Evidence of Deprotonation of Aromatic Acids and Amides Adsorbed on Silver Colloids by Surface-Enhanced Raman Scattering

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• 作者：Jos茅 L. Castro ; Maria R. Lopez-Ramirez ; Juan F. Arenas ; Juan Soto ; Juan C. Otero
• 刊名：Langmuir
• 出版年：2012
• 出版时间：June 19, 2012
• 年：2012
• 卷：28
• 期：24
• 页码：8926-8932
• 全文大小：336K
• 年卷期：v.28,no.24(June 19, 2012)
• ISSN：1520-5827

文摘

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;谓_ring when adsorbed on the metal surface (ca. +50 cm^鈥?). 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鈥?4. 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.