The boronic acid-amine (B-N) interaction was applied to the molecular design of a visualized sensing system for saccharides. 3-Nitrophenyl-boronic acid (3b) interacts with the pyridine nitrogen of 4-(4-dimethylaminophenyl-azo) pyridine (2) in methanol and changes its color from yellow to orange. Added saccharides form complexes with 3b and enhance the acidity of the boronic acid group. As a result, the B-N interaction becomes stronger and the intensified intramolecular charge-transfer band changes the solution color to red. By combined 1H-NMR and absorption spectroscopic studies, four association constants governing the present ternary complex system have been determined. In case of D-fructose, the 3b-D-fructose complex can interact with 2 240 times more strongly than 3b and the 2-3b complex can associate D-fructose 241 times more strongly than 3b. Clearly, the B-N interaction not only enables the colorimetric saccharide sensing but also enhances the association ability (i.e., sensitivity). Thus, the present paper offers a novel basic concept for molecular design of visualized saccharide sensing systems.
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