Gold nanoparticles were prepared by reduction of HAuCl4 with Sodiumborohydride. The expected size of particles are around 5-10 nm.
Probably, it is due smaller size of the NPs. When size is nearly 5 nm or below , the SPR effect vanishes and now due to modification of band structure molecular like behaviors starts. We can search papers on absorption of gold nanoclusters.
What is the emission range of your gold particles.
It is common for Gold nanoparticles to have a color. This is caused by the Localized Surface Plasmon Resonance effect as stated by Ashmin. This effect is caused by conduction electrons on the surface of the nanoparticle which oscillate in resonance with incident light. The red color suggestes indeed that you have very small particles. If they become larger the energy at which they show this LSPR effect also shifts leading to other colors. You may observe black and blue as well.
Next to the size, also the environment of the nanoparticles matters. This is if the dielectric properties or refractive index of the solvent changes, it will affect the observed color as well. Finally, color is a nice tool to judge the stability of your nanoparticle dispersion. The moment the color starts to change it is most likely due to aggregation of the nanoparticles. For more info, just look at wikipedia "Colloidal gold" it has a more elaborate explanation and shows also the color range colloidal gold can have, as well as the parameters affecting that.
Kind regards, Leo
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