Properties and Uses of Nanoparticles

More surface for the same amount of material

Surface area to volume ratio compares a particle’s exposed surface area with the space it occupies. Imagine cutting one cube into many smaller cubes without losing any material. Their combined volume stays the same, but the cuts reveal new surfaces. The smaller cubes therefore have a greater total exposed surface area.

For a cube of side length $l$, surface area is $6l^2$ and volume is $l^3$. Therefore,

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

If the side length becomes ten times smaller, the surface area to volume ratio becomes ten times greater. When length is measured in nm, area divided by volume has units of nm⁻¹. A large ratio matters when a process happens at a surface: for the same amount of substance, smaller particles expose more surface where that process can occur.

Nanoparticles in sunscreen

Some sunscreens contain nanoparticles of zinc oxide or titanium oxide. These substances help stop ultraviolet (UV) radiation reaching the skin by absorbing or scattering it. Their tiny particle size allows many particles to be distributed through a layer of sunscreen, where incoming UV can encounter them. This is a link between the particles’ size, their interaction with UV and the sunscreen’s protective use; a high surface area to volume ratio alone is not an explanation of UV protection.

Sunscreen protection is often described by its sun protection factor (SPF). A higher SPF means less UV passes through, although the relationship is not linear: successive increases in SPF have a diminishing effect on the amount transmitted.