Development of color-change indicators using inclusion complex formation

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Cyclodextrins (CDs) are spectroscopically inert, but they can be converted into spectroscopically active compounds by modification with cheomophores. In this study, we prepared several dye-modified CDs to construct color-change indicators for molecules.We found that the chromophore-modified CDs change the location of the chromophore from their own hydrophobic cavity to the outside water environment upon guest accommodation. This environmental change around the chromophore moiety usually causes perturbation in its spectroscopic property. On this basis, methyl-red modified CD changes color from yellow to red upon guest binding in acidic media. On the other hand, p-nitrophenol-modified alpha-CD and beta-CD undergo guest-induced color (yellow)-to-colorless change under neutral pH conditions. Futhermore, we prepared phenolphthalein-modified beta-CD,which also undergoes color (purple)-to-colorless change upon guest binding. All these hosts exhibit different affinities for guest species, and consequently they can be used as color-change indicators for molecules.