Nanosensors: The Next Step Forward

10 min readJul 31, 2019

The human species prides itself for its brainpower. It’s what makes us unique and different from the other animals we share this Earth with. We’re constantly pushing forward with research, always searching for that next step in technology, that next breakthrough, that will change the world. That next step might just be nanotechnology. Specifically, nanosensors.

What is a nanosensor?

According to nature.com, “Nanosensors are chemical or mechanical sensors that can be used to detect the presence of chemical species and nanoparticles, or monitor physical parameters such as temperature, on the nanoscale.”

Basically, nanosensors are sensors that work on the nanoscale level, meaning they’re capable of detecting very small things. One common misconception is that all nanosensors must be on the nanoscale of 10–100 nanometers. However, though many are, nanosensors are just sensors that can operate on the nanoscale and detect nano-particles and materials. They themselves may or may not be on the nanoscale, but they’re designed to be much smaller than normal sensors.

So just how small are we talking? Just to put this into perspective, a human hair has a diameter of around 60,000 nm. The targeted nanoscale level is a mere fraction of the size of a human hair, less than 1/600th. This tiny scale allows nanosensors to do things regular sensors can’t.

Why use nanosensors?

Nanosensors are more efficient, sensitive, and accurate than normal sensors. This is mainly because nanosensors are usually extremely small, allowing them to have a greater surface area to volume ratio, which is extremely important. A bigger surface area means the exposure to target compounds is greater, allowing the sensor to detect particles at very low concentrations. Nanosensors are capable of detecting even just a few molecules of inorganic, organic, or biological substances, making them extremely accurate. Nanosensors also operate with high-efficiency thanks to their compact size. They are able to function faster because information and electrons have less distance to travel.

Real-life applications

Nanosensors can be applied to all sorts of fields and research is currently expanding on and increasing the ways they can be used as a part of everyday life. Their ability to detect molecules, even at low concentrations, can be applied to fields such as consumer products and safety. For example, researchers at the University of Utah are developing nanosensors that are able to detect even trace amounts of explosives or dangerous chemical vapors.

Nanosensors also are able to detect small bacteria and viruses, making them especially useful for the medical field. They can be used to detect cancer at very early stages, making it possible to treat the devastating sickness earlier and potentially saving millions of lives. These are only a few examples of the countless ways nanosensors could be incorporated into daily life.

Nanosensors and neuroscience

Scientists at the University of California are currently working on developing nanosensors that can detect neurotransmitters in the brain. Neurotransmitters are chemical messengers that transmit signals from one nerve cell to another when the neurons fire. They affect both physical and psychological functions, from heart rate to emotions to pain.

link

Berkeley researchers have been able to construct a nanosensor designed to track and image the neurotransmitter dopamine.

Dopamine is a neurotransmitter well known for establishing pleasure, motivation, and reward-seeking behavior. It also plays a role in focus, emotional resilience, and cognitive flexibility (the ability to switch between thinking about two different topics). In addition, it’s one of the main regulators of motor control and movement. Dopamine plays a major role in drug addiction because, when drugs enter the body, they trigger surges of endorphins and dopamine, making the person more likely to do it again resulting in addiction. However, not enough dopamine can trigger mental illnesses such as depression, schizophrenia, and psychosis. Loss of dopamine also can directly cause Parkinson’s disease, as the decrease of this vital neurotransmitter leads to issues with movement.

Tracking dopamine is crucial to gaining a better understanding of these illnesses so we can eventually find cures and treatments. Nanosensors provide an efficient, effective solution for imaging neurotransmitters. Made by combining carbon nanotubes and biomimetic synthetic polymers, the nanosensor produces a fluorescent signal when they detect dopamine. Using the intensity of the fluorescent signals, the neurotransmitter levels are able to be directly quantified. Microscopes are able to observe the signals and process them, making it possible to image the nanosensors in living brain tissue.

This is big. Scientists are already using these sensors to observe the effects of antidepressants on the brain which can help the development of better, more efficient antidepressants with fewer side effects. These sensors can also help us understand mental illnesses and dementia better in order to find a cure.

“Developing sensors for brain chemistry is an exciting area of research that could transform how we diagnose diseases based on imbalances in brain chemistry, such as depression and anxiety,” says Landry, a scientist at Berkeley.

Also, if scientists can already map dopamine, they can modify the nanosensors to map other neurotransmitters, giving us another piece of the puzzle that is the human brain. Gaining a better understanding of neurotransmitters is crucial to understanding how we think and react the way we do and why we are who we are.

link

How nanosensors work

There are many different types of nanosensors that function in different ways including optical nanosensors and biological nanosensors. For the sake of simplicity, I’m just going to address two of the main types of nanosensors: chemical nanosensors and mechanical nanosensors. Chemical nanosensors can detect and identify chemicals while mechanical nanosensors detect and measure outside physical forces.

link

Chemical nanosensors work by monitoring electrical changes in the sensor material as it comes into contact with target molecules. They use a variety of different materials such as carbon nanotubes or zinc oxide nanowires to detect the different chemical properties of molecules by coating them with different materials, making them sensitive to reacting with certain molecules and immune to others. The target molecules can then react with the sensors, changing the electrical conductivity and making it possible to detect and identify the molecules. For example, a molecule of nitrogen dioxide (NO2) reacting with the carbon nanotube would strip away an electron, causing the nanotube to be less conductive. On the other hand, ammonia (NH4) reacting with water vapor would actually cause the nanotube to gain an electron, making it more conductive. Because nanotubes and nanowires are so small, even just a few molecules are enough to change the electric properties, allowing the nanosensor to detect chemicals at extremely low concentrations.

