Place-exchange reactions of thiols on electrochemically roughened SERS-active silver

The thiolate-thiolate exchange reactions between sodium 2-mercaptoethanesulfonate (MES) and 3-mercaptopropionic acid (MPA) and between MES and 2-aminoethanethiol (cysteamine, CYS) are analysed. The monolayers formed on the electrochemically roughened silver substrates from one thiol were exposed to the solution of the other thiol, resulting in rapid replacement of some molecules in the monolayer. The place-exchange reaction of thiols has been studied using surface-enhanced Raman scattering (SERS). SERS measurements revealed that in the rapid initial replacement approximately one-third of chemisorbed thiolate moieties are exchanged. According to the previous investigations of monolayers formed on the flat metal surfaces also about one-third of chemisorbed thiolate moieties exchange rapidly. This phenomenon was ascribed to replacement of thiol moieties at defect sites. Thus, our results suggest that the electrochemical roughening of the metal surface does not significantly increase the number of molecules adsorbed on defects sites in the self-assembled monolayers. Therefore, one can conclude that an average size of “atomically flat” faces on electrochemically roughened SERS active silver surfaces is not smaller than an average size of an individual domain in the self-assembled monolayers formed on the flat metal surfaces. These results also show that thiolates-thiolates exchange procedures used for the preparation of mixed monolayers, which were initially developed and tested on the “flat” metal surfaces, should give similar mixed monolayers if rough SERS-active substrates are used.