**4. Rare earth chelate based luminescence probes**

Rare-earth based chelate luminescence probe can also serve significant role in the detection of singlet oxygen. In comparison to organic fluorescence probe rare earth based chelate probes exhibits markedly different advantages including long luminescence lifetime, sharp emission profile, large Stokes shift, which makes them suitable to use in time-resolved luminescence detection of singlet oxygen [36]. Large Stokes shift helps to reduce the influence caused by excitation light effectively. Time resolved luminescence measurement using rare-earth-chelated probe can efficiently reduce the interference originated from the background noise associated with biological sample, scattering lights (Tyndall, Rayleigh and Raman scattering) and the optical components [16]. Rare earth chelate probes which are successfully applied for the detection of singlet oxygen including Eu, Tb and Re based chelate complexes are well-known [16, 31, 37].
