Ratiometric fluorescence sensing of d-allulose using an inclusion complex of γ-cyclodextrin with a benzoxaborole-based probe

Abstract

Because D-allulose has been attracting attention as a zero-calorie sugar, the selective sensing of D-allulose is desired to investigate its health benefits. We report herein a novel fluorescence chemosensor that is based on an inclusion complex of γ-cyclodextrin (γ-CyD) with a benzoxaborole-based probe. Two inclusion complexes, 1/γCyD and 2/γCyD, were prepared by mixing γ-CyD with their corresponding probes in a water-rich solvent, where γ-CyD encapsulates two molecules of the probes inside its cavity to form a pyrene dimer. Both 1/γCyD and 2/γCyD exhibit monomeric and dimeric fluorescence from the pyrene moieties. By the reaction of 1/γCyD with saccharides, the intensities of monomeric and dimeric fluorescence remained unchanged and decreased, respectively. We have demonstrated that 1/γCyD has much higher affinity for D-allulose than for the other saccharides (D-fructose, D-glucose, and D-galactose). The conditional equilibrium constants for the reaction systems were determined to be 498 ± 35 M⁻¹ for D-fructose, 48.4 ± 25.3 M⁻¹ for D-glucose, 15.0 ± 3.3 M⁻¹ for D-galactose, and (8.05 ± 0.59) × 10³ M⁻¹ for D-allulose. These features of 1/γCyD enable ratiometric fluorescence sensing with high sensitivity and selectivity for D-allulose. The limits of detection and quantification of 1/γCyD for D-allulose at pH 8.0 were determined to be 6.9 and 21 μM, respectively. Induced circular dichroism spectral study has shown that the reaction of 1/γCyD with D-allulose causes the monomerisation of the dimer of probe 1 that is encapsulated by γ-CyD, which leads to the diminishment of the dimeric fluorescence.