Maria Vega Cañamares

In this work, a structural analysis of the polyphenol complexes with iron and copper at several conditions is reported. The investigated polyphenols were tannic acid, gallic acid, pyrogallol, and syringic acid, being components and molecular models of the gallnuts usually employed in the past in fabrication of iron gall inks (IGIs). Commercial tannic acid extracted from gallnuts, which is a complex mixture of different gallotannins and simpler galloylglucoses, was also employed in this analysis. This analysis comprised the use of Raman, Fourier-transform infrared (FTIR), UV-vis absorption, and fluorescence spectroscopy. The complexation of iron with these molecules leads to a strong change in color due to the deep restructuring of the polyphenol that can be clearly seen by Raman and FTIR spectra. Three main Raman bands appeared at 1450-1490 cm^-1 (ν₁), 1320-1345 cm^-1 (ν₂), and 400-650 cm^-1 (ν₃), which are characteristic of the metal complexes. The structural changes of the polyphenol complexes with iron were also investigated at different pHs and different polyphenol/iron stoichiometries. Other effects of the interaction of polyphenols with iron are the pH decrease of the mixture upon metal complexation and fluorescence quenching induced by the interaction of iron. This quenching is important since it facilitates the Raman inspection of manuscripts since polyphenols show a strong fluorescence emission that overlaps the Raman spectrum. Furthermore, DFT calculations were performed for the first time on the gallic acid complex with iron in order to elaborate a detailed assignment of the vibrational modes of polyphenols and their metal complexes, something that was missed in previous applications of Raman to IGIs.

Bufotenine (5-hydroxy-N,N-dimethyltryptamine) is a natural tryptamine derivative with hallucinogenic activity. In this paper, we present novel chemical and molecular conformational analyses of bufotenine based on an experimental and theoretical approach integrating surface-enhanced Raman scattering (SERS) and density functional theory (DFT). For the first time, low concentrations of bufotenine in acetonitrile solutions were analyzed by SERS using two types of silver nanoparticle substrates synthesized via one- or two-step reduction processes. The vibrational characteristics of this molecule were verified by molecular dynamics simulations of Raman bands based on DFT. Here we demonstrate the potential of this integrated approach for the identification of bufotenine, a prominent hallucinogenic agent, establishing an innovative rapid and accurate sensing and characterization method of the identification of controlled substances at trace amounts.

In this work, we have developed a simple method to carry out the quantitative analysis of deethylhydroxyatrazine (DEHA), the most abundant metabolite of atrazine herbicide (ATZ), based on the surface-enhanced Raman scattering technique. Since this ATZ product can undergo pH-dependent tautomerization, a previous characterization of the DEHA vibrational spectrum was accomplished. This study consisted of the Raman scattering study, both experimental and theoretical, of the enolic and ketonic tautomers of this molecule. SERS spectra were recorded at different pH in order to assess the effect of the metal surface in nanoparticles along with the pH on the structure of DEHA and to find the optimal experimental conditions of the quantitative detection of DEHA. Additionally, the interaction of DEHA with two types of humic acid reference standards, the Elliot humic and leonardite humic ones, was also performed by SERS. This interaction was conducted with two different objectives: to evaluate the interaction mechanism of the ATZ degradation product with humic substances and to check if this interaction can modify the sensitivity of the SERS detection of DEHA. The results presented in this study have clearly demonstrated that SERS spectroscopy is a very powerful technique for characterizing DEHA and other triazine sub-products at a very low concentration in water and also for analyzing the interaction of these important pollutants with humic substances.

Acid Organge 20 is a synthetic monoazo dye, which molecular structure consists of a naphtol unit and a sulfonated benezene ring linked together by an azo chromophore group. As a phenylazonaphthol with the OH group in para position respect to the azo group, it undergoes an azo‐hydrazone tautomerism. In this work, the study of the preponderance of one tautomer over the other in different pH conditions was carried out by means of UV–Vis, Raman, and surface‐enhanced Raman scattering (SERS) spectroscopies. Raman spectra of powder and aqueous solutions of the dye were obtained at different excitation lines at the natural pH of the AO20 solutions. The best results were obtained at 633 nm, and the observed Raman bands suggested the presence of the hydrazone tautomer. The molecular structure and the theoretical Raman spectra of the two tautomers were calculated by density functional theory (DFT) methods. The obtained results were used for the assignment of the Acid Orange 20 vibrational modes. The SERS analysis of the dye solutions were carried out employing Ag nanostars, pH below and above 4.7, and excitation at 442, 532, and 633 nm. The hydrazone tautomer was prevalent in most conditions. The azo tautomer was only detected at pH ~ 5 under excitation with the 532 nm wavelength due to resonant Raman conditions. Finally, the effect of pH on the morphology of metal nanoparticles employed as SERS substrates was evaluated by TEM.

Safflower red is a natural dye extracted from the petals of Carthamus tinctorius L. Its main coloring matter is carthamin. Despite its poor lightfastness, safflower was greatly appreciated in Asia for its bright red color. It was particularly important in nishiki‐e multicolor Japanese woodblock printing. In the present work, safflower red was analyzed by Fourier transform (FT)‐Raman and surface‐enhanced Raman scattering (SERS) spectroscopies and the band assignment was carried out with the support of density functional theory calculations. Different pH values were tested in order to determine the optimal conditions for SERS analysis. Changes in the relative intensity of the Raman bands at neutral and acid pH suggest a deprotonation of the acidic groups in the molecule. From the obtained results, the adsorption geometry of the dye molecule on Ag nanoparticles was deducted. Finally, SERS was successfully applied to the detection of safflower red in a paper fiber from a Japanese woodblock print.

Silver nanoparticles were fabricated in situ in the hydroxypropyl cellulose (HPC) matrix by irradiation at different wavelengths (365 and 254 nm) following the principles of green chemistry. HPC is involved in the silver reduction process and in the stabilization as well. Characteristics of the dispersions depend on the weight ratio between silver ions and HPC, irradiation time, and energy of the incident irradiation. Interaction at higher energies leads to higher degradation of the HPC matrix, resulting also in lower pH values of the dispersion. The latter is even more pronounced when using lower initial concentrations of silver nitrate. Different formulations were tested as surface‐enhanced Raman scattering (SERS) substrates with alizarin solutions and several other probes. Different spectral responses are related to different surface availability and/or to differences in microenvironment or macroenvironment of the nanoparticles. Substrates irradiated at lower energies show superior properties in terms of stability, whereas the substrates with a lower concentration of silver ions and irradiated with higher energies are more suitable for the detection of compounds with acidic properties (such as carminic acid and lac dye) due to lower electrostatic repulsion between the compounds and nanoparticles.