Optical fluoride sensor based on monomer–dimer equilibrium of scandium(III)-octaethylporphyrin in a plasticized polymeric film

A fluoride-selective optical sensor based on scandium(III)-octaethylporphyrin (Sc(III)OEP) as an ionophore within a plasticized PVC film is described. The presence of fluoride ion in the aqueous sample phase increases the formation of a difluoro-bridged Sc(III)OEP dimer species in the polymer film. The ability of the Sc(III) porphyrin to form the dimeric structure in the presence of fluoride is confirmed by UV–vis spectroscopy and X-ray crystallography. For more practical sensing applications, a pH chromoionophore (ETH 7075) is added to the plasticized PVC film along with Sc(III)OEP and the observed optical response is based on coextraction of protons with sample phase fluoride to create the dimeric porphyrin and a protonated chromoionophore species. The selectivity pattern observed is F− ≫ ClO4−, SCN−, NO3− > Br−, Cl−. Only organic salicylate is a significant interferent. Fast and reversible fluoride response is observed over the range of 10−4 to 10−2 M fluoride, allowing use of the sensing film in a waveguide configuration for flow-injection measurements.