A device that turns something in its surroundings – a gas, a magnetic field, a pressure, a molecule in blood – into an electrical signal. make unusually sensitive sensors because every atom is on the surface: a few molecules landing on a change how well it conducts. Graphene also makes very good magnetic-field sensors. The hard part is not sensitivity but selectivity – responding to one thing and ignoring everything else – and staying stable for years.
Going deeper
In a single layer every atom is exposed, so a few molecules landing on it measurably change its resistance. Graphene also makes Hall sensors whose voltage tracks a magnetic field from room to cryogenic temperature. The difficulty is selectivity: humidity and other gases move the signal too, so receptors, arrays and calibration are needed.
All surface
In a bulk sensor most atoms sit inside, untouched by what is being measured; in a single layer every atom is exposed. A molecule landing on graphene or MoS2 gives or takes a little charge, and because the layer holds few carriers to begin with, the change in its resistance is easy to measure. In 2007 a graphene device recorded the arrival and departure of individual nitrogen dioxide molecules as steps in its resistance – the ultimate sensitivity.
The same principle works for humidity, for light, for in a stretched film and, with receptor molecules attached to the surface, for proteins or DNA in a liquid.
Magnetic-field sensors
Graphene is also an excellent Hall sensor. Its electrons are few and fast, so a magnetic field produces an unusually large for a given current, and because its properties change little with temperature, the same sensor works from room temperature down to the cryogenic conditions of quantum computers and magnets. in hBN, graphene Hall sensors outperform silicon ones, and sensors grown directly on are among the first 2D-material electronic products on sale.
Selective, stable, calibrated
Sensitivity is the easy half. A sensor that responds to everything tells you little: humidity, temperature and other gases shift the signal of a 2D gas sensor as much as the gas of interest, and surfaces slowly change as they age. Practical designs coat the layer with receptors or filters, combine several differently treated sensors in an array whose pattern of responses identifies the gas, encapsulate whatever should not respond, and calibrate against drift. For medical tests the bar is higher still: the sensor must be cheap, reproducible from one device to the next and approved like any other diagnostic.
For specialists
Transducers in which a stimulus changes an electrical property of a 2D layer: chemiresistive and field-effect gas and humidity sensors, in which adsorbates transfer charge and shift the conductance (graphene registered single NO2 molecules in 2007); Hall sensors, in which graphene’s low and high give a large, temperature-stable Hall voltage; biosensors with receptors on a graphene or MoS2 channel that bind target molecules; and strain, pressure and light sensors. Sensitivity comes from the all-surface geometry and the low ; the limits are selectivity, drift and from humidity and contamination, recovery time, device-to-device variation and calibration, addressed by functionalisation, encapsulation and arrays read by pattern recognition.