Gold surface enhanced Raman spectroscopy of amphiphilic sulfur viologens during electron transfer

Abstract

Surface enhanced Raman spectroscopy (SERS) was used to characterize electron transfer through mediators present as self-assembled monolayers on gold electrode surfaces. The surface active species were three alkyl sulfides with the general structure: [octadecyl]-S-[CH 2] n -[viologen]-[CH 2] m dibromide, in which n was 5, 7 and 10, and the sum of n and m held constant . These structures thus present the electroactive moiety at varying distances from the surface upon which they are adsorbed. Cyclic voltammetry was used to confirm results of previous measurements on silver, after which SERS spectroelectrochemistry was used to identify the electroactive species formed following the electron transfer steps. Electrochemistry imdicated that electron transfer was inversely proportional to the distance of the viologen from the gold surface; spectroelectrochemistry using SERS corroborated this, as the intensity of Raman scattering in SERS is a function of the distance of a scattering chromophore from the metal surface with which it is associated. Spectra of viologens were obtained for the neutral and radical species. Analysis of these data indicates agreement with the previous purely electrochemical studies of these compounds. These studies extend the utility of SERS spectroscopy for monolayer characterization by using gold surfaces to provide enhancement; silver surfaces have usually been used in previous SERS spectroelectrochemical studies, SERS employing gold surfaces may be especially significant in the case of biological molecules, because excitation in the red spectral regions greatly reduces background fluorescence, and a significantly decreased risk of photodecomposition is assured. For example, observation of gold SERS of chlorophyll or heme proteins by excitation into their Q-bands will permit acquisition of new information regarding these important compounds [cf. 6].