Reactive Ag+ Adsorption onto Gold

Proposed mechanisms of monolayer silver formation on gold nano-particle (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/cm(2) for the AuNPs preaggregated with KNO3 to 4.3 +/- 0.2 nmol/cm(2) 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 x 10(3) M-1 and 2.1 x 10(5) 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.