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A sensor is an instrument designed to detect some change in the surroundings, such as temperature, pressure, a flow of some kind, movement and moisture. It then responds by some type of output, such as an optical, mechanical or electrical signal. For instance, a lamp may dim in response to touch, because of a surface sensor, or an elevator button may respond to touch by means of a touch-sensitive sensor. Again, a mercury thermometer responds to temperature changes by an increase in the volume of mercury.
There are both active and passive sensors, depending on whether they require a source of power. Active sensors include light detection sensors, while passive ones include film photography sensors.
What is Graphene?
Graphene is a two-dimensional sheet material, composed entirely of carbon atoms. It is just one atom in thickness. This means that each of the atoms in this material is able to respond completely to any change in the environment. Sensors intended to detect chemicals need to be able to sense just a single molecule of a hazardous substance. This level of sensitivity is being realized with the use of graphene.
Graphene-based sensors are not only micron-scale but can pick up each event in terms of molecular changes.
Why is Graphene Ideal for Sensor Applications?
There are many reasons why graphene is ideal for use in sensors:
- At the microscale, it has 200 times the strength of steel with a tensile modulus of 1 Tpa.
- It has the highest conductivity of any material in the world, for both heat and electricity; this enables good transport of electrons between the surface of the graphene electrode and a biomolecule.
- It is extremely dense, but also light and transparent.
- Its surface to volume ratio is very high which makes it easy to attach biomolecules of interest to its surface, by simple adsorption or by covalent crosslinking between reactive groups (such as carboxyl, sulfonate, acid chloride, hydroxyl, and amine) and the graphene surface, to functionalize it.
- It has unique optical properties like transparency.
- It has a high carrier density of over 1013 cm-2 as well as high carrier mobility, more than 20000 cm2V-1s-1.
- It has a small band gap, which means the energy required for electrons to shift from the top of the valence band to the bottom of the conduction band is small.
- It produces a voltage following the application of a magnetic field perpendicular to the direction of current flow (the Hall effect) at room temperature.
- In comparison to carbon nanotubes, graphene has twice the surface area available for use, and the amount of uniform surface is also higher, making it ideal for even and effective functionalization.
What are the Advantages of Using Graphene in Sensors?
Graphene and its derivatives such as graphene oxide, reduced graphene oxide, multi-layer graphene, and graphene nanoplatelets are produced by several techniques. Graphene-nanocomposites are also available and are prepared by using any of a variety of methods to link graphene to nanomaterials or polymers. Other forms of graphene, such as graphene nanoribbons and graphene quantum dots, are even more sensitive to the field effect and respond to chemical changes at the edges.
Multiple Sensing Mechanisms
All these are able to sense low concentrations of molecules or of physical changes by a variety of mechanisms. These may include being able to flex under pressure, responding to thermal changes, adsorption of molecules at the surface, or adsorption by functionalized graphene which has been treated to respond to certain molecules with high specificity.
The high charge mobility and electrical conductivity at the surface of graphene lend it the capability to produce a ready and measurable response to any of these changes. This efficient response lends graphene a great advantage when compared to other sensor materials.
Ease of Use
The use of graphene will help to reduce the size and weight of sensors, as well as imparting greater sensitivity. They will also provide rapid sensing capabilities while being more cost-effective. For instance, using graphene as a charge transduction layer in short-wave infrared detectors will help make these sensors widely available for use in sorting and inspection of semiconductors, spectroscopic imaging, and process surveillance.
Some Examples of Graphene Sensor Applications
Graphene is likely to be of extensive use in biosensors that are used to detect a variety of analytes in the body fluids, including hemoglobin, glucose, and cholesterol. They are suited especially for electrochemical sensors which make use of electron transfer directly between the biomolecule and the surface of the electrode.
New biosensors could look for specific molecules to analyze the composition of a person’s breath, such as lung cancer biomarkers, thus diagnosing the condition affecting the individual. Current ‘electronic nose’ devices could be completely transformed by the use of graphene.
Graphene can detect magnetic changes at a scale of a hundred times lower than is possible with silicon sensors.
Graphene-based functionalized sensors have been developed to detect chemical and biological fluctuations at unparalleled sensitivity and with high efficiency compared to traditional instruments. Functionalization of graphene electrodes also improves the specificity and avoids fouling of the electrode.
Non-enzymatic graphene electrodes are excellent at detecting several biological molecules.
Sometimes a Field Effect Transistor (FET) including a graphene channel is utilized in the sensor, to take advantage of the low noise and high carrier mobility. When an analyte binds to it, there is a change in the current flow, producing a signal that can be analyzed to provide several variables. These can be used to detect DNA hybridization and bacteria possessing a negative electrical charge.
DNA sensors based on nanoscale graphene-based devices are being explored to detect nucleic acid components. Other applications of these miniaturized devices include gas sensors, pH sensors, pressure sensors, strain sensors, and eco-sensitive sensors.
Graphene-based light detectors and humidity sensors are faster than conventional instruments while being flexible and transparent. These may be very important in liquid crystal displays, touchscreens and organic LEDs, as well as organic solar cells.
Smart food packaging is being created by incorporating graphene oxide. This can detect gases emitted by rotting food, thus preventing food wastage while also avoiding food poisoning.
Similarly, graphene sensors could be used in military applications to detect harmful chemicals and explosives at extremely low concentrations, providing early warning of ambushes or encampments to soldiers in war while still far away from the spot.
Many similar graphene-based sensor applications are on the anvil. Depending on the availability of high-quality graphene with reliable properties, this could well signal a transformation of the world of sensors.
Sources and Further Reading