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I put a drop of aqueous red colloidal gold on a glass. After the drop dried I saw that remaining nanoparticles were black instead of red. Why black? Does it mean that plasmon resonance is so strong and extends over all optical wavelengths? Is the effect due to something other than plasmon resonance?

This is picture of said structure:

said structure

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  • $\begingroup$ in what liquid were the particles? $\endgroup$
    – scrx2
    Commented Jan 10, 2016 at 21:06
  • $\begingroup$ @fpdx Nanoparticles were in water. $\endgroup$
    – user46147
    Commented Jan 10, 2016 at 21:23
  • $\begingroup$ you could be forming anisotropic shapes, 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. webcache.googleusercontent.com/… $\endgroup$
    – bandybabboon
    Commented Jan 11, 2016 at 3:20

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"Black Gold: Plasmonic Colloidosomes with Broadband Absorption Self-Assembled from Monodispersed Gold Nanospheres by Using a Reverse Emulsion System" describes a similar phenomena of red gold nanoparticles assembling into black 3D structures, except they had to go through more steps than just drying them.

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