Development of modified SERS substrates for chromatographic analysis

Surface enhanced Raman scattering (SERS) techniques provided a sensitivity advantage over normal Raman scattering. This sensitivity was further improved by applying molecular specific coatings onto a SERS substrate. Specifically, these modified SERS substrates were prepared by coating roughened silver substrates with alkanethiols that form self-assembled monolayers (SAMs). Alkanethiol coatings function to protect the silver surface and to preconcentrate analytes from solution to the SERS interfacial region. These analyte specific modified SERS substrates were integrated with novel flow cell designs to develop Raman-based detection under dynamic conditions.; The development of new flow cells has made it possible to use SERS under dynamic conditions both in the gas and aqueous phases. For gas phase detection, SERS was coupled with gas chromatography (GC-SERS). Detection limits for aromatic compounds were ca. 50 ng. Another application for SERS involved detection of aromatic compounds in liquid chromatography (LC-SERS) and flow-injection analysis (FIA-SERS). The solvent conditions required for separating aromatic compounds with LC, including a 45:55 MeOH:H{dollar}sb2{dollar}O mobile phase, reduced the LC-SERS sensitivity to ca. 4000 ppm. Decreased analyte adsorptivity onto SERS substrates was attributed to the presence of organic modifiers (e.g., methanol), which limited development of LC-SERS. Under dynamic aqueous conditions, the detection limit for benzene using FIA-SERS was approximately 200 ppm. To analyze complex mixtures, applications of multicomponent analysis to FIA-SERS were investigated using water-only conditions. The results show that Raman and SERS analyses can be performed with multicomponent analysis techniques.; The final topic of this dissertation was a SERS investigation to evaluate alkanethiol modified SERS substrates under varying solvent and experimental conditions. Monolayer conformations change with solvent exposure (e.g., from air to water). These changes were evaluated by monitoring the C-S trans/gauche stretching modes between 650-750 cm{dollar}sp{lcub}-1{rcub}.{dollar} With solvent exposure, the monolayers both degrade and experience conformational changes. A possible mechanism for the decreased stability of monolayer systems was solvation of the alkanethiol coatings under varying solvent conditions. Due to inconsistency in the peaks used for an internal standard, these conformational changes presented difficulties for developing dynamic SERS detection systems. However, these results do suggest further research areas such as the development of polymerizable SAMs to create more stable coatings.