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