Study of the Interaction of Pollutant Nitro Polycyclic Aromatic Hydrocarbons with Different Metallic Surfaces by Surface-Enhanced Vibrational Spectroscopy (SERS and SEIR)

In this work, we have applied surface-enhanced Raman scattering (SERS) and surface-enhanced infrared spectroscopy (SEIR) to study the interaction of nitro polycyclic aromatic hydrocarbons (NPAHs) with metal surfaces. The nitro group confers the polycyclic aromatic hydrocarbons (PAHs) moiety additional electronic and structural characteristics which verify NPAHs - surface interactions. SERS and SEIR spectra suggest that the NPAHs 1-nitropyrene (1NP) and 2-nitrofluorene (2NF) at concentrations of < 10^-6 M (∼200 ppb) interact with metal surfaces through the nitro group, adopting a perpendicular orientation onto the surface. In both cases, the interaction seems to occur through the O atoms of the nitro groups, which are coplanar to the PAHs fragment. The most efficient interaction was obtained using silver metal colloidal surfaces. The mechanism of interaction with the metal surface is related to the structure of NPAHs: although a monodentate interaction is deduced in the case of 1NP, a bidentate one seems to occur in the case of 2NF. To complete this study, theoretical Raman vibrational spectra of both free 2NF and their complexes with silver metal clusters were obtained using the local spin density approximation (LSDA) methods with the LAN12DZ basis set. The theoretical calculations were compared to the experimental results, obtaining a good agreement with the SERS and SEIR data.