Nanoparticles are between 1 and 100 nanometres across
Definition
Nanoparticle
A particle between 1 and 100 nanometres across.
- One nanometre (nm) is 1×10−9 m1 \times 10^{-9}\,\text{m}1×10−9m.
- A typical atom is about 0.2 nm0.2\,\text{nm}0.2nm across, so a 1 nm1\,\text{nm}1nm nanoparticle is only about five atoms wide.
- A 100 nm100\,\text{nm}100nm nanoparticle is about 500 atoms wide.
- Nanoparticles are therefore much larger than simple molecules but far smaller than grains of dust or sand.
Example
- Compare the width of a 50 nm50\,\text{nm}50nm nanoparticle with an atom 0.2 nm0.2\,\text{nm}0.2nm across.
- Divide the widths: 50 nm0.2 nm=250\dfrac{50\,\text{nm}}{0.2\,\text{nm}} = 2500.2nm50nm=250.
- The nanoparticle is about 250250250 atoms across, more than two orders of magnitude wider than one atom.
Smaller particles have larger surface area to volume ratios
Definition
Surface area to volume ratio
The surface area of a particle divided by its volume.
- For a cube of side LLL, the surface area is 6L26L^26L2 and the volume is L3L^3L3, so the ratio is 6L\dfrac{6}{L}L6.
- When the side of a cube decreases by a factor of 10, its surface area to volume ratio increases by a factor of 10.
- A nanoparticle therefore has a large fraction of its atoms at the surface compared with a lump of the same material.
- Surface atoms are the ones that react or act as a catalyst, so nanoparticles often behave differently from the bulk material.
Example
- Calculate the surface area to volume ratio of a cube with sides of 10 nm10\,\text{nm}10nm and of a cube with sides of 1 nm1\,\text{nm}1nm.
- For the 10 nm10\,\text{nm}10nm cube: 6×102103=0.6 nm−1\dfrac{6 \times 10^2}{10^3} = 0.6\,\text{nm}^{-1}1036×102=0.6nm−1.
- For the 1 nm1\,\text{nm}1nm cube: 6×1213=6 nm−1\dfrac{6 \times 1^2}{1^3} = 6\,\text{nm}^{-1}136×12=6nm−1.
- The ratio for the smaller cube is 101010 times larger.
Large surface area gives nanoparticles their useful properties
Definition
Catalyst
A substance that increases the rate of a reaction by providing an alternative pathway with a lower activation energy without being used up.
- Less material is needed for the same effect, because so much of each nanoparticle is surface.
- Nanoparticles of platinum and other metals make efficient catalysts, for example in car catalytic converters.
- Silver nanoparticles kill bacteria, so they are added to wound dressings, socks and some food containers.
- Titanium dioxide and zinc oxide nanoparticles in sun creams absorb ultraviolet light but are too small to leave a white layer on the skin.
- Nanoparticles are also being developed to deliver drugs to particular cells and for use in electronics and sensors.
Note
Gold is unreactive as a lump but becomes an active catalyst as nanoparticles, which shows how strongly size can change a substance's behaviour.
The risks of nanoparticles are not fully understood
Definition
Toxicity
The ability of a substance to cause harm to living organisms.
- Nanoparticles are small enough to be breathed in deeply and may pass into the bloodstream and cells.
- Their large surface area makes them more reactive, so they could be toxic in ways the bulk material is not.
- Silver nanoparticles washed out of clothing could harm useful bacteria in soil and water.
- Nanoparticle products are fairly new, so their long-term effects on health and the environment are still being studied.
- This uncertainty is why nanoparticle products need thorough testing and clear labelling before sale.
Exam technique
When a question asks you to evaluate a use of nanoparticles, give a benefit linked to surface area and a risk linked to size or reactivity, then weigh them for that product.
Self review
- How many nanometres are there in one metre?
- Roughly how many atoms wide is a 20 nm20\,\text{nm}20nm nanoparticle?
- What happens to the surface area to volume ratio as a particle gets smaller?
- Why are silver nanoparticles used in wound dressings?
- Give one possible risk of nanoparticles.
