A benzimidazole-based turn-off fluorescent sensor for selective detection of mercury (II)

In this study, potential fluorescent sensing ability of a donor-acceptor-donor type conjugated molecule over various metal ions (Ni(II), Zn(II), Cu(II), Cd(II), Pb(II), Fe(II), Fe(III), As(III), Se(IV), Hg(I) and Hg(II)) was investigated. The sensor basically includes benzimidazole and 3,4-propylenedioxythiophene as the acceptor and donor units, respectively. Solution of the sensor in tetrahydrofuran exhibits an emission with having a maximum at 632 nm. Since having insolubility of some ions in tetrahydrofuran, ion solutions were prepared in deionized water and titrations were performed by stepwise additions of ions into sensor solution in tetrahydrofuran. The control experiment including only deionized water resulted in 7 nm red-shift and 71 % quenching in the emission band. On the other hand, upon 1.07 eq. addition of Hg(II), 25 nm red-shift and 88 % fluorescent quenching were observed, exhibiting a difference among other ions. The corresponding spectral-shift and quenching amounts in the emission were found to be time-dependent and reach to equilibrium after 10 min. Studies showed that Hg(II) in aqueous solutions can be selectively detected even in the presence of other ions. Moreover, presence of Hg(II) (0.36 eq or beyond) changed the color of the sensor solution from pink to pale violet, which can be followed by the naked eye. In the light of these results, the reported sensor can be assigned as a fluorescent and a chromogenic sensor for Hg(II) detection. Limit of quantification was found to be 39.2 nM, which is comparable to the most of the fluorescent Hg(II) sensors in the literature.