Chemisorption of Cysteamine on Silver Studied by Surface-Enhanced Raman Scattering

The chemisorption of cysteamine (2-aminoethanethiol) on silver, giving monolayers often used for adsorption of proteins, was studied using surface-enhanced Raman scattering (SERS). Raman measurements revealed that the ratio of gauche−trans conformers of adsorbed cysteamine molecules significantly changed during the incubation. Layers with a higher ratio of trans conformers are formed when the time of incubation is relatively short (less than 1 h). Long incubation leads to reconstructed layers with a higher portion of cysteamine molecules in gauche conformation. The results of SERS experiments reveal that cysteamine molecules during chemisorption do not replace the large part of preadsorbed Cl- anions and that the structure of the obtained monolayer strongly depends on the surface concentration of preadsorbed chlorides. The effect of prolonged treatment of cysteamine layers by various electrolytes was also investigated. Raman spectroscopy revealed that after the transfer of the monolayer of cysteamine on silver to the solution containing ClO4- or SO42- anions, a relatively large amount of these anions is coadsorbed. However, after about 1 day, the bands of ClO4- and SO42- anions disappear, whereas the spectrum of adsorbed cysteamine is still well visible. Possible explanations for these experimental findings are discussed. Our results provide some suggestions that may help to obtain more uniform monolayers, terminated by amino groups, which are desired for various applications.

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