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SERS analysis of selected nitrogen- and sulfur-containing compounds on silver and gold colloidal nanoparticles

Surface-enhanced Raman spectroscopy (SERS) is a highly sensitive and selective method of detecting molecules located very close to a rough metal surface [1]. By adsorbing the molecule on metal substrate, the scattered radiation is amplified up to a million times, and fluorescence is extinguished, allowing structural investigations of molecules at micromolar concentrations. For more efficient adsorption of analyte and enhancing the scattering, various metal SERS substrates are designed and prepared, mostly in the form of silver and gold nanoparticles, which can be dispersed in suspension or immobilized on a solid support. It is known that gold atoms preferably bind to the sulfur atom of the observed molecule, while silver atoms form a covalent bond with the nitrogen atom, which opens the way to the preparation of a substrate that will preferably bind with a specific part of the observed molecule [2]. In this work, colloidal suspensions of silver and gold nanoparticles were used as substrates for the SERS study of methylene blue and 4-aminotiophenol, in concentration range 1×10–7–1×10–4 mol/dm3. Both molecules contain sulfur and nitrogen atoms in their structure as part of different functional groups, which makes them good candidates for testing the binding path of the observed molecules to different metals in SERS substrates. Three different colloids were used as substrates, which differ in the type of used metal and anions that stabilize the nanoparticles. Silver colloids were prepared by reduction of silver nitrate with trisodium citrate [3] and hydroxylamine hydrochloride [4], respectively, while gold nanoparticles were prepared by citrate reduction of tetrachloroauric(III) acid [5]. When gold colloidal nanoparticles were used as the metal substrate for SERS analysis of methylene blue, a decrease in dye concentration resulted in spectral differences. Few new bands appeared in the spectra and the 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 of the aromatic system faced the gold surface, hence a stretching vibration of Au–S bond was observed. Using silver colloids as the SERS metal substrate, only small variations in intensity of methylene blue vibrational bands was observed, implying slight changes in position of the dye molecules on the silver surface. In case of 4-aminotiophenol and regardless of using either gold or silver nanoparticles as the metal substrates, concentration-dependent SERS spectra of the compound showed a very similar trend, including only band intensity changes with lowering the dye concentration. It can be assumed that 4-aminotiophenol molecules were very similarly oriented on both metal surfaces.

SERS, nanoparticles, gold, silver

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