A ratiometric luminescence probe for highly reactive oxygen species based on lanthanide complexes

Reactive oxygen species (ROS) are important mediators in a variety of pathological events, but the oxidative stress owing to excessive generation of ROS is implicated in many human diseases. In this work, we designed and synthesized a novel dual-functional chelating ligand, [4'-(p-aminophenoxy)methylene-2,2':6',2''-terpyridine-6,6''-diyl]bis(methylenenitrilo)tetrakis(acetic acid) (AMTTA), that can strongly coordinate with both Eu(3+) and Tb(3+) in aqueous solutions for the recognition and time-gated luminescence detection of highly ROS (hROS), hydroxyl radical ((•)OH), and hypochlorite (ClO(-)). The complexes AMTTA-Ln(3+) (Ln = Eu and Tb) are almost nonluminescent because of the photoinduced electron transfer from the electron-rich aminophenyl group to the terpyridine-Ln(3+) moiety but can rapidly react with hROS to afford highly luminescent complexes (4'-hydroxymethyl-2,2':6',2''-terpyridine-6,6''-diyl)bis(methylenenitrilo)tetrakis(acetate)-Ln(3+) (HTTA-Ln(3+)). Interestingly, when the AMTTA-Eu(3+)/Tb(3+) mixture (AMTTA/Eu(3+)/Tb(3+) = 2/1/1) was reacted with hROS, the intensity ratio of its Tb(3+) emission at 540 nm to its Eu(3+) emission at 610 nm, I(540)/I(610), showed a ratiometric response toward hROS, and the dose-dependent increase of the ratio displayed a double-exponential correlation to the concentration of hROS. This unique luminescence response allowed the AMTTA-Eu(3+)/Tb(3+) mixture to be used as a ratiometric probe for the time-gated luminescence detection of hROS.