When you prepare gold nanoparticle by citrate method, after the addition of citrate, first, the solution changes to blue or black and subsequent heating changes the color to regular wine red. What is the reason of blue or black color formation
There is quite a lot of work on this topic. One interpretation is the formation of gold "nonowires" in the early stage of the reduction (grey-blue color) from which the gold nanoparticles are generated:
B. K. Pong, H. I. Elim, J. X. Chong, J. Phys. Chem. C, 2007, 17, 6281.
I would assume you are forming some anisotropic shapes like wires and triangles etc. I make silver triangles using citrate. You can see the color progression im my picture. I think you may be able to loosly confirm this with UV-vis.
colour change occurs with reducing size of particles as it approaches the nano range. it happens because of changed properties of material due to increased surface area of small nano-sized particles. the properties of all materials change on reducing particle size as through this, the surface to volume ratio of atoms increases and as we know that the atoms at the surface act differently than those within, this changes the physical and chemical properties as well.
You may please study the effect of adding 1% of Gold concentration of PVP-K at the end of the process
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