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A novel optical rhodamine-quinoline type chemosensor (REQ) was successfully synthesized and characterized as a reversible molecular switch. It displayed high selectivity toward Al 3+ among 19 metal ions and the resultant complex [REQ-Al 3+ ] acted as a sequential chemosensor toward F À among 16 anions in neutral aqueous media (ethanol-water, v/v ¼ 3/7, Tris-HNO 3 , 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 À1 , 211 mM) could be achieved with a detection limit (LOD) estimated to be down to 8.0 mM. The reversible ring-open mechanism of the rhodamine spirolactam induced by Al 3+ binding and the 1 : 1 stoichiometric structure between REQ and Al 3+ were adequately supported by Job plot evaluation, optical titration, and 1 H NMR study. Finally, it could be conveniently used for the determination of Al 3+ and F À with the "naked eye" due to the obvious color change of the testing solution, showing its potential practical application.

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