Surface Chemistry of Gold Nanoparticles Produced by Laser Ablation in Aqueous Media

Abstract

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.

Interaction of colloidal nanoparticles with their local environment: the (ionic) nanoenvironment around nanoparticles is different from bulk and determines the physico-chemical properties of the nanoparticles

“…rsif.royalsocietypublishing.org J. R. Soc. Interface 11: 20130931 measurements verified the presence of Au-O bonds on the surface [103]. An indirect verification of a pI value in ligand-free noble metal systems was recently found for Au 50 Pt 50 -alloy NPs which were laser-ablated at different pHs.…”

“…Specific accumulation of chaotropic anions in the nanoenvironment of NPs does not only affect colloidal stability, it also interferes with the growth mechanism of ligand-free NPs. As a result, these ions induce a size quenching effect during NP growth which was reported for gold [31,97,103] and silver NPs [102,[104][105][106]. This means that in a micromolar concentration regime increasing ionic strengths significantly reduce NP size and their NP size distribution.…”

“…For NO 3 2 , the findings in the literature are inconsistent. While generally a destabilizing effect is reported for gold [95,103], at lower ionic strengths [101] and for silver [102], however, stabilization was found. It is believed that this ion adsorption significantly alters the NP's nanoenvironment, as the ions transfer their charges to the NP surface and hence increase electrostatic stabilization (figure 3a).…”

In-situ bioconjugation in stationary media and in liquid flow by femtosecond laser ablation

“…Pulsed laser ablation in liquids (PLAL) is a recently emerged alternative single step approach enabling sizecontrolled generation of stable nanoparticle colloids [7][8][9][10] with outstanding purity and novel surface chemistry not reproducible by any other conventional manufacturing route. Recently, Sylvestre et al revealed the presence of the oxidation states Au + and Au 3+ besides the metallic Au 0 due to partial oxidation of the nanoparticles in aqueous media [9].…”

“…Recently, Sylvestre et al revealed the presence of the oxidation states Au + and Au 3+ besides the metallic Au 0 due to partial oxidation of the nanoparticles in aqueous media [9]. Owing to this partial oxidation, laser-generated gold nanoparticles act as electron acceptors; hence, they are easily coordinated by molecules bearing electron donor moieties as thiol, amine or carboxyl groups added to ablation media prior (in-situ conjugation) or after (ex-situ conjugation) the laser process.…”