Raman studies on the coverage integrity of monolayers formed on silver from various ω-functionalised alkanethiols

Silver substrates were modified with monolayers formed from: 2-aminoethanethiol (cysteamine, CYS), sodium 2-mercaptoethanesulfonate (MES), 3-mercaptopropionic acid (MPA), 11-mercaptoundecanoic acid (MUA) and 16-mercaptohexadecanoic acid (MHA). Then, such modified silver substrates were kept in the aqueous solutions of laccase (LAC) from Trametes versicolor and bovine serum albumin (BSA). The composition and structure of formed organic adlayers have been determined from their surface-enhanced Raman scattering (SERS) spectra. The analysis of measured spectra was based on the assumption that, for studied systems, SERS spectrum is dominated by the contribution from molecules directly interacting with the metal surface. In situ SERS measurements showed that the modification of the silver surface with a linkage monolayer formed from CYS, MES or MPA practically prevents the direct interaction of BSA molecules with the metal substrate. However, such monolayers do not prevent the direct interaction of LAC with the silver surface. Thus, the coverage integrity of the linkage monolayer can be destroyed by the adsorption of one peptide (in this case LAC), whereas, during the adsorption of the other peptide (in this case BSA) the coverage integrity is not destroyed. It means that the coverage integrity of a linkage monolayer should be independently determined in the solution of individual peptide. The modification of silver surfaces with monolayers formed from long-chain compounds (MUA and MHA) prevents the direct interaction of both LAC and BSA with the metal surface.