Nanosensors: A Disruptive Innovation in Nanotechnology
“Nanotechnology has given us the tools to play with the ultimate toy box of nature–atoms and molecules. Everything is made from it…The possibilities to create new things appear limitless” says Nobel laureate Horst Stormer.
Nanosensors are transforming, even replacing, existing sensor technology. So what is a nanosensor? To understand this, we should consider sensor history and it’s defining differences from nanotechnology.
Sensors are essential to modern life, humans rely on them. From traffic stops to water refill stations, sensors are all around us. A sensor is basically any object that uses an active physical or chemical component to produce a signal from analyzing something. Nanosensors work similarly to conventional sensors, but their defining factor is that they work on a nanoscale (10–100nm)and can detect minute particles or minuscule quantities of anything in this range using nanomaterials (as their active sensing element). In fact, the nanoscale is so tiny that if a human were shrunk down to nano size, they would be 1000 times smaller than a single red blood cell.
Nanosensors are divided by the form of energy signal they detect. I.e. physical, chemical, biosensors, thermal, optical, magnetic nanosensors, etc.
There are two main kinds of nanosensors:
- Mechanical
- Chemical
Nanosensors generally work by detecting a change (physical or chemical) in the electrical conductivity of the sensor material (a nanomaterial). For instance, a chemical sensor would determine the concentration or identify chemical substances. Whereas a mechanical nanosensor would be used to measure properties like temperature, pressure, force, etc. Nanosensors then convert those observed materials into electrical signals that can be analyzed. Carbon nanotube-based sensors work in this way, a nitrogen dioxide molecule interacting with a carbon tube nanosensor will strip an electron from the nanomaterial, making it less conductive. The nanosensor will detect this change and send electrical signals. Nanosensors detect changes from external interactions and communicate them to other nanocomponents.
Chemical:
Chemical nanosensors detect chemicals by measuring the change in the electrical conductivity of the nanomaterial. Many nanomaterials have high electrical conductivity, which reduces as molecules are absorbed or bound together. This binding/absorption is a detectable and measurable change. Specifically, 1D materials like nanowires are great for chemical sensors because they act as a transducer(turns one kind of energy into another) and electronic wires once the analyte is detected.
Mechanical:
Like chemical, mechanical nanosensors work by measuring the electrical changes. However, the method in which they do so is different. Mechanical nanosensors change their electrical conductivity when the material is physically manipulated, causing a detectable reaction. The response is measurable using an attached capacitor (component storing energy) where the physical change makes a quantifiable change in the capacitance (ratio of change in the electric charge to the corresponding change in its electrical potential). For instance, the system utilized by car airbags depends on monitoring changes in the capacitance. The minuscule weighted shaft attached to the capacitor of the airbag will be triggered to bend with changes in the acceleration measured in the capacitance from sensors.
Nanosensors are important to modern life for numerous reasons. They yield more efficient, portable, accurate, low power and responsive results. But, the most important aspect of nanosensors is that they are tiny! Nanosensors use tiny nanomaterials that inhibit better sensitivity, power, speed, and range of detection due to the surface area to volume ratio. The minuscule nanoparticles have a very ratio. To visualize this imagine spreading jam on toast. The bread’s exposed crust (not covered with jam) will make the toast less tasty. However, if you cut the same amount of bread into smaller pieces, the surface area increases, meaning you can put more jam on your toast! More surface area means more exposure to the compounds the sensor is trying to detect. As a result, there is an improved ability to detect the compounds at lower concentrations. This allows the sensors to conduct multiple functions or detect multiple targets simultaneously.
The heightened sensitivity causes low response time, making it a great tool for real-time analysis. For example, nanosensors are very useful in the medical field. Using nanosensors to conduct tests and blood analysis for early diagnosis would be more effective because of their fast response time, heightened sensitivity and efficacity. The fast response time eliminates weeks of waiting for results because of the high sensitivity which can detect analytes at a low concentration. Plus, the small size and lightweight physical properties of nanosensors make them perfect for integrating into portable device manufacturing.
The Many Applications of Nanosensors
Nanosensors have an infinite application due to their incredible sensitivity. Currently, nanosensor technology is being applied to environmental pollutant detection, early onset detection of cancer, diabetes blood diagnostics, food quality control, auto, aircraft, and spacecraft safety, monitoring health, water quality, mechanical strain, and the list goes on…
For example, NASA develops nanosensors, that consume low power, to detect chemical and biological species. They apply these sensors to monitoring crew health, water quality in the ISS and spacecraft safety. Currently, NASA is working on developing nanosensors to measure mechanical strain and detect early onset damage in materials for monitoring the structural health of their aircrafts/spacecrafts.
…So, how are these devices built?
Nanofabrication.
