A supramolecular carbohydrate sensor was prepared by self-assembly of unilamellar bilayer vesicles of amphiphilic β-cyclodextrin with a boronic acid-adamantane conjugate (PBA-AD), which binds strongly to β-cyclodextrin through host-guest interactions (Ka ≈4×104 m-1 ), so the vesicle surface exhibits multiple boronic acid receptors for carbohydrates. The binding of diols to the functionalized vesicles was tested with alizarin red S (ARS) as a reporter dye by using fluorescence and UV/Vis spectroscopy. Analysis of the competitive binding of monosaccharides revealed pH-dependent (pH 7.4-10.1) binding constants in the range of 100-3000 m-1 for d-fructose and 5-400 m-1 for d-glucose. Interestingly, the self-assembled sensor showed fluorescence intensity enhanced by about fivefold and a binding affinity to the reporter dye increased by about eightfold in comparison to PBA-AD and ARS without vesicles. This is attributed to increased local concentration of ARS and PBA-AD on the surface of the vesicles. Detection of d-fructose and d-glucose was possible in the physiologically relevant range in dilute aqueous solution.
Keywords: boronic acids; carbohydrates; host-guest systems; sensors; vesicles.
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