Structures of monolayers formed from different HS—(CH₂)₂—X thiols on gold, silver and copper: comparitive studies by surface‐enhanced Raman scattering

Abstract

Molecules of 2-aminoethanethiol (cysteamine, CYS), 2-mercaptoethanol (MET), 3-mercaptopropionic acid (MPA), sodium 2-mercaptoethanesulphonate (MES) and 1-propanethiol (PTH) were spontaneously chemisorbed on electrochemically activated silver, copper and gold surfaces. The structure of monolayers formed was studied with surface-enhanced Raman scattering (SERS). In CYS and MPA monolayers the relative surface concentration of gauche conformers was higher than in MET, PTH and MES monolayers. This is probably due to double bonding to the metal surface by a fraction of adsorbed MPA and CYS molecules (via the sulphur moiety and the terminal carboxylic or amino group). In CYS, MET, MPA and PTH monolayers the surface concentration of trans conformers is significantly higher on Ag than on Au or Cu. The structures of MES monolayers on Ag, Au and Cu are similar, with very high surface concentrations of trans conformers. Statistically, the wavenumbers of n(C-S) bands of both gauche and trans conformers are the highest on Au, slightly lower on Ag and the lowest for Cu. However the positions of n(C-S) bands are also sensitive to the other parameters (e.g. C -C -S -metal torsion angle, the overall ordering of the monolayer). Therefore, the wavenumbers of n(C-S) bands are not good indicators of differences in the interaction between chemisorbed thiols and the metal substrates. Desorption measurements suggest that part of the adsorbed molecules is bonded significantly weaker than the rest. Thiolate monolayers on Cu decomposed relatively easily, forming a layer of copper sulphide. The strength of adsorbed MPA acid is similar on all substrates.

Raman study on the structure of adlayers formed on silver from mixtures of 2-aminoethanethiol and 3-mercaptopropionic acid

“…As follows from our previous results, comparable intensities of the 906 and 928 cm 1 bands are observed 32 at pH values in the range 5.8-6.2 (on the basis of the relative intensities of the 906/928 cm 1 bands one can roughly estimate the degree of dissociation of chemisorbed MPA -for details see Ref. 32). Two bands at about 700 cm 1 , which are also clearly seen in the SERS spectra of MPA, are due to stretching vibrations of the C-S bonds.…”

“…As has been previously shown, desorption of the part of adsorbed CYS molecules leads to a significant increase of the ratio of surface concentrations of gauche to trans conformers 32,37 (CYS molecule in the gauche conformation takes more 'surface space' than in the trans conformation; see Fig. 2).…”

The Porter-Whitesides Discrepancy: Revisiting Odd-Even Effects in Wetting Properties of n-Alkanethiolate SAMs

“…This self-assembly process is a mechanism by which disordered small building units spontaneously arrange into large, ordered phases [5]. Many self-assembled systems have been investigated, with monolayers of alkanethiolates on gold or silver being the most studied SAM systems to date [6][7][8][9][10][11][12][13][14][15].…”

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

“…655 cm -1 . [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.…”