Why aren't Gold Nanoparticles Gold?

Why do gold nanoparticles appear red or purple? Let's make some and find out! But please don't drink this 😉

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The color of gold nanoparticles depends on their physical size, ranging from light red to a dark bluish/purple. This phenomenon is caused by Plasmon Resonance. There is a cloud of delocalized electrons flying around a metal, and these electrons can react in a coordinated manner to electromagnetic fields (like light). But when the surface of the metal is very small -- like in a nanoparticle -- the electron cloud is can begin to oscillate across the surface.

If the wavelength of light is just right it will resonate with the electron cloud, and that wavelength of light is reflected while all others are transmitted or absorbed. This causes the particle to look a specific color.

This property mostly applies to noble metals like Gold or Silver, and can be seen in the famous Lycurgus Cup.

The nanoparticles were generated using a technique known as Laser Ablation in Liquid, or sometimes Laser Synthesis of Colloids. It uses a pulsed laser to ablate metal into a liquid, where the plasma and superheated material is rapidly quenched to form nanoparticles. I synthesized nanoparticles from a range of materials: gold, silver, nickel, copper, titanium, graphite, silicon, and platinum.

==== References ====
Handbook of Laser Synthesis of Colloids: https://www.researchgate.net/publicat...
Carbon nanocube synthesis: Liu, P., et al. "Micro-and nanocubes of carbon with C8-like and blue luminescence." Nano letters 8.8 (2008): 2570-2575. https://pubs.acs.org/doi/abs/10.1021/...
Salt effect on Gold nanoparticles: Rehbock, Christoph, et al. "Current state of laser synthesis of metal and alloy nanoparticles as ligand-free reference materials for nano-toxicological assays." Beilstein journal of nanotechnology 5.1 (2014): 1523-1541. https://www.beilstein-journals.org/bj...
Lycurgus Cup: https://en.wikipedia.org/wiki/Lycurgu...
More about Gold/Silver Dichroicism: Dekker, Floris, et al. "Syntheses of gold and silver dichroic nanoparticles; looking at the Lycurgus cup colors." Chemistry Teacher International 3.1 (2021). https://www.degruyter.com/document/do...

==== Equipment and techniques ====
nGauge AFM from ICSPI: https://www.icspicorp.com/
Scans are post processed in Gwyddion (http://gwyddion.net/) and 3D images rendered in Blender
Generic 50W fiber laser

==== Timeline ====
0:00 Gold and silver nanoparticles
1:10 Laser Ablation in Liquid (LAL)
4:04 LAL advantages and parameters
6:53 Gold particle sizes
7:29 Plasmons
9:36 Plasmon Resonance
11:00 Mie scattering
12:18 Lycurgus Cup
12:59 Noble metals and Conclusion