The interaction of Hg(2+) 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(+), Ca(2+), Co(2+), Ni(2+), Cu(2+), Zn(2+), Cd(2+), Pb(2+), Hg(2+), Al(3+), Fe(3+), Ga(3+)and Cr(3+) 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 Hg(2+) 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 Al(3+), Fe(3+), Ga(3+), Cr(3+), and Hg(2+). Theoretical calculation using Density Functional Theory was performed in order to obtain atomistic insights into the coordination geometry of Al(3+) and Hg(2+) 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.