Cell viability imaging in tumor spheroids via DNA binding of a ruthenium(II) light-switch complex.

The famous ''light-switch'' ruthenium complex [Ru(bpy) 2 (dppz)](PF 6 ) 2 (1) has been long known for its DNA binding properties in vitro . However, the biological utility of this compound has been hampered by its poor cellular uptake in living cells. Here we report a bioimaging application of 1 as cell viability probe in both 2D cells monolayer and 3D multi-cellular tumor spheroids of various human cancer cell lines (U87, HepG2, A549). When compared to propidium iodide, a routinely used cell viability probe, 1 was found to enhance the staining of dead cells in particular in tumor spheroids. 1 has high photostability, longer Stokes shift, and displays lower cytotoxicity compared to propidium iodide, which is a known carcinogenic. Finally, 1 was also found to displace the classical DNA binding dye Hoechst in dead cells, which makes it a promising dye for time-dependent imaging of dead cells in cell cultures, including multi-cellular tumor spheroids.