CHEMICAL indicators that change colour in response to the presence of neutral organic molecules are valuable for qualitative chemical analysis. Although some ionophores are known to exhibit colour changes on binding metal or ammonium cations1-3, however, the detection of neutral organic species in solution by this means remains problematic. We have shown recently4-7 that some fluorophore-modified cyclodextrins exhibit variations in fluorescence intensity on binding organic guests. Here we report guest-selective binding to a cyclodextrin that has been modified in such a way as to undergo a colour change on host-guest complexation. We attach the pH indicator methyl red to the wall of a β-cyclodextrin: in acidic solution, the modified cyclodextrin remains yellow owing to binding of the methyl red group inside the cavity, protecting it from protonation. When an organic guest displaces the methyl red group from the cavity, a colour change to red is observed. It should be possible to exploit the molecular recognition capability of cyclodextrins to develop a range of such 'molecular indicators'.
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