In Situ Non-DLVO Stabilization of Surfactant-Free, Plasmonic Gold Nanoparticles: Effect of Hofmeister’s Anions

Abstract

Specific ion effects ranking in the Hofmeister sequence are ubiquitous in biochemical, industrial, and atmospheric processes. In this experimental study specific ion effects inexplicable by the classical DLVO theory have been investigated at curved water-metal interfaces of gold nanoparticles synthesized by a laser ablation process in liquid in the absence of any organic stabilizers. Notably, ion-specific differences in colloidal stability occurred in the Hückel regime at extraordinarily low salinities below 50 μM, and indications of a direct influence of ion-specific effects on the nanoparticle formation process are found. UV-vis, zeta potential, and XPS measurements help to elucidate coagulation properties, electrokinetic potential, and the oxidation state of pristine gold nanoparticles. The results clearly demonstrate that stabilization of ligand-free gold nanoparticles scales proportionally with polarizability and antiproportionally with hydration of anions located at defined positions in a direct Hofmeister sequence of anions. These specific ion effects might be due to the adsorption of chaotropic anions (Br(-), SCN(-), or I(-)) at the gold/water interface, leading to repulsive interactions between the partially oxidized gold particles during the nanoparticle formation process. On the other hand, kosmotropic anions (F(-) or SO4(2-)) seem to destabilize the gold colloid, whereas Cl(-) and NO3(-) give rise to an intermediate stability. Quantification of surface charge density indicated that particle stabilization is dominated by ion adsorption and not by surface oxidation. Fundamental insights into specific ion effects on ligand-free aqueous gold nanoparticles beyond purely electrostatic interactions are of paramount importance in biomedical or catalytic applications, since colloidal stability appears to depend greatly on the type of salt rather than on the amount.

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

“…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).…”

“…Owing to this emerging synthesis route, ultrapure curved NP surfaces are nowadays available which enable studies of NP -ion interactions in the Hü ckel regime at extremely low ionic strengths [31,101]. These effects will be thoroughly reviewed in the following and complemented by some recent findings.…”

“…These findings seem to indicate that Hofmeister effects, thoroughly described in the preceding paragraph, may also appear on curved gold interfaces. A Hofmeister series of anions for this system could be deferred, by successfully correlating the stability of gold colloids to the polarizability of the anions present during synthesis [101] (figure 3b). Interestingly, these effects already occur in highly diluted electrolytes in a micromolar to millimolar concentration range [31,101,102].…”

“…is described by the law of mass action by the respective [101], to which we also refer for more details. rsif.royalsocietypublishing.org J. R. Soc.…”

Colloidal Stability of Metal Nanoparticles in Engine Oil under Thermal and Mechanical Load

“…In this study, the relative gold concentration is calculated from the absorbance ratio at 480 nm since the engine oil contains different kind of additives [37], which absorb the UV-vis light < 400 nm. For the same reason, the primary particle index is calculated from the absorbance ratio at 480 and 800 nm instead of 380 and 800 nm [38,39].…”