The interaction of Hg²⁺ and trivalent ions with two new fluorescein bio-inspired dual colorimetric/fluorimetric probes

Two new luminescent compounds containing fluorescein-amino acid units have been designed and synthesized via an ester linkage between a fluorescein ethyl ester and Boc-Ser(TBDMS)-OH or Boc-Cys(4-MeBzl)-OH, and their photophysical properties have been explored. The optical response of both compounds (2 and 3) towards the metal ions Na⁺, K⁺, Hg⁺, Ag⁺, Ca²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Pb²⁺, Hg²⁺, Al³⁺, Fe³⁺, Ga³⁺and Cr³⁺ was investigated in pure acetonitrile and in acetonitrile/water mixtures. A strong CHEF (Chelation-Enhanced Fluorescence) effect was observed with all the trivalent metals and Hg²⁺ ions in both solvents. UV-vis absorption, steady state and time resolved emission spectroscopy methods were employed. The results show the formation of mononuclear complexes with Al³⁺, Fe³⁺, Ga³⁺, Cr³⁺, and Hg²⁺. Theoretical calculation using Density Functional Theory was performed in order to obtain atomistic insights into the coordination geometry of Al³⁺ and Hg²⁺ to the fluorescein 3, which is in accordance with the experimental stoichiometry results obtained in the Job's plot method. Among the active cations, the minimum detectable amount is under 1 μM for most of the cases in both absorption and fluorescence spectroscopy methods.