Cellular Membrane-Targeted Europium(III) Probe for Selective and Time-Resolved Detection of Peroxynitrite

Peroxynitrite (ONOO^-) is an extremely short-lived and highly reactive species whose biological effects depend on its local concentration at the site of formation. Consequently, detecting and quantifying ONOO^- requires probes that combine high selectivity with well-defined subcellular localization. The cell membrane is a particularly important yet underexplored interface, as it is the first barrier encountered by extracellularly generated ONOO^- and determines how much of this species penetrates into the cell. Such membrane-proximal ONOO^- formation occurs during inflammatory processes and oxidative stress-based therapies, and is especially relevant in cold atmospheric plasma treatment, where complex RONS mixtures interact with the membrane before intracellular effects arise. Here, we report Eu.Chol, a luminescent europium(III) complex that integrates a phenylboronic acid ONOO^- reactive motif with a cholesterol anchor to localize the probe within lipid bilayers. We demonstrate that Eu.Chol incorporates efficiently into liposomes and Jurkat cells, enabling selective, membrane-localized ONOO^- detection with minimal response to H₂O₂. These results establish a modular europium(III)-based scaffold for targeted ONOO^- sensing, enabling potential incorporation of organelle-specific targeting groups to probe RONS dynamics at defined sub-cellular sites.