To direct the self-assembly of AIEgens by three-gear switch: Morphology study, amine sensing and assessment of meat spoilage

Abstract In this study, we synthesized a class of positional isomers by attaching a carboxylic group in different sites (para-, meta- and ortho-position) of an aromatic core. The isomers undergo both aggregation-induced emission (AIE) and intramolecular charge transfer (ICT) mechanisms, as proved by their photoluminescence (PL) behaviour. Changing the position of the carboxylic group will adjust the dipole–dipole direction, which further controls the self-assembled architecture: The morphology undergoes a transition from 1D nanowire to 2D microsheet, and even to 3D microcube. The benzoic acid moiety of the AIE isomers has strong affinity to amines, and adsorbing amines leads to spectral changes including hypochromatic shift and emission quenching. The sensing mechanism was studied by density functional theory (DFT), which reveals that the amine-responsive fluorescence can be ascribed to the weakened ICT effect. A prototype of amine sensor based on m-DB self-assemblies was developed accordingly, showing a remarkable quenching efficiency as high as 85.4%. A linear relation was obtained from the quantitative determination of amine, offering a low detection limit of 2.02 Pa. It can also respond to other organic amines such as diisopropylamine (DIPA), triethylamine (TEA), isopropylamine (IPA) and cyclohexylamine (CHA). Finally, the practicality of the sensor was also demonstrated: When placing the sensing film beside the spoiled meat, the spectrum shows a remarkable fluorescence quenching; in contrast, fresh meat samples have induced a much lower quenching efficiency. This demonstrates that the AIE isomers have a great potential to serve as indicators for meat spoilage.