A rhodamine–quinoline type molecular switch as a highly selective sequential sensor for Al³⁺and F⁻in aqueous solution

Abstract

A novel optical rhodamine–quinoline type chemosensor (REQ) was successfully synthesized and characterized as a reversible molecular switch. It displayed high selectivity toward Al3+ among 19 metal ions and the resultant complex [REQ–Al3+] acted as a sequential chemosensor toward F− among 16 anions in neutral aqueous media (ethanol–water, v/v = 3/7, Tris–HNO3, 20 mM, pH 7.0). Moreover, detecting fluoride at concentrations below the maximum contaminant level in drinking water defined by the U. S. Environmental Protection Agency (4.0 mg L−1, 211 μM) could be achieved with a detection limit (LOD) estimated to be down to 8.0 μM. The reversible ring-open mechanism of the rhodamine spirolactam induced by Al3+ binding and the 1 : 1 stoichiometric structure between REQ and Al3+ were adequately supported by Job plot evaluation, optical titration, and 1H NMR study. Finally, it could be conveniently used for the determination of Al3+ and F− with the “naked eye” due to the obvious color change of the testing solution, showing its potential practical application.