Proposed
mechanisms of monolayer silver formation on gold nanoparticle (AuNP)
include AuNP-facilitated under-potential reduction and antigalvanic
reduction in which the gold reduces Ag+ into metallic atoms
Ag(0). Reported herein is the spontaneous reactive Ag+ adsorption
onto gold substrates that include both as-obtained and butanethiol-functionalized
citrate- and NaBH4-reduced gold nanoparticles (AuNPs),
commercial high-purity gold foil, and gold film sputter-coated onto
silicon. The silver adsorption invariably leads to proton releasing
to the solution. The nominal saturation packing density of silver
on AuNPs varies from 2.8 ± 0.3 nmol/cm2 for the AuNPs
preaggregated with KNO3 to 4.3 ± 0.2 nmol/cm2 for the AuNPs prefunctionalized with butanethiol (BuT). The apparent
Langmuir binding constant of the Ag+ with the preaggregated
AuNPs and BuT-functionalized AuNPs are 4.0 × 103 M–1 and 2.1 × 105 M–1, respectively. The silver adsorption has drastic effects on the
structure, conformation, and stability of the organothiols on the
AuNPs. It converts disordered BuT on AuNPs into highly ordered trans conformers, but induces near complete desorption of
sodium 2-mercaptoethanesulfonate and sodium 3-mercapto-1-propyl sulfonate
from AuNPs. Mechanically, the Ag+ adsorption on AuNPs most
likely proceeds by reacting with molecules preadsorbed on the AuNP
surfaces or chemical species in the solutions, and the silver remains
as silver ion in these reaction products. This insight and methodology
presented in this work are important for studying interfacial interactions
of metallic species with gold and for postpreparation modulation of
the organothiol structure and conformation on AuNP surfaces.