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The aim of this work is to evaluate the vibrational and structural properties of N-acetyl- L -cysteine (NAC), and its molecular structure and electronic properties in relation to the action of thiol and amine groups at different pH. Raman and Surface Enhanced Raman Spectroscopy (SERS) spectra were measured in aqueous solution. The influence of an aqueous environment on the NAC spectra was simulated by means of an implicit (polarizable continuum model) method. SERS spectra indicate that the S atom is interacting with the surface through the sulfur atom. One of the consequences of the interaction with the surface is the deprotonation of the SH group, as revealed by the disappearance of the ν(S–H) band. The calculations performed for the Ag–NAC complex confirm the experimental data obtained by SERS, where the S–Ag interaction is the most important. These results are very interesting when one can formulate the drug feasibility of NAC using silver nanoparticles as a carrier. Raman spectra were measured to compare the behavior of different functional groups in the molecule, both in the solid phase and in aqueous solution at different pH. Apparent ionization constants (pK′ values) for the S–H group at high ionic strengths were calculated from the intensity of the 2580 cm−1 frequency as a function of pH. UV and circular dichroism spectra were also measured in aqueous solution at different pH. Finally, the study was completed with natural bond orbital (NBO) analysis to determine the presence of hyper-conjugative interactions. It is important to observe the behavior of the C2–N bond with the delocalization effect; as the pH increases the hyperconjugative interaction of this bond decreases in the same way as in the case of νCN. The way in which the LP πO1 → σ*C2–N interaction and νCN decrease is an inverse reflection of the fractional ionization αSH. [ABSTRACT FROM AUTHOR]

Abstract: Organochlorine pesticide endosulfan has been detected for the first time by using surface-enhanced Raman scattering (SERS) at trace concentrations. The bis-acridinium dication lucigenine was successfully used as a molecular assembler in the functionalization of metal nanoparticles to facilitate the approach of the pesticide to the metal surface. From the SERS spectra valuable information about the interaction mechanism between the pesticide and lucigenin can be deduced. In fact, endosulfan undergoes an isomerization upon adsorption onto the metal, while the viologen undergoes a rotation of the acridinium planes to better accommodate the pesticide molecule. An interaction between the N atom of the central acridinium ring and the pesticide Cl–CCl fragment is verified through a charge-transfer complex. The present study affords important information which can be applied to the design of chemical sensor systems of persistent organic pollutants based on the optical detection on functionalized metal nanoparticle. [Copyright &y& Elsevier]

Abstract: A humic acid (HA) extracted from lignite was successfully used in surface-enhanced Raman scattering SERS experiments as molecular occlusion assembler deposited onto a Ag colloidal surface to detect the polycyclic aromatic hydrocarbons PAHs chrysene and pyrene. Thus, this is the first time that HA compounds are employed in the design of metal nanoparticles-based sensors for pollutants detection. Chrysene (Chr) and pyrene (Pyr) were detected at low concentration. A charge transfer between HA and the analyte characterizes their interaction. The interaction resulted to be more significant in the case of the pyrene molecule. Extended Hückel calculations based on a molecular model for the interacting PAHs/HA/Ag system support the experimental conclusions. The PAHs–HA distance is about 3.5Å and the most probable orientation for both analytes is plane parallel to the aromatic fragments of HA. An energy transfer, from the silver surface to HA and from the analyte to HA, is concluded. [Copyright &y& Elsevier]

Abstract: Aromatic amino groups can be attached through different binding points to the surface. In a previous work, we have demonstrated that α-phenylglycine (PGly) interacts with a silver surface through its functional groups. In addition, a large charge-transfer (CT) contribution was found in the overall SERS intensification. In the present work we studied the SERS of PGly on Ag metal colloids prepared by different methods in order to investigate the influence of the chemical nature of the metal substrate on the adsorption and on the CT processes taking place between this amino acid and the metal. [Copyright &y& Elsevier]

Abstract: FT-Raman and surface-enhanced Raman scattering (SERS) spectroscopy were applied in the vibrational characterization and study of the adsorption and acidity behaviour of the highly fluorescent anthraquinone red pigment carminic acid (CA) on Ag nanoparticles prepared by chemical reduction with citrate and hydroxylamine. The SERS spectra were obtained at several pHs and excitation wavelengths and on Ag nanoparticles prepared by two different methods: chemical reduction with citrate and hydroxylamine in order to find the best experimental conditions for the vibrational study and detection of CA. Three CA forms can be identified on the metal surface, corresponding to the mono-, di- and tri-anionic CA species, whose relative intensity in the SERS spectra can markedly vary depending on the chemical properties of the offered surface and the excitation wavelength. This study could be very helpful for researchers trying to apply surface-enhanced techniques in the detection of small amounts of anthraquinone molecules, both under a cultural heritage and under a biological point of view. [Copyright &y& Elsevier]

Surface-enhanced Raman scattering (SERS) spectroscopy was used for the vibrational characterization of the highly fluorescent anthraquinone molecules danthron and quinizarin, by using Ag nanoparticles prepared by citrate reduction. The adsorption of these two molecules is studied in this work on the basis of the results found by varying the pH of the medium, and the excitation wavelength. On the other hand, interesting structural findings where deduced by comparing the SERS results reported here with those found for the parent molecules emodin and naphtazarin which could be of interest in the assessment of the biological activity of these important drugs. [Copyright &y& Elsevier]

Surface-enhanced Raman scattering (SERS) spectroscopy is applied in this work to the study of the adsorption of naphthazarin (NZ) on Ag nanoparticles. Spectra recorded at different excitation wavelengths and pHs revealed that this molecule is adsorbed on the metal through several mechanisms. Two main types of adsorbed molecules can be identified that correspond to neutral and ionized NZ, which may be physisorbed or chemisorbed on the metal. The existence of these different forms can be due to different binding sites on the surface or to the formation of a multilayer architecture on the metal surface giving rise to different adsorbate states. Although the amount of the ionized molecule attached on the surface is higher at neutral pH, the neutral molecule may also exist even at very high pH. The amount of neutral NZ increases with the time and also as the NZ concentration is raised or as the dimethylsulfoxide (DMSO) concentration existing in the medium is increased. [Copyright &y& Elsevier]

The FT-IR and Raman spectra in the solid state, the reflection–absorption IR spectra (RAIRS) and the surface enhanced IR (SEIR) and surface enhanced Raman spectra (SERS) of the Ni(II) (trans-7,14-dimethyl-5,12,-diphenyl-1,4,8,11-tetra-aza-2,3-9,10-dinaphthyl-cyclotetradeca-5,7,12,14-tetraene) and Cu(II) (5,7,12,14-tetramethyl-1,4,8,11-tetra-aza-2,3-9,10-dinaphthyl-cyclotetradeca-5,7,12,14-tetraene) are reported. The RAIRS analysis and differences observed in the relative intensities of complexes deposited onto KBr and Cu surfaces suggest that the organisation of each macrocycle is different on both surfaces. SERS data allow to propose for both complexes an adsorbate–substrate interaction between the complex and the colloidal Ag surface. Complexes are oriented face-on to the surface being the number of Cu macrocycles oriented face-on to the surface larger than that of the Ni macrocycles. The organisation of the complexes on Au film is different to that found on Ag colloids and rather random. The phenyl groups avoid a better organisation of the Ni complex on the surface. SEIR data analysis of samples deposited on Au film suggests that the Ni complex is less organised than the Cu complex. The dynamic and energetic of the adsorbate–substrate interaction is interpreted by using a simple molecular model and INDO/1 calculations. [Copyright &y& Elsevier]