Graphene sensors are, as the name suggests, sensors which employ graphene as the active sensing material or sensing surface. Graphene has emerged as a highly efficient sensing surface for many reasons, and its sensing properties are currently being exploited for a wide range of sensors, from strain gauges to biosensors, and are now starting to be produced commercially. Here, we look in more depth at why graphene has the potential to disrupt a very established industry.
Sensors rely on an active material to provide a response to the conditions within a given environment. How the active material works and how it detects and senses the chosen parameter in a given environment varies between sensor types. This can take the form of stretching the material (such as in stress/strain gauges) to detect gaseous molecules or even changes in temperature. Regardless of how the parameter is sensed, there must be a change across the active material, i.e. an active response, and it is this response that is detectable and readable to the surrounding components. This active response usually takes the form of a conductivity change across the material as the stimulus interacts with the material.
Regardless of the type of sensor, this active sensing material is the most important aspect, and the rest of the components are there to help convert the initial sensing mechanism into a readable output that can be used by the operator. This is why graphene is being widely trialled in sensors, as it possesses a significant number of properties that can be utilized and exploited to produce a measurable response, and with a much greater degree of sensitivity than the status quo—perfect for companies trying to find ways of creating more efficient and sensitive sensors.
Types of Graphene Sensor
When we talk about a ‘graphene sensor’ there is no single type, and the versatility of graphene means that it can be used in almost every sensor type. Its inherent thinness provides an excellent surface to interact with molecules, and because it has such a high electronic conductivity and charge carrier mobility, even the smallest change in conductivity across the graphene sheet (due to stimulus interaction) can be detected—hence, extremely high sensitivities are possible with graphene sensors.
Moreover, graphene has a high relative surface area, meaning it offers a larger relative sensing area compared to other materials. Overall, this means that a smaller concentration of molecules can be detected using graphene compared to other materials. This ability to be sensitive to the smallest concentration of molecules that encounter the graphene sensing surface has made graphene an excellent choice for gas, volatile organic compound (VOC), humidity sensors, and for a wide range of biosensors (for measuring glucose, DNA and proteins to names a few).
Aside from these fundamental properties, graphene is thermally conducting and very stable to high temperatures, hence it has found use in temperature sensors. Graphene also has a very high mechanical strength and an ability to flex under strain without breaking. Once the stimulus has been removed, graphene will also return to its original orientation. These properties have led to the development of various pressure, stress, strain, piezoelectric and piezoresistive sensors.
In practice, all forms of graphene have been used, from single and few layer unfunctionalized graphene to graphene oxide, reduced graphene oxide and even more complex architectures such as graphene nanowalls, as well composite, hybrid and conjugated graphene materials.
Regardless of the type of sensor, graphene-based sensors can be used in many different environments, be they high temperature and pressure or harsh chemical environments, as graphene has a high stability to all the harmful stimuli. Therefore, graphene not only has versatility in the types of sensor it can be used in, but the environments in which graphene-based sensors can be used is versatile; and the combination of these two factors is why there is a lot of promise for graphene-based sensors.
References and Further Reading
Graphene Biosensors – CheapTubes
“Graphene-Based Glucose Sensors: A Brief Review”- Li J. W. et al. IEEE Transactions on NanoBioscience. 2015. DOI: 10.1109/TNB.2015.2475338
“Graphene Sensors”- Novoselov K. et al. IEEE Sensors Journal. 2011. DOI: 10.1109/JSEN.2011.2167608
“Graphene sensors: a review of recent developments”- Bogue R., Sensor Review. 2014. DOI: 10.1108/SR-03-2014-631
“Review on the graphene based optical fiber chemical and biological sensors”- Zhao Y. et al, Sensors and Actuators B: Chemical. 2016. DOI: 10.1016/j.snb.2016.03.026
“Graphene Based Electrochemical Sensors and Biosensors: A Review”- Lin Y. et al, Electroanalysis, 2010. DOI: 10.1002/elan.200900571
“Review of graphene-based strain sensors”- Jing Z. et al, Chinese Physics B, 2013. DOI: 10.1088/1674-1056/22/5/057701