In Situ FTIR-ATR Studies of Functionalized Self-Assembled Bilayer Interactions with Metal Ions in Aqueous Solutions

Abstract

The affinity of selected metal ions for palmitic (PAL) and hexadecanesulfonic acid (HDSA) monolayers self-assembled onto an octadecyl silane (ODS) monolayer 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 metal-ion 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.