Apart from deprotonation-induced inhibition of ESIPT, a ratiometric sensor can also be achieved by reaction of the proton donor group of ESIPT molecules with analyte-¬cleavable protecting groups.52e,f Based on this mechanism, Yang and colleagues reported a fluoride sensor BTTPB (Figure 8d) whose hydroxyl group was protected by tert-butyldiphhenylsilyl (TBDPS) group and thus ESIPT process was inhibited.52e BTTPB showed only blue-violet fluorescence in aqueous micellar solution of cetyltrimethylammonium bromide (CTAB), which was almost identical to the E* emission of 3-BTHPB (Figure 8d). Upon fluoride addition, the Si-O bond of BTTPB was immediately cleaved and 3-BTHPB was generated, which exhibits a yellow K* emission. As a result, both the absorption and emission spectra of the solution showed ratiometric responses. It should be noted that aqueous fluoride anion was detectable down to 100 ppb level without any significant interference from other anions such as H2PO4−, AcO−, or CN−.