Study of the azo‐hydrazone tautomerism of Acid Orange 20 by spectroscopic techniques: UV–Visible, Raman, and surface‐enhanced Raman scattering. (30th June 2020)

• Record Type: Journal Article
• Authors: Vannucci, Giulia; Cañamares, Maria Vega; Prati, Silvia; Sanchez‐Cortes, Santiago
• Is Part Of: Journal of Raman spectroscopy. Volume 51:Number 8(2020)
• Page Start: 1295
• Page End: 1304
• Publication Date: 2020-06-30
• Subjects: azo dyes -- DFT calculations -- pH -- SERS -- silver nanostars; Raman spectroscopy -- Periodicals; 535.846
• DOI: 10.1002/jrs.5906
• Languages: English
• ISSNs: 0377-0486

Abstract

Abstract: 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. Abstract : Acid Orange 20 is an acidic monoazo dye that can undergo an azo‐hydrazone tautomerism. In this work, this synthetic dye was studied by UV–Vis, Raman, and SERS spectroscopies. Analysis of the dye solutions were carried at different pH conditions, in order to evaluate the preponderance of one tautomer over the other as a function of the pH. Theoretical DFT calculations of the azo and hydrazone tautomers were also performed.