A ratiometric fluorescent probe for fluoride ion employing the excited-state intramolecular proton transfer

A ratiometric fluorescent probe 1 for fluoride ion was developed based on modulation of the excited-state intramolecular proton transfer (ESIPT) process of 2-(2'-hydroxyphenyl)benzimidazole (HPBI) through the hydroxyl group protection/deprotection reaction. The probe I was readily prepared by the reaction of HPBI with tert-butyldimethylsilyl chloride (TBS-Cl) and shows only fluorescence emission maximum at 360 nm. Upon treatment with fluoride in aqueous DMF solution, the TBS protective group of probe 1 was removed readily and ESIPT of the probe was switched on, which resulted in a decrease of the emission band at 360 nm and an increase of a new fluorescence peak around 454 nm. The fluorescent intensity ratio at 454 and 360 nm (I(454)/I(360)) increases linearly with fluoride ion concentration in the range 0.3-8.0 mu mol L(-1) and the detection limit is 0.19 mu mol L(-1). The proposed probe shows excellent selectivity toward fluoride ion over other common anions. The method has been successfully applied to the fluoride determination in toothpaste and tap water samples. (C) 2009 Elsevier B.V. All rights reserved.