**4.3 [4′-(10-Methyl-9-anthryl)-2,2′:6′,2**′′**-terpyridine-6,6**′′**-diyl] bis(methylenenitrilo)tetrakis(acetate)-Eu3+, abbreviated as MTTA-Eu3+**

To improve the selectivity, sensitivity and rapid reaction with singlet oxygen species, Bo Song et al. introduced a new chelate complex of Eu3+, [4′-(10-methyl-9 anthryl)-2,2′:6′,2′′-terpyridine-6,6′′-diyl]bis(methylenenitrilo) tetrakis(acetate)- Eu3+, abbreviated as MTTA- Eu3+ as a 1 O2 probe [39]. MTTA- Eu3+ complex is highly water soluble and the reaction rate with 1 O2 is much faster in comparison to previously mentioned ATTA-Eu3+ (**Figure 10a** and **c**) with reaction rate constant of 1010 M−1S−1. Apart from that this complex can also be applied for a wide pH range from 3 to 10 accompanied by the large enhancement of luminescence quantum yield from 0.90% to 13.8%. The complex is almost non luminescent and when specifically reacts with 1 O2 to form the endoperoxide it becomes highly fluorescent with luminescent lifetime changes from 0.8 to 1.29 ms which makes the complex suitable for time gated luminescent measurement. The quantitative detection limit of 1 O2 using MTTA- Eu3+ complex is found to be 3.8 nM/L which is very much comparable to the detection limit of ATTA- Eu3+ complex [31, 39].
