Electrochemical Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy: Bonding, Structure, and Ion-Pairing of the Positive Charge Bearing Pyridinium Ring Terminated Monolayer at Smooth Gold Electrode

Abstract

Electrode potential effect on the molecular structure of adsorbed species at an interface is one of the most important issues in physical electrochemistry. In this study, shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) was used to probe the electrochemical potential effect on bonding with surface, ion-pairing, and molecular structure of a positive charge bearing self-assembled monolayer formed from N-(6-mercapto)hexylpyridinium (MHP) at a smooth gold electrode in aqueous perchlorate solutions. In situ electrochemical SHINERS results show that the Au–S stretching frequency exhibits near linear blue-shift as the electrode potential was tuned to more positive values. The frequency tuning rate was found to be as high as 18.6 ± 0.9 cm–1/V. Analysis of ion-pairing at the interface revealed a decrease in intensity of the vibrational spectrum of electrostatically attracted perchlorate anions as the electrode potential shifts progressively to more negative values. Spectroscopic evidence for potential-driven conformational changes in the structure of MHP monolayer was revealed. It was found that the negatively charged electrode surface attracts terminal positively charged pyridinium groups, resulting in loss of all-trans conformation in hydrocarbon chains of MHP and forces some methylene groups into direct contact with the metal surface. Surface attracted pyridinium ring groups are not able further to attract perchlorate anions from the solution phase, and the intensity of perchlorate bands in SHINERS spectra decreases.