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 Cland OHto 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.

Femtosecond laser radiation has been used to ablate a gold target in pure deionized water to produce colloidal gold nanoparticles. We report evidence for two different mechanisms of material ablation in the liquid environment, whose relative contributions determine the size distribution of the produced particles. The first mechanism, associated with thermal-free femtosecond ablation, manifests itself at relatively low laser fluences F<400 J/cm2 and leads to very small (3–10 nm) and almost monodispersed gold colloids. The second one, attributed to the plasma-induced heating and ablation of the target, takes place at high fluences and gives rise to a much larger particle size and broad size distribution. The fabricated nanoparticles exhibit plasmon-related optical absorption peak and are of significance for biosensing applications.

An analytical model for the prediction of the dielectric properties of gold–silver alloys is developed. This multi‐parametric model is a modification of the usual Drude–Lorentz model that takes into account the band structure of the metals. It is fitted by a genetic algorithm to the dielectric function of thin alloy films of different gold–silver ratio obtained by ellipsometry. The model is validated for arbitrary alloy compositions by comparing the experimental extinction spectra of alloy nanoparticles with the spectra predicted by Mie theory.