Reactive Ag⁺ Adsorption onto Gold

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 <i>trans</i> 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

Organothiols Self-Assembled onto Gold: Evidence for Deprotonation of the Sulfur-Bound Hydrogen and Charge Transfer from Thiolate

Organothiol (OT) adsorption onto gold nanoparticles (AuNPs) and gold powder was studied in 50% aqueous ethanol and in water. The OT solution rapidly acidifies upon addition of AuNPs or Au powder, and the number of protons released into the solution is proportional to the amount of OT adsorbed onto the gold surface. Theoretical calculations and normal Raman and surface-enhanced Raman spectroscopic (SERS) measurements show that the p<i>K</i>_a of the OTs adsorbed onto AuNP can be more than 10 p<i>K</i>_a units smaller than the p<i>K</i>_a of OT in solution. The pH measurements suggest that there is a substantial fraction (up to 45%) of the protons derived from the surface-adsorbed OTs retained close to the gold surface, presumably as the counterion to the negatively charged, thiolate-covered AuNPs. Charge transfer between the surface-adsorbed thiolate and the AuNPs is demonstrated by the quenching of the OT UV–vis absorption when the OTs are adsorbed onto the synthesized AuNPs or bovine serum albumin-stabilized AuNPs

Contradictory Dual Effects: Organothiols Can Induce Both Silver Nanoparticle Disintegration and Formation under Ambient Conditions

Using propanethiol (PrT), 2-mercaptoethanol (ME), glutathione (GSH), and cysteine (Cys) as model thiols, we demonstrated herein that organothiols can induce both silver nanoparticle (AgNP) disintegration and formation under ambient conditions by simply mixing organothiols with AgNPs and AgNO₃, respectively. Mechanistically, organothiols induce AgNP disintegration by chelating silver ions produced by ambient oxygen oxidizing the AgNPs, while AgNP formation in AgNO₃/organothiol mixtures is the result of organothiols serving as the reducing agent. Furthermore, surface-plasmon- and fluorescent-active AgNPs can be interconverted by adding excess Ag⁺ or ME into the AgNP-containing solutions. Organothiols can also reduce gold ion in HAuCl₄/organothiol solutions into fluorescence- and surface-plasmon-active gold nanoparticles (AuNPs), but no AuNP disintegration occurs in the AuNP/organothiol solutions. This work highlights the extraordinary complexity of organothiol interactions with gold and silver nanoparticles. The insights from this work will be important for AgNP and AuNP synthesis and applications

Desulfurization of Mercaptobenzimidazole and Thioguanine on Gold Nanoparticles Using Sodium Borohydride in Water at Room Temperature

Organosulfur compounds are known to poison metallic nanoparticle catalysts. Herein NaBH₄ is shown to desorb and desulfurize 2-mercaptobenzimidazole (2-MBI) and 6-thioguanine (6-TG) adsorbed on 10, 15, and 50 nm diameter gold nanoparticles (AuNPs). The desulfurization rates decrease significantly with increasing AuNP sizes. Isotope labeling experiments, conducted with NaBD₄ in H₂O, indicate that this desulfurization reaction proceeds through a pathway requiring hydrogen uptake onto AuNP surfaces prior to the 2-MBI or 6-TG desulfurization reaction, rather than direct hydride attack from BH₄^– on the sulfur-bearing carbon in 2-MBI or 6-TG, or H₂ reaction with 2-MBI or 6-TG . In addition to serving as the hub for electron charge transfer between hydride and proton, AuNPs capture the cleaved sulfide, facilitating sulfur separation from the desulfurized products

NaHS Induces Complete Nondestructive Ligand Displacement from Aggregated Gold Nanoparticles

Ligand displacement from gold is important for a series of gold nanoparticle (AuNP) applications. Complete nondestructive removal of organothiols from aggregated AuNPs is challenging due to the strong Au–S binding, the steric hindrance imposed by ligand overlayer on AuNPs, and the narrow junctions between the neighboring AuNPs. Presented herein is finding that monohydrogen sulfide (HS^–), an anionic thiol, induces complete and nondestructive removal of ligands from aggregated AuNPs. The model ligands include aliphatic (ethanethiol­(ET)) and aromatic monothiols, methylbenzenethiol (MBT), organodithiol (benzenedithiol (BDT)), thioamides (mercaptobenzimidazole (MBI) and thioguanine (TG)), and nonspecific ligand adenine. The threshold HS^– concentration to induce complete ligand displacement varies from 105 μM for MBI and TG to 60 mM for BDT. Unlike using HS^–, complete ligand displacement does not occur when mercaptoethanol, the smallest water-soluble organothiol, is used as the incoming ligand. Mechanistically, HS^– binding leads to the formation of sulfur monolayer on AuNPs that is characterized with S–S bonds and S–Au bonds, but with no detectable S–H spectral features. The empirical HS^– saturation packing density and Langmuir binding constant on AuNPs are 960 ± 60 pmol/cm² and (5.5 ± 0.8) × 10⁶ M^–1, respectively. The successful identification of an effective ligand capable of inducing complete and nondestructive removal of ligands from AuNPs should pave the way for using AuNP for capture-and-release enrichment of biomolecules that have high affinity to AuNP surfaces

Structures and Conformations of Alkanedithiols on Gold and Silver Nanoparticles in Water

Organodithiols with two distal thiols have been used extensively in gold and silver nanoparticle (AuNP and AgNP) applications. However, understanding the structures and conformations of organodithiols on these nanoparticles is challenging. Reported in this work is a combined surface enhanced Raman spectroscopy (SERS), transmission electron microscope (TEM), inductively coupled plasma mass-spectrometry (ICP-MS), and localized surface plasmonic resonance (LSPR) study of alkyldithiol (ADT, (HS-(CH₂)_<i>n</i>-SH, <i>n</i> = 2, 4, and 6) interactions with AuNPs and AgNPs in water. These complementary techniques revealed a series of new insights that would not be possible using individual methods. A large-fraction of ADTs lies flat on AuNP surfaces. The upright ADTs are dimerized horizontally through disulfide-bond, or remain as monothiolates on the AuNP surfaces. The possibility of a significant amount of vertically disulfide-linked organodithiol on the surface is excluded on the basis of ICP-MS and AuNP LSPR experiments. ADTs induced significant AgNP disintegrations in which ADTs are predominantly in dithiolate forms. This work highlights the extraordinary complexity of organodithiol interactions with plasmonic nanoparticles. The insights provided in this work will be important for enhancing fundamental understanding of the structure and properties of organothiol-functionalized AgNPs and AuNPs