Fiber-Optic Raman Spectroscopy for in Situ Monitoring of Metal-Ion Complexation by Ligands Immobilized onto Silica Gel

Fiber-optic Raman spectroscopy was adapted to measurements of metal-ion complexation by silica-immobilized 8-hydroxyquinoline (8HQ) under conditions typical of metal-ion preconcentration or recovery applications. The extraction of metal ions from aqueous sources onto immobilized ligands is useful in environmental cleanup initiatives and in the preconcentration of metal ions for ultratrace detection. In situ vibrational spectroscopy of metal complexation by silica-immobilized ligands can assist in the development of these applications. In the present work, in situ Raman spectroscopy and flow methods allowed the reactivity of silica-immobilized 8HQ with Cu²⁺ to be investigated. The structure of the complexed and uncomplexed neutral and protonated forms of the ligand could be resolved and quantified in Raman spectra taken under different solution conditions. Spectra of the ligand bound to different metal ions could be distinguished as well. Raman spectra taken under batch sampling conditions were able to confirm a 1:1 metal/ligand stoichiometry of the silica-immobilized 8HQ complex, which was reasonable in light of the surface coverage by the ligand. In situ monitoring under flowing solution conditions allowed changes in the state of binding to be followed in real time.