nanotechnology - Why do nanoparticles have a different color than their macro counterparts?

It astonished me when I learned that at nanoscale, gold is no longer "gold",rather it's red. Other elements' nanoparticles also have a different color than their macro counterparts.

Why does this phenomenon occur?

Answer

Disclaimer: I am not an expert on this

Gold nanoparticles interaction with light is strongly dictated by their environment, size and physical dimensions.

Small (~30nm) monodisperse gold nanoparticles absorb light in the blue-green portion of the spectrum (~450 nm) while red light (~700 nm) is reflected, yielding a rich red color. This absorption is due to surface plasmon resonance.

From wikipedia

Surface plasmon resonance (SPR) is the resonant oscillation of conduction electrons at the interface between a negative and positive permittivity material stimulated by incident light. The resonance condition is established when the frequency of incident photons matches the natural frequency of surface electrons oscillating against the restoring force of positive nuclei

As particle size increases, the wavelength of absorption shifts towards longer, redder wavelengths. Red light is then absorbed, and blue light is reflected, yielding solutions with a pale blue or purple colour.

As you approach the bulk limit, the surface plasmon resonance wavelengths move into the IR portion of the spectrum.

The surface plasmon resonance phenomenon is not only sensitive to size, but also to the morphology and shape of the nanoparticles. This can be used to tune the optical properties of the gold nanoparticles.

For other kinds of nanoparticles, for example, semi-conductor nanoparticles, quantum confinement (e.g. quantum dots) might come into play, and this affects the wavelength of absorption of electromagnetic radiation, and hence the perceived colour.

The matrix the nanoparticles are in itself will also play a role in the perceived colour. Light scattering events taking place at the nanoparticle interface, vis-à-vis single or multiple light scattering events also need to be considered. To learn more about light scattering phenomenon read Mie theory and T-matrix method (again, size and shape of the scattering particles influence the solution of maxwell's equations).