A water-soluble europium(III)-based probe, EuL, has been designed and synthesized for selective recognition of cysteine (Cys) over other structurally similar thiols and amino derivatives due to formation of a more stable and preferable seven-membered ring. Addition of Cys shows a significant re- [a] 814 Titrations with the Hcys and GSH have also been performed and are in the supporting information. (Figure S10 and S11) Relative quantum yields were determined for all three analytes Eur. J. Inorg. Chem. 2019, 813-820 www.eurjic.org