Synthesis of novel squaraine–bis(rhodamine-6G): A fluorescent chemosensor for the selective detection of Hg2+

• A novel squaraine–bis(rhodamine-6G) (SR) chemosensor was synthesized in high yield.
• Squaraine–bis(rhodamine-6G) (SR) can be used to detect Hg2+ ions with improved selectivity.
• The selective binding of Hg2+ leads to a colorless to pink change could make it a suitable “naked eye” indicator.
• The SR–Hg2+ complex structure was confirmed via FAB-MS, FT-IR and molecular modeling studies.

We report the design and synthesis of a novel squaraine–bis(rhodamine-6G) (SR)-based indicator for selective detection of Hg2+ ion. Sensing behavior toward various metal ions (M = Li+, Na+, Mg2+, K+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Ag+, Cs+, Hg2+, and Pb2+) was investigated by UV–vis and fluorescence spectroscopy in acetonitrile solution. The indicator exhibited highly selective and sensitive colorimetric and “turn-on” fluorescent responses compared with the other investigated metal ions because of the soft-acid nature and large size of mercuric ion. The obvious change from colorless to pink upon the addition of Hg2+ could make it a suitable “naked eye” indicator for Hg2+. Finally, we proposed a reversible ring-opening mechanism (OFF–ON) of the rhodamine spirolactam induced by Hg2+ binding and forming a 1:1 stoichiometric complex of SR–Hg2+ which is supported by the density functional theory calculations. More significantly, the sensor displayed a remarkable blue-to-green fluorescence switch in the presence of Hg2+ ions.

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