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.

A new approach to measure melamine, cyanuric acid, and melamine cyanurate using surface enhanced Raman spectroscopy coupled with gold nanosubstrates

“…The most intense peak at 676 cm -1 is assigned to the ring breathing II mode and involves in-plane deformation of the triazine ring. The second highest peak at 984 cm -1 arises from the ring breathing mode I of the triazine ring [17]. No peaks were observable in the SERS spectra obtained from the melamine sample with a concentration of 10 -7 mol L -1 or lower.…”

Effect of processing temperature on structure and photocatalytic properties of g-C3N4

“…The most intense peak in the melamine Raman spectrum appears at 675 cm -1 and is assigned to the breathing 2 mode of the triazine ring [41,42]. The other strong peak at 980 cm -1 represents the breathing 1 mode of the triazine ring [41,42].…”

Rapid detection of melamine with 4-mercaptopyridine-modified gold nanoparticles by surface-enhanced Raman scattering

“…The enhancement associated with SERSup to 10 14 times [23]-is widely attributed to two primary mechanisms: the short-range chemical effect and the longrange classical electromagnetic effect. Recently, several groups have reported the determination of melamine using SERS by detecting the signal from melamine directly with roughened gold substrates [24], silver colloid substrate [25] or other SERS substrates [26][27][28]. However, the signal from melamine itself is relatively weak, and the detection limits of these direct sensing methods are unsatisfactory (most are more than 10 ppb).…”