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Silver and gold colloids as substrates for SERS study of methylene blue and rhodamine B

Surface-enhanced Raman scattering (SERS) spectroscopy provides a vibrational spectrum of a molecule located very close to a rough metal surface, where Raman scattering intensity increases up to a million times compared to classical Raman spectroscopy [1]. Organic dyes have been very often analyzed by SERS spectroscopy due to their aromatic structural motifs which strongly scatter radiation and fluorescence quenching of the molecules adsorbed onto the metal substrate, allowing structural studies of dyes at micromolar concentrations [2]. In this work, colloidal suspensions of silver and gold nanoparticles, synthesized by citrate reduction of silver nitrate and tetrachloroauric(III) acid, respectively, were used as substrates for SERS study of organic dyes, methylene blue and rhodamine B, in concentration range 1×10–7–1×10–4 mol/dm3. When gold colloid was used as the metal substrate for SERS analysis of methylene blue, a decrease in dye concentration resulted in spectral differences. New bands appeared in the spectra and relative intensity of SERS bands, originated from phenothiazine ring vibrations, changed. Most probably, the dye molecules changed their position on the metal nanoparticles, in a way that sulfur atom from the aromatic system faced the gold surface, hence a stretching vibration of Au–S bond was observed. Using silver colloid as the SERS metal substrate, only small variations in intensity of methylene blue vibrational bands were observed, implying slight changes in position of the dye molecules on the silver surface. In case of rhodamine B and regardless of using either gold or silver nanoparticles as the metallic substrates, concentration-dependent SERS spectra of dye showed very similar trend, including only band intensity changes with lowering dye concentration. It can be assumed that rhodamine B molecules were very similarly oriented on both metal surfaces.

gold, silver, SERS

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