Design and synthesis of a methylation-modified bifunctional fluorescent probe with Tunable ACQ-AIE transition for ultrasensitive detection of trace water and polarity in lipid droplets and lysosomes.

AIE-based biological probes have attracted extensive research attention owing to their remarkable merits, such as high fluorescence brightness, long-term in-situ retention capacity, outstanding photostability and low cytotoxicity. Meanwhile, lipid droplets (LDs) have been proven to participate in a variety of vital biological processes. Accordingly, we rationally designed and synthesized an intramolecular charge transfer (ICT) fluorescent probe Py-TM featuring typical aggregation-caused quenching (ACQ) characteristics, and further modified its molecular skeleton to construct an AIE-active probe Py-TM-Me. The as-synthesized Py-TM-Me exhibits excellent polarity responsiveness, a large Stokes shift, favorable photostability and superior biocompatibility. It also possesses high detection sensitivity and precise lipid droplet-targeting capacity, enabling high-contrast fluorescence imaging of intracellular lipid droplets, and simultaneously shows specific lysosome-targeting performance. Moreover, Py-TM-Me can be applied to monitor intracellular microenvironmental polarity fluctuations and in vivo fluorescence imaging in zebrafish, which offers a novel tool for future investigations on human lipid droplet-related diseases.