contributed by Melanie Mastronardi
Over the past few years, you may have started to hear the words ‘nanotechnology’ or ‘nanomaterials’ pop up in the media and everyday conversation, particularly now that commercial TV displays are starting to be constructed with nanotechnology. Nanomaterials are fragments of a material that are so small we need the most advanced electron microscopes to see them. For example, nanocrystals (often called quantum dots) are particles that range from 1 to 100 nanometers (nm) in diameter, where 1 nm is 0.000001 mm.
What makes nanomaterials interesting is that regardless of the material they are made of, their small size gives them properties very different from the bulk material. For nanocrystals, one of the most interesting of these properties is the ability to emit light when energy is put into the system; the specific colour of light emitted depends on the size of the particle.
While cadmium nanocrystals are safe in this particular application, there are many potential uses where toxicity may be a bigger concern, such as medical applications. Nanomaterials have the potential to be useful in delivering drugs to targeted areas of the body, imaging from within the body using their light emission abilities, and killing cancerous cells by converting the light energy they absorb into heat. All of these possible applications are still a long way from actual use, but it is unlikely they will ever get there if the nanomaterials being developed are highly toxic. Consequently, a lot of research is currently being focused on nanomaterials that are made up of non-toxic materials, for example, silicon nanocrystals, which are my area of study.
Even though they’re made of a completely different material, silicon nanocrystals possess light-emitting properties very similar to the well-established cadmium-containing nanocrystals. The process through which the light is emitted is slightly more complex, but silicon nanocrystals still show vivid light emission when energy is put into the system, and the colour changes depending on the particle size. Below, you can see an example of samples of silicon nanocrystals I’ve prepared that emit red, orange and yellow light when they are exposed to UV (“black”) light as an energy source. The red particles are the largest, with a diameter of about 3 nm, and as the particle size decreases to around 1 nm, the colour shifts from red to orange to yellow.
By making silicon nanocrystals even larger than those shown above, the colour of the light can be shifted to near-infrared (NIR), which is not visible to the human eye. This type of light is particularly interesting for medical imaging applications because it is one of the few colours that human tissue doesn’t absorb. This means that, in theory, silicon nanocrystals could be introduced into the body and excited to emit NIR light that can pass through the body and be easily detected outside it. This, along with the low toxicity of silicon, makes silicon nanocrystals very attractive for medical imaging applications.
In addition to being non-toxic, silicon has the advantage of being the second most abundant element in the earth’s crust (oxygen is the first). In fact, almost 30% of the earth’s crust is made up of silicon, whereas the average concentration of cadmium is between 0.1 and 0.5 parts per million—that’s .00001 to .00005%. This means that silicon is far more accessible and available than cadmium, and as a result it is a lot cheaper and likely to remain so. Based on these advantages, silicon nanocrystals could be a greener alternative to cadmium in display technologies, but there is still a lot of work required for silicon to compete with cadmium in light-emitting efficiency and colour purity.
The field of nanotechnology is a young one, and needs a great deal of work before commercial applications can take full advantage of the unique properties of nanomaterials. Aside from the toxicity of the bulk material, there are concerns about the small size and novel shape of the nanomaterials, and what harm they may cause to us or the environment. Since nanomaterials have been developed so recently there hasn’t yet been time to really understand their hazards. Even so, the fact remains that nanomaterials possess some truly unique properties, which if harnessed safely could be responsible for some of the greatest innovations of our generation.