Lanthanide probes for bioresponsive imaging

Two classes of lanthanide probes, luminescent and magnetic lanthanides, that are subsets of the larger area of metalloimaging, are reviewed. Lanthanide-based luminescent probes are particularly attractive for their long luminescence lifetimes. Lanthanides possess intrinsic luminescence that originates from electron transitions in the shell of the [Xe]5s ²5p⁶ configuration and offer unique properties for optical imaging contrast agents that address current limitations of their organic counterparts. Lanthanides are typically chelated with multidentate ligands to attenuate the toxicity of free lanthanide ions. In luminescent applications, chelation serves an additional role of protecting the metal center from solvent coordination. For use as cellular probes, the chelate must form a complex that stably saturates the Ln(III) coordination sphere over a wide pH range and resists hydrolysis.