Surface-enhanced Raman spectroscopy as a probe for orientation of pyridine compounds on colloidal surfaces

Surface-enhanced Raman spectroscopy (SERS) has been used to characterize three isomeric pyridine compounds adsorbed to silver colloids. The three structural isomers, which all adsorb in the carboxylate form, to the silver colloids, bind in two different geometries to the silver surface. The differences in the binding geometry correlate with Raman resonance shifts and intensity enhancements that can be probed by SERS. For picolinic acid, the ortho isomer, we observed blue shifts of many of the SERS peaks attributed to the pyridine ring in comparison to the normal Raman vibrations of the non-adsorbed crystalline material. For this molecule, steric hindrance between the carboxylate and nitrogen groups causes the pyridine ring and the carboxylate group to tilt from the flat geometry, and hence both groups can form σ bonding to the silver surface. On the other hand, nicotinic and isonicotinic acid, the meta and para isomers (respectively), that have less steric hindrance, adsorb to the silver colloids with the pyridine ring in a flat geometry, where only the carboxylate functional group tilts in order to bind coordinatively to the silver colloids. In this case, π bonding between the pyridine ring and the metal colloids causes small red shifts between the Raman and SERS peaks.