A ratiometric fluorescent probe for bisulphite anion, employing intramolecular charge transfer

4-(1H-benzimidazol-2-yl)benzaldehyde (1) has been developed as a new ratiometric fluorescent probe for bisulphite, based on the modulation of intramolecular charge transfer (ICT). Upon mixing with bisulphite in aqueous ethanol, an aldehyde-bisulphite adduct was formed and the ICT of the probe was switched off, which resulted in a ratiometric fluorescence response with an enhancement of the ratios of emission intensities at 368 and 498 nm. The detection range of the probe for bisulphite is in the 2.0-200 µmol/L concentration range and the detection limit is 0.4 µmol/L. Probe 1 produces a ratiometric fluorescent response to bisulphite with a marked emission wavelength shift (130 nm) and displays high selectivity for bisulphite over other anions.