The adsorption of paraquat on silver electrode surfaces: a SERS microprobe study

Abstract

Surface-enhanced Raman microprobe spectroscopy (micro-SERS) has been used to study, in situ, the adsorption process of the bipyridylium herbicide paraquat (1,1′-dimethyl-4,4′-dipyridylium dichloride, PQ 2 +) on roughened silver electrode surfaces. The electrogeneration of the paraquat cation PQ + . and its interaction with the silver surface was investigated. The micro-SERS method permits the acquisition of surface Raman spectra from electrode surface spots down to 1 μm in size. Vibrational assignment was achieved by comparison of observed band position and intensity in the FTIR and FT-Raman spectra with wavenumbers and intensities from Hartree-Fock (6-13G ∗ level) and Density Functional Theory (DFT) calculations. Information on the monolayer adsorption properties at the charged surface can be obtained on the basis of this assignment. On the positively charged oxidised surface (potential of −0.2 V/ sce), the PQ 2 + is strongly adsorbed in the first monolayer with coadsorbed halide ions. Corresponding results are found by means of Colloid-FT-SERS investigations of PQ 2 + adsorbed on silver sols. Evidence for the monolayer adsorption of the PQ + . monoradical formed by one-electron reduction at potentials between −0.6 and −0.9 V/ sce has been obtained by in situ SERS microprobe investigations. By spectral mapping the electrode surface with the 1 μm 2 laser spot special surface active reaction areas are located which interact strongly with the monomeric PQ + . radical as compared with other surface regions with PQ + . monomer and dimer adsorption sites. In addition it is shown that the use of electrode micro-SERS spectroscopy is a sensitive analytical technique for trace analysis by detecting paraquat on a dried electrode surface in the sub-nanogram range. In order to reveal the optimal condition for this kind of SERS microprobe spectroscopy, atomic force microscopy was applied to investigate the surface morphology of silver electrodes.