Mechanical sensors also work by measuring electrical changes. However, unlike chemical nanosensors, these changes happen when the sensor is physically manipulated or subjected to an outside force. They can be used to measure information about temperature, pressure, flow, stress, strain, position, displacement, acceleration or force. One major way these sensors are used is in the MEMS systems that control car airbags. MEMS systems have tiny, weighted shafts attached to captors which are devices with the ability to store energy in the form of electric charge. When the car’s acceleration changes, the shaft bends which can be measured as a change in capacitance which is the ability of a system to store electrical charge. These changes are monitored by mechanical nanosensors. The speed and efficiency of their signals make it possible to fully deploy airbags within 55 milliseconds after the collision, ultimately saving lives. In fact, airbags have saved 50,457 lives in the 30 years from 1987 to 2017.

How nanosensors are built

Nanofabrication is the design and build of devices on the nanoscale, such as nanosensors and other nanostructures. There are two main methods of nanofabrication: top-down and bottom-up.

The top-down approach generally uses nanolithography techniques to either “carve” out the desired device from a larger block of material or to “print” the device out onto a surface. Nanolithography is the science of etching, writing or printing to modify structures under 100 nm.

One of the most common types of “etching” techniques is photolithography. It’s the method that’s often used by websites to define “top-down” nanofabrication. Using this technique, materials are first covered with a chemical called photoresist. Then selected areas are covered with a “mask” and the exposed areas harden. The covered, soft areas are then chemically “etched” or “carved” away and the process is repeated to build up multiple layers. This method can produce features as small as 50 nm.

Another common technique is called Electron Beam Lithography (EBL). This method uses a highly focused beam of electrons to draw custom shapes on a disc coated with an electron-sensitive layer called a resist. The disc is then placed in a solvent and, depending on how the specific resist material reacted with the electron beam, the exposed parts will dissolve or the non exposed parts will dissolve, leaving a mold. A machine called spinner coats the surface with a layer of metal, or a layer of the desired material. This material coats the entire surface, forming a layer over the resist and directly contacting the surface of the disc, binding to it. Finally, another solution removes the resist and the layer of material on top of it, leaving behind only the material directly bound to the surface. This technique can be used to make electrodes and the Harvard CNS lab can already produce features as small as around 4–10 nm.

link

The bottom-up approach is the opposite and builds devices from atoms and molecules, shaping them to create the desired nanostructure. Chemical and physical forces are applied to mimic the natural biological systems found in nature. There are two main types of bottom-up assembly: molecular assembly and self-assembly. Molecular assembly is when scientists build nanostructures through chemical or physical procedures. Self-assembly is when the nanostructure essentially builds itself in a controlled environment.

One method of molecular assembly is vapor deposition. There are two types: Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD). Both of these techniques are used to deposit thin layers of materials called films onto a base wafer, called a substrate. In an isolated chamber with a controlled environment, the substrate is exposed to vaporized sources which, with the help of stimuli, develop a thin film that coats the surface. Properties of the film are determined by the chemistry of the vapors. Using the PVD technique, solid or liquid sources are placed inside a chamber with the substrate. They’re then subjected to physical processes such as heating or sputtering, causing them to form vapors that coat the substrate. However, PVD techniques could result in a non-even layer of film on geometrically complex surfaces, such as surfaces with trenches and bumps. This is because PVD vapors generally have a defined path of how the molecules hit the surface.

link

The CVD method is different. Instead of solid or liquid sources and physical processing, the CVD technique works by pumping gas into a chamber containing a substrate. The gas reacts with the surface of the substrate, creating a film that coats the surface. This process results in a uniform layer of film because the vapors are multi-directional and are able to coat everything. CVD is one of the main techniques for making carbon nanotubes, which are a crucial part of the construction of nanosensors.

Link

Self-assembly is another method of bottom-up nanofabrication. Through chemical forces between molecules, matter can organize itself into desired structures. Self-assembly must take place in highly controlled environments with just the right chemical settings and temperature in order to actually work. Scientists, use the chemical properties of molecules to predict how they would react and bind with each other in their process of designing a way to construct specific nanostructures.

Problems with nanofabrication

Top-down, Bottom-up, and self-assembly techniques are definitely not perfect. Nanotechnology is a relatively recent field of study and we need to keep progressing.

Top-down photolithography is generally able to produce greater quantities of product at faster speeds. However, the machines and clean rooms necessary to use this technique are pretty expensive. Photolithography also gets increasingly more difficult as nanostructures get smaller and are unable to create structures under 50nm.

Top-down Electron Beam Lithography (EBL) is more precise than photolithography and is able to generate features down to 4–10 nm. However, it’s also very expensive and it’s time-consuming. For example, a process that would take 5 minutes using photolithography may take up to 5 hours using EBL. This makes it unsuitable for commercial production.

Bottom-up nanofabrication is able to produce smaller products and overall, they have fewer defects. However, molecular assembly is very time consuming and is, therefore, unable to commercially produce products.

Self-assembly has the potential to be an extremely efficient, cost-effective, and accurate method of nanofabrication. However, research hasn’t yet advanced to that point. We are still taking the first steps into understanding and using this technique,

Moving forward

Nanotechnology and nanosensors will definitely play a major role in the future in a variety of fields such as healthcare and safety. However, in order for this to happen, we need efficient techniques for nanofabrication. Currently, top-down nanofabrication is the most commonly used technique. However, top-down nanofabrication wastes too many resources and is relatively clumsy compared to bottom-up methods. Moving forward, we need to dedicate for research and effort to bottom-up techniques, especially self-assembly. I believe self-assembly is the eventual future of nanofabrication and is essential for producing large numbers of precise nanostructures we need in order to move forward in innovation.