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While watching the latest computerphile video (https://www.youtube.com/watch?v=FGiBHsUkVzU) I came across a solution of nanoparticles of gold

enter image description here,

but to my surprise, they appeared as red, shouldn't gold be golden color?

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One thing to note : red back background. How much is that affecting the appearance ? Certainly their will be a great amount of red light to be reflected by the water, glass and particles in the water. Hard to know if this effect is significant or not in this case.

Part of the color (the spectral response) you see in metallic gold is due to the energy levels structure of the metal structure. The spectrum of atomized gold is a different thing. In a liquid suspension, you are seeing another set of complex interactions and the metallic structure is not likely as significant.

So this appearance is a combination of multiple factors.

This webpage does describe some of the properties of gold nanoparticles, including the varied color appearance and why that is. One statement is :

A major determinant of the optical properties of gold nanoparticles is their shape. By synthesizing gold nanoparticles of different shapes, the surface plasmon resonance can easily be tuned to give absorption maxima from around 500nm into the near-infrared part of the spectrum.

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  • $\begingroup$ It turns out that gold nanoparticles remain metallic, and it is exactly this metallic character (along with the confined geometry) which is responsible for the plasmonic resonance. $\endgroup$ – Gilbert Jun 11 at 11:07

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