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. Moreover, detecting fluoride at concentration below the maximum contaminant level in drinking water defined by the USEPA (4.0 mg L−1, 211 μM) could be achieved with a detection limit (LOD) estimated down to 1.6 μM. The reversible ring-open mechanism (OFF-ON-OFF) 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 “real-world applications” such as the determination of fluorides in toothpastes with “naked eye” due to the obvious color change.