Nanoparticles

(Separate Chemistry only) Nanoparticles are assessed in AQA GCSE Chemistry, not in Combined Science: Trilogy. If you are taking Combined Science you do not need this topic.

Nanoscience is the study of structures that are 1 to 100 nm in size. A nanometre (nm) is one billionth of a metre ($1\text{ nm}=1\times 10^{-9}\text{ m}$), so a nanoparticle is only about a few hundred atoms across. This places nanoparticles on a scale between individual atoms and the particles we normally see.

Nanoparticles are particles in this 1–100 nm range. They are far smaller than the particles you would think of as dust or smoke. The reason they matter is that at this tiny size, a material can behave very differently from the same material in a large lump, which opens up new uses.

Object	Approximate size
A single atom	About 0.1 nm
A nanoparticle	1 to 100 nm
A fine particle (PM2.5)	100 to 2500 nm
A coarse particle (PM10)	2500 to 10 000 nm
A human hair (width)	About 80 000 nm

A nanoparticle is only a few hundred atoms across. Being able to compare nanoparticle sizes to those of atoms and molecules is a stated requirement of the specification.

Alongside nanoparticles, the specification defines two larger categories of particle by their size. These are used when talking about air pollution.

• Fine particles, written PM2.5, have diameters between 100 nm and 2500 nm (that is $1\times 10^{-7}$ m to $2.5\times 10^{-6}$ m).
• Coarse particles, written PM10, have diameters between 2500 nm and 10 000 nm (that is $2.5\times 10^{-6}$ m to $1\times 10^{-5}$ m). Coarse particles are often called dust.

The labels "2.5" and "10" refer to the particle diameter measured in micrometres (µm): PM2.5 means up to 2.5 µm across, and PM10 means up to 10 µm across. Recall that $1\text{ µm}=1000\text{ nm}$, so 2.5 µm is 2500 nm and 10 µm is 10 000 nm.

Category	Symbol	Diameter range	Also called
Nanoparticles	—	1 to 100 nm	—
Fine particles	PM2.5	100 to 2500 nm	—
Coarse particles	PM10	2500 to 10 000 nm	dust

Nanoparticles are smaller than fine particles, which are smaller than coarse particles. Keep the order straight: nano < fine (PM2.5) < coarse (PM10).

The key to why nanoparticles behave differently is their surface area to volume ratio. As particles get smaller, a much greater fraction of their atoms sits on the surface, so the surface area to volume ratio rises sharply.

For a cube of side length $L$:

$\text{surface area}=6L^2\text{volume}=L^3\frac{\text{surface area}}{\text{volume}}=\frac{6L^2}{L^3}=\frac{6}{L}$

The ratio is $6/L$, so it gets bigger as the side $L$ gets smaller. This is the exact quantitative rule the specification requires:

As the side of a cube decreases by a factor of 10, its surface area to volume ratio increases by a factor of 10.

Worked example — a cube of side 10.

• Surface area $=6\times 10^2=6\times 100=600$
• Volume $=10^3=1000$
• Ratio $=600÷1000=0.6$

Worked example — a cube of side 1 (side reduced by a factor of 10).

• Surface area $=6\times 1^2=6$
• Volume $=1^3=1$
• Ratio $=6÷1=6$

The side length dropped by a factor of 10 (from 10 to 1), and the ratio rose by a factor of 10 (from 0.6 to 6). This confirms the rule.

Because nanoparticles have such a high surface area to volume ratio, a very large proportion of their atoms are exposed at the surface where they can interact with other substances. This has two important consequences the specification expects you to know.

First, nanoparticles may have different properties from the same material in bulk (in large pieces). A property such as strength, melting point, or how well the material acts as a catalyst can change dramatically at the nanoscale, even though the substance is chemically the same.

Second, because so much of the material is at the surface and available to react, smaller quantities of a nanoparticle may be needed to be effective compared with using the bulk material. For example, a catalyst made of nanoparticles exposes far more of its surface, so less material achieves the same effect.

Feature of nanoparticles	Consequence
Very high surface area to volume ratio	More atoms exposed at the surface
Different properties from the bulk	New uses not possible with the bulk material
Highly effective per unit mass	Smaller quantities can do the same job

The link runs: small size → high surface area to volume ratio → different properties and greater effect per gram. Route your explanation through the surface area to volume ratio.

Nanoparticles already have a wide range of uses, which you should be able to give and evaluate.

Uses. The specification lists applications in:

• Medicine (for example delivering drugs to specific parts of the body)
• Electronics
• Cosmetics, including sun creams
• Deodorants
• Catalysts

Nanoparticles in sun creams give better protection from ultraviolet light and go on more thinly and clearly than older creams. As catalysts, their huge surface area makes them very effective, so less material is needed.

Risks. Because nanoparticles are so small and their widespread use is relatively new, their effects on health and the environment are not yet fully understood. Their tiny size means they may be able to enter cells or pass into the bloodstream, and it is not always clear what happens once they do. For any given use, the benefits must be weighed against these possible risks, and this is why some people are cautious about products such as sun creams and cosmetics that contain them.

The exam may ask you to evaluate the use of nanoparticles for a purpose: give the benefits (such as effectiveness or new properties) and set them against the risks (unknown long-term effects on health or the environment), then reach a judgement.

1. Getting the size ranges the wrong way round

Nanoparticles are 1 to 100 nm, fine particles (PM2.5) are 100 to 2500 nm, and coarse particles (PM10) are 2500 to 10 000 nm. Nanoparticles are the smallest, not the largest. Learn the order nano < PM2.5 < PM10.

2. Confusing nanometres and micrometres

$1\text{ µm}=1000\text{ nm}$. PM10 means 10 µm, which is 10 000 nm, not 10 nm. Check the prefix before comparing sizes.

3. Saying the ratio increases when the cube gets bigger

The surface area to volume ratio is $6/L$ for a cube, so it increases as the cube gets smaller. As the side decreases by a factor of 10, the ratio increases by a factor of 10.

4. Explaining a use without mentioning surface area to volume ratio

Most special properties of nanoparticles come from their very high surface area to volume ratio. An "explain" answer that does not mention this ratio is missing the central idea.

5. Giving only benefits when asked to evaluate

"Evaluate" needs both sides: the benefits of the nanoparticles and the risks (their effects on health and the environment are not fully known), followed by a judgement. Listing only the advantages will not gain full marks.