Believe it or not, when you suspend gold in water, it can turn a lilac-blue color. You just need the right amount of salt. And plasmons. See samples of this blue-gold liquid, and learn how a famously warm material can turn cool.

Gold is frequently used as a way to add color. It’s used in jewelry, of course. It’s also used in stained glass windows and goblets. If you have churches in your area, gold nanoparticles may be responsible for the deep red color of some parts of their windows. That makes some intuitive sense. Gold may not be red, but it is a yellow-orange color. What throws many people for a loop is the fact that a suspension of gold particles can turn items blue as well as red.

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Here we see vials of liquid, each of which contain different-sized gold nanoparticles. As the particles get bigger, the liquid turns from a warm red color, to darker purple-red, and finally to a light lilac-blue. How does that happen?

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The gold particles can come together due to something as simple as salts being added to the liquid. Gold nanoparticles are charged, and since they are charged the same way, they all repel each other and stay small. Adding salt to the liquid can remove this charge and turn the particles neutral. This allows the particles to aggregate, making each “lump” of gold bigger.

As the gold gets bigger, the wavelength of the plasmons on its surface gets longer. Plasmons are what happens when photons couple to electrons bound to the surface of a metal. When the gold nanoparticles are small, so are the plasmons. The gold absorbs smaller, bluish wavelengths of light and we see reddish wavelengths. As the plasmons get bigger, the wavelengths that the gold absorbs get bigger and move into the red spectrum. The gold absorbs red light and we see blue light. And that’s how gold turns blue in suspension.

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Top Image: Classical Numismatic Group, Inc. Second Image:Aleksandar Kondinski