Identification of Al₁₃ on the Colloid Surface Using Surface-Enhanced Raman Spectroscopy

Al₁₃ is the most active polymeric Al species responsible for coagulation at the solid-liquid interface, whereas the detection techniques for Al₁₃ at the interface are currently limited. In this study, for the first time, the identification of Al₁₃ on the silicon dioxide-based colloid surface was realized by using surface-enhanced Raman scattering (SERS), which is an ideal surface method sensitive for single-molecule detection. The high purity Al₁₃ salts were prepared by an electrolysis procedure followed by precipitation or metathesis. Al₁₃-Cl_n was determined to be feasible for the Raman detection as it exhibited more noticeable signals in comparison to Al₁₃-(SO₄)_p and Al₁₃-(NO₃)_m. The peak of Al₁₃-Cl_n at 635 cm^-1 could be the major characteristic peak of Al₁₃, and the other two peaks at 300 and 987 cm^-1 could be accessorial evidence for the identification. Further, the identification of Al₁₃ adsorbed on the surface of Ag and gold-core/silica-shell colloids was confirmed by the SERS response at the above three wavenumbers with a higher signal-to-noise ratio than the normal Raman scattering. According to the least-squares fitting computed Raman spectra, each of the characteristic peaks was associated with specific vibrational modes.