Spherical and planar gold(0) nanoparticles with a rigid gold(I)-anion or a fluid gold (0)-acetone surface

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-coated particles irreversibly. It is concluded that Au(I) salts make up the surface of the spherical citrate- or sulfide-covered particles and stabilize it byAu(I)-Au(I) binding interactions (aurophilic effect). If a large amount of acetone is added to the citrate gold solution, the electroneutral acetone replaces the citrate and the 20 nm particles dissociate to form 3 nm particles. They tend to fuse quickly to form Au(O) platelets with the same thickness of about 3 nm. The mixture of acetone-covered platelets and spheres remains in solution upon addition of either sodium borohydride or HCl. No reduction or charge neutralization occurs. It is concluded that acetone is adsorbed to Au(0) on the surface of the particles in solution. Dried material tightly adsorbs acetone in a eta(1)-orientation (IR: 1635 cm(-1)) perpendicular to the gold surface where only the oxygen is bonded to the metal surface. Acetone-covered platelets were then also coated with lipoate, and an Au(I)SR layer was again formed. This was indicated again by irreversible precipitation of the blue colloid in aqueous solution by sodium borohydride and formation of a black metal powder. The Au(I)-lipoate-coated particles produced indefinitely stable blue colloidal solutions at PH 11, coagulated at, PH values below 4, and redissolved at PH 11. Analogies between curved or planar gold nanoparticles and molecular assemblies of amphiphilic lipids are briefly discussed.