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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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There is an increasing interest in developing surface enhancement Raman spectroscopy methods for intracellular biomolecule and for in vitro protein detection that involve dye or protein-dye conjugates. In this work, we have demonstrated that protein adsorption on silver nanoparticle (AgNP) can significantly attenuate the surface-enhanced Raman spectroscopy (SERS) signal of dye molecules in both protein/dye mixtures and protein/dye conjugates. SERS spectra of 12 protein/dye mixtures were acquired using 4 proteins [bovine serum albumin (BSA), lysozyme, trypsin, and concanavalin A] and three dyes [Rhodamine 6G, adenine, and fluorescein isothiocyanate (FITC)]. Besides the protein/dye mixtures, spectra were also obtained for the free dyes and four FITC-conjugated proteins. While no SERS signal was observed in protein/FITC mixtures or conjugates, a significantly reduced SERS intensity (up to 3 orders of magnitude) was observed for both R6G and adenine in their respective protein mixtures. Quantitative estimation of the number of dye molecules absorbed onto AgNP implied that the degree of R6G SERS signal reduction in the R6G/BSA sample is 2 to 3 orders of magnitude higher than what could be accounted for by the difference in the amount of the absorbed dyes. This finding has significant implications for both intracellular SERS analyses and in vitro protein detection using SERS tagging strategies that rely on Raman dyes as reporter molecules.