Adsorption and Displacement of Melamine at the Ag/Electrolyte Interface Probed by Surface-Enhanced Raman Microprobe Spectroscopy

Abstract

Surface-enhanced Raman spectroscopy (SERS) is evaluated as a quantitative analytical tool for low concentrations of melamine and melamine derivatives in solution. Substantial variations in absolute and relative intensities of SERS bands were encountered using silver sols, which cannot be controlled. Alternatively, it was shown that SERS using a roughened silver electrode, while conditioning the applied potential, permits the acquisition of Raman spectra from electrode spots down to 1 μm in size, and the results of multiple measurements using a hard cathodic cleaning step in between each adsorption experiment gave a relative standard deviation of 15%. The high enhancement factor of the electrode micro-Raman scattering intensity creates a new trace analytical technique for obtaining high-resolution spectra of melamine from dilute aqueous solution (detection limit ∼ 10-7 mol L-1) in the opto-electrochemical cell. As an alternative for the hard cathodic cleaning step, we demonstrated that the cationic surfactant molecule cetylpyridinium chloride is able to remove preadsorbed melamine within a few seconds. The surfactant molecules can subsequently be removed from the surface by switching to a negative applied potential. This procedure results in a relative standard deviation of 10%. The effects of electrode potential on the observed SERS spectra are consistent with current ‘SERS surface selection rules'. The electrode potential and the surface concentration of the chloride counterions strongly affect the intensity of the out-of-plane modes in the adsorbed state. However, additional experiments using various excitation lines showed that an alternative theory, surface complex formation combined with charge transfer resonance Raman processes with Herzberg−Teller contributions, plays an important role.