Azobenzene-caged sulforhodamine dyes: a novel class of 'turn-on' reactive probes for hypoxic tumor cell imaging

New sulforhodamine-based fluorescent 'turn-on' probes have been developed for the direct imaging of cellular hypoxia. Rapid access to this novel class of water-soluble 'azobenzene-caged' fluorophores was made possible through an easily-implementable azo-coupling reaction between a fluorescent primary arylamine derived from a sulforhodamine 101 scaffold (named SR101-NaphtNH(2)) and a tertiary aniline whose N-substituents are neutral, cationic, or zwitterionic. The detection mechanism is based on the bioreductive cleavage of the azo bond that restores strong far-red fluorescence (emission maximum at 625 nm) by regenerating the original sulforhodamine SR101-NaphtNH(2). This valuable fluorogenic response was obtained for the three 'smart' probes studied in this work, as shown by an in vitro assay using rat liver microsomes placed under aerobic and then under hypoxic conditions. Most importantly, the probe namely SR101-NaphtNH(2)-Hyp-diMe was successfully applied for imaging the hypoxic status of tumor cells (A549 cells).