Surface chemistry of gold nanoparticles produced by laser ablation in aqueous media

The femtosecond laser ablation of a gold target in aqueous solutions has been used to produce colloidal Au nanoparticles with controlled surface chemistry. A detailed chemical analysis showed that the nanoparticles formed were partially oxidized by the oxygen present in solution. The hydroxylation of these Au - O compounds, followed by a proton loss to give surface Au - O ^-, resulted in the negative charging of the nanoparticles. The partial oxidation of the gold nanoparticle surface enhances its chemical reactivity and consequently has a strong impact on its growth. In particular, the oxidized surface reacted efficiently with Cl^- and OH^- to augment its net surface charge. This limited the coalescence of the particles, due to electrostatic repulsion, and led to a significant reduction of their size. Taking advantage of the repulsion effect, efficient size control was achieved using different salts (7 ± 5 nm for 10 mM KCl, 5.5 ± 4 nm for 10 mM NaCl, 8 ± 5 nm for NaOH, pH 9.4), a considerable improvement comparatively to particles prepared in deionized water, using identical ablation conditions, where particles of 1-250 nm were produced. The partially oxidized gold surface was also suitable for surface modification through both covalent and electrostatic interactions during particle formation. Using solutions of N-propylamine, we showed an efficient control of nanoparticle size (5-8 ± 4-7 nm) by the involvement of these interactions. The results obtained help to develop methodologies for the control of laser-ablation-based nanoparticle growth and the functionalization of nanoparticle surfaces by specific interactions.