A ratiometric fluorescent chemosensor for iron: discrimination of Fe2+ and Fe3+ and living cell application

A newly designed probe, 6-thiophen-2-yl-5,6-dihydrobenzo[4,5]imidazo-[1,2-c] quinazoline (HL1) behaves as a highly selective ratiometric fluorescent sensor for Fe2+ at pH 4.0-5.0 and Fe3+ at pH 6.5-8.0 in acetonitrile-HEPES buffer (1/4) (v/v) medium. A decrease in fluorescence at 412 nm and increase in fluorescence at 472 nm with an isoemissive point at 436 nm with the addition of Fe2+ salt solution is due to the formation of mononuclear Fe2+ complex [Fe-II(HL)(ClO4)(2)(CH3CN)(2)] (1) in acetonitrile-HEPES buffer (100 mM, 1/4, v/v) at pH 4.5 and a decrease in fluorescence at 412 nm and increase in fluorescence at 482 nm with an isoemissive point at 445 nm during titration by Fe3+ salt due to the formation of binary Fe3+ complex, [Fe-III(L)(2)(ClO4)(H2O)] (2) with co-solvent at biological pH 7.4 have been established. Binding constants (K-a) in the solution state were calculated to be 3.88 x 10(5) M-1 for Fe2+ and 0.21 x 10(3) M-1/2 for Fe3+ and ratiometric detection limits for Fe2+ and Fe3+ were found to be 2.0 mu M and 3.5 mu M, respectively. The probe is a naked eye chemosensor for two states of iron. Theoretical calculations were studied to establish the configurations of probe-iron complexes. The sensor is efficient for detecting Fe3+ in vitro by developing a good image of the biological organelles.