Near-Infrared Surface-Enhanced Raman Spectra of 3-Picoline and 3-Chloropyridine on a Copper Electrode

Abstract

SERS is well suited to detecting and fingerprinting environmental contaminants such as polychlorinated biphenyls. For this reason, we have been investigating SERS for identifying and quantifying the individual components of mixtures of these and related compounds. Initially, very simple monosubstituted aromatic compounds are being studied. One of the simplest of these is 3-chloropyridine (CP). This compound has been found to give very intense NIR-SER spectra on copper colloids and on copper electrodes; these results have been previously reported. In this paper we report the NIR-SER spectrum of 3-picoline on a copper electrode as well as the NIR-SER spectra of a mixture of 3-picoline and 3-chloropyridine on a copper electrode at several different potentials. Electrode potential effects on SER spectra have been mentioned in many reports, but there have been very few studies that have dealt specifically with these effects, and fewer still that have dealt with the SER spectra of mixtures. Most of the qualitative features of SERS are explained by two models, an electromagnetic model and a charge-transfer model. Variation of SER spectra with electrode potential is predicted by the charge-transfer model and is a result of the shift of the energy of the charge-transfer band in the metal with respect to the photon energy from changes in the applied voltage. Therefore, for near-infrared excitation, the optimal electrode potential might be different from that for visible-wavelength excitation. Variation of electrode potential might also have analytical uses. We are investigating the use of electrode potential for obtaining additional information about the SER spectra of mixtures. Eventually, we hope to make these measurements in contaminated aquifers using optical fibers. Near-infrared excitation is being used for these studies because of its high transmission in optical fibers and because it minimizes fiber background fluorescence.