Negative‐Imaging of Citrate Layers on Gold Nanoparticles by Ligand‐Templated Metal Deposition: Revealing Surface Heterogeneity

Abstract

Surface heterogeneity of a metal nanoparticle is typically regarded as boundary defects and various crystalline facets. While organic capping ligands of a single type are assumed to be homogeneously distributed on the nanoparticle surface, heterogeneous surface coverage of citrate molecules on individual facets of gold nanoparticles (AuNPs) is revealed. Pt metallic clusters with 2 nm in diameter, epitaxially grown on the surface of AuNPs by chemical reaction and imaged by high‐resolution transmission electron microscopy, are utilized as negative‐imaging probes for densely packed adlayers where the underneath gold surface may not be accessible for Pt deposition. At pH > 5.0, citrate anions form only a loosely packed layer. At pH 4.5, citrates and citric acids form both loosely packed and densely packed layers that appear phase separated, and the densely packed domain as small as 5 nm × 5 nm is likely composed of fully protonated citric acids. IR spectra indicate that citric acid binds to a surface Au adatom through the oxygen atom of the central hydroxyl group, and similarly, citrate anions bind to Au adatoms through the carboxylate oxygen atom. This study also reveals the role of Au adatom in the adsorption of citrate species on the metallic surface of AuNPs.

Dispersion state phase diagram of citrate-coated metallic nanoparticles in saline solutions

“…1c ). This is in agreement with the experimentally observed agglomeration of gold NPs at pH ~ 4, conditions at which dihydrogen citrate becomes the dominating species 41 .…”

“…1d ). This range is in good agreement with the lower limit of 35% citrate surface coverage, experimentally estimated for ~40 nm gold NPs 21 , 41 . This means that ~70–80% of the surface area of such soluble citrate-coated NPs remains in direct contact with the solvent, fully available, for example, for surface catalysis 43 – 46 .…”

Conjugates of Zinc Tetramethylpyridylporphyrinate with Colloidal Gold Nanoparticles Stabilized with Citrate

“…[64,65] Это обстоятельство предполагает, что прочность связи лигандов с поверхностью золотой наночастицы изменяется в том же порядке, а центральный карбоксилат является ключевой якорной группой. [66][67][68][69] В качестве иллюстрации на Рисунке 3a,b показан фрагмент поверхности Au{100} с плотностью заселения…”

Dispersion State Phase Diagram of Citrate-Coated Metallic Nanoparticles in Saline Solutions

“…This range is in good agreement with the lower limit of 35% citrate surface coverage, experimentally estimated for ~40 nm gold NPs. 29,53 This means that ~70-80% of the surface area of such soluble citrate-coated NPs remains in direct contact with the solvent, fully available, for example, for surface catalysis. [55][56][57][58] Effect of NP charge and solution's ionic strength on the ζ-potential.…”