In situ FTIR-ATR studies of functionalized self-assembled bilayer interactions with metal ions in aqueous solutions

The affinity of selected metal ions for palmitic (PAL) and hexadecanesulfonic acid (HDSA) monolayers self-assembled onto an octadecyl silane (ODS) mono1ayer covalently bound to a Ge internal reflection element has been examined in situ in aqueous electrolytes by attenuated total reflection Fourier transform infrared spectroscopy. Marked changes in the bands associated with the symmetric and asymmetric stretches of the carboxylate groups induced by metalion binding could be discerned for PAL/ODS self-assembled bilayers (SABs) in contact with Cd²⁺ solutions of pH = 6 even at concentrations in the sub-/μM range. Subsequent exposure of Cd²⁺/PAL/ODS SABs to solutions of pH < 4 devoid of Cd²⁺ led to the removal of Cd²⁺ from the bilayer leaving behind a better ordered structure, as judged by the shift of the C-H stretching modes toward lower wavenumbers. Spectral evidence for strong metal-terminal sulfonate group interactions was also obtained for HDSA/ODS SABs immersed in solutions containing Cd²⁺, Fe³⁺, and Fe²⁺, for which the effects on the characteristic symmetric and asymmetric modes of the terminal sulfonate moieties were found to be unique to the nature of each of the metal ions.