Monitoring the conditions of the environment and particularly how human activity affects different parts of the ecosystem is increasingly important as we gain more understanding into the extent of human impact. The rise of connected devices and the internet of things is making it easier than ever to collect and combine information about the environment around us.
Environmental monitoring is important not only in measuring the effects of industrial and personal human activity, but also in changing people’s habits to reduce their environmental impact. Consumer access to the data collected by continuous monitoring of air and water quality can enable people to change their behaviour, for example, reducing their personal carbon footprint, or selecting transport modes that will reduce their exposure to pollution.
As well as passive monitoring, environmental sensors also have a key role to play in reducing the harmful impact of industrial activity on the environment. Careful monitoring of air and water quality in industrial production facilities can provide alerts for leaks or process failures that would release harmful emissions into the environment, or before exposure limits are exceeded. Such alerts allow preventative actions to be taken in a real-time basis, and decisions and actions are increasingly being automated. New technologies are enabling betterthan- ever measurements of trace amounts of pollutants.
“new technologies are enabling better-than- ever measurements of trace amounts of pollutants”
Active monitoring is also increasingly being adopted for consumer electronics, like health devices and smart infrastructure, including intelligently controlled cities and buildings. At the heart of this development are accurate and responsive miniaturised sensors that provide the relevant knowledge.
Graphene – a material of intense research interest since its isolation in 2007 and the subject of the 2010 Nobel prize in physics – has exceptional promise as a sensing material due to its unique combination of properties. It is a two-dimensional form of carbon, with atoms arranged in an sp2 bonded hexagonal lattice that gives rise to excellent electrical, optical and mechanical properties.
Other new materials, such as PtSe2, could also be used for environmental sensing. PtSe2 has excellent response and recovery times for gas adsorption. (Credit: Trinity College Dublin/Graphene Flagship)
Graphene is an excellent electrical and thermal conductor. It has very high charge carrier mobility and density, which can be tuned electrically, with very little temperature dependence. Pristine graphene is chemically inert, but its electrical properties are highly sensitive to surface effects such as the presence of molecules. Graphene also has a very high level of interaction with light, absorbing approximately 2.3% of light despite being only a single atom thick. It’s hexagonal lattice structure makes it flexible and extremely strong.
Applications of graphene
These exceptional properties are of interest for a wide range of applications: from flexible electronic and photonics to composite materials and energy generation and storage. The Graphene Flagship is an EU research consortium founded in 2013 to take advantage of graphene’s potential and develop new technologies based on graphene and the new class of related materials. Among several different research avenues, the Flagship is particularly interested in how graphene’s novel properties could improve sensors – particularly in development of high-volume, low-cost sensing solutions for environmental monitoring, health monitoring and biosensing.
There are several mechanisms by which graphene can be used for sensing. As a two-dimensional material, graphene is the extreme limit of specific surface area, which makes it ideal for chemiresistive sensing of surface-adsorbed molecules. Such sensors can be used for gas monitoring, as gas molecules attach to the surface of the graphene and influence the resistance of the sensor. In fact, due to graphene’s intrinsically low noise, it can be used to detect the adsorption and desorption of individual gas molecules1.
“sensors can be used for gas monitoring, as gas molecules attach to the surface of the graphene and influence the resistance of the sensor”
Additionally, graphene can be used in a field-effect transistor or capacitive configuration for sensing of liquid analytes. This is especially interesting for sensing of biological molecules, for example, disease markers in biological samples. Fast, label-free sensing of biological markers is increasingly desired for point-of-care diagnostics and on-the-ground infection monitoring. Also interesting for biological sensing is graphene’s strong interaction with light, particularly in the IR range. Biological molecules have complex vibrational fingerprints in the IR spectrum, which can be detected with graphene.
“due to graphene’s intrinsically low noise, it can be used to detect the adsorption and desorption of individual gas molecules”
Producing graphene gas sensors
The challenges in the development of industrially viable graphene sensors relate to obtaining high-sensitivity in combination with selective response to the desired stimulus, appropriate response and recovery rates, and reasonable lifetime in a range of operating and storage conditions.
