SERS Spectra of the Pesticide Chlorpyrifos Adsorbed on Silver Nanosurface: The Ag₂₀ Cluster Model

Surface enhanced Raman scattering (SERS) experiments and quantum chemical calculations (using density functional theory) on the interactions of chlorpyrifos (CPF), which is an intensively used pesticide, with a roughed silver nanoparticle surface were thoroughly investigated to study the inherent molecular mechanism. Ligand–cluster interaction geometries show that the CPF molecule is mainly adsorbed on the silver surface via both S atom and pyridine ring involving a covalent Ag···S coordination as well as van der Waals physisorption. Raman vibrational modes of CPF are centered at 474, 632, 678, 1277, and 1551 cm–1 characterizing the P–O–C bending, P═S stretching, Cl-ring mode, and pyridine ring stretching, respectively, which are all enhanced when CPF is adsorbed on a silver surface. The concentration-dependent effect of CPF on silver substrates has been reproduced for the first time by coordinating 2 and 3 CPF molecules on an Ag20 silver cluster model simulated by DFT computations. The intensities of the characteristic peaks of CPF as shown in the calculated SERS spectra are increased by 2 and 3 times with respect to those of the CPF–Ag20 complex, which indicate a positive influence of high analyst concentration on the SERS signal. This observation can be explained by the electron-donating effect of CPF upon adsorption. The latter donates an electron from its lone pair on S and Cl atoms and a π electron on the S═P bond to silver atoms on the surface, and then the positive charge of silver surface is displaced to the CPF moiety via Ag···S and Ag···Cl contacts. The information obtained from the adsorption of CPF on silver by SERS is helpful to understand the molecular mechanism of adsorption process involving chlorpyrifos ligand coordinated on silver nanoparticle surfaces. It also contributes to design field detection methods for rapid screening and monitoring of pesticides in environment or agricultural products by using portable detection systems such as paper-based or fiber-based SERS sensors.