We designed a ratiometric carbohydrate sensor consisting of the boron dipyrromethene fluorophore substituted with boronic acid at the 2-position, based upon the strong substituent dependency of the absorbance/fluorescence wavelengths of BODIPY. The substituent is in equilibrium between the boronic acid B(OH)2 and boronate (B(OH)3 −) forms, which have different absorbance/fluorescence wavelengths in the visible region. Reaction of the boronic acid moiety with hydroxy groups of carbohydrate affords a cyclic ester and shifts the equilibrium in favor of the boronate (B(OR)3 −) form, resulting in a carbohydrate-concentration-dependent change of the fluorescence ratio. Thus, the sensor, BA-BODIPY, can ratiometrically detect carbohydrate at a pH near the pKa of cyclic ester formation.
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