Ligand designing and synthesis: The ESIPT phenomenon is linked to the presence of an intramolecular hydrogen bond, usually between a proton donor (-OH/-NH) and a proton acceptor (-C=O/heterocyclic N). The fluorophore 2-hydroxy phenyl benzoxazole and its derivatives could provide on-off switching properties of the ESIPT signal through metal complexation, leading to metal induced fluorescence quenching. Decomplexation of the metal through the analyte would then lead to recovery of the ESIPT signal. Provided the decomplexation can be selective, as previously shown in disassembly strategies, e.g. by the Zelder group for PPi, one could design a polyphosphate responsive ESIPT sensor. To confer solubility in aqueous medium, a charged group, such as a sulfonate, would be beneficial. Based on the above considerations, fluorophore 4 was synthesized in four steps starting from 3-chloro-2-hydroxybenzaldehyde (Scheme 2).