Silver nanoparticles are used as an antibacterial coating on some medical instruments. Describe two properties of silver nanoparticles that make them more useful for this purpose than bulk silver, and explain why silver nanoparticles have these properties.

AQA GCSE Chemistry (8462) — 4.2.4 Bulk and surface properties including nanoparticles (Chem only) · Describe · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Silver nanoparticles have diameters between 1 nm and 100 nm. Bulk silver consists of much larger particles.

Model answer (4 marks)

1. A much larger surface area to volume ratio – the small size gives a higher proportion of atoms on the surface.
2. Greater surface exposure of silver atoms – more atoms can interact with bacteria, enhancing the antibacterial effect.

The larger surface area means that a greater number of silver atoms are available to release ions or directly contact bacterial cells, which increases the rate and extent of bacterial killing. The small size also allows the particles to penetrate surfaces and reach bacteria that bulk silver cannot access, further improving their effectiveness.

Examiner tips

  • Use the exact phrase ‘surface area to volume ratio’ and ‘more silver atoms exposed’ – these are the key terms the mark scheme looks for.
  • Explain the link between size, surface area and antibacterial action – this shows understanding of the underlying principle.

Common mistakes

  • Confusing surface area with total surface area without mentioning the ratio; or omitting the explanation of how the ratio leads to more exposed atoms.
  • Failing to mention that the small size allows penetration of surfaces or that nanoparticles can exhibit different properties from bulk material.

Mark scheme (4 marks)

  1. Silver nanoparticles have a much larger surface area to volume ratio than bulk silver
  2. Because of this larger surface area to volume ratio, more silver atoms are exposed / in contact with bacteria, making the antibacterial effect more effective
  3. Nanoparticles can exhibit different properties to the same material in bulk / nanoparticles may have different chemical or physical properties to bulk silver
  4. Nanoparticles are very small (1–100 nm) so they can penetrate or access areas / surfaces that larger bulk particles cannot reach

Key terms in this question

nanoparticles · bulk

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