, pressure, force, or displacement based on the principle that the electronic properties of both nanotubes and nanoribbons change when these are bent or deformed. For example, a CNT can be used to build a field-effect transistor (FET) nano in size, whose on/off threshold depends on the tube dimensions, shape and temperature, amongst others. A local deformation of the tube creates a change in the on/off threshold voltage of the transistor. Starting from this simple principle, different types of nano-electromechanical systems (NEMSs) have been proposed in the literature with different applications, such as pressure nanosensors, force nanosensors or displacement nanosensors.
Chemical nanosensors
Chemical nanosensors are used to measure magnitudes such as the concentration of a given gas, the presence of a specific type of molecules, or the molecular composition of a substance. This is working on the principle that the electronic properties of CNTs and GNRs change when different types of molecules are adsorbed on top of them, which locally increase or decrease the number of electrons able to move through the carbon lattice.
Similarl to physical sensors, when a nanotube or a nanoribbon is used in a transistor configuration, the presence of a specific type of molecules changes the on/off threshold voltage of the transistor. For the time being, hundreds of chemical nanosensors based on this simple principle have been manufactured with different specific detection targets.
ZnO nanowires
Pure and doped ZnO is one of the first suitable semiconductors widely used for sensing O2, H2, NOx, ethanol and humidity. They show instant change in electrical resistance when exposed to inflammable materials like organic solvent vapors, petrol vapors and methane gas that pose a threat to safety when they cross their thresholds limit in air.
Biodetection by ZnO Nanowire
Optical methods are used for the biological molecule detection, and these methods suffer from bleaching effects while electronic sensors such as Si-based transistor structures suffer from fundamental instability problems due to reaction with the ionic solutions endemic to bio-sensing.ZnO nanowires also offer many advantages over alternative strategies such as carbon nanotubes for sensing, because of being chemically inert in most electrolyte solutions (low linear drift), the availability of heterostructures which greatly enhances sensitivity due to the presence of 2D carrier gases, transparency to wavelengths up to the UV and greater control of conductivity. The nanowires bio-sensors have increased detection sensitivity of almost three orders of magnitude compared to thin film bio-sensors.
Chemically-functionalized ZnO nanowires
Functionalized ZnO nanowires are used in the detection and identification of biological pathogens. The mechanism for detection is the change in electrical conductance as chemical species absorb on the metal oxide surface. Identification of selected biomolecules will be possible via the adsorption of antibody groups on the ZnO surface that target specific molecules of interest. The high surface area to volume ratio of a free-standing nanowires is near-ideal for developing high sensitivity nano-sensors for gas- and/or bio-detection. ZnO nanowires can be synthesized with diameters as small as 15 nm and length greater than 1 µm. Conductance bridges can be fabricated using e-beam lithography. These structures, when functionalized with selective surface chemistry, are ideal for detection of molecular species based on changes in low dimensional transport.
Schottky type ZnO UV nanosensor
ZnO nanostructures have high photon conductance and the sensitivity of the ZnO NW UV nanosensor (NS) can be improved five orders of magnitude by functionalizing the surface of ZnO nanobelts using polymers that have a high absorption at the UV range. By fabricating Schottky type (ST) devices instead of Ohmic type (OT) devices, the UV sensitivity of ZnO NW NS has been improved for four orders of magnitude, and the reset time has been decreased. By further surface coating with positive charged poly (diallydimethylammonium chloride) (PDADMAC) and negative charged poly (sodium 4-styrenesulfonate) (PSS), the reset time has been further decreased even without correcting the electronic response of the measurement system. The ZnO NWs for the NS fabrication can be synthesized by thermal evaporation of ZnO powders without using any catalyst. The fast UV response speed, high spectrum selectivity combined with high photosensitivity suggest the possibility of using ZnO NWs as visible-blind UV sensors for commercial, military, and space applications. Beside the UV sensors, the performance of gas sensors, strain sensors and biosensors can also be improved dramatically by the Schottky contacts introduced in device fabrication, which is distinctly different from the conventionally designed devices with Ohmic contacts.