Development and Comparison of Surface-Enhanced Raman Scattering Gold Substrates for In Situ Characterization of 'Model' Analytes in Organic and Aqueous Media

Surface-Enhanced Raman Spectroscopy (SERS) is a technique that provides high enhancement of Raman scattering from molecules adsorbed on a rough noble metal surface. The aim of this study was the development of convenient and reproducible in situ SERS methods suitable for the detection and the quantification of analytes in organic or aqueous media. For this purpose, we used a signal acquisition technique which simply consisted of recording the SERS signal in the bulk solution by using a Raman immersion probe close to the surface of the immersed solid SERS substrate. This method should be useful for on-line process analysis and more robust than conventional acquisition techniques that are generally based on a drying step which may induce heterogeneous analyte repartition onto the substrate surface, thus often requiring the use of SERS mapping technique to improve the signal reproducibility. In this study, two types of gold SERS substrates (metal nanostructures on a solid substrate and metal nanoparticles in suspension) were investigated and compared for the in situ characterization of two 'model' analytes, Rhodamine 6G (R6G) and 1,2-bis(4-pyridyl)ethylene (BPE), in aqueous and organic media. The solid substrate developed by sputtering deposition of a nanometric gold film onto a glass slide provided reproducible and stable SERS signals of BPE in organic media at concentration down to 10(-12) M. But it appeared unusable in aqueous solutions due to the removal of the gold deposit. Despite an improvement of the deposit adhesion onto the substrate by using tetraethoxysilane/(3-mercaptopropyl) trimethoxysilane sol or the use of an electroless deposition technique, the developed solid substrates did not allow to reach satisfying R6G SERS signal in aqueous solutions. Therefore, both star-like and spherical gold nanoparticles were finally developed and used as SERS substrates. After aggregation, colloids induced the best enhancement of R6G Raman signal with a possible quantification at concentrations down to 5.10(-9) M.