Density functional theory study on surface-enhanced Raman scattering of 4,4'-azopyridine on silver

Raman and SERS spectra of thiamethoxam and the Ag3-thiamethoxam complex: an experimental and theoretical investigation

The insecticide thiamethoxam (TMX) is one of the most important neonicotinoid pesticides. The chromatographic methods currently employed to detect TMX require multiple operational steps. This study proposes a simple method that detects TMX via surface-enhanced Raman scattering (SERS) spectroscopy with Ag nanoparticles (NPs) as the SERS active substrate. Density functional theory (DFT) was used to calculate the structures and vibrational modes of the Ag- and Ag₃-TMX complexes at the B3LYP/6-311++G(d,p)(C,H,N)/LanL2DZ(Ag) level of theory. The results reveal that the atoms in the thiazole ring all lie in the same plane, while the six-membered ring is perpendicular to the thiazole ring. Data from both Ultraviolet-visible and Raman spectroscopy indicated that TMX bonds to Ag through its nitro group, vertically. A weak intramolecular (N22-O23…H26) hydrogen bonding and Ag-O bands shift N-O symmetrical vibration to down to lower wavenumber. This was supported by the appearance of a strong 866 cm^-1 band in the SERS spectrum assigned to the N-O symmetrical vibration coupled with the N-N stretching vibrational mode of different excitation wavelength. Notably, a good linear relationship was observed in the TMX concentration range 1.0 × 10^-6-1.0 × 10^-4mol·L^-1 (R² = 0.9667). SERS is an extremely simple and rapid technique that requires little sample for analysis.

Sensitive SERS nanotags for use with a hand-held 1064 nm Raman spectrometer

This is the first report of the use of a hand-held 1064 nm Raman spectrometer combined with red-shifted surface-enhanced Raman scattering (SERS) nanotags to provide an unprecedented performance in the short-wave infrared (SWIR) region. A library consisting of 17 chalcogenopyrylium nanotags produce extraordinary SERS responses with femtomolar detection limits being obtained using the portable instrument. This is well beyond previous SERS detection limits at this far red-shifted wavelength and opens up new options for SERS sensors in the SWIR region of the electromagnetic spectrum (between 950 and 1700 nm).

1064 nm SERS of NIR active hollow gold nanotags

NIR active hollow gold nanotags have been developed to provide effective SERS when excited at 1064 nm.

Surface-enhanced Raman scattering and density functional theory study of 1,4-benzenedithiol and its silver complexes

This paper experimentally and theoretically investigated Raman and surface-enhanced Raman scattering (SERS) of 1,4-benzenedithiol (1,4-BDT). Density functional theory methods were used to study Raman scattering spectra of isolated 1,4-BDT and 1,4-BDT-Agn (n=2,4,6) complexes with B3LYP/6-311+g(d)(C,H,S)/Lanl2dz(Ag) basis set. A full assignment of the Raman spectrum of 1,4-BDT has been made based on the DFT analysis. The calculated data showed good agreement with experimental observations. The adsorption sites, metal cluster size, and HOMO-LUMO energies are discussed to give insight in the SERS mechanisms for 1,4-BDT molecules.

Interaction of melamine molecules with silver nanoparticles explored by surface-enhanced Raman scattering and density functional theory calculations

Recently, unethical manufacturers illegally adulterated foods to increase the nitrogen content of protein through the use of melamine. Although surface-enhanced Raman scattering (SERS) methods have been widely used to detect melamine, the details of the interaction mechanism between a melamine molecule and silver colloids are still unclear. In this paper, we present the adsorption behavior of melamine on the surface of silver nanoparticles, which we explored by using SERS and density functional theory calculation methods. The calculated results demonstrate that the melamine molecule interacts with the silver surface mainly via the heterocyclic nitrogen. When comparing the predicted spectra of the complexes of melamine-Ag(+) and melamine-Ag(0), we found that the spectrum of the former agrees better with the experimental spectra than that of the latter. In the structure of the melamine-Ag(+) complex, we found that the melamine molecule "stands" on the surface of nanoparticles, with a small tilt angle. The concentration-dependent SERS spectra reveal that the melamine molecule stands on the silver surface with a small tilt angle in high concentration, and the tilt angle becomes larger when the concentrations are diluted.

Surface-enhanced Raman scattering of trans-1,2-bis (4-pyridyl)-ethylene on silver by theory calculations

Surface-enhanced Raman spectra of trans-1,2-bis (4-pyridyl)-ethylene (t-BPE) on silver foil were detected at laser line of 514.5, 633, 785 and 1064 nm, respectively. The structure of Ag-t-BPE, Ag4-t-BPE, Ag6-t-BPE, Ag10-t-BPE and Ag20-t-BPE complexes has been calculated using a local version of the Amsterdam density functional program package. The Raman spectra and electronic polarizability of t-BPE-Ag at 514.5, 633, 785 and 1064 nm excitation lines were calculated. The Raman bands of t-BPE were assigned according to the calculation of potential energy distribution. The experimental and calculated Raman spectra of t-BPE-Ag at 514.5, 633, 785 and 1064 nm were compared. The relative Raman intensities change at different excitation lines were discussed based on the Raman enhanced mechanism and surface selection rules.