Naphthalene Based Azo Dyes and Its Substituted Derivatives Containing Mono Boronic Acid - Saccharide Sensors

Three novel naphthalene-based azo dyes incorporating boronic acid functional groups were synthesized. Their sugar-sensing behavior toward glucose and fructose was systematically investigated using UV-visible, fluorescence, time-resolved fluorescence, pH titration, and cyclic voltammetry techniques. Upon increasing the concentration of the sugars, both the absorbance and fluorescence intensities of the dyes decreased, indicating effective interaction. In the excited state, the dyes exhibited stronger sensing responses to fructose compared to glucose. The fluorescence lifetime measurements further confirmed the compounds' capability to detect sugars. At elevated pH levels, the boronic acid groups exist predominantly in their anionic form [B(OH)<sub>3</sub><sup>-</sup>], which induces a change in the boron atom's hybridization from sp<sup>2</sup> to sp<sup>3</sup>, facilitating binding with sugar molecules. Among the three compounds, compound 1 exhibited the highest association constant with fructose, suggesting a stronger binding affinity is higher than compared to glucose. To further validate the sugar-sensing behavior, the quantum yields of the compounds were measured in pure water, glucose, and fructose solutions. When higher concentrations of the sensor were introduced into the sugar solution, the oxidation peaks current (Ipa) decreased while the reduction peak current (Ipc) increased. In contrast, Ipa increased in the sensor-only solutions. Based on these observations, a plausible sensing mechanism has been proposed.