8-TQEN (N,N,N′,N′-tetrakis(8-quinolylmethyl)ethylenediamine) analogs as fluorescent cadmium sensors: strategies to enhance Cd2+-induced fluorescence and Cd2+/Zn2+ selectivity

In order to exploit the untapped sensing potential of the TQEN (N,N,N',N'-tetrakis(2-quinolylmethyl)ethylenediamine) family of fluorescent probes, 8-TQEN (N,N,N',N'-tetrakis(8-quinolylmethyl)ethylenediamine) analogs were designed and characterized. Although 8-TQEN lacks practicality owing to poor solubility in both aqueous media and organic solvents, 6-MeO-8-TQEN (N,N,N',N'-tetrakis(6-methoxy-8-quinolylmethyl) ethylenediamine, 1b) exhibits Cd2+-selective fluorescence enhancement at 395 nm in DMF-HEPES buffer (1 : 1), (I-Cd/I-0 = 25, lambda(ex) = 332 nm, phi(Cd) = 0.025). Zn2+ induces weaker fluorescence (I-Zn/I-0 = 9, I-Zn/I-Cd = 0.35). In contrast, both the parent probes TQEN (I-Zn/I-0 = 23, I-Cd/I-Zn = 0.64) and 6-MeOTQEN (I-Zn/I-0 = 11, I-Cd/I-Zn = 1.29 exhibit higher sensitivity toward Zn2+. When the quinoline groups were replaced with 8-hydroxyquinoline different responses were observed. The propanediamine derivative, 8-TQOEPN (N,N,N',N'-tetrakis(8-quinolyloxyethylene)propanediamine, 2c) exhibits significant fluorescence enhancement at 423 nm upon Cd2+ binding (lambda(ex) = 325 nm, I-Cd/I-0 = 19, phi(Cd) = 0.31). Fluorescence enhancement is Cd2+-specific as Zn2+ induces only more modest emission increases (I-Zn/I-0 = 8.8, I-Zn/I-Cd = 0.45).