A ratiometric fluorescent chemosensor for fluoride ions based on a proton transfer signaling mechanism

We report an easy-to-prepare fluorescent chemosensor, 4-benzoylamido-N-butyl-1,8-naphthalimide, which is sensitive to fluoride ions. The sensor shows spectral shifts and intensity changes in the presence of fluoride, in a wavelength-ratiometric and -colorimetric manner, which can detect fluoride concentrations in a range of 20-100 mu M at visible wavelengths. The striking colorless-to-yellow color change and blue-to-orange emission color change are thought to be due to the deprotonation of the 4-amido moiety of the naphthalimide fluorophore. The sensor compound shows great promise for the selective detection of fluoride in the presence of other halides because none of those are likely to deprotonate the amide.