A new synthetic method produces a bioresponsive near-infrared molecular probe that undergoes “turn-on” fluorescence for microscopic imaging of hypoxia.

Two new classes of near‐infrared molecular probes were prepared and shown to exhibit “turn on” fluorescence when cleaved by the nitroreductase enzyme, a well‐known biomarker of cell hypoxia. The fluorescent probes are heptamethine cyanine dyes with a central 4‘‐carboxylic ester group on the heptamethine chain that is converted by a self‐immolative fragmentation mechanism to a 4‘‐caboxylate group that greatly enhances the fluorescence brightness. Each compound was prepared by ring opening of a Zincke salt. The chemical structures have either terminal benzoindolinenes or propargyloxy auxochromes, which provide favorable red‐shifted absorption/emission wavelengths and a hyperchromic effect that enhances the photon output when excited by 808 nm light. A fluorescent probe with terminal propargyloxy‐indolenines exhibited less self‐aggregation and was rapidly activated by nitroreductase with large “turn on“ fluorescence; thus, it is the preferred choice for translation towards in vivo applications.

A high-affinity supramolecular dye-capture system with “turn on” visible fluorescence is used to create a new assay for phospholipase-mediated liposome leakage.

The Front Cover illustrates the action of a cyanine‐based fluorescent probe inside a hypoxic A549 cell. Nitroreductase enzyme, a well‐known biomarker of hypoxia, transforms a carboxylic ester group on the central heptamethine chain through a self‐immolative fragmentation mechanism into a brightly fluorescent carboxylate group, resulting in a “turn on” fluorescent response. The cell elements were provided by Reactome.org under the CC BY 4.0 license. More information can be found in the Research Article by P. Štacko, B. D. Smith et al.