Abstract
We propose here a new model for gold nanoparticles, which are thought to be solubilized by either a Au(I)−citrate or Au(I)−sulfide layer. In the case of pure Au(0) particles, stabilization of the colloidal solutions occurs by inverse micelles or by tight adsorption of solvent covers, in particular acetone. The main evidence for this model is as follows: Standard 20 nm citrate gold particles precipitate from the red colloidal solution in water in the form of a black metal powder after addition of sodium borohydride or as blue coagulate after acidification with HCl. Both processes are irreversible. If the same particles are carrying a lipoate coating, they dissolve in water at pH 11, coagulate at pH 3, and redissolve quantitatively at pH 11. This cycle has been repeated several times without any change of the 20 nm particles. No irreversible color change of the colloidal solution or fusion of the particles takes place upon acid−base treatment. Sodium borohydride, on the other hand, precipitates the lipoate...
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