By printing graphene oxide on top of graphene-based RFID antennas, researchers from the UK have produced wireless humidity sensors that do not require a power source. (Credit: University of Manchester)
Due to dangling bonds, the edges of graphene are more reactive than the flat surface. One way to enhance the reactivity of graphene is to introduce edge defects into the graphene sheet.
Researchers working at the Technical University of Denmark, Denmark reported an efficient method to produce graphene gas sensors with a high volume of edge defects via nanoimprint lithography2,3. To produce these sensors, graphene is selectively etched using a resist patterned with a reusable nanoimprint template. One benefit of this method is that the electrical properties of the graphene are preserved to a high degree. Nanoimprint lithography provides the advantage that it is a simple, low-cost and scalable production method for obtaining patterned nanostructures on wafer scale, making it ideal for chipintegrated sensors.
“graphene doped with boron has been shown to be extremely sensitive to gas adsorption, detecting NO2 and NH3 at levels of 1 ppb and 1 ppm respectively”
Testing their devices as sensors for NO2, they found that the nanopatterned sensors detected 50 ppb NO2 in air, below the World Health Organization recommended hourly average of 100 ppb. The unpatterned devices did not display any response at this concentration.
Though these sensors showed sensitivity for NO2 due to its exceptionally high affinity to graphene defect sites, pristine graphene is highly non-specific. For gas-sensing, it is important to be able to distinguish the target molecules – such as pollutants like NO2, NH3, CO and so on – from ambient gases, as well as to distinguish between the different pollutants.
Many different methods have been explored to improve the selectivity of graphene-based sensors, including introducing dopants and functional groups into the graphene layer. Graphene doped with boron – in which boron atoms take the place of some of the carbon atoms – has been shown to be extremely sensitive to gas adsorption, detecting NO2 and NH3 at levels of 1 ppb and 1 ppm respectively4.
Surface functionalisation is an important avenue for producing graphene with specificity to different gas molecules. One promising method is the functionalisation of reduced graphene oxide (rGO), which is more reactive than pristine graphene and can be more easily functionalised through covalent bonding of functional groups. These can be directed towards target molecules; however, the electrical properties of graphene oxide are much reduced compared to graphene.
Sputtering and pulsed laser deposition
An alternative is to deposit more reactive atoms or nanoparticles onto the graphene surface using thin film deposition techniques such sputtering5 or pulsed laser deposition6,7. These provide tailorable adsorption centres and can improve the sensitivity as well as selectivity. For example, pristine graphene does not exhibit a sensing response towards benzene or formaldehyde, while TiO2- or Fe3O4-corenanoparticle- decorated graphene showed detection response towards low ppb concentrations of the volatile organic compounds (VOCs) depending on the size of the nanoparticles5.
“researchers at the University of Tartu, Estonia are using these ideas to develop an “electronic nose” based on an array of differently functionalised sensors”
Though this tailorable response is not true selectivity, researchers at the University of Tartu, Estonia are using these ideas to develop an “electronic nose” based on an array of differently functionalised sensors, inspired by the working of the human nose.
The human nose has around 400 different types of odour receptor but can detect many more smells. Differentiation of smells is achieved by combined function of the odour receptors, producing a unique odour profile for each smell. The researchers aim to use this principle to develop an electronic nose that can distinguish between several types of gases based on the different responses of an array of differently functionalised sensors. By calibrating the different sensor responses to a range of gases, different individual gases present in a mixture can be identified. The researchers plan to use such a device towards smart cities and pollution monitoring, envisioning a smartphone app that could be used to check pollution hotspots.
Researchers in Estonia are developing an “electronic nose” using an array of differently functionalised graphene sensors. Different gases are distinguished by the respnse profile of the sensor array. (Credit: Graphene Flagship)
Enhancing sensor response
While graphene devices tend to respond quickly to the introduction of gases, recovery of the devices needs to be accelerated. At room temperature, the gas molecules remain adsorbed onto the surface. These can be removed by heating to temperatures above 150 °C, or irradiation with UV light. In fact, continuous irradiation with UV light enhances both the absorption and desorption process, making sensor response much faster.
For biomolecular sensing, surface functionalisation with biomolecular receptors can give highly selective results. Combined with the high sensitivity of the graphene sensors, this can provide means for rapid point-of-care (PoC) diagnostics, and in monitoring the spread of infections and diseases, with on-chip sensors identifying disease markers without the need for time-consuming culturing and labelling.
