Surface-enhanced resonance Raman spectroscopy of synthetic dyes and photosynthetic pigments in monolayer and multilayer assemblies

Abstract

The applicability of surface-enhanced resonance Raman scattering (SERRS) spectroscopy to the study of transferred Langmuir-Blodgett (LB) monolayers was investigated. Three surface-active cyanine dyes, as well as chlorophyll a, bacteriochlorophyll a, and bacteriopheophytin a were incorporated into monolayers with inert matrix diluent molecules and the monolayers were transferred to glass slides which had previously been coated with 50 Å silver films. Good-quality SERRS spectra were obtained of the compounds studied, at monolayer concentrations of less than 10 mole-per cent. Appreciable quenching of intrinsic fluorescence was evident for those systems in which the chromophore was in immediate contact with the metal surfaces. When the chromophore was removed a distance of two spacer layers (about 56 Å) from the silver surface, a reduction of approximately 40% in SERRS signal intensity was observed for each of the dyes. These latter results indicate that an electrodynamic enhancement mechanism is operative for these systems. Significant photobleaching occurred in all of the samples studied and photooxidation occurred in the bacteriochlorophyll monolayers. These problems were minimized by the use of low laser power and short exposure times through the use of diode array detection. The present results illustrate that SERRS is a highly sensitive technique for the study of transferred LB monolayers arrays.