Nanoparticles

Relative Sizes of Particles 

Once particles shrink to a certain size, it becomes more practical for scientists to begin use smaller units of measurement, instead of using metres. The particles studied in nanoscience are typically around $0.00000001$ to $0.000001$ metres in size. To save themselves from having to write out all these zeros over and over and over again, researches instead define a smaller unit, the nanometre (nm). $1text{ nm}$ is equal to  $1 text{ billion}^{text{th}}$  of a metre.

There are $3$ categories of small particles that scientist usually refer to. These are:

• Coarse particle ranging from $2,500$ to $1text{ million nm}$  in size. These are often what we think of as dust and can even be big enough to see with the naked eye. These particles are referred to as $text{PM}_{10}$, meaning particulate matter that is less than $10$ micrometers in size.
• Fine particles ranging from $100$ to $2,500text{ nm}$ in size. These particles are referred to as $text{PM}_{2.5}$, meaning particulate matter that is less than $25$ micrometers in size.
• Nanoparticles, ranging from $1$ to $100text{ nm}$ in size.

The diagram below demonstrates just how tiny nanoparticles really are. Roughly to scale, it shows a blown up representation of a particle of dust at the very bottom limit of what we can see unaided, next to this (highlighted inside the black circle) is a nanoparticle. To further illustrate its size, the nanoparticle has been blown up and and compared to the size of at atom (the red circle).

Nanoparticle Properties

An interesting feature of particle size is that, as the side of a cube decreases in length by a factor of $10$, its surface area to volume ratio increases by a factor of $10$. This has important consequences for nanoparticles. The most important result of this is that nanoparticles have a very high surface to volume ratio; the majority of the atoms in a nanoparticle are contained at the surface.

This is a property unique to nanoparticles, as in most other particles, the majority of the atoms are found on the inside of the particle.

One result of this is that nanoparticles of a substance have different chemical and physical properties to that of the substances in bulk. For example, nanoparticles will react much more efficiently than bulk reactants, as there is a far greater proportion of surface layer atoms available for reaction. 

Uses of Nanoparticles

The high surface area to volume ratio of nanoparticles means that they tend to make good catalysts. Because substances made up of nanoparticles have a higher surface area than equal masses of substances made up of larger particles, they are able to be used in much smaller quantities when catalyzing reactions. 

Nanoparticles may also have medical applications. Nanoparticles of silver have antibacterial properties. Added to the polymer fibers of face masks or bandages, they can be used to reduce the spread of diseases. Nano particles are also absorbed more easily in the body, making them good drug delivery vehicles. However, the effects of nanoparticles on human health are not yet fully understood. Before they can be used more widely they must be thoroughly tested.

Some nanoparticles are able to conduct electricity. This means they can be used in the development of tiny circuits for use in miniaturized computing.