Effect of the Surface Chemical Composition and of Added Metal Cation Concentration on the Stability of Metal Nanoparticles Synthesized by Pulsed Laser Ablation in Water

Metal nanoparticles (NPs) made of gold, silver, and platinum have been synthesized by means of pulsed laser ablation in liquid aqueous solution. Independently from the metal nature, all NPs have an average diameter of 10 +/- 5 nm. The zeta-potential values are: -62 +/- 7 mV for gold, -44 +/- 2 mV for silver and -58 +/- 3 for platinum. XPS analysis demonstrates the absence of metal oxides in the case of gold and silver NPs. In the case of platinum NPs, 22% of the particle surface is ascribed to platinum oxidized species. This points to a marginal role of the metal oxides in building the negative charge that stabilizes these colloidal suspensions. The investigation of the colloidal stability of gold NPs in the presence of metal cations shows these NPs can be destabilized by trace amounts of selected metal ions. The case of Ag(+)is paradigmatic since it is able to reduce the NP zeta-potential and to induce coagulation at concentrations as low as 3 mu M, while in the case of K(+)the critical coagulation concentration is around 8 mM. It is proposed that such a huge difference in destabilization power between monovalent cations can be accounted for by the difference in the reduction potential.