Adsorption of Rhodamine 6G on SiO2 and Ag@SiO2 Porous Solids: Coupling Thermodynamics and Raman Spectroscopy

Abstract

The sensitive detection of small molecules analytes at very low concentrations is of great interest, notably in the medical and environmental fields. In this study, two porous solids, namely SiO2 (dpore ≈ 5 nm) and Ag@SiO2, were prepared. In the latter, the incorporation of Ag nanoparticles in the porous matrix was highlighted using different techniques. Around 0.25 wt % of nanoparticles were inserted with a mean diameter close to 3 nm. Then, the adsorption of rhodamine 6G (R6G) on these two materials was studied coupling thermodynamics and Raman spectroscopy. From a thermodynamic point of view, it was shown that whereas the adsorbed amounts of R6G (Nads) were not affected by the presence of Ag nanoparticles, the adsorption enthalpies in the case of Ag@SiO2 are doubled compared to those of SiO2, highlighting the strong interaction of R6G with the Ag nanoparticle surface. Furthermore, for Nads > 1 μmol g–1, it was evidenced that the Raman response of R6G, in terms of band positions and intensities, is directly related to the amounts of R6G adsorbed on the solid surface and not to the presence of Ag. This result highlights the preconcentration properties of porous solids. For Nads = 0.05 μmol g–1, the intensities of Raman bands that are detected, corresponding to bending modes of ethylamine groups, are enhanced in the case of Ag@SiO2 by a factor of roughly 10. This value corresponds to the order of magnitude generally assumed for the chemical enhancement mechanism of surface enhanced Raman spectroscopy effect. This result and the high values of integral enthalpies of adsorption determined for Ag@SiO2 highlight strong interactions between amine groups and the Ag surface.