ATR-FTIR spectroscopy as a probe for metal ion binding onto immobilized ligands

ATR-FTIR spectroscopy is shown to be useful in probing metal ion binding to ligands immobilized on polymer beads. Lu(III) is the metal ion, phosphinic and sulfonic acids are the immobilized ligands, and polystyrene beads crosslinked at low and high levels are the support. Analysis is based on ligand band positions and intensities before and after complexation as a function of time. ATR spectra of the phosphinic acid show two types of binding sites with different accessibilities: surface ligands that bind Lu(III) from solution and interior ligands that gain ions by diffusion from the surface. Initially bound Lu(III) ions are mobile within the solid phase and their movement is accompanied by the breaking of hydrogen bonds among the phosphinic acid ligands to form Lu(III) phosphinate complexes. Competition between hydrogen bonding and bond-making with Lu(III) contributes to slow sorption rates compared to results from the sulfonic acid ligand whose strong acidity and ionizability allows rapid Lu(III) sorption. Comparing ATR spectra of the phosphinic acid bound to lightly- and highly-crosslinked polystyrene shows how the extent of crosslinking affects the transport mechanism.