Vibrational analysis of herbicide diquat: A normal Raman and SERS study on Ag nanoparticles

A surface-enhanced Raman scattering (SERS) study of diquat (DQ) is reported in this work for the first time. Previously, we have accomplished the assignment of the Raman spectra of DQ and its radical cation DQ on the basis of theoretical DFT calculations. SERS spectra on Ag colloid were obtained at different conditions: varying the added anion, the surface coverage and the excitation wavelength. Structural marker bands related to the molecular planarity and the environment of N atoms were identified on the basis of the calculated frequencies and the SERS spectra. The analysis of SERS spectra at different conditions revealed that two DQ species are present on the metal surface: DQ attached to the surface through the formation of charge-transfer (CT) complexes with adsorbed anion, bearing a structure similar to that of DQ, and other DQ species which are adsorbed on the metal forming multilayers.