A reversible switch as highly selective sequential chemosensor for Al3+ cation followed by F− anion

A novel optical Rhodamine-Furan type chemosensor (RF) was easily synthesized and characterized as a revisable switch. It dispalyed high selectivity toward Al3+ among 19 metal ions and the resultant complex [RF–Al3+] as a sequential chemosensor toward F− among 16 anions in neutral aqueous media (ethanol/water, v/v = 2/3, Tris–HNO3, 10 mM, pH 7.0). Moreover, detecting fluoride at concentration 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 down to 1.6 μM. Both sensors were very easily to construct and only required simple synthesis. The reversible ring-open mechanism of the rhodamine spirolactam induced by Al3+ binding and the 1:1 stoichiometric structure between RF 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 fluorides in dental products such as toothpastes with “naked eye” due to the obvious color change of testing solution, showing its potentially practical application.