Sara Khan
The term "color less" is absent in the method of spectroscopy among scientists. Artists and writers have it. Scientists use a spectrophotometer, not their eyes. During centrifugation, large nanoparticles can settle, while small ones remain. In the visible spectrum, there may be a shift in wavelength and the color of the solution changes. If the intensity of the flattening band maximum changed at the same wavelength, this means that the concentration of nanoparticles of the same size has decreased. Thus, when discussing color, the terms "wavelength", "maximum flattening of the spectrum", "absorption band changed from one wavelength to another" are used.
You have settled all the nanoparticles and their plasmonic absorption in the visible spectrum is gone.
Hello you meant the color of supernatant was colorless. If yes, this is the normal when you centrifuge the red suspension at high speed the particles precipitate in the bottom of Ependorph and Supernatant becomes colorless or light pink depends on the size of the NPs.
Thus, may be NPs are unstable and convert to grey ppt which stick to the wall of
the Ependorph and you cant see it.
there is no solution of nanoparticles
https://drive.google.com/file/d/1z7nM0U8vOJ2uKuoDGIZyf07WnWK8xOUo/view?usp=drive_link
so where might the particle be, maybe they form a film on the centrifuge tube?