Indole-Bipyridyl Fluorophores as dual-functional probes for endoplasmic reticulum imaging and Zn2+ sensing in live cells

The endoplasmic reticulum (ER) plays a central role in protein folding, lipid biosynthesis, and calcium regulation, and its dysfunction is closely linked to diseases such as cancer, neurodegeneration, and metabolic disorders. Here, we report a new class of indole–bipyridyl (IBP) fluorophores designed for high-contrast visualisation of ER architecture and dynamic ER-associated processes in living cells. Three derivatives (IBP-1, IBP-2, IBP-3) were synthesised through a condensation–alkylation strategy and exhibit strong absorption around 420–430 nm, large Stokes shifts, and pronounced environment-sensitive emission. All derivatives display excellent biocompatibility and rapid cellular uptake, producing a distinct reticular fluorescence pattern with high colocalization to commercial ER trackers (PCC = 0.92–0.96) in HeLa and COS-7 cells. The probes sensitively report ER stress and reticulophagy, revealing treatment-dependent ER fragmentation, enhanced ER-lysosome interactions, and increased lipid-droplet formation following DTT or CCCP exposure. In addition to organelle selectivity, the bipyridyl core imparts reversible Zn²⁺ responsiveness, with the probes showing pronounced fluorescence response accompanied by spectral shifts attributed to coordination-induced electronic modulation upon Zn²⁺ binding in both solution and cellular environments. Together, these findings establish the fluorophores as versatile dual-function tools for ER imaging and Zn²⁺ sensing, offering a valuable platform for interrogating ER homeostasis, organelle crosstalk, and metal-ion–associated cellular processes.