Concentration-dependent Surface-enhanced Raman Scattering Research Articles

Concentration dependent SERS, DFT and molecular docking studies of a ureido derivative with antitubercular properties

Spectroscopic analysis, density functional theory (DFT) studies and surface enhanced Raman scattering (SERS) of antimycobactetial 4-[3-(4-acetylphenyl)ureido]-2-hydroxybenzoic acid (AUHB) have been studied on different silver sols. For Raman and SERS wavenumbers, very large changes are observed. Observed variations in the modes of ring may be due to surface π-electron interactions and presence of this indicated that poly substituted ring is more inclined than para substituted phenyl ring and assumes a inclined position for concentration 10−3 M. Changes in orientation are seen in SERS spectra depending on concentration. In order to find electron-rich and poor sites of AUHB, molecular electrostatic potential was also constructed. The molecular docking results show that binding affinity and interactions with the receptor DprE1 may be supporting evidence for further studies in design further AUHB pharmaceutical applications. Based on antitubercular activity of 4-aminosalicylic acid (PAS) and urea derivatives we designed, synthesized and investigated mutual PAS-urea derivatives as potential antimycobacterial agents.

Spectroscopic investigations, concentration dependent SERS, and molecular docking studies of a benzoic acid derivative

Spectroscopic analysis, density functional theory (DFT) studies and surface enhanced Raman scattering of 4-((3-bromo-5-chloro-2-hydroxybenzylidene)amino)benzoic acid (BCHB) have been studied on different silver colloids concentrations in order to know the particular chemical species responsible for the spectra. For Raman and surface enhanced Raman scattering (SERS) wavenumbers, changes are observed. Observed variations in the modes of ring may be due to interaction of the π-electrons and presence of this indicated that RingII is more inclined than RingI and the BCHB assumes inclined orientation for concentration 10-3 M. Changes in orientation are seen in SERS spectra depending on concentration. In order to determine the electron-rich and poor sites of BCHB, the molecular electrostatic potential was also constructed. The molecular docking studies show that the bindings and interactions with the receptors may be supporting evidence for further studies in design further BCHB pharmaceutical applications.

Adsorption and trace detection of pharmacologically significant 5-methylthio-1, 3, 4-thiadiazole-2-thiol molecule adsorbed on silver nanocolloids and understanding the role of Albrecht’s “A” and Herzberg–Teller contributions in the SERS spectra

The surface enhanced Raman scattering (SERS) spectra of biologically and industrially significant 5-methylthio-1, 3, 4-thiadiazole 2-thiol molecule have been investigated. The SERS spectra at various concentrations of the adsorbate are compared with the Fourier transform Infrared (FTIR) and normal Raman spectra (NRS) of the probe molecule recorded in different environmental conditions. The optimized molecular structures of the most probable thione and the thiol forms of the molecule have been estimated from the density functional theory (DFT) calculations. The vibrational signatures of the molecule have been assigned from the potential energy distributions (PEDs). The detail vibrational analyses reveal that ∼54% of the thione form of the molecule is prevalent in the solid state and its population increases to ∼65% in ACN solvent medium. Concentration dependent SERS, together with the 2-dimensional correlation spectra (2D-COS), corroborate the presence of both the thione and the thiol forms of the molecule even in the surface adsorbed state. The orientations of the thione and the thiol forms of the molecule on the nanocolloidal silver surface have been predicted from the surface selection rule. The selective enhancement of Raman bands in the SERS spectra have been explored from the view of the Albrecht’s “A” and Herzberg–Teller (HT) charge transfer (CT) contribution.

Conformations of Morpholine in Liquid and Adsorbed on Gold Nanoparticles Explored by Raman Spectroscopy and Theoretical Calculations

Morpholine is a typical six-membered saturated heterocycle with the molecular formula HN(CH2CH2)2O. In this work, the conformations of Morpholine in liquid and adsorbed on the surface of gold nanoparticles were studied by means of Raman spectroscopy and theoretical calculations. Ab initio calculations indicate that the energy of the chair conformers of Morpholine is ca. 7.5 kcal/mol lower than the skew-boat conformers, which implies that the chair conformers would be favorable in liquid Morpholine. Comparison of the observed Raman spectra of liquid Morpholine, its solution, and the predicted spectra of the chair conformers (equatorial and axial) revealed that both of the chair conformers coexist in its liquid, and the content of the equatorial-chair conformer may reduce in the solution. Considering the concentration-dependent Surface-Enhanced Raman Scattering (SERS) spectral profile, the surface selection rule, and the theoretical calculations, it has been inferred that at higher concentrations Morpholine is vertically chemisorbed on gold nanoparticles through the N atom of the ring, and the dominant conformation adsorbed is the axial-chair conformer. However, at the dilute concentrations, Morpholine is gradually flatly chemisorbed on the gold nanoparticles through the N atom, and the Morpholine may be deprotonated. Furthermore, the predicted spectra agree with the experimental ones very well, which confirms the results above.

Concentration-Dependent Orientational Changes of 2-Amino-2-thiazoline Molecule Adsorbed on Silver Nanocolloidal Surface Investigated by SERS and DFT

The concentration-dependent surface-enhanced Raman scattering (SERS) study of the biologically important, 2-amino-2-thiazoline (2ATH) molecule adsorbed on silver nanocolloids has been investigated. The SERS spectra of the molecule at different adsorbate concentrations are compared with its FTIR spectrum and normal Raman spectra (NRS) in varied environments. The optimized structural parameters and computed vibrational wavenumbers of the tautomeric amino and the isomeric imino forms of the molecule in the gas phase and in methanol solvent have been estimated from density functional (DFT) calculations. The observed Raman signals along with the corresponding FTIR bands have been assigned for the first time from the potential energy distributions (PED) in terms of internal coordinates of the molecule estimated from the output of the DFT calculations. Vibrational analyses reveal that imino species are prevalent in the solid state while the amino species of the molecule are prevalent in methanol solution. The adsorbate concentration-dependent SERS spectra of the molecule adsorbed on a nanocolloidal silver surface are also reported. Considering the entire concentration-dependent SERS spectral profile and the surface selection rule, it has been predicted that at higher adsorbate concentrations the amino and the Z-isomeric imino form of the molecule attain face-on adsorption geometry. Alternatively, at dilute concentrations, the amino form of the molecule has edge-on adsorption geometry with the molecular plane lying nearly orthogonal to the nanocolloidal silver surface.