Water-soluble hydroxypropyl β-cyclodextrin was converted to acetonyl hydroxypropyl β-cyclodextrin via selective oxidation. Ethylenediamine was used as a flexible chain as well as a connecter to get water-soluble methyl red-modified hydroxypropyl β-cyclodextrin. When hydrophobic molecules such as cyclohexane were added in the diluted solution of the methyl red-modified hydroxypropyl β- cyclodextrin (6 * 10~(-8) to 2 * 10~(-5) mol·dm~(-3)), the methyl red moiety could be pressed out of the cavity of the β-cyclodextrin. This variation would be accompanied with color changing from yellow to red in an acidic solution due to the diazenediyl protaonation of the methyl red moiety. The color changing is influenced by the structure, bulkage, polarity etc. of the guest molecules, the methyl red-modified hydroxypropyl β-cyclodextrin can thus be used as a water-soluble color changing molecular indicator.

Key words: CYCLODEXTRIN, WATER SOLUBILITY, molecular indicator