Synergism between Specific Halide Anions and pH Effects during Nanosecond Laser Fragmentation of Ligand-Free Gold Nanoparticles

Abstract

Gold nanoclusters (AuNCs) with diameters smaller than 3 nm are an emerging field of research because they possess interesting optical properties, such as photoluminescence. However, to date, it is still difficult to distinguish whether these properties originate from the cores of the nanoparticles or from the adsorbates on their surfaces. Hence, there is a high demand for ligand-free, ultra-small particles because they make it possible to study ligand and core effects separately. Pulsed laser fragmentation in liquids (LFL) is a convenient route for the synthesis of ligand-free AuNCs. The influence of physical parameters, such as melting and evaporation, on the LFL process is well understood both theoretically and experimentally. However, the impact of the chemical composition of the medium during LFL, which critically affects the particle formation process, has been less well examined. Therefore, in this work, we elucidate the extent to which the ionic strength, the pH value, and the nature of the halide anion that is present, that is, F–, Cl–, Br–, or I–, influence the particle size distribution of the LFL product and the mean yield of small particles (<3 nm) of the product. We showed that the yield of small particles can be enhanced by the synergism between pH and specific ion effects, which probably is attributable to the adsorption of specific anions. In addition, our findings indicated that anion-based stabilization depends critically on the type of anion. A direct Hofmeister effect was observed for anions in the neutral pH regime, whereas an indirect Hofmeister series was reported in alkaline solution, which probably was due to the more hydrophilic surfaces of the AuNCs that were formed.

In situstructural kinetics of picosecond laser-induced heating and fragmentation of colloidal gold spheres

“…Recently, Ziefuß et al studied the role of anions during LFL and found synergism between specific halide anions and basic pH on yielding narrow nanocluster size distributions. 44 The profile of powder scattering in Fig. 2 as well suggests that apart from the scattering loss due to molten educt particles the additional scattering on the wings of the powder peaks indicates that smaller fragments (o2 nm) have formed and have rapidly crystallized as soon as getting in contact with water.…”

“…41,43 Fragmentation has been performed either in ultrapure water or in a solution of 0.3 mM NaCl and 0.3 mM NaOH. 44 Transmission electron micrographs (TEM) have been In an isochoric situation within the first tens of nanoseconds the expanding bubbles demand for a corresponding compression of the water bulk. Possible scenarios of particle destruction include a partial ablation for fragments around a remaining (size-reduced) parent particle or a complete explosion.…”

“…This would be disturbed when adding surface charge delivering additives for preventing ripening. 44 In the instantaneous process of educt particle explosion, the product particles are directly formed and could be size-quenched, if desired. The process would also be biased by external parameters, such as educt concentration, if further secondary processes would define the products.…”

Ion-specific Adsorption on Bare Gold (Au) Nanoparticles in aqueous Solution: Double-Layer Structure and Surface Potentials

“…18 Recently, laser-fragmentation (or laser-ablation) in liquids has become a viable route to synthesize surfactant-free/ligand-free AuNPs in water. [19][20][21][22] This technique makes it possi-ble to study ligand and bare AuNP effects separately and control functionalization using post-synthesis chemistry, important for biological applications and catalysis in aqueous systems. It was demonstrated, for example, that during the laser-fragmentation process the type of halide anion that is present can be used to control the particle size distribution of the AuNP, attributed to the adsorption of the specific anions, 21 probably involving selective binding on crystal facets.…”

“…[19][20][21][22] This technique makes it possi-ble to study ligand and bare AuNP effects separately and control functionalization using post-synthesis chemistry, important for biological applications and catalysis in aqueous systems. It was demonstrated, for example, that during the laser-fragmentation process the type of halide anion that is present can be used to control the particle size distribution of the AuNP, attributed to the adsorption of the specific anions, 21 probably involving selective binding on crystal facets. 23 In addition, it was shown that colloidal stabilization and dispersion of the laser-ablated NPs also depends critically on the type of anion in solution,…”