Organothiol (R-SH) (OT) adsorption onto silver nanoparticles (AgNPs) in water was studied for a series of aromatic OTs including p-methylbenzenethiol (p-MBT), p-benzenedithiol (p-BDT), and 2-mercaptobenzimidazole (2-MBI). Unlike the common view that OT forms monolayer adsorption on AgNPs, we found that these aromatic OTs continuously reacted with AgNPs through formation of RS–Ag complexes until complete OT or AgNP consumption occurred. The RS–Ag complex can remain on the AgNP surface, converting the AgNPs into core–shell structures. The OT adsorption onto AgNPs occurs predominately through reaction with silver oxide present on the AgNP surfaces before the OT addition or formed from environmental oxygen in the presence of OT. The RS–H protons are completely released when both p-MBT and 2-MBI reacted with AgNP, Ag2O, and AgNO3. However, a substantial fraction of S–H bonds remained intact when p-BDT, the only dithiol used in this work, is adsorbed on AgNPs or reacted independently with Ag2O and AgNO3. The new insights from this work should be important for understanding OT interaction with AgNPs in water and the SERS spectra of the OT adsorbed onto AgNPs.

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 pK a of the OTs adsorbed onto AuNP can be more than 10 pK a units smaller than the pK 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.

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Organosulfur compounds are known to poison metallic nanoparticle catalysts. Herein NaBH 4 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 NaBD 4 in H 2 O, 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 BH 4 − on the sulfur-bearing carbon in 2-MBI or 6-TG, or H 2 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.

Ion-pairing, the association of oppositely charged ionic species in solution and at liquid/solid interfaces has been proposed as a key factor for a wide range of physicochemical phenomena. However, experimental observations of ion pairing at the ligand/solid interfaces are challenging due to difficulties in differentiating ion species in the electrical double layer from that adsorbed on the solid surfaces. Using surface enhanced Raman spectroscopy in combination with electrolyte washing, we presented herein the first direct experimental evidence of ion pairing, the coadsorption of oppositely charged ionic species onto gold nanoparticles (AuNPs). Ion pairing reduces the electrolyte concentration threshold in inducing AuNP aggregation and enhances the competitiveness of electrolyte over neutral molecules for binding to AuNP surfaces. The methodology and insights provided in this work should be important for understanding electrolyte interfacial interactions with nanoparticles.