Nanofabrication is the process where structures -like nanosensors- are made at the nanoscale, typically 1 to 100nm, which can be implemented into more complex systems. There are two primary approaches to nanofabrication:
1. Top-down
2. Bottom-up
Top-down Fabrication:
The top-down strategy consists of miniaturizing blocks by cutting or etching them down to form a nanopattern on the material. The top-down approach is often described as being similar to carving a block of wood. Whereas the bottom-up method would be comparable to building a house brick by brick. One of the most common top-down techniques is etching. Etching is a way to remove material chemically or physically. An example of etching at the nanoscale is reactive ion etching (RIE). This is a method that uses many ions in plasma form to etch the nanomaterial (instead of a liquid etchant). This causes some of the lighter ions in the plasma to interact with the negatively charged nanomaterial. In this collision, the nanomaterial is being etched through kinetic and chemical interactions. An advantage of the top-down approach is that there is no assembly step needed since the parts are patterned and built in place, in the fabrication of the integrated circuit. In general, the top-down strategy is about starting with a base material and gradually working down, removing unrequired material to achieve the desired shape.
Bottom-up Fabrication:
The bottom-Up or the self-assembly method builds nanostructures up one atom at a time. This technique is very time consuming and is only used when required. This fabrication process assembles simple units into larger structures, mimicking a natural process. Individual atoms pile atop one another to form molecules, which then arrange themselves into their desired form for the nanostructure. The process begins with a nucleation stage where the nuclei are activated from supersaturation of reagents, temperature or external electric field application. From here, a number of techniques including, molecular self-assembly, occur. Molecular self-assembly uses the organizational ability of matter to create homogenous layers that are one molecule thick.
The most common method used is the top-down nanofabrication (where the integrated circuits get scaled down by removing a single atom after another until the desired form is attained). The top-down method is used when creating nanostructures that require long range for connections.
Nanofabrication is amazing, but there’s much to improve…
Despite the many advantages of nanosensors, there are many difficulties concerning both fabrication methods. The top-down method is more efficient and less time consuming, but the equipment required is expensive. The bottom-up strategy is cost-effective but takes much longer to execute. On the whole, nanosensors are hard to mass produce because they are so small. There is no quick or easy way to place tiny wires at precise locations on a surface. Plus, the equipment and facilities are very expensive and rare. Another issue with the production is the interference of dust particles. They are on the nanoscale and can easily ruin nanofabrication by clogging it. Finally, to create nanosensors you need people who are highly educated and comfortable working with delicate nanomaterials, which is hard to find.
In regards to biosensors, the behavior of carbon nanotubes and nanoparticles in the body is undergoing risk research, funded by the National Science Foundation. Looking at the big picture, nanosensor markets are still comparatively small, so certain nanosensor fabrication may not be economically justifiable because of the extensive resources it requires.
But there is still hope! There are many groundbreaking leads on how to solve these issues:
- To solve the issue of dust particles, researchers suggest the use of fully sterilized labs
- To solve the lack of educated individuals for jobs in this grossing market, the learning of nanotechnology engineering and designing should be encouraged to secondary/post-secondary youth, in order to expose them to the industry (and it’s potential)
- to solve inefficient nanofabrication methods, scientists are working on ways to combine elements of both top-down and bottom-up techniques to make an accurate and faster process
“Now researchers at Penn State University have come up with a way to guide single nanowires into place on a silicon chip using an electric field. Once the nanowires are in place, the researchers deposit electrodes on top to make arrays of sensing devices. This is a step toward affordable, sensitive handheld sensors that could quickly screen for hundreds of pathogens and toxic chemicals or catch the first signs of disease…The new technique is simple, fast, and compatible with conventional silicon-chip fabrication.” (MIT Technology Review)
- To reduce economic costs of diverse, rare, materials, researchers are experimenting with sustainable and cheaper resources
- To review the ethical perspectives of nanotechnology, engineering students discuss the ethical dilemmas in class
Evidently, the use of nanosensors and all nanotechnology is a game-changer that will undoubtedly impact everyone’s lives in the future. With important nanotechnology breakthroughs being announced left and right, the future of technology is quickly approaching.
My Final Thoughts and Further Reading
The amount of progress and the sheer number of breakthroughs that are challenging the roadblocks of nanotechnology is inspiring. Though there are many obstacles in nanofabrication restricting the integration of nanosensors and other nanotechnology into everyday life, and mass production. The constant strides from student researchers and future world-changers, who are working to formulate new standards for the principles of nanofabrication, provide hope that this generation will push ingenuity to create a disruptive market of nanosensors that can be sustainable and able to replace all sensor technology. The numerous examples of researchers who work to ensure that technology to better the future is on the way, serve as a testament to the determination and hopefulness of this generation.
If you are interested in learning more about the groundbreaking revelations in nanotechnology, and the future of nanosensors/nanotechnology, I’d recommend the following:
https://uwaterloo.ca/institute-nanotechnology/nano-101
http://web.pdx.edu/~pmoeck/nanosensors.pdf
https://www.technologyreview.com/s/411695/nanosensors-made-easy/
https://www.nasa.gov/home/hqnews/2007/jun/HQ_07140_Nanotech_Sensor_Test.html
