Selective sensing of Hg2+ by a proton-ionizable calix[4]arene fluoroionophore

A fluorogenic derivative of a calix[4]arene withtwo proton-ionizable N-(phenyl)sulfonyl carboