Exploring Self‐Assembly and Viscoelastic Behavior of Pyrene‐Based Fluorescent Hydrogel: Designing Paper Sensors for Water‐Soluble Explosives

Recently organic‐doped polyethyleneimine based hydrogels have emerged as smart responsive materials for optical sensing applications. To this end, herein we have designed and synthesized a biocompatible fluorescent hydrogel based on polyethyleneimine (PEI), incorporating pyrene as the polyaromatic signaling moiety. The self‐assembly properties of the composite materials were primarily influenced by the nature of the organic dopant, the degree of substitution, and the microenvironment. The presence of the polyaromatic scaffold, such as pyrene, not only affects the fluorescence properties of the molecules but also modulates the extent of cross‐linking by controlling π‐π stacking interactions. The hydrogel formation was observed in a methanol‐water (3% v/v) mixture. Additionally, the pyrene‐attached hydrogel demonstrated promising potential for the highly sensitive detection of trinitrophenol (TNP), a toxic, water‐soluble explosive. Spectroscopic investigations revealed that the electrostatic interaction between the ammonium units of PYR‐PEI and picrate anions in aqueous medium facilitated charge transfer from pyrene to the picrate anion, resulting in a fluorescence turn‐off response. Along with fluorometric response, using rheological experiments we show that the viscoelastic behaviour of the polymer gels also varied significantly upon addition of TNP.