Electric-Field Control of the Tautomerization and Metal Ion Binding Reactivity of 8-Hydroxyquinoline Immobilized to an Electrode Surface

Surface-enhanced Raman scattering (SERS) spectra of a metal-complexing ligand, immobilized to a silver electrode surface, exhibits significant structural changes upon application of modest potentials. A detailed spectroscopic investigation shows that the potential applied to the electrode surface governs the tautomerization equilibrium of the immobilized ligand, p-((8-hydroxyquinoline)azo)benzenethiol (SHQ). Potential-dependent SERS spectra reveal that SHQ exists predominantly in a keto-hydrazone tautomeric form at applied potentials that are negative of -300 mV (vs Ag/AgCl), while the enol-azo tautomer is strongly favored at potentials positive of this value. The observed switching of the tautomer population occurs within a narrow range of applied potentials, approximately 200 mV (Ag/AgCl). Electrical control over the tautomerization equilibrium of immobilized SHQ governs the reactivity of the ligand toward metal ion complexation, where the enol-azo tautomer exhibits much greater affinity for metal ion binding compared to its keto-hydrazone counterpart. Accordingly, the potential applied to the electrode can be used to influence metal ion binding of immobilized SHQ through control over the tautomerization equilibrium, to produce an electrically switchable surface for metal ion complexation. Large differences in the electric dipole moment of the two tautomers, estimated from density functional theory calculations, suggested a model where the potential dependence arises from the interaction of the ligand dipole with electric fields that exist at a polarized electrode surface. This model accurately predicts the relative tautomer populations versus applied potential, at interfacial electric fields that are consistent with previous measurements of the vibrational Stark effect at polarized interfaces. Potential applications of this technology to several areas of analytical chemistry are considered.