A fluorescein-based chemosensor for “turn-on” detection of Hg2+ and the resultant complex as a fluorescent sensor for S2− in semi-aqueous medium with cell-imaging application: experimental and computational studies

Abstract

A fluorescein hydrazone based conjugate (H₂L³) was synthesized by coupling fluorescein hydrazide (L¹) with 2-(pyridin-2-ylmethoxy)-naphthalene-1-carbaldehyde (L²). H₂L³ was well characterized by several spectroscopic methods such as IR, ¹H, ¹³C-NMR and ESI-MS. The probe H₂L³ exhibited high selectivity and high sensitivity towards toxic Hg²⁺ ions in semi-aqueous medium (pH 7.2, 10 mM HEPES) over other metal ions. The significant enhancement in fluorescence emission centered at 520 nm was attributed to the Hg²⁺-induced ring opening of the spirolactam moiety in the fluorescein structure. The 1 : 1 binding of H₂L³ to Hg²⁺ was established by Job's method and confirmed by ESI-MS⁺ (m/z) studies and the binding constant was calculated as (0.43 ± 0.04) × 10⁴ M⁻¹ with a detection limit of 1.24 μM. Then again, the [Hg(HL³)]⁺ ensemble could be utilized as a reversible fluorescent sensor for S²⁻. On addition of S²⁻ to the [Hg(HL³)]⁺ complex, the fluorescence intensity was totally quenched because Hg²⁺ in the complex was grabbed by S²⁻ because of the stronger binding force between Hg²⁺ and S²⁻. The tentative coordination environment in the [Hg(HL³)]⁺ complex was established by DFT studies. The fluorescence “OFF–ON–OFF” mode of H₂L³ was examined in the presence of Hg²⁺ and S²⁻ and finds applications in devices with logic gate functions. H₂L³ also exhibits bio-compatibility and negligible cytotoxicity and is suitable for fluorescence cell imaging of Hg²⁺ ions in live HepG2 cells.