Adsorbed states of substituted α-aminophosphinic acids on a silver electrode surface: comparison with a colloidal silver substrate

Abstract

We investigated the interfacial structures of various aromatic (each compound contains one or two phenyls) di-α-amino (L¹–L³) and α-amino-α-hydroxyphosphinic (L⁴–L⁶) acids immobilized onto an electrochemically roughened silver electrode surface in an aqueous solution using surface-enhanced Raman scattering (SERS). These structures were compared to those on a colloidal silver surface to determine the relationship between adsorption strength and geometry. The presence of an enhanced ν_19a ring band in the SERS spectra of L⁶, L², and L³ on the electrode indicated that the benzene rings of those molecules interact with the electrode surface through localized CC bond(s). We observed significant band broadening of the benzene ring modes for all α-hydroxyphosphinic acids on both substrates, except for L¹ deposited onto the electrode surface. This suggests the possibility of direct interaction between the ring and Ag, although the benzene ring–surface π overlap is weaker for the colloidal silver than for the Ag electrode. The downward shift in wavenumber and alternations in the enhancement of a ν₁₂ ring band indicate a general increase of tilt angle on both silver substrates in the order L³ < L⁴ < L⁵ < L⁶. The altered enhancement of the bands due to the vibrations of the NH₂ and OPO fragments, a finding observed on both silver substrates, strongly suggests that the groups interact with different strength and geometry with these substrates. Copyright © 2009 John Wiley & Sons, Ltd.