What is nanotechnology?

Nano is the ancient Greek word for dwarf. In scientific terms it has been used to identify length scales that are one billionth of a unit. This is typically a meter and so you often here things that are nanometers in size. In terms of nanotechnology it has been defined as anything that has a unique property or function resulting from the size of the artifact being in the nano regime, and that the size regime is between 0.1 and 100 nm. This size range is rather broad; encompassing simple molecules to more complicated molecules like enzymes. However, these items can be looked at from many points of view, from a chemist that considers molecules, to that of an engineer that would look at how each of the molecules interacts in the bigger system and creates new materials from these building blocks. For this reason there are many disciplines that are interested in the study of nanotechnology such as Chemistry, Physics, Engineering, Biological sciences, Material Sciences, Computer Science and many more besides. For this reason nanotechnology is not a strict discipline and many people use their skills and backgrounds from other areas to contribute to research in this particular field.

Why care about nanotechnology?

There are many effects that occur at the nanoscale that we do not notice on a larger macro scale. Most of nature actually works at the nanoscale, and by understanding the forces that are at work using knowledge from chemistry, physics and engineering one can better understand the working of organic life. Enzymes are very large molecules that are too large to consider in terms of chemistry alone, other effects come into play In order to understand the full picture we need to borrow from physics and computer modeling to gain a better understand of what is happening.

There are many effects that occur at the nanoscale that we do not notice on a larger macro scale. Most of nature actually works at the nanoscale, and by understanding the forces that are at work using knowledge from chemistry, physics and engineering one can better understand the working of organic life. Enzymes are very large molecules that are too large to consider in terms of chemistry alone, other effects come into play In order to understand the full picture we need to borrow from physics and computer modeling to gain a better understand of what is happening.

When we cross from the small scale as in molecules and atoms, to the large scale that we see with our own eyes, we travel through the nanoscale. In that scale we go from quantum physics to classical physics and a lot of very interesting effects can be used to our benefit, and actually nanoparticles are an excellent example of this. Just by virtue of their size they are able to absorb four times more light than is even shone on them! This is very different from the bulk material, it is difficult to understand in one sitting, but let’s just say that there is a coupling between the light energy and the matter of the nanoparticles that is best explained through quantum mechanics, but we won’t go into that now.

When you make something very large, there is lots of room for error, the more parts you have in a system the more chances there are that some of those parts can be faulty. However when you make something in the nanoscale you have far less parts in the system and each part has to be virtually perfect. Material scientists are concerned with the defects that are created in materials, because these are the parts that cause a material to break down often and stop functioning correctly. As you get into the nanoscale there are less defects and you get enhanced effects from the purer material, that don’t occur on the larger scale. One example of this is carbon nanotubes, by virtue of there shape and size they are 6 times lighter than steel, but almost 100 times stronger. There is great potential for using these in new materials in the future that are ultra lightweight and extremely strong.

When we make things with modern technology we have for centuries been using a top down approach, and this brings us down to a fine limit but not as fine as that on which nature works. Nanotechnology is more about understanding the fundamental forces in nature by physics, and seeing their interaction through chemistry, and then making something larger from our engineering skills. And we can always take examples from biology that has been doing this for far longer than we have. So really what we do is take a bottom up approach, so that we can create large materials that we can use, that has every part of the interaction tailored all the way from how the atoms interact and how the molecules are formed and bonded together to make building blocks for new materials and applications. This bottom up approach is a change in the way things have been done and for this reason nanotechnology is a very potent discipline, with an immense capacity for expansion.

In all we have only really begun to scratch the surface of what could be possible when we create things using nanotechnology, and we should be aware of this because nanotechnology is finding its way into every corner of life, from health studies, medicine, robotics, materials and maybe even food and many many more.

What are nanoparticles and how are they made?

A simple way of seeing this is by imagining tennis balls that are squeezed down to a few billionths of a meter. The particles are rounded because they try to minimize the surface energy as much as possible; any edges will make things more energetic since typically nature follows the path of least resistance the particles tend to form colloids, or spheres with as few edges as possible. It is possible though, to direct the growth of nanoparticles into various shapes such as cubes, and tetrahedrons. We will concern ourselves with only colloidal nanoparticles for the moment.

The nanoparticles have a large surface area compared with the total volume. The surface area to volume ratio is interesting because chemical reactions typically occur on surfaces, so nanoparticles that have a high surface to energy ratio can be used in many interesting ways, such as in catalysis. One teaspoon of nanoparticles might weigh only 200 mg, but because of their shape and the large amount of surface area the tea spoon could have the same surface area as a whole football field! This gives them huge potential and potency compared to the bulk material. Imagine laying out a football field with a thin layer of silver, think how much silver that would need, and then compare that with the amount that is in the spoon! This high surface area to volume ratio is one of the most important properties about nanoparticles.

With all that surface area and the energy that exists, the nanoparticles need to be held together ‘somehow’. That is where the furry parts of the tennis ball come into play. Imagine them as small molecules that hold on to the surface of the particle and stop it from breaking up under its own energy. It is like a tree whose roots can prevent soil erosion because the soil is bonded to the root in the ground. The chemical we use in this lab is mercaptosuccinic acid, and this helps to hold the nanoparticles in shape by bonding to the surface of the particles.

There are a few basic points to remember about making nanoparticles:

1) You need a nucleation point, a place for the metal (silver in this case) to start bonding to one another and start growing into a larger particle. For this you often need some ingredient that can break down a metal salt, in this case silver nitrate, which is accomplished by using sodium borohydride. This reduces the silver nitrate into silver ions that are free then to bond with each other.

2) You need some mechanism to keep the particles at the nanoscale and stop them from ripping and growing into something much larger, this is accomplished using the capping agent mentioned earlier (mercaptosuccinic acid). A great deal of cutting edge research revolves around varying the capping agent in order to control the size of your nanoparticles and tailor them for specific tasks. But not only can you change the size of particles in this way, you can also change the shapes.

Why silver nanoparticles?

Silver is a very easily oxidized material; it has been used already commercially for its anti-microbial properties from athletic wear to sterilizing water. It has a very interesting interaction with light due to a dielectric constant that makes the light response occur in the visible regime. Notably silver is one of the only metals that can be tailored to respond across the full visible spectrum.

Their light interaction can then be used in various fields such as photonics where new materials can be made to transport light in a similar fashion to the optical cables that we use now, but with a higher yield. These waveguides act like wires and could be made smaller and lighter than present day wires, but carry more light.

Another use of this light interaction can be used for imaging in biological systems, where the nanoparticles can be used as vectors to carry drugs to specific sights because of specific capping agents being used, and the internal core can be used to image the delivery and ensure the cargo is delivered correctly to the correct location in the body. One group at Rice that is working on this exciting research is the Barron lab (http://python.rice.edu/~arb/Barron.html).