Monolayers of sulfur‐containing molecules at metal surfaces as studied using SERS: 3, 3′‐thiodipropionic acid and 3‐mercaptopropionic acid adsorbed on silver and copper

Abstract

The modification of metal surfaces with self-assembled thiol monolayers is the subject of intensive studies owing to both its fundamental interest in surface chemistry and its potential technological significance. In this work, the applicability of surface-enhanced Raman scattering (SERS) to determine the conformation and pK a values of !-terminated thiol molecules was investigated. 3, 3 -Thiodipropionic acid (TDPA) and 3-mercaptopropionic acid (MPA) monolayers were spontaneously formed on silver and copper surfaces by adsorption from TDPA and MPA solutions, respectively. The structure of the monolayers formed was determined from SERS measurements. The SERS investigations showed that molecules forming TDPA monolayers prefer to adopt a gauche conformation of the -S -C -C chains, whereas in MPA monolayers formed from 1 mM MPA aqueous solution a significant part of the adsorbed molecules adopts a trans conformation. Formation on the metal surface of an MPA monolayer with high surface coverage of MPA makes dissociation of the carboxylic groups of MPA significantly more difficult. Analogous changes in the strength of similar adsorbed compounds have been reported by other groups. In contrast to those results, we found that the formation of monolayers from TDPA increases its acidic strength even at high surface coverage when monolayers are formed from relatively concentrated TDPA solutions. Probably the interaction of carboxylic groups with the metal surface is easier for TDPA than for MPA for geometric reasons. We consider that ionization of carboxylic groups increases the strength of this interaction.

Recent Advances in linear and nonlinear Raman spectroscopy I

“…Osaki et al 25 have investigated the adsorption of pyridine, Ypicoline, and isonicotinonitrile on silver colloidal particles by SERS. A vibrational characterization of E102 food additive by Raman and SERS, including theoretical studies was performed by Peica et al 28 Kudelski et al 29 report on their study of monolayers of the sulfur-containing molecules 3,3 0thiodipropionic acid and 3-mercaptopropionic acid adsorbed on silver and copper surfaces. Vo-Dinh et al 27 describe the successful application of SERS for medical diagnostics and biological imaging.…”

Self‐assembled monolayers of mercaptosuccinic acid on silver and gold surfaces designed for protein binding. Part I: structure of the monolayer

“…65 In the presence of metals like silver, copper, and gold, the SH group loses a proton easily even in strongly acidic pH, forming stable metal mercaptans. 66 The lowering of the pKa 1 value for adsorbed MSA molecules can be explained by the interaction of the carboxylic groups with the silver or gold substrate, which probably possesses a number of metal cations, which are able to coordinate with COO groups, thus facilitating deprotonation of COOH groups and/or by electric field in the electrochemical double layer. As follows from the SERS spectra on both silver and gold surface, which exhibit the band around 1575 cm 1 , assigned to the antisymmetric stretching vibration of the COO group, a portion of carboxylic groups of MSA is dissociated even in acidic solutions (pH 1.0-2.0; Figs 4(a) and 5(a) as example).…”

Surface‐enhanced Raman spectroscopy: advancements and applications

“…47 Kudelski et al demonstrate the applicability of SERS to determine the conformation and the pK a values of thiol molecules consisting of selfassembled monolayers formed on silver and copper surfaces, respectively. 48 Finally, with regard to the extension of electrochemical applications to more complicated and applied systems, Gewirth and Li report an in situ SERS study of hydrogen peroxide electroreduction on a lead-modified gold electrode; the electroreduction mechanism was examined in detail. 49 Cao and Gu describe some advanced studies with the objective of illustrating the virtues of SERS as a versatile tool to examine not only aqueous but also non-aqueous interfaces for transition metals.…”