2.7 Silver nanoparticles: a case study in cutting edge research  (Page 3/6)

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.