Time-gated luminescent probes for lysosomal singlet oxygen: Synthesis, characterizations and bioimaging applications

Single oxygen (¹O₂), the molecular oxygen at its excited state, plays a crucial role in the photodynamic therapy (PDT) of some diseases owing to its strong oxidizing property to destroy malignant cells. Although the fluorescent probe technique has proven its powerful application abilities for detection of ¹O₂ in biological systems, most of the reported fluorescent probes suffered from the interference of background autofluorescence of biological samples. It is clear that the real-time and in situ, background-free fluorescent detection of ¹O₂ generated in live cells, especially in some organelles, is of great significance for understanding the action mechanism of PDT drugs.

By introducing a lysosome-anchoring motif, a morpholine moiety, into a ¹O₂-specifically-reactive terpyridine polyacid ligand, [4'-(9-anthryl)-2,2':6',2″-terpyridine-6,6″-diyl] bis(methylenenitrilo) tetrakis (acetic acid) (ATTA), and chelating with lanthanide ions (Eu³⁺ or Tb³⁺), two lanthanide complex-based "turn-on" luminescent probes that can be used for the background-free time-gated luminescent (TGL) detection of lysosomal ¹O₂, Lyso-ATTA-Eu³⁺ and Lyso-ATTA-Tb³⁺, have been developed. The probes exhibit fast luminescence responses (within 2.5 min) towards ¹O₂ with high selectivity and sensitivity (<0.75 μM) in a wide pH range (4-11). And the excellent lysosome-localization performance of the probes allowed them to be used for the monitoring of endogenous ¹O₂ in lysosomes, which enabled the variability of lysosomal-¹O₂ concentrations induced by different photosensitizers to be successfully discriminated. Furthermore, by doping Lyso-ATTA-Eu³⁺ into the polyethylene glycol (PEG) hydrogel, the smart luminescent sensor film, PEG-Lyso-ATTA-Eu³⁺, was prepared, and successfully used for the detection of the on-site ¹O₂ production during the PDT process of psoriatic disease in model mice.

Two lysosome-targetable background-free TGL probes for ¹O₂ were firstly reported. The developed smart luminescent sensor film could be a powerful tool for the clinical monitoring of PDT on skin diseases without using sophisticated and expensive instruments.