Nanosensors and Nanofabrication

In this article we will explore what nanosensors are, why they are important, different classifications of nanosensors, how they work, some applications, the process through which they are built (nanofabrication), the problems with nanofabrication, and ideas to improve the nanofabrication process. First let’s explore what nanosensors are.

What are Nanosensors?

Nanosensors can be broadly classified into 2 groups: those that have nanoscale dimensions, and those that perform nanoscale measurements but do not necessarily have nanoscale dimensions. Basically, the nanosensor is a sensor that either has to have a size in the nanoscale (10 nanometers to a 100 nanometers, in order for you to truly understand how small this is, a human hair is approximately 80,000- 100,000 nanometers wide) or the distance between the nanosensor and the object it is observing has to be in the nanoscale. Therefore a nanosensor does not have to be nanosized to be classified as a nanosensor. So now we know what a nanosensor is, but why are they important?

Why are they important?

Nanosensors open opportunities to build new generations of devices. They are capable of interacting easily at the nanolevel (e.g. biomolecules) and can observe unique processes at the nanolevel which are completely different from the macro level.

• Nanosensors have high sensitivity which allows for more accuracy
• Are small, durable, light and portable
• Nanosensors have low power consumption
• Require less sample volume for analyzing and cause minimum disturbance to the observed material or process
• They have low response time and more speed than other sensors which allows them to do real time analysis
• It can detect multiple things at the same time allowing for multiple functions

Because of all these things nanosensors have a lot of potential in many fields. Now let’s look at different classifications of nanosensors.

Classifications of Nanosensors

There are several classifications of Nanosensors. Let’s take a look at some of them:

• Active nanosensors: Requires an external source of power in order to operate.
• Passive nanosensors: Uses external factors to work.
• Absolute nanosensors: Absolute nanosensors have a set reference point. For example an absolute pressure nanosensor measures pressure with reference to zero pressure.
• Relative nanosensors: Relative nanosensors have a chosen reference point.

Nanosensors are also classified according to the form of energy signal they detect: physical, chemical, or biological. All nanosensors fall under one of these three but there are also more specific classifications based on the form of energy:

• Physical nanosensors: Physical sensors are used for measuring properties like temperature, pressure, flow, stress, strain, position, displacement, or force
• Chemical Nanosensors: Chemical sensors are meant for determining concentration or identity of a chemical substance
• Biosensors:Biosensors are useful for dealing with biologically active substances
• Nanobiosensors: Based on the interfacing of nanomaterials with biomolecules
• Thermal Nanosensors: Used to detect heat and changes in heat.
• Optical Nanosensors: They are capable of constantly monitoring chemical or biological processes, and are capable of converting the available information into signals useful for analytical purposes. They are considered better than other nanosensors for analytical purposes.
• Magnetic Nanosensors: Used to detect magnetic fields and nonmagnetic items inside a magnetic field.

Now we know what nanosensors are, why they are important, and some classifications, but how exactly do they work.

How Nanosensors Work

Nanosensors work by converting the observed material or a process into electrical signals which can then be analyzed. Most nanosensors work by measuring electrical changes in the sensor materials (nanomaterials). For example if a molecule of nitrogen dioxide comes in contact with a chemical carbon tube nanosensor, it will strip an electron from the nanotube (nanomaterial), which will make the nanotube less conductive. The nanosensor will notice this and send electrical signals. Nanosensors detect changes from the external interactions and communicate to the other nanocomponents.

Applications

Applications of nanosensors span across many fields. Nanosensors can be equipped to do real time quick analysis themselves using AI, and in the future even decide on a course of action which can then be quickly approved by humans. Scientists are working on something similar for healthcare to create analytical instruments capable of diagnosing various medical conditions by assessing a patient’s breath using nanosensors and AI. Scientists were able trace the internal temperature of a living animal on the nanoscale for the first time. With a nanoparticle-based thermosensor, researches have revealed minute variation in temperature along the muscle fibers of a beetle as it prepares for flight. Another prominent application is using the fluorescence characteristics of cadmium selenide quantum dots to discover a tumor in the body.

Researchers have developed a system capable of detecting 17 different diseases from a patient’s breath alone. Image was taken from https://www.technology.org/2016/12/29/breath-test-based-nanosensors-ai-sniffs-17-different-diseases/

Next, we will look at the process through which nanosensors are built (nanofabrication).

Nanofabrication

Nanofabrication is the process through which nanosensors, and other nanostructures are made. There are 2 approaches to nanofabrication. The first one is top down fabrication. Top down fabrication is a method that can be compared to sculpting from a block of stone. The base material is sculpted to the desired shape. The second one is bottom up fabrication. Bottom up fabrication can be compared to building a brick house, but instead of placing bricks one by one, atoms or molecules are placed one at a time to build a nanostructure. The most common top down fabrication technique is nanolithography. In this process, the material required for the nanostructure is protected by a mask, and the exposed material is etched away. There are many different methods to etching. Depending upon the level of resolution required for the nanostructure etching of the base material can be done chemically using acids or mechanically using ultraviolet light, x-rays or electron beams. There are also many other chemical and mechanical methods to etching. Most top down fabrication methods involve etching.

Next lets discuss what are the problems with nanofabrication

The problems with Nanofabrication

Currently there are problems with both the top down and bottom up fabrication methods. The top down fabrication method is more efficient and takes less time than the bottom up fabrication method, but the equipment is expensive. The bottom up fabrication method is cost effective, but is not currently effective and takes much longer than the top down fabrication method. Now let’s look at ideas to improve this process.

Ideas to improve nanofabrication

Many scientists are working on using the bottom up fabrication method except making it self assembling in order to make it efficient, but there are significant obstacles that needs to be overcome. Other ways to improve the nanofabrication process in a smaller time scale include finding better materials for the nanostructures, and combining certain elements from the top down fabrication method to the bottom up fabrication method.