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

Abstract

Silver surfaces were covered with adlayers formed from mixtures of 2-aminoethanethiol (cysteamine, CYS) and 3-mercaptopropionic acid (MPA). The composition and structure of adlayers thus formed were investigated by surface-enhanced Raman scattering (SERS). SERS measurements revealed that the layer formed from the 1 : 1 mixture of 10-mM aqueous solutions of CYS and MPA is composed of co-adsorbed CYS and MPA molecules. From the SERS spectrum of such an adlayer, one can also deduce that practically all MPA molecules are dissociated. pH-controlled experiments showed that MPA is also dissociated when mixed MPA + CYS adlayer is soaked in the solution buffered to pH = 5.0, whereas in the case of one-component adlayer, almost all MPA molecules are protonated at the same pH value. It means that the presence of CYS molecules induces dissociation of neighbouring MPA molecules and that co-adsorbed MPA and CYS probably form mixed salt-like structures under these conditions. At lower pH values (3.2), the layer is mainly composed of non-dissociated MPA molecules, while at slightly alkaline solution (pH = 9) strong preference of CYS molecules at the surface adlayer is observed. This phenomenon may be ascribed to strong stabilization of the adlayer structure by the network of hydrogen bonds between COOH (at lower pH) or NH 2 (at higher pH) groups of adjacent molecules. Formation of mixed adlayer is possible in the limited range of pH values that enable protonation of NH 2 groups with simultaneous deprotonation of the COOH groups. Our results also demonstrate that the mixed salt-like MPA + CYS adlayers could be formed from the MPA + CYS solutions only if the concentrations of both compounds are of the same order of magnitude. This suggests no strong thermodynamic preference for such adlayers.

New insights on surface‐enhanced Raman scattering based on controlled aggregation and spectroscopic studies, DFT calculations and symmetry analysis for 3,6‐bi‐2‐pyridyl‐1,2,4,5‐tetrazine adsorbed onto citrate‐stabilized gold nanoparticles

“…On the other hand, considering that citrate is a relatively ‘hard’ ligand, the description of its interaction with the gold atoms should not involve relevant MO interactions, thus implying in the predominance of the EM‐SERS mechanism. The assignment of the main citrate SERS bands (Table 3) was carried out in comparison with the Raman spectra of sodium citrate55 and literature data for related species 56–68…”

Recent Advances in linear and nonlinear Raman spectroscopy I

“…Reference 26 reports on the analysis of 5-fluorouracil in saliva using SERS. 30 The electrosorption of 1,2,3-triazole on gold 31 and NO on cold-deposited Cu 32 as well as imidazole adsorbed on electrode and colloidal surfaces of Cu, Ag, and Au 33 were further interesting studies. 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.…”

Tuning the Surface-Enhanced Raman Scattering Effect to Different Molecular Groups by Switching the Silver Colloid Solution pH

“…6 Addition of halide ions such as chloride considerably increases the SERS signal, although the mechanisms behind this effect are not entirely clear. 7 The ''pH effect'' on surface charge 8 and the resulting SERS spectra has been investigated using numerous chemical species, such as thiol molecules, [9][10][11][12][13] imid azole, 14 purine derivatives, 15 cytosine, 16 pyridines and acridines, 17,18 isonicotinic acid, 19 mercaptones, 20 humic substances, 21 lysozyme, 22 pigments, 23,24 dyes, 25 pesticides, 26 vitamins, 27 food additives, 28 nucleic acids, 29 amino acids, [30][31][32] proteins, 33 ligands, 34 lipids, 35 and drug components. [36][37][38][39][40][41] There are also some studies on bacteria 4,42 that have investigated the ''pH effect'' using different reduction agents (sodium borohydride and citrate) but that mainly discussed the change in the SERS spectral intensity of adsorbates.…”