Assessment of ternary iron-cyclodextrin-2-naphthol complexes using NMR and fluorescence spectroscopies

Recent research has indicated that ternary complexes can be formed among carboxymethyl-β-cyclodextrin, certain polycyclic aromatic hydrocarbons (PAHs) (e.g. anthracene and 2-naphthol), and Fe2+ in aqueous solution. The formation of these ternary complexes has been suggested as the reason for improved reaction efficiency in iron catalyzed Fenton degradation (H2O2 + Fe2+ → OH + OH− + Fe3+) of PAHs and other pollutants. In the present work, several other cyclodextrins were examined to determine their ability to form similar ternary complexes with 2-naphthol and Fe2+. Fluorescence and NMR techniques were employed in this study. Results showed that hydroxypropyl-β-cyclodextrin, β-cyclodextrin, and α-cyclodextrin were able to encapsulate 2-naphthol molecules, but their binding with Fe2+ was weak. On the contrary, sulfated-β-cyclodextrin has significant binding with Fe2+, but it showed little inclusion of 2-naphthol molecules. Consequently, none of these four cyclodextrins formed significant amounts of ternary complexes in aqueous solution. The techniques used in this study provide useful methods for assessing the ability of cyclodextrins to form ternary complexes with guest compounds and metal ions.