No 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.

Direct and indirect antioxidant activities of rosmarinic acid (RA) based on HOO˙/CH3OO˙ radical scavenging and Fe(iii)/Fe(ii) ion chelation were theoretically studied using density functional theory at the M05-2X/6-311++G(2df,2p) level of theory.

Surface-enhanced Raman spectroscopy (SERS) is an efficient analytical method to detect the chemical compound at ultralow concentration. In this work, chemical enhancement phenomenon during the interaction of fungicide thiram (THR) on silver nanoparticles (AgNPs) is studied by combining experimental and density functional theory approaches. Limit of detection value for THR being 1 lM is achieved. Structural and electronic properties of THR and its complexes with pyramidal Ag 20 clusters as a surface model are investigated at the PBE/cc-pVDZ-PP//cc-pVDZ level of theory. The calculated normal Raman and SERS spectra are in good agreement with the experimental ones. Analysis of the most stable complex indicates that THR interacts with the silver cluster via two S(sp 2 ) atoms at the top of the cluster. By increasing the adsorbed THR molecules from 1 to 3, the SERS peak intensities increase from 4 to 10 times, respectively. The spectral fingerprints of THR are also analyzed via the adsorption of thiram-chlorpyrifos mixture. Consequently, the peaks at 1496, 1350 and 956 cm -1 are recommended as the three most characteristic SERS peaks for quantifying THR in a real mixture. Finally, natural population analysis (NPA) charges, NBO analysis, charge density difference (CDD) maps allow providing more insight into charge transfer process THR and AgNPs.

Hydroxyl radical (HO·) formation initiated by the Fenton-type reactions of Fe and Cu complexes of l -leucine (Leu) amino acid as well as its oxidation reaction by HO· was computationally investigated by using the density functional theory method at the M05-2X/6-311++G(3df,2pd)//M05-2X/6-311++G(d,p) level of theory in the aqueous phase. The results showed that dipole-salt is the main form of Leu in the physiological condition. Leu exhibits high chelating potential towards both Fe(III)/Fe(II) and Cu(II)/Cu(I) ions with the most favourable coordinating positions at two oxygen atoms of the –COO functional group. Furthermore, the Leu-ions complexes show a high risk of HO· formation via Fenton-like reactions, especially when ascorbate anion exists in the environment as a reducing agent. Finally, the oxidation reaction of l -leucine by HO· demonstrated a relatively high overall apparent reaction rate, k overall , being 1.18 × 10 9 M −1 s −1 , in which formal hydrogen transfer reactions of the dipole-salt form occur as the primary mechanism. Consequently, the Leu oxidation by HO· radical can be promoted by the Fenton reaction enhancement of its transition metal complexes.