Nanoparticles (NPs) synthesized through chemical routes are stabilized by a surface layer of capping agents.
These molecules, beside avoid the infinite growth of the solid phase, impart steric or electrostatic repulsive inter- particle interactions.
The technique known as “Laser ablation in liquid” (LAL) is an alternative technique to synthesize capping agents-free metal nanoparticles.1 LAL involves focused laser pulsed irradiation of a bulk metal target in a liquid and consist of four stages . Laser-matter interaction, plasma induction, cavitation bubble formation and particle release in solution. Strikingly, LAL leads to the formation of very stable “naked” NPs that are long standing for
months. It is worth emphasizing that the stabilization of noble metal colloids in water is challenging because of the large Hamaker constant.
Noble metal NPs prepared by LAL have a large negative zeta-potential and therefore their stability should be electrostatic in nature and it is due to the presence negative surface charges.
The question is what is the origin of these surface charges?
Common explanations for this phenomenon involve the presence of gold oxides and/or the anion adsorption.2, 3
However, the presence of oxidized gold species on the surface of NPs prepared in water has been recently questioned on the basis of XPS analysis.4
Very recently we have accumulated evidences that, in the case of gold NPs prepared by LAL, the metal oxidation and anion adsorptions have only a minor role on building the negative surface potential and we proposed that excess electrons formed within the plasma phase could charge the gold particles.5 The figure below describes an experiment that points in this direction: the addition of macroscopic metallic objects induce the loss of charge (as seen in the temporal evolution of the zeta-potential) and eventually NPs aggregation pnly
the case of gold NP synthesized by LAL while it is ineffective in the case of NP synthesized by the classical
Turkevitch chemical reduction of HAuCl4 reduction (see the picture of the cuvettes after 4 days).
Please click Additional Files below to see the full abstract
