Ion specific effects on the immobilisation of charged gold nanoparticles on metal surfaces

Abstract

Since the pioneering work of F. Hofmeister, Arch. Exp. Pathol. Pharmakol., 1888, 24, 247, ion specific effects have been steadily reported in the context of colloidal or protein stabilisation in electrolyte solutions. Although the observed effects are omnipresent in chemistry and biology, their origin is still under ferocious discussion. Here, we report on ion specific effects affecting the self-assembly of amine and carboxylic acid functionalised gold nanoparticles on metal surfaces as well as in electrolyte solution as a function of the monovalent cations Li⁺, Na⁺, K⁺ and Cs⁺. Mercaptooctanoic acid and 1,8-amine-octanethiol functionalised gold nanoparticles were adsorbed on structured AuPd/Pt substrates under addition of the respective chloride salts. Furthermore, the influence of the same salts on the salt induced aggregation of these AuNP was investigated. Our results demonstrate that the assembly processes on the metal surface as well as in electrolyte solution are influenced by the addition of different cations. We attribute the observed effects to ion pairing of the functional end groups with the added cations. With these findings we introduce a new parameter to control the self-assembly of 2D AuNP arrays on solid supports or of 3D AuNP networks in solution, which could be of relevance for the fabrication of new tailor-made functional materials or for biomedical applications.

Fig. 1. (a) Kosmotropic cations form strong ion-pairs with COO⁻ groups, chaotropic ions adsorb to neutral NH₂-groups. (b) Proposed binding model: adsorption to the metal surface by coordinative bonds of the AuNPs' end groups. (c) For Au-AOT decreasing coverage density and increasing stability against precipitation is expected for more chaotropic cations, while for Au-MOA an inverse trend in coverage density and similar stability against precipitation is expected.

Fig. 2. SEM images of adsorption experiments on substrates consisting of a 100 nm Pt layer with 15 nm AuPd structures on it without salt addition, (a) Au-MOA in HEPES/TRIS at pH 9; (b) Au-AOT in diluted acetic acid at pH 3; (c) Au-AOT in diluted HCl at pH 3. Scale bar represents 500 nm.

Fig. 3. (a) Representative SEM images of adsorption experiments of Au-MOA with added monovalent salts MCl (M = Li, Na, K, Cs, the size of the schematic ions correlates to the sizes of the corresponding ionic radii) and (b) the determined covering densities, where the error bars show the mean of three experiments and the nine different spots which were analysed on each substrate, scale bar represents 500 nm.

Fig. 4. (a) Representative SEM images of adsorption experiments of Au-AOT with added monovalent salts MCl (M = Li, Na, K, Cs, the size of the schematic ions correlates to the sizes of the corresponding ionic radii) and (b) the determined covering densities, where the error bars show the mean of three experiments and the nine different spots which were analysed on each substrate, scale bar represents 500 nm.

Fig. 5. (a) Time dependent absorption spectra of Au-MOA in HEPES/TRIS at pH 9 after adding NaCl (final concentration 50 mM), spectra were taken every 3 minutes over 1 h. (b) Progression of the ratio of absorbance R (symbols) for NaCl concentrations from 40 mM to 80 mM and the respective fit curves (lines) fitted by a first order exponential decay function. (c) Semi-logarithmic plot of the aggregation constant k as a function of electrolyte concentration for the time dependent aggregation of Au-MOA. The higher the value of k, the faster the aggregation proceeds.

Fig. 6. (a) Time dependent absorption spectra of Au-AOT in H₂O/HCl at pH 3 after adding NaCl (final concentration 200 mM), spectra were taken every 3 minutes over 1 h. (b) Time course of the ratio R of absorbance for NaCl concentrations from 75 to 225 mM (symbols) and the respective fit curves (lines) fitted by a first order exponential decay function. (c) Semi-logarithmic plot of k as a function of electrolyte concentration for the time dependent aggregation of Au-AOT. The higher the value k, the faster the aggregation proceeds.