Molecular design of amphiphiles for Microenvironment-Sensitive kinetically controlled gelation and their utility in probing alcohol contents

Abstract

Pyrene-laced ‘fluorescent’ amphiphiles have been designed with a C4-alkyl chain as ‘hydrophobic spacer’ and sugar moiety as hydrophilic residue. The amphiphile containing acetylacetohydrazide as a bridging unit (1) showed gel formation both in water and ethanol medium, while no gelation was observed with compound (2) with acetohydrazide as bridging unit. Accordingly, the self-assembly behavior of 1 was thoroughly investigated both in water and EtOH medium (as well as in their mixtures) in addition to their physicochemical properties. Though the gel formation kinetics was slower in water medium, the hydrogel showed superior mechanical strength (and thixotropic behavior) and improved thermal stability compared to the ethanolic gel. In water, the percolating H-bonding network with high cohesive force restricts the dynamics of the probe molecules, leading to some extent limited intermolecular interactions. However, hydrophobic interactions in water medium can result in the formation of rather ‘compact’ nano-assembly (H-type aggregation) with prominent π-π stacking interactions. Such unique solvent-dependency of 1 was further utilized to screen commercially available beverages in terms of their alcohol contents and probe adulteration in beverages. Besides, the hydrogel also showed excellent stimuli responsiveness, a gel-to-sol transition, accompanied by fluorescence color change, was observed in the presence of insulin.