The mechanism of nanoparticle formation during the reduction of HAuCl4 with sodium citrate under reflux has been studied. We have found that the redox potential in the nucleating colloid remains very positive, around +0.75V vs. Ag/AgCl, until the reaction is almost complete. It then shifts suddenly to around +0.1V vs. Ag/AgCl at the end of the reaction, which is close to the final open-circuit potential in the sol. AuCl4- continues to be reduced slowly over the course of the reaction, but is suddenly depleted as the potential shifts near the end of the reaction. The change in potential is correlated to the peptization of aggregates to form the final monodisperse gold particles. There is also a blue-shift of the surface plasmon band associated with charging of the gold particles.
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