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.