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

Abstract

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 Ag 20 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−Ag 20 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.

Insight into SERS Chemical Enhancement Mechanism of Fungicide Thiram Adsorbed on Silver Nanoparticles

“…Although the SERS chemical enhancement mechanism has widely been studied by DFT approach, there are scarcely computational works on the SERS phenomenon of pesticides. Recently, Ngo et al analyze the SERS CE mechanism of the pesticide chlorpyrifos (CPF) adsorbed on the Ag 20 pyramidal cluster [40]. The authors reported that the Ag 20 model successfully reproduces the SERS spectra of CPF as well as the CE mechanism, and its adsorption configurations are in good agreement with the ones obtained by the periodic slab model.…”

“…Characteristics of the Ag nanoparticles used in this study were previously presented elsewhere [40]. Field emission scanning electron microscopy (FE-SEM) (CZ/Mira II LMH Tescan, Czech) was used to determine the surface morphology of the AgNPs samples.…”

“…An optimal scale factor for harmonic frequencies being 1.0353 was applied [53]. The Ag 20 pyramidal cluster which has been recommended by several computational works in literature is used to reproduce the AgNPs surface model [37][38][39][40][41][42][43][44]52]. Geometries of all possible interaction configurations between the THR and Ag 20 clusters are optimized in the gas phase with tight convergence criteria and an ultrafine integration grid and loose symmetry.…”

“…Several papers have been published on this topic, including studies on large and complicated molecules like Tinuvin P [32], crystal violet [33], paroxetine [34], amoxicillin [35], and L-valine phosphonate dipeptides [36]. Many authors have utilized pyramidal Ag 20 or Au 20 clusters as models to reproduce the NPs surfaces [37][38][39][40][41][42][43][44], although it has recently been reported that these models are not the most stable cluster isomers [45]. Despite this harsh simplification, these works provide valuable insight into the interaction of different molecules onto SERS substrates.…”

Adsorption/Desorption Behaviors and SERS Chemical Enhancement of 6-Mercaptopurine on a Nanostructured Gold Surface: The Au20 Cluster Model

“…For example, the Raman enhancements of pyridine on an Ag electrode were measured and then analyzed with the aid of DFT calculations using the tetrahedral Ag 20 cluster as a model representing the silver nanoparticles [88]. More recently, similar approaches were also applied to probe the SERS phenomenon of the chlorpyrifos, which is a pesticide using the Ag 20 cluster as a model for the silver surface [89]. Likewise, the tetrahedral gold Au 20 cluster was also used as a model to validate the SERS spectrum of pramipexole molecules adsorbed on the gold surfaces [75].…”