Label-free 3D cell imaging using hydrogels functionalized with switchable iridium complexes.

The use of fluorescent labels is the most common tool to visualize cells. However, the internalization of dye molecules often modifies the cell behavior. In this paper we demonstrate that it is possible to transiently label cells using a 3D scaffold, a hydrogel, covalently functionalized with luminescent cyclometalated iridium(III) complexes. The unique feature of our design is that the complexes are emissive only when they interact with the cell membrane while their emission is quenched in water. We exploited this feature to perform real-time and staining-free cell visualization and imaging. The hydrogels immerged in culture media are non-luminescent, however, when cells are added to it, they interact with the iridium complexes, covalently linked to the gel, and their lipophilic membrane "switches on" the luminescence enabling a clear and dynamic, real-time 3D visualization of cell proliferation. A complete photophysical and biological study of the materials is presented which demonstrates the potential of our methodology for 3D-realtime cell tracking.