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.

中文翻译：

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光谱技术研究酸性橙20的偶氮hydr互变异构现象：紫外可见，拉曼和表面增强拉曼散射

Acid Organge 20是一种合成的单偶氮染料，其分子结构由萘酚单元和通过偶氮发色团连接在一起的磺化苯环组成。与在OH基phenylazonaphthol对相对于偶氮基团而言，它经历了偶氮hydr互变异构现象。在这项工作中，通过UV-Vis，拉曼和表面增强拉曼散射（SERS）光谱学研究了一个互变异构体在另一个pH条件下的优势。在AO20溶液的天然pH值下，在不同的激发线下获得了染料粉末和水溶液的拉曼光谱。在633 nm处获得了最佳结果，观察到的拉曼谱带表明存在t互变异构体。通过密度泛函理论（DFT）方法计算了两种互变异构体的分子结构和理论拉曼光谱。获得的结果用于分配酸橙20振动模式。染料溶液的SERS分析是使用银纳米星进行的，pH值低于和高于4.7，并在442、532和633 nm处激发。most互变异构体在大多数情况下普遍存在。由于共振拉曼条件，偶氮互变异构体仅在pH〜5，激发波长为532 nm的波长下检测到。最后，通过TEM评估pH对用作SERS底物的金属纳米颗粒形态的影响。