Two novel lanthanide complex-based luminescent probes, ANMTTA-Eu(3+) and ANMTTA-Tb(3+) {ANMTTA, [4'-(4-amino-3-nitrophenoxy)methylene-2,2':6',2"-terpyridine-6,6"-diyl] bis(methylenenitrilo) tetrakis(acetic acid)}, have been designed and synthesized for the highly sensitive and selective time-gated luminescence detection of hypochlorous acid (HOCl) in aqueous media. The probes are almost nonluminescent due to the photoinduced electron transfer (PET) process from the 4-amino-3-nitrophenyl moiety to the terpyridine-Ln(3+) moiety, which quenches the lanthanide luminescence effectively. Upon reaction with HOCl, the 4-amino-3-nitrophenyl moiety is rapidly cleaved from the probe complexes, which affords strongly luminescent lanthanide complexes HTTA-Eu(3+) and HTTA-Tb(3+) {HTTA, (4'-hydroxymethyl-2,2':6',2"-terpyridine-6,6"-diyl) bis(methylenenitrilo) tetrakis(acetic acid)}, accompanied by the remarkable luminescence enhancements. The dose-dependent luminescence enhancements show good linearity with detection limits of 1.3 nM and 0.64 nM for HOCl with ANMTTA-Eu(3+) and ANMTTA-Tb(3+), respectively. In addition, the luminescence responses of ANMTTA-Eu(3+) and ANMTTA-Tb(3+) to HOCl are pH-independent with excellent selectivity to distinguish HOCl from other reactive oxygen/nitrogen species (ROS/RNS). The ANMTTA-Ln(3+)-loaded HeLa and RAW 264.7 macrophage cells were prepared, and then the exogenous HOCl in HeLa cells and endogenous HOCl in macrophage cells were successfully imaged with time-gated luminescence mode. The results demonstrated the practical applicability of the probes for the cell imaging application.
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.
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