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

Abstract

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 × 10⁵ M⁻¹ for Fe²⁺ and 0.21 × 10³ M^−1/2 for Fe³⁺ and ratiometric detection limits for Fe²⁺ and Fe³⁺ were found to be 2.0 μM and 3.5 μM, 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.