Sensors are used in many areas of science and everyday society, from monitoring the upstream and downstream processes in a chemical plant to controlling automatic doors, computers, and autonomous vehicles. It is safe to say that sensors are an integral part of everyday life. There’s always a need to improve the accuracy and precision of sensors to provide more reliable data. The need to further optimize sensor accuracy and precision becomes even more important as many areas of manufacturing move toward automated processes supported by the Internet of Things (IoT) and big data as the full-scale implementation of Industry 4.0 approaches.
Because sensors have many different application areas, sensors can measure changes in a localized environment through many different mechanisms. In any case, the design will include an active sensing component to detect changes in the environment. In terms of the mechanisms, some will detect an analyte in the local area through the molecules temporarily binding to the sensors surface—which can be a gaseous molecule (including water for humidity sensing), a liquid, or a specific chemical—whereas some mechanisms rely on a physical deformation of the sensing material—such as stress and strain sensors—and others will rely on an optical or thermal change in the local environment to invoke a detectable response.
One thing that is common throughout all sensing mechanisms is that the sensing mechanism causes a change throughout the sensing material, and this enables the change to be detected and recorded. In many cases, the sensing mechanism causes the electronic properties of the sensing material to change. It is this change that is outputted by the sensor readout in a more usable and readable format. This electrical change can take the form of increasing the conductivity across the sensing material (thus increasing the voltage), or via an increase in the resistivity across the material.
Nanomaterials are inherently thin in nature, and this is a big positive when it comes to sensing applications. In recent years, sensors that use 2D and 1D materials have proven to produce high sensitivities. Because nanomaterials are so thin, their relative surface area is usually high. So, nanomaterials not only make sensors smaller, but they provide a much higher sensing surface area than when bulk materials are used. The higher sensing surface area means that more ‘sensing points’ on the surface are possible compared to other materials. Because the materials are so thin, defects—and specifically charged cavities—can be introduced to the surface of a nanomaterial, and this is a way that nanomaterials can make sensors selective to a certain type of molecule. This can be specific gases—such as ammonia, methane, or water vapor—or specific chemicals within a flowing liquid. In addition, designers can use some surfaces to create defined regions that are specific to one molecule and others that target different molecules. This enables nanomaterial-based sensors to have multi-sensing capabilities.
There is another aspect to their thinness, and that is flexibility. Not all nanomaterials are flexible, but those that are—such as graphene—can be deformed by a large degree without breaking, and this again changes the electrical conductivity across the nanomaterial (which is detected). Many flexible nanomaterials also have a high tensile strength—just look at graphene with the highest known tensile strength of any single material. Therefore, the flexibility of some nanomaterials can become a sensing mechanism with the ability to return to its original conformation and have a long useful life. In many cases, nanomaterials can also behave the same under pressure and provide a detectable response. There are various piezoelectric and piezoresistive nanomaterials that will deform under a strain and invoke a change in their electrical current—much like bulk piezoelectric and piezoresistive material but on a much smaller scale—which makes them more accurate to small strain deformations.
Some nanomaterials are also thermally conductive and can be exposed to high amounts of heat, which is an ideal property for temperature sensors. In these instances, when the local temperature is increased, it can be detected by the drop in thermal resistivity across the nanomaterial.
Another property that is beneficial from some nanomaterials is their optical properties. Some nanomaterials possess photo-absorption properties, which when coupled with a high electrical conductivity and charge carrier mobility, can act as highly sensitive photodetectors. In some cases, this can extend beyond visible light into other areas of the electromagnetic spectrum, such as for UV radiation.
We’ve talked about how the different mechanisms and properties of nanomaterials help to induce a change in the electrical conductivity of the nanomaterial and/or other sensing surfaces. But, the electrical conductivity and charge carrier mobility—the ability for charged particles such as electrons and holes to move through the atomic lattice—are two properties in themselves that many nanomaterials excel at. Many nanomaterials possess highly conducting or semiconducting electronic properties, which alongside a high-charge carrier mobility, makes the electrical change across the nanomaterial significantly more sensitive through being considerably more responsive to minor changes.
In the case of those nanomaterials that exhibit semiconducting properties, they can be used to detect molecules that have both acceptor and donor electronic properties. Semiconducting nanomaterials can employ mechanisms that cause holes to deplete from the valence band—thus increasing the resistivity across the nanomaterial—or mechanisms which cause electrons to migrate to the conduction band—thus increasing the conductivity. Both mechanisms are easily detectable via the change in applied voltage across the nanomaterial.
We’ve talked above about nanomaterials on their own, but designers can incorporate many nanomaterials into hybrid materials (such as composites) and bring about benefits in this form. When they are incorporated into a hybrid matrix, the nanomaterial will bind intermolecularly with the other materials. Intermolecular bonding can be through hydrogen bonding (if the nanomaterial contains polar groups), van der Waals forces, and π-π stacking. These intermolecular interactions enable efficient charge transfer mechanisms to take place where there are delocalized electrons (particularly where π-electron networks are formed) within the hybrid material. This provides a more efficient conduction mechanism compared to when they’re not included in the matrix, which results in a higher degree of sensitivity.
Not all nanomaterials are suitable for sensing applications, but those that are can provide a significant improvement in the sensing capabilities of a sensor over other materials. Overall, there are a range of beneficial properties—from a high surface area to thermal conductivity, a high electrical conductivity, and charge transfer properties—that designers can use to provide more accurate sensing mechanisms over other sensing materials.
There are many different areas where sensors use nanomaterials, and these include, but are not limited to, stress/strain gauges, various types of biosensors, temperature and humidity sensors, pressure sensors, optical sensors, capacitance sensors, piezoelectric sensors, and piezoresistive sensors.
Liam Critchley is a writer, journalist and communicator who specializes in chemistry and nanotechnology and how fundamental principles at the molecular level can be applied to many different application areas. Liam is perhaps best known for his informative approach and explaining complex scientific topics to both scientists and non-scientists. Liam has over 350 articles published across various scientific areas and industries that crossover with both chemistry and nanotechnology.
Liam is Senior Science Communications Officer at the Nanotechnology Industries Association (NIA) in Europe and has spent the past few years writing for companies, associations and media websites around the globe. Before becoming a writer, Liam completed master’s degrees in chemistry with nanotechnology and chemical engineering.
Aside from writing, Liam is also an advisory board member for the National Graphene Association (NGA) in the U.S., the global organization Nanotechnology World Network (NWN), and a Board of Trustees member for GlamSci–A UK-based science Charity. Liam is also a member of the British Society for Nanomedicine (BSNM) and the International Association of Advanced Materials (IAAM), as well as a peer-reviewer for multiple academic journals.