María Rosa López-Ramírez

Evidence of deprotonation of aromatic acids and amides adsorbed on silver colloids by surface-enhanced Raman scattering.

The surface-enhanced raman scattering (SERS) of benzoic acid/benzamide and salicylic acid/salicylamide on silver colloids show important wavenumber shifts with respect to the Raman spectrum of the band assigned to mode 1;ν(ring) when adsorbed on the metal surface (ca. +50 cm(-1)). In the case of the acids, this shift is originated by the deprotonation of the carboxylic group in agreement with the well-known fact that aromatic acids are adsorbed on silver as carboxylates. However, the main conclusion of this work is that a similar behavior is found for the respective amides that do not behave as acids in water solution. The here studied aromatic amides are adsorbed as azanions on silver nanoparticles even at pH 7 and link to the metal through the nitrogen and oxygen atoms of the ionized carboxamide group. This is a very surprising result given that amides are not significantly ionized even at pH 13-14. The deprotonation of these amides is not determined exclusively by the pH, but it is mainly caused by the strong affinity of the anionic species to the metal. Therefore, the SERS must be cautiously used as a universal pH sensor if the adsorption occurs through the ionizable group. In order to support this conclusion, theoretical DFT force field calculations have been carried out, confirming that deprotonated benzamide and salicylamide interact with the metallic surface.

Surface-enhanced Raman scattering of hydroxybenzoic acids adsorbed on silver nanoparticles.

Surface-enhanced Raman scattering (SERS) of hydroxybenzoic acids has been studied on silver sols in H(2)O and D(2)O solutions. The adsorption behavior of 4-hydroxybenzoic acid (4HBA) is different from that of salicylic (2HBA) and 3-hydroxybenzoic (3HBA) acids. It was concluded that 4HBA is adsorbed on silver nanoparticles (Ag(n)) as either oxidobenzoate (A(2-)) or hydroxybenzoate (A(-)), depending on the pH of the solution, given rise to a flat orientation. Both 2HBA and 3HBA acids are always adsorbed as hydroxybenzoates anions (A(-)) at pH >or=5 and link to the metal through the carboxylate group (Ag(n)--A(-)), standing more or less perpendicular to the metal surface. In the case of these monoanions, the selective enhancement of the bands is due mainly to a resonant electron or charge transfer process (ET or CT) from the metallic nanoparticles to the adsorbates, yielding the transient formation of the respective radical dianions (Ag(+)(n)--A(2-)). It is found that the enhanced bands, and especially the mode 8a;nu(ring), are related to the difference between the equilibrium structures of the adsorbate in its ground (A(-)) and CT-excited (A(2-)) states. In the SERS spectrum of 4HBA dianion, the contribution of CT mechanism is not observed.

Surface-enhanced Raman scattering of picolinamide, nicotinamide, and isonicotinamide: unusual carboxamide deprotonation under adsorption on silver nanoparticles.

Surface-Enhanced Raman Scattering (SERS) of picolinamide, nicotinamide, and isonicotinamide has been studied on silver colloids at pH⩾7. The wavenumbers of the SERS bands assigned to 1; νring and ν(C-X) vibrational modes show important blue-shifts (ca. +50cm(-1)) with respect to the Raman spectra, whereas the Amide III bands undergo red-shifts up to -50cm(-1). We demonstrate that these shifts are originated by the deprotonation of the carboxamide groups which link to the metal through the nitrogen and oxygen atoms of the respective azanion groups. In order to support this conclusion, theoretical DFT force field calculations have been carried out, confirming that the pyridinecarboxamides interact with the metallic surface in their deprotonated forms as benzamide does.

Study of modern artistic materials using combined spectroscopic and chromatographic techniques. Case study: painting with the signature “Picasso”

A painting bearing the signature “Picasso” was investigated and its components – pigments and binders – were characterised using micro-Raman spectroscopy (mRS) combined with other analytical techniques such as Scanning Electron Microscopy with Energy Dispersive X-ray analysis (SEM/EDX), Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR) and Gas Chromatography-Mass Spectrometry (GC-MS). We identified the pigments in order to assess chronological inconsistency (anachronistic pigments) with the work of the Spanish artist Pablo Picasso (1881–1973) by means of in situ and cross-section analyses by mRS. Using mRS we identified the pigments ‘phthalocyanine blue’, ‘phthalocyanine green’, ‘pyrazolone orange’, ‘anatase’ and ‘rutile’ (titanium dioxide). These results were corroborated by SEM/EDX elemental analysis, which detected Ba, S and Zn among others, so prompting discussion about the use of ready-mixed house paints (Ripolin®). The ground layer was also investigated by mRS, which confirmed the use of rutile (titanium dioxide), calcite and phyllosilicates (Al, Fe). mRS on the white area identified the binding media as beeswax, the most important wax used in art. ATR-FTIR and GC-MS analysis support the presence of a wax and an oil–resin mixture as the binder, described in previous research as being used by Picasso. All the artist materials identified were widely used in art at the beginning of the twentieth century according to the available bibliography and are therefore consistent with the Picasso era. To the best of our knowledge this is the first attempt to investigate a painting related to Picasso using the combination of analytical techniques proposed here.

DMABI tripod structures with sensing capabilities: synthesis, characterization and fluorescence analysis

We present herein the synthesis, and the structural and spectroscopic analysis of a non-planar tripod-shaped p-(N,N′-dimethylamino)benzyliden-1,3-indandione (DMABI) chromophore. This novel molecule is composed of a Si core with three incorporated arms, each of them contains a 1,3-indandione derivative with an electron donating (−NMe2) group, thus providing fluorescence capabilities. We prepared a DMABI arm by coupling a p-(N,N′-dimethylamino)benzaldehyde (DMAB) tripod substituted molecule with 1,3-indandione via aldol condensation. The structures of DMAB-tripods were confirmed by spectroscopic data and studied by quantum chemical calculations. Fluorescence spectroscopy was used for optical characterization. Quantum yields and the corresponding lifetimes reveal typical characteristics of conjugated derivatives. Finally, we monitored the enhancement in fluorescence intensity of compound 1 in the presence of 4-chloro-2,6-dinitroaniline (4CDNA) in the range between 0 and 20 mg L−1. We justify this enhancement by calculated energies and the distribution of the HOMO and the LUMO for DMABI-tripod and 4CDNA.