The phenylboronic acid azoprobe (BA-Azo)/gamma-cyclodextrin (gamma-CD) complex exhibits a selective response for D-glucose by forming a supramolecular 2:1 inclusion complex of the azoprobes with D-glucose inside the gamma-CD cavity.

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Sugars are essential biological molecules that play fundamental roles in metabolism and the maintenance of cell structure. They are also related to cell differentiation and immunity.1 Because of their important properties, researchers need simple methods that can quickly recognize sugars in aqueous solution. Enzymebased sensors are highly selective, but of limited use for continuous monitoring and in vivo analysis because of their poor stability and because they consume their substrate in the recognition process. Although chemosensors based on boronic acid are not as selective, they have attracted much attention because of their greater stability. 2,3 Phenylboronic acid can readily form stable cyclic esters with the diol moiety of sugars in water, 4 and various types of the boronic acid chemosensors with response mechanism based on internal charge transfer (ICT), 5-8 photoinduced electron transfer (PET), 9-11 and fluorescent resonance energy transfer (FRET) 12 have been developed. The versatile designs of chemosensors based on supramolecular chemistry are another approach to construct novel sugar sensors. [13][14][15] We reported that the complex consisting of the boronic acid fluorophore (C4-CPB) and β-cyclodextrin (β-CD) acts as a supramolecular saccharide sensor (Fig. 1). 16 The β-CD solubilizes the water-insoluble C4-CPB by forming an inclusion complex. The fluorescence response of this complex is based on the PET mechanism. The inherent fluorescence of C4-CPB is quenched by the internal PET from pyrene to the trigonal form of phenylboronic acid, but saccharide binding converts the boronic acid to the tetrahedral boronate, thereby inhibiting PET quenching and increasing the fluorescence intensity. In a previous study, the spacer effect of the boronic acid probe was examined in detail by using the fluorophores C1-CPB, C4-CPB, and C1-APB (Fig. 1). We found that the C1-APB/β-CD complex was a desirable fluorescent chemosensor for saccharide recognition because of its high affinity to saccharides as well as its relatively high fluorescence recovery upon saccharide binding. Since C1-APB is more soluble than C1-CPB and C4-CPB, because of possessing an amide group, it can be dissolved in water without adding β-CD at the level for the fluorescent measurement. This makes a precise comparison of the CD effect feasible. In the present work, therefore, we focused on C1-APB/CD complexes, and investigated how the sensing function of C1-APB for saccharide recognition in water was affected by differences in An inclusion complex consisting of a fluorescent phenylboronic acid (C1-APB) and β-cyclodextrin (β-CD) acts as a supramolecular saccharide sensor whose response mechanism is based on photoinduced electron transfer (PET). This study evaluated four kinds of cyclodextrins (α-CD, β-CD, γ-CD, and NH2-β-CD) by comparing their pH profiles, and confirmed that β-CD was the best host for C1-APB because the C1-APB/β-CD complex exhibited high affinity for saccharides as well as high fluorescent recovery upon saccharide binding. An ...

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