Enhancement and selectivity strategies for Raman spectroscopy.

Abstract

Several strategies for the observation of Raman scattering using enhancement mechanisms, surface selectivity, and electrophoretic preconcentration techniques are presented. Surface-enhanced Raman scattering (SERS) has been used to determine the orientation of the indoleamine neurotransmitters serotonin and melatonin at a silver electrode. Both molecules form a bidentate-like binding site with the ring mostly perpendicular to the surface at potentials more negative than $-$0.6 V vs. Ag/AgCl. Only the ethylamine side chain remains bound at $-$0.4 V with the ring more parallel or further from the surface. Surface-enhanced resonance Raman scattering (SERRS) has been used for the observation of resting level concentrations (nanomolar) of dopamine in Ringer's solution. Resonance enhancement in the green is achieved through complexation of the catechol functionality with ferric ion. A carefully controlled protocol is critical for complex formation in the presence of iron chelating anions found in biological buffers. Some selectivity is introduced by electrostatic interactions between the iron complex and the colloidal silver particles. The use of functionalized self-assembled thiol monolayers on silver SERS substrates is discussed for the selective partitioning of charged neurotransmitters to the surface. Structural changes in the monolayer are observed in the presence of the analyte. However, no Raman bands can be directly assigned as neurotransmitter vibrations. Finally, the use of normal Raman spectroscopy as a detection method for capillary electrophoresis is presented. The use of electrophoretic preconcentration techniques is necessary for the observation of Raman signals from analyte zones less than $1\times10\sp{-3}\ M.$ Using field-amplified injections along with moving boundary stacking, sub-ppm concentrations of nitrate and perchlorate ($1\times10\sp{-5}\ M$) can be detected and separated in under 3 minutes.