Graphene has many applications, and sensor technology is perhaps one of the areas where graphene has found the most use, especially from an academic perspective. There are many different types of sensors that can be made from graphene and its derivatives, and in this article we will look at the unique properties make graphene such a useful material for sensor applications, as well as showcasing the most prominent sensor areas that use graphene.
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Useful Properties of Graphene for Sensor Applications
Graphene, as many people now know, has a number of properties that make it a useful material which enhances the efficiency of many applications; one of which being sensors. There are two ways in which graphene is used in sensors. The first is as graphene (or one of the graphene derivatives), and the second is as part of a composite or conjugate. These sensors can employ a wide range of graphene forms and graphene derivatives, including pure graphene, graphene oxide, reduced graphene oxide, graphene polymer composites, graphene-nanoparticle conjugates and doped-graphene sheets, to name a few of the many varieties that have been implemented.
Sensors respond to changes in an environment, and, at least in the case of graphene sensors, most work by adsorbing the molecules onto the surface of the graphene sheet, which invokes a measurable change that is then detected. As far as properties go, the planar sheet of graphene provides an even and large surface for molecules to interact with. In addition to this, the high conductivity of graphene changes when molecules interact, and this measurable change is one of the most common sensing mechanisms. For functionalized graphene sheets, the groups can act as a binding site and the sp2 hybridized nature of graphene can enhance the sensitivity when paired with another material, as it can form efficient π-π charge transfer networks that facilitate the movement of electrons, which, in turn, invokes a more detectable response.
There are many different types of sensor where graphene has found some use. In this article, we will focus on prominent areas of graphene sensor technology that have been widely-researched, namely: gas sensors, biosensors, temperature sensors and humidity sensors. Despite these areas being the focus of this article, other areas where graphene can be used for sensor applications is in piezoelectric sensors, piezoresistive sensors, capacitance sensors, pressure sensors and optical sensors.
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Graphene sensors can be used to detect many different small gaseous molecules and volatile organic compounds (VOCs). To date, pure graphene, doped-graphene, graphene-nanoparticle conjugates, graphene oxide and reduced graphene oxide-based sensors have been used to detect ammonia, nitrogen dioxide, methane, hydrogen, water, ethanol, methanol, isopropanol, DNT, hydrogen cyanide and DMMP (all in gaseous form). The precise mechanisms used to detect these molecules differ from sensor to sensor, but, because a lot of non-pure graphene sheets are used, most of the sensing mechanisms work by the adsorption of molecules into a cavity on the surface (specific to the molecule of interest), which then changes the conductivity of the graphene sheet. This change can then be detected. Many graphene-based sensors can also detect gaseous molecules at the parts per billion (ppb) level.
Graphene-based sensors can be used to detect a wide range of biomolecules, the most common of which being glucose, proteins and DNA. The mechanical, optical and electrical conductivity properties are extremely useful for biosensor applications, as a molecule is sensed by transferring electrons from the biomolecule to the sensor. The high charge carrier mobility of graphene helps to facilitate an efficient transfer of electrons from the biomolecule, which causes an electrical response in the graphene sheet that can then be detected. In biosensors, graphene can be used in a number of ways, including as impedance materials, electron transfer material, for transferring phonon/photons and as FETs.
Most temperature sensors in use today use a semiconductor material, and, given that graphene has semiconducting properties, it is a natural progression for graphene to be used in this type of sensing. Of all the forms of graphene, reduced graphene oxide is the most widely-used graphene type for temperature sensing, as it undergoes a detectable thermal resistivity drop when the temperature of the surrounding environment is increased. However, many different forms can be used (e.g. non-conventional architectures, such as graphene nanowalls), and this is mainly due to the large surface area and high thermal conductivity of graphene, which is 3000-5000 W m-1 K-1 in its stand-alone non-functionalized form, and 600 W m-1 K-1 when in a composite, both of which are important properties of any temperature sensor.
As mentioned in the gas sensors section, graphene can be used to measure gaseous forms of water, and this can be adapted for measuring the relative humidity within a given atmosphere. For this type of sensing, graphene is generally not presented as part of a composite or other conjugate. However, CVD-grown graphene and exfoliated graphene and graphene oxide have been used to determine the relative humidity of an environment. Many of these sensors can detect relative humidities from as little as 1%, up to 95%+ (upper values depend on the individual sensor).
These sensors work by adsorbing water molecules onto the surface of the graphene sheet. The interaction between the water molecule(s) and the graphene sheet changes the electronic structure of the graphene sheet, which invokes a detectable change in the resistivity. This resistivity change is caused by opening the band gap of graphene, and in turn, temporarily turning it into an insulating material. This process also invokes a change in the electrical current output, which is the measurable parameter. Monolayered graphene sheets are often used in this type of sensing, as they possess a much higher sensitivity and precision than multi-layered graphene. High-quality graphene is key for this type of sensor.
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