When I prepare gold nanoparticles coated with negatively charges phosphine ligands (BSPP), they are stable, but aggregate upon addition of NaCl. However, this aggregation is reversible upon dilution of the mixture. Why is this kind of aggregation reversible?
Thanks for Your help!
It most likely has to do with having negative phosphine molecules on the surface of the nanoparticles, these molecules through the phosphine group could be interacting with gold on the surface; becoming part of the nanoparticle and not only being adsorbed on the surface such as citrate ions. When you add salt the particles come together but I do not think that the salt will be able to remove the phoshine molecules. What concentrations of salt have you tested?
Thank you so far.
I tested concentrations of 200 to 500 mM NaCl for circa 60nm AuNP. Within this range I could see reversibility of the agglomeration upon dilution.
It is pretty interesting, the electrostatic repulsion is still surpassing any agglomeration tendency.
Most probably that is the effect of electrolyte on the electrical double layer (EDL) of colloidal particles. NaCl compress EDL and allows attractive forces aggregate such particles. 0.2-0.5 mol.L-1 is quite concentrated !! Did you measure zeta potential to know the aggregation tendency? If it is less than 25-30 mV the system is electrotatically stabilized and then responds to electrolyte. Seach in colloid science literature (Robert Hunter, Foundations of Colloid and Interface Science) how colloid particles are stabilized (electrostatic, steric, ...)
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