Colorimetric sensing of Cu2+ using a cyclodextrin–dye rotaxane

A dye rotaxane having 8-hydroxyquinoline as coupling component was prepared in the presence of α-cyclodextrin and its structure identified using NMR. The dye rotaxane displayed high color strength regardless of the solvent used and responded to changes in pH based on tautomeric equilibrium, without any interference by the cyclodextrin macrocycle. Compared to the non-rotaxanated forms, the dye rotaxane displayed a characteristic spectral response to the presence of Cu2+ which was attributed to the formation of a polymeric, metal–dye rotaxane complex which was of high solubility because of the presence of cyclodextrin. Thus, this approach can clearly be used as a facile and efficient sensor for copper ions.