Surface-Enhanced Raman Scattering Study of the Kinetics of Self-Assembly of Carboxylate-Terminated n-Alkanethiols on Silver

Abstract

Adsorption of 11-mercaptoundecanoic acid (MUA) on silver from methanol and aqueous solutions was monitored in situ by surface-enhanced Raman scattering (SRES) spectroscopy. While adsorption of MUA from methanol is a one-step formation of a thiol-bound monolayer, SERS spectra reveal that monolayer formation from aqueous solution involves interactions of both carboxylate and thiol groups of MUA with the silver surface. Several Raman scattering bands, including the ν(C-S), ν(s)(COO(-)), and ν(C-C), were used to investigate the evolution of the structure of adsorbed MUA on silver surfaces. The time-dependent profiles of these bands for assembly from aqueous solution indicate a multistep process, which is initiated by the binding of both carboxylate and thiol groups to silver, producing a mixture of gauche and trans conformations. In a subsequent step, the COO-Ag interactions are displaced by stronger S-Ag bonds, leading to ordering of the resulting monolayer with formation of a complete SAM with all-trans conformations. The results also showed that the adsorption process depended strongly on the solution pH and surface potential of the metal. These factors can significantly affect the participation and displacement of -COO(-) during self-assembly of MUA from aqueous solution.

Selective characterization of proteins on nanoscale concave surfaces

“…It is notable that for the 11-MUA capped AuNG, the C-S band is in trans instead of gauche conformation, and the stretching modes of the carboxylic group are much less pronounced, indicating that the molecule bonds to the surface via its thiol head group, with the carbonyl tail group dangling free in solution [29][30][31]. Theoretically, 11-MUA can bond to Au/Ag surfaces via either its thiol or carbonyl group; however, in this particular ligand exchange process, the thiol group is preferred to replace the carbonyl group of PVP [27,29,32], and the prolonged ligand exchange process renders a close packing of 11-MUA on the nanoparticle surface [33].…”

Efficient Passivation of Ag Nanowires with 11‐Mercaptoundecanoic Acid Probed Using In Situ Total‐Internal‐Reflection Surface‐Enhanced Raman Scattering Spectroscopy

“…Two marker bands are selected, namely the C=O stretching vibration centered at 1766 cm −1 for PVP and the C−S stretching vibration centered at 630 cm −1 for MUA. The wavenumber of the C=O mode of PVP can be assigned to the PVP bound to the surface of AgNWs when it ranges between 1750 and 1769 cm −1 . In contrast, free PVP molecules exhibit a C=O stretching vibration centered at 1776 cm −1 due to the increased force constant of the C=O bond.…”

“…The wavenumber of the C=O mode of PVP can be assigned to the PVP bound to the surface of AgNWs when it ranges between 1750 and 1769 cm À 1 . [31][32][33] In contrast, free PVP molecules exhibit a C=O stretching vibration centered at 1776 cm À 1 due to the increased force constant of the C=O bond. The wavenumber of the CÀ S stretching vibration of MUA nicely matches the value expected for the Gauche conformation of the molecule adsorbed on a silver surface.…”

Surface‐Enhanced Raman Spectroscopy as a Probe of the Surface Chemistry of Nanostructured Materials

“…[11] Indeed this effect has been used to observe the evolution of adsorbed layers, from initially formed highly disordered systems to much more ordered layers. [12] The final type of information that SERS can provide is orientation of the adsorbed molecules with respect to the metal surface. Due to the directionality of the plasmonic adsorption the selection rules for SERS are different from those of the same molecules in solution; the modes with polarizability changes perpendicular to the surface are preferentially enhanced and dominate the SERS spectrum.…”