Analysis of squaric acid and its complex with copper(II) ions adsorbed on silver nanoparticle surfaces by surface-enhanced Raman spectroscopy

Surface-enhanced Raman scattering (SERS) spectra of squaric acid (SA) and its complex with Cu(II) ion have been obtained on silver nanoparticle (AgNP) surfaces. The spectral deconvolution of the band assigned to symmetric carbonyl stretching showed the presence of two distinct coordination modes in the surface complex on AgNP even at high concentrations of Cu(II) ion. Additionally, in the presence of Cu(II), two bands have been observed in the low wavenumber range assigned to the AgO and Cu(II)O stretching vibrations. An increase of the squaric acid-Cu(II) complex signal has also been shown with the increase of the cation concentration up to a molar ratio of 1:1 (SA:Cu(II)). At higher concentrations of Cu(II) ions the intensity ratio of the features assigned to the symmetric CO stretching for SAAg and Cu(II)-SA-Ag species was kept approximately constant. A relative population analysis suggests that an equilibrium between the species contributing to the SERS signal has been established, with the saturation of the surface with the Cu(II)-SA-Ag surface complex. The SERS analysis used here is a facile method for the detection and structural characterization of coordinated systems derived from squaric acid adsorption on SERS-active silver surfaces.