Researchers from VTT Technical Research Centre of Finland, Finland, have developed an on-chip graphene-based sensor to detect biological molecules in solution8. Graphene is functionalised by biological hydrophobin molecules, which self-assemble into a dense monolayer in the presence of a hydrophobic surface such as graphene. The hydrophobin molecules can be functionalised with specific receptors, which will bind only the desired biomolecules to give highly selective sensing capabilities.
The same device can be used to detect different biomolecules, as the receptor molecules attached to the hydrophobin layer can be exchanged in-situ. As well, the sensors can be dried with the receptor molecule attached, which means that functionalised sensor chips could be stored for later use at PoC contexts. This reusable biosensor platform could be promising for a strongly selective, programmable sensor chip for diagnostic systems.
Graphene is also promising as a biomolecular sensing platform using surface plasmonics9, 10. Biomolecules have complex vibrational spectral that act as an identifiable optical fingerprint. These optical signatures can be revealed through infrared spectroscopy, but the interaction between the molecules and infrared light is weak due to the small size of the molecules. Graphene, in contrast, has a relatively high interaction with light, and a very strong plasmon response that can be tuned electrically to resonate with the molecular vibrations. Compared to state-of-the-art metal sensors, graphene nanoribbons demonstrate a much greater resonance shift, leading to a much higher sensitivity. These plasmon sensors could provide a platform for highly sensitive – down to single molecules – label-free detection and identification of proteins.
Other layered materials
Graphene was the first to emerge in the new class of layered materials with excellent electrical properties and a very high surface ratio. Among others are the transition metal dichalcogenides (TMDCs), which are layered semiconductors. While not strictly two-dimensional, the TMDCs have a cleavable layered structure similar to graphene’s monolayers. These materials are also currently being investigated for their potential in new technologies such as flexible electronics. Like graphene, their strong surface response is also highly promising for new sensors.
PtSe<sub>2</sub> for gas sensing
Little studied thus far, PtSe2 has been shown to be a very good candidate for gas sensing11, with extremely short response and recovery times without the need for UV illumination. With excellent gas sensing performance when tested at low NO2 concentrations of 10 ppb, this platform is highly promising for real-time environmental monitoring.
Graphene’s strong interaction with light makes it ideal for plasmonic biosensing. (Credit: ellamarustudio/Adobe Stock)
Furthermore, growth of the PtSe2 material is directly compatible with back-end-of-line semiconductor processes, making it possible for such sensor chips to be fabricated as integrated circuits. As a relative newcomer to the family of layered materials, further work is needed to determine a suitable approach for selectivity.
As well as gas sensors and biosensors, the new layered materials also have potential in other types of environmental sensors. Graphene oxide is a very good water absorber and can be used as an effective and fast humidity sensor, with no further functionalisation.
“many experts see sensors as one of the most promising applications for graphene”
Researchers working at the University of Manchester, UK have recently demonstrated battery-free wireless radio-frequency identification (RFID) humidity sensors based on graphene oxide-coated RFID antennas made from graphene12. The permittivity of the graphene oxide changes with humidity, affecting the amplitude and phase response of the antenna and allowing passive wireless sensing. The fully printed device result opens up the possibility of low-cost and widely available humidity sensors that could be used in health monitoring devices and food packaging.
While much more work is needed for specific applications and devices, so far, studies have shown that graphene is biocompatible and not harmful to the environment. As such, graphene is an ideal material for a wide variety of environmental sensing applications, due to its excellent electronic properties, possibilities for integration with silicon electronics and versatility.
Considerable work is being done across Europe under the umbrella of the Graphene Flagship to translate these promising qualities into fully-developed technologies for applications across industrial and consumer applications. Many experts see sensors as one of the most promising applications for graphene, and this is an area of intense market growth worldwide. Though much of the research still in the early stages, it will not be too long before graphene-integrated sensors are helping us monitor the world around us.
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Main Picture: Graphene is a flexible, two dimensional form of carbon with excellent electronic, optical and mechanical properties. (Credit: artemegorov/Adobe Stock)