The Nitty-Gritty of Nanosensors
If you were an avid Poptropica fan back in the day like me (still am to be honest), you would have likely came across Shrink Ray Island. Now in this island, a child genius invents a ray gun that shrinks anything it touches. However, on the day of the Science Fair, both the scientist and her invention go missing. To be the hero that you are and solve a problem this big, you must “think small.”
So, you might be thinking, “What does that have to do with anything you’re going to talk about?” Well, when things are as small as nanosensors, they start acting differently, so in order to fully comprehend nanosensors, you’ll have to think just like a Poptropican again.
Okay, so what are nanosensors?
In short, nanosensors work quite similarly to normal sensors, measuring physical quantities and converting them into signals that can be detected and analyzed. Our lives today rely quite heavily on sensors, without us even giving thinking about it. Sensors on the streets detect cars at traffic lights and adjust the flow through intersections accordingly. Sensors at shopping malls detect your presence and open doors to allow you to enter. Sensors are everywhere.
However, the main difference is that this occurs on the nanoscale. The nanoscale simply refers to anything from 1 to 100 nanometers (nm), equivalent to a billionth of a meter. To put that into perspective, a sheet of paper is approximately 75,000 nm thick. That’s crazy!
As mentioned before, objects in the nanoscale behave quite differently to everyday objects, and they start to be governed by quantum effects. At this scale, with such a small volume, surface area has a larger effect on material behaviour than it does for larger objects, so properties such as conductivity, reflectivity, magnetism, fluorescence, chemical reactivity, and melting point can change. For example, nanoscale gold particles aren’t the typical yellow colour that you would expect, rather, it could appear as a red or purple.
What makes them so important?
Sensors will help us better understand the world we live in. — Aleksandra Lobnik, Founder of Centre for Sensor Technology at University of Maribor
We already have regular-sized sensors. Why make them nano-sized? In comparison to traditional sensors, nanosensors have increased sensitivity and specificity. This comes from the high surface area to volume ratio of nanomaterials that was explained earlier, as it allows for large signal changes upon coming in contact with the object in question. Think of it like this. With an increased surface area, a larger part of it is being “exposed” to the outside world, allowing for greater interaction with objects. Its size also allows it to go places conventional sensors can’t, and see things from a different perspective, much like how the size of your Poptropica avatar allowed you to travel through the bloodstream in Virus Hunter Island. Many other benefits stem from the sensor being such a small size, some of which include:
- Faster response
- More accurate data
- Less impact on the phenomenon being measured
- Provide real-time monitoring
With the current trend of portability and downsizing, nanosensors will be increasingly integrated into future technologies. Currently, they have applications in many different fields, for example, healthcare, military, food, the environment, any many more. In the medical industry, nanosensors are mainly being used to monitor organ health. The nanosensors are inserted into the bloodstream to detect certain biomarkers — a substance that is produced when something bad occurs in the body — and sends signals to an external device to report its findings. Some other commendable uses include:
- Detecting chemicals in gases for pollution monitoring
- Monitoring physical parameters such as temperature, displacement and flow
- Monitoring metabolism to understand plant biology
- Accelerometers in MEMS devices (e.g. airbag sensors)
- Determining the quality and freshness of food
- Assessing the vitals of soldiers in combat
How do they work?
Though all sensors measure different things, they share the same basic workflow. They key components of a sensor system are the analyte, sensor, transducer, and detector. The detector selectively binds to an analyte — the substance of interest. A signal is then generated from the interaction of the two and processed into useful metrics through the transducer — a device that converts changes in physical quantities into an electrical signal. This information is then displayed on the sensor to be interpreted by humans.
There are two types of nanosensors: chemical and mechanical.
Chemical nanosensors work by measuring the change in the electrical conductivity of the nanomaterial once an analyte has been detected. Carbon nanotube-based sensors are excellent examples of this. These nanotubes are treated with coating materials, causing them to be more sensitive to certain molecules. For example, a molecule of nitrogen dioxide will strip away an electron from the nanotube, causing it to be less conductive, while ammonia will donate an electron, making it more conductive.
Mechanical nanosensors also work by measuring the change in the electrical conductivity, but do so by detecting a physical change to the material itself. For example, the nanosensors that car airbags depend on measure this by determining how far the material bends with changes in acceleration.
How are they built?
Now onto my favourite part. We now know that nanosensors will revolutionalize the future, but someone has to build them before all of this can happen.
The process of producing nanosensors is called nanofabrication. There are typically 3 methods of nanofabrication: top-down fabrication, bottom-up fabrication, and molecular self-assembly.
Top-down fabrication involves starting out with a larger block of material and carving out the desired form, similar to the wood carving projects that you might have done in the boy scouts. The technique used most commonly is called optical lithography, involving optical sources such as light rays, ultraviolet rays, and x-rays. A wafer substrate (the bottom surface), usually a silicon chip, is coated with a photoresist, which can be either positive or negative. A mask is then applied over certain areas to selectively expose it to the rays. The exposed areas will become softer using a positive photoresist, but will harden if using a negative photoresist. The softer areas are then chemically etched away, and the process is repeated for each layer. There are multiple variations on this, with some even using mirrors to burn the pattern into the silicon! The benefit of this method is that hundreds of chips can be built on a single wafer of silicon simultaneously, since the rays all beam down at once.
Another technique is nanoimprint lithography, which involves making a “master stamp” first with one of the other techniques, then applying the stamp to the surface to create the pattern. This method is ideal for mass production, but is limited to the features of the original stamp. Another technique is the scanning beam technique, using either focused beams of electrons or ions to carve patterns into the silicon. The disadvantage of this technique is its impracticality for mass production, as you must start from scratch for each sensor. However, it can produce patterns of much higher resolution, down to 20 nanometers.
Bottom-up fabrication is quite the opposite, as you have to assemble the already incredibly small nanosensors out of even tinier components, most likely individual atoms, and can be compared to bricklaying. Using tools such as atomic force microscopes, scientists are able to move atoms and nanoparticles one by one into their positions. This level of precision ensures that the nanosensors have fewer defects than if they were to be produced using top-down fabrication. One of the most common techniques is vapour-phase deposition. The building-block materials called monomers are first vapourized through heat, then passed into a vacuum chamber that contains the wafer. The monomers then link up to form polymers on the surface of the cold wafer, eventually condensing and forming a coating, just like how there’s a misty coating of water vapour on your bathroom mirror after taking a shower.
Molecular self-assembly can be considered a subset of bottom-up fabrication, but differs in the sense that a set of components automatically assembles itself into the finished product. This would be similar to taking a break from working on a 1000 piece puzzle, only to return and find out that it assembled itself. This might seem absolutely insane, but it was actually inspired by nature itself, as chemical forces have essentially created all life structures. One example of this is the growth of quantum dots. To dumb this down a bit, a thin layer of a metal compound is placed on top of a similar, but slightly different metal compound. Due to forces present in the quantum realm, repulsive forces within the top layer cause dots of the first metal to isolate from each other. After this process is repeated multiple times, a fairly uniform spacing of the dots can be achieved. The problem with this is that there is quite a limited amount of control over where these dots end up.
Another technique that especially struck my eye was the liquid-phase technique. To explain this concept, I’m going to turn the attention towards the biggest issue in our world today: the COVID-19 pandemic. We’re constantly being told to wash our hands thoroughly for 20 seconds with soap and water. But what is the science behind it? Soap has two-sided molecules. The head, made up of phosphates, is attracted to water, while the tail, made up fatty acids, is attracted to fats. The virus is simply made up of a bit of material on the inside, contained by a layer of proteins and fats. When soap molecules come in contact with the virus, the tail attaches to the fat layer. However, the head is attracted to the surrounding water, pulling and breaking apart the layer of fat. The water then rinses out the leftover shards of virus.
So what does that have to do with molecular self-assembly? It actually works the exact same way. By having the opposite ends of a molecule attract what the other repels, small building-block materials can be attracted to one end, while the other end drives it to a new place in trying to repel from its other half. Think of it as if you are trying to run away from your shadow. It’s still going to stay with you, but your running would have brought you to a different place.
What problems are there with nanofabrication?
Don’t get me wrong, nanosensors are great and all, but perhaps the reason why they haven’t had their big break yet is due to the inefficiencies in being able to produce them.
The problem with top-down fabrication is that while the techniques work well at the microscale (1000 times the size of a nanometer), it becomes increasingly difficult to apply them the smaller you go. This is because in etching away material with precision, one small mistake will spoil the entire batch. Aside from this, it uses planar techniques, meaning that the structures are made from repeatedly adding and removing layers, which becomes difficult in creating more arbitrarily-shaped nanosensors. It is also quite wasteful, as nothing is being done about the material that is etched away and it uses high-intensity rays and beams that must be powered with high voltages.
Just like building a house of cards, bottom-up fabrication takes an extreme amount of precision, control, and most importantly, time. Although it is able to produce smaller nanosensors than top-down fabrication, the attention to detail required for each makes it extremely hard to mass produce, decreasing the supply, increasing the price, and ultimately making it less accessible to the public.
With molecular self-assembly, we can be hopeful that it will reduce the amount of time required for bottom-up fabrication. By letting molecules assemble themselves, scientists could manufacture nanosensors much more quickly and cheaply. However, much of this is still in research and development, and it will be quite a while more before quantum dots or the liquid-phase technique are perfected.
How might we be able to overcome these challenges?
In order to have all the benefits and applications of nanosensors eventually come to revolutionize our world, we must make an effort to continue developing and improving what we currently have. Here are some of my ideas on how we might be able to do this:
- I strongly believe there is potential in combining the top-down approach with molecular self-assembly. As contrasting approaches, they each have their own benefits and drawbacks. Perhaps we can have the best of both worlds by using these methods in conjunction, reducing both time and costs. Using nanofabricated templates, we might be able to guide and control the positioning of nanoparticles on the substrate. In fact, there is some research being done that involves trapping the nanoparticles with the forces of attraction, induced by the nooks and crannies of the substrate.
- DNA molecules could prove to be quite effective in the liquid-phase technique or molecular self-assembly. Given that we are looking for molecules with complementary components, the nature of DNA — having two strands that are complementary to each other in regards to its nitrogenous bases — could prove to be beneficial. Aside from that, DNA has its own copying mechanism, meaning that DNA molecules can be sequenced and replicated in large quantities.
- We may be able to leverage artificial intelligence in the bottom-up approach. Currently, human intelligence is being used to operate the atomic force microscopes, which is one of the main reasons why bottom-up is so time-costly. With artificial intelligence, we will be able to get the same amount of precision, but save loads of time.
While I’m still no expert in nanosensors, I certainly have learned a lot in the past two days spent researching all this information. To those who have read up to this far, I hope I have opened your eyes to such a fascinating, new topic. On a personal note, what captivates me the most in researching exponential technologies like nanosensors is how we often resort to mimicking nature’s biological concepts in order to maximize their potential.
While nanosensors are currently still in its early stages of development, our current findings allow me to remain hopeful for a large future in nanotechnology, as there is a big world of small things waiting out there to be explored.