2.8 - Nanoparticles

Nanoparticle size and surface area to volume ratio

• A nanoparticle is a structure between 1 and 100 nanometres (nm) across - roughly the size of a few hundred atoms.
• Nanotechnology is the use and control of structures of that size.
• A nanometre is a billionth of a metre: 1 nm = 1 × 10^-9 m.

How nanoparticles compare in size

• Nanoparticles are smaller than the two categories used to classify particulate matter in air:
  • Fine particles (PM2.5) - Diameters between 100 and 2,500 nm.
  • Coarse particles (PM10) - Diameters between 2,500 and 10,000 nm (often referred to as dust).
• A single atom is about 0.1 nm (1 × 10^-10 m) across, so nanoparticles are roughly 10 to 1,000 times larger than an individual atom.

Surface area to volume ratio

• The surface area to volume ratio is a particle's surface area divided by its volume.
• As particles decrease in size, their surface area to volume ratio increases.
• As the side length of a cube decreases by a factor of 10, its surface area to volume ratio increases by a factor of 10.

Why this matters

• Because so much of a nanoparticle is exposed at its surface, a much higher proportion of its atoms can interact with surroundings.
• Nanoparticles can show different properties from the same material in bulk - including reactivity, strength, or electrical conductivity.
• A much smaller mass of nanoparticulate material is often needed to achieve the same effect as bulk material (e.g. as catalysts).

Uses and risks of nanoparticles

Nanoparticles have a very high surface area to volume ratio, meaning smaller quantities are often needed compared to bulk materials.

Uses of nanoparticles

• Catalysts - Far less material is needed to expose the same reactive surface area.
• Medicine - Nanoparticles are absorbed into the body more readily and can deliver drugs directly to target cells.
• Electronics - Used in miniature electrical circuits and computer chips because some conduct electricity efficiently.
• Cosmetics and sun creams - Titanium dioxide nanoparticles provide better UV protection and skin coverage without leaving a visible white mark.
• Antibacterial products - Silver nanoparticles have antibacterial properties and are added to wound dressings, surgical masks, and deodorants.
• Composite materials - Added to materials to make them stronger and lighter without significantly adding mass.

Risks of nanoparticles

• The long-term health and environmental effects are not yet fully understood.
• Their high reactivity and tiny size may allow them to:
  • Pass through the skin and enter the bloodstream, where they could damage cells in the body.
  • Cause lung damage or inflammation if inhaled.
  • Accumulate inside living organisms rather than breaking down safely.
  • Wash into waterways and damage aquatic ecosystems.
• Because these risks aren't fully known, products containing nanoparticles require careful safety testing and clear labelling.