Sugar binding enhances fluorescence by strengthening the boron–nitrogen. Lewis acid–base interaction, which suppresses photoinduced electron transfer and quenched emission. As a result, these systems exhibit enhanced fluorescence in neutral aqueous solutions upon sugar binding. The use of substituted aryl boronic acids as chelating agents for saccharides has drawn significant interest in designing novel sugar sensing approaches. Over the past decade, numerous fluorescent probes for monosaccharides based on boronic acids have been developed. Three new diphenylazo-based boronic acid compounds (1–3) were synthesized. The sensing performance of the synthesized compounds toward glucose and fructose was investigated using UV-visible spectroscopy, fluorescence, time-resolved fluorescence, pH studies, cyclic voltammetry, and 1H NMR techniques. Upon the addition of sugars, changes were observed in both the absorption and fluorescence intensities of compounds 1, 2, and 3. The Benesi–Hildebrand plots yielded straight lines, indicating the formation of complexes between the sugars and the sensor molecules. The quantum yield and fluorescence lifetime values of the compounds in glucose and fructose media were higher than those in water, suggesting their sugar-sensing capabilities. At higher pH, the boronic acid group exists in its anionic form B(OH)-3, leading to a change in the boron atom’s configuration from sp² to sp³, thereby facilitating binding with sugar molecules. A possible sugar-sensing mechanism is proposed. The conversion of the boron atom from an sp² to an sp³ configuration upon sugar binding is believed to cause the enhancement in fluorescence. Additionally, the observed increase in quantum yield and fluorescence lifetime confirms the interaction between the saccharide and probe molecules in the excited state.
Suresh et al. (Tue,) studied this question.