Citrate capped gold nanoparticle colloids show an abrupt change in colour from ruby red to purple color upon dilution with PBS(Phosphate buffer saline).
The buffer concentration must be too high. you might need to reduce it to 10mM NaCl PBS. The salt effect and its ionic charge will disrupt the electron charge over the gold nanoparticles surface leading to an non-stabilization of the nanoparticles in suspension. Which in its turn will lead to aggregation between nanoparticles. Therefore the change in colour from red to purple and if you continue to add more ionic strength in the buffer, it will go from purple to a translucide solution (with a small black aggregate in solution that are the agglomerated gold nanoparticles). Alternatively to avoid that you could cap the gold nanoparticles with some protective coating such as a polymer layer.
Just measure optical spectra and you will observe the longitudinal plasmon resonance absorption if your particles are agglomeration (as noted by Lionel, indeed it seems to)
Dear Sibashish,
PBS contains NaCl and generally due to the salt effect AuNPs tends to aggregate. The surface negative charge neutralizes by either Na+ or K+ ions. For the same reasons in acid medium also due to the presence of H+, AuNPs can get aggregation.
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