Explain why nanoparticles of gold can appear red or purple in solution, whilst bulk gold appears yellow, and suggest one reason why this difference in properties is useful to scientists.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Gold nanoparticles with diameters between 1 nm and 100 nm are used in medical diagnostic tests. Bulk gold, such as gold jewellery, has a familiar yellow colour. Gold nanoparticles in solution can appear red or purple.
Model answer (4 marks)
Nanoparticles have a much larger surface area to volume ratio than bulk gold.
The increased surface area to volume ratio means that the electrons in the gold nanoparticles interact with light differently to bulk gold, giving a different colour.
Gold nanoparticles therefore exhibit properties that are different to bulk gold.
The colour change can be used as a visual indicator in diagnostic tests, allowing scientists to detect or identify substances.
The increased surface area to volume ratio means that the electrons in the gold nanoparticles interact with light differently to bulk gold, giving a different colour.
Gold nanoparticles therefore exhibit properties that are different to bulk gold.
The colour change can be used as a visual indicator in diagnostic tests, allowing scientists to detect or identify substances.
Examiner tips
- Use the term 'surface area to volume ratio' to gain a point. Show that this ratio changes the interaction with light. Mention that the colour change is useful as a diagnostic indicator.
- common_mistakes
- :
- Confusing bulk gold colour with nanoparticle colour. Failing to mention surface area to volume ratio. Giving a vague reason for usefulness without linking to diagnostics.
Mark scheme (4 marks)
- Nanoparticles have a much larger surface area to volume ratio than bulk gold
- The increased surface area to volume ratio means nanoparticles interact with light differently to bulk gold, causing the different colour
- Nanoparticles can exhibit properties different to those of the same material in bulk (so gold nanoparticles have different properties to bulk gold)
- The colour difference can be used to detect or identify substances / used as an indicator in diagnostic tests (any sensible suggestion that the colour change is a useful signal)
Key terms in this question
Related
- All AQA GCSE Chemistry (8462) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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