Sorry - didn't mean to make a statement of the obvious! There's no reason to think that the cells modified the shape or size of the particles so their optical behavior in the cell could reasonably be expected to be like that of the colloid itself since the optical properties are primarily a function of size and geometry, secondarily environment. Might mean that the cells have taken up many particles each?

Yes - colloidal gold is well know to produce a red color. See http://en.wikipedia.org/wiki/Colloidal_gold for example.

Thanks.. I know the gold suspension is red in colour.. but can cell pellet show red colour after nanoparticles uptake. 

I found the answer. It is possible that cell pellet color be red with gold nanoparticle uptake. Reference: British J Radiol. Jun 2011; 84(1002): 526–533, DOI: 10.1259/bjr/42612922.

Sorry - didn't mean to make a statement of the obvious! There's no reason to think that the cells modified the shape or size of the particles so their optical behavior in the cell could reasonably be expected to be like that of the colloid itself since the optical properties are primarily a function of size and geometry, secondarily environment. Might mean that the cells have taken up many particles each?

Thanks John...

yes, the cell pellet can change its color. It is mainly due to the nano-uptake inside the cells. If your color intensity of AuNPs is more wash the cell after a certain point of time of treatment assume 6-8 hours. At that time Aunp can internalize inside the cells.

Your nanoparticles will appear red if the plasmon resonance is not lost. If your PEG density is very high, then their organization will be like a brush. Generally this will prevent surface plasmon coupling giving your cells a red appearance. If you use lower densities of PEG, then the will organize like a mushroom on the surface of the nanoparticle. These nanoparticles can still be stable in PBS, but when internalized in cells they will appear gold (due to loss of surface plasmon resonance).