Adsorption mechanism and acidic behavior of naphthazarin on Ag nanoparticles studied by Raman spectroscopy

Surface-enhanced Raman scattering (SERS) spectroscopy is applied in this work to the study of the adsorption of naphthazarin (NZ) on Ag nanoparticles. Spectra recorded at different excitation wavelengths and pHs revealed that this molecule is adsorbed on the metal through several mechanisms. Two main types of adsorbed molecules can be identified that correspond to neutral and ionized NZ, which may be physisorbed or chemisorbed on the metal. The existence of these different forms can be due to different binding sites on the surface or to the formation of a multilayer architecture on the metal surface giving rise to different adsorbate states. Although the amount of the ionized molecule attached on the surface is higher at neutral pH, the neutral molecule may also exist even at very high pH. The amount of neutral NZ increases with the time and also as the NZ concentration is raised or as the dimethylsulfoxide (DMSO) concentration existing in the medium is increased.