Chemistry
The red line on a rapid test is not a dye. It is gold — and gold that small is not gold-coloured
19 Sept 2026 · 10 min read
Hundreds of millions of people have looked at gold nanoparticles without knowing it. At a few tens of nanometres gold turns red, the colour shifts with size and shape, and it changes again when particles come close — which is why a colour you can read with your eyes has become a serious measurement.
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