Three-dimensional self-assembly of gold nanocolloids in spheroids due to dialysis in the presence of sodium mercaptoacetate

Elemental gold nanocolloids were synthesized by reducing an aurate aqueous solution in the presence of sodium mercaptoacetate. The nanocolloids, 4.6 nm in diameter, self-assembled in highly monodispersed spherical aggregates, 30 nm in diameter, when the nanocolloid suspension was dialyzed in water. Elimination of excess ions, due to dialysis, caused the three-dimensional self-assembly of the nanocolloids. The spheroid and also the elemental nanocolloids before dialysis did not produce the plasmonic absorption peak that is characteristic of conventional gold colloids. Chemisorption of mercaptoacetate onto the nanocolloids caused the peak to disappear. The features of the spheroid structure and assembly mechanism are presented.