Gold Nanoparticles

[77a7ba75b6e14ae990cbb09879dcaefd]

Hi! I am working on a research project in which I am trying to figure out why gold nanoparticles are red. I went through some resources online where I came across topics like surface plasmon resonance and plasma oscillations. I haven't yet learnt about topics like dielectric function of metals so I was finding it difficult to digest the information I had collected. I request you to explain it to me in simple terms as if the explanation was meant for a ten-year old which would help me to explain it to the ones who would go through this project. The other thing that was bothering me was the question if surface plasmon resonance takes place in bulk gold and if it does why does bulk gold appear yellow?

[SciB]

Hi there and welcome to scibuddies. Your questions might get more answers if you post them on the physical science forum, but I am somewhat familiar with physics at the nanoscale so I will try to answer you.

The reason that larger pieces of gold appear yellow is because their atomic structure, chiefly the electrons, causes them to absorb blue light. Here's a good explanation of this: https://www.sciencefocus.com/science/wh ... ld-yellow/

Gold nanoparticles, however, are molecule-sized structures, which changes the properties of their electrons in terms of light absorption and emission. The diameter of a nanoparticle is measured in nanometers (nm) which are billionths (10 to the power minus 9) of a meter. A 4 nm gold (chemical symbol Au for the Latin, aurum, meaning gold) nanoparticle contains 2000 gold atoms. If you want to know how that number was determined, you can read here: http://sustainable-nano.com/2016/07/28/ ... oparticle/

Metals like gold, silver, copper and platinum conduct electricity very well and this is because their outer electrons are free to move from atom to atom. In chemical terms, these electrons are delocalized and do not belong to any one atom. Here's more about this: https://en.wikipedia.org/wiki/Delocalized_electron

Electrons can move at a fraction of the speed of light, so they are subject to Einstein's relativistic equations linking mass and speed. Now here is where it gets complicated and my knowledge is limited. I know that the color of gold nanoparticles (AuNPs) varies with their size and shape and that it is the movement of the outer electrons, and possibly their surface resonance that determines the color of the reflected light--orange to red: http://www.webexhibits.org/causesofcolor/9.html

My best guess is that plasmon resonance does not occur in solid gold because the electrons have a large field to move in compared to AuNPs. Resonance is a function of movement, and electrons can act as waves as well as particles. Waves interact to form peaks and troughs that, I think, can change the properties of light photons hitting them: https://www.cytodiagnostics.com/pages/g ... properties

I think this is mostly correct, but other experts please weigh in with your better knowledge, and you can check the info on the links to understand the physics better. It is pretty amazing to think of metals like gold that are so heavy and solid covered in a sea of moving electrons that we cannot feel. That is why gold is the best conductor of electricity.

Nanomaterials are also amazing and worth reading about. I have worked with graphene which is a carbon-based NP that has some unique physical and electrical properties because it has delocalized electrons that can zip around and not stay tied down to one carbon atom.

I hope this will be helpful and stimulate your interest in this ultra-cool field. If you have more questions, let us know. Are you thinking of a project involving NPs?

Scibee

[77a7ba75b6e14ae990cbb09879dcaefd]

I am supposed to explain a complex topic from the sciences in the easiest way possible so this project will be more of informative than practical. Thank you for your help.

[SciB]

OK, now I understand you. Was my information helpful? Did you go to the websites and read more?

Explanations of complex scientific subjects like this can only be reduced to simple terms to some degree, otherwise the whole meaning is lost. The idea of differences in size and a sea of electrons is pretty simplistic, but still accurate. How electrons absorb light photons and re-emit visible light is not so easy for a beginning scientist to grasp. If a person knows nothing about atoms and electrons then I don't think you can explain this topic in a way they could understand.

Scibee

[77a7ba75b6e14ae990cbb09879dcaefd]

Yes, I went through the websites and the information helped me build upon my knowledge. Thank you for your help.

[SciB]

You are welcome--glad I could help. This is a really exciting and useful area of research, and if you decide to do a project based on nanoparticles, let me know and I can give you some tips.

Stay safe,

Scibee

[77a7ba75b6e14ae990cbb09879dcaefd]

Hey!
On digging in further through the topic, I came across another phenomenon called as the localized surface plasmon resonance. What exactly is the difference between localized surface plasmon resonance and surface plasmon resonance?

[SciB]

Thanks for the new question. I am learning more about plasmon resonance in trying to help you. I still don't understand exactly what a 'plasmon' is. It seems to be a product of the interaction of a light wave of a certain wavelength and an electron, or cloud of electrons. Localized plasmon resonance, LPR, seems to refer to what takes place at a single NP while surface plasmon resonance, SPR, takes in the entire conductive structure of nanoparticles (NPs).

Did you read the wikipedia entry on LPR?
https://en.wikipedia.org/wiki/Localized_surface_plasmon

I can't say if the wiki info is correct, but at least it does offer some explanation of the phenomenon. The scale of plasmon resonance is in the quantum range which is several orders of magnitude smaller than NPs. This region is smaller than the wavelength of the light that is used to irradiate the particles and this seems to be necessary to set up the resonance. By tweaking the size and shape of the NPs, as well as the type of metal used, their color can be changed.

Did you post your question on the physical sci forum? There should be some experts there who can help. I have had a course in quantum physics, but it was some time ago and new data keep coming out to refine and improve our knowledge of the subatomic world. I think it is a great area for research. Remember what Richard Feinman said--"There's always room at the bottom", meaning the quantum scale region where matter and energy become interchangeable and all kinds of seemingly weird things can happen.

I'm glad you are interested in this as it is making me relearn some topics I had forgotten.

Good luck!

Scibee

[77a7ba75b6e14ae990cbb09879dcaefd]

Hi!
As per my research, plasmon is the particle name for plasma wave(similar to photon which is particle name for a light wave). I have posted my questions on a few forums and am waiting for their replies.Thanks for your help and time.