Abstract

beta-Cyclodextrin (beta-CD) is a nontoxic cyclic oligosachcharide that can encapsulate all or part of organic molecules of appropriate size and specific shape through noncovalent interaction. Herein, we report the influence of beta-CD complex formation of an antipyrine derivative on its metal ion sensing behavior. In aqueous solution, the antipyrine shows a turn-on fluorescence sensing of vanadyl ion, and in cyclodextrin medium it senses aluminum ion. The compound shows an unusual fluorescence quenching on binding with beta-cyclodextrin (log K-SV = 2.34 +/- 0.02). The differential metal ion sensing is due to the partial blocking of the chelating moiety by the cyclodextrin molecule. The structure of the antipyrine-cyclodextrin complex is optimized by two-dimensional rotating-frame Overhauser effect spectroscopy. The binding constant is determined by isothermal titration calorimetry (log K = 2.09 +/- 0.004). The metal ion binding site is optimized by quanutm mechanical calculations. The lower limit of detection of vanadyl and aluminum ions, respectively, are 5 X 10(-8) and 5 X 10(-8) mol dm(-3). This is the first report of selectivity of two different cations by a chemosensor in water and in beta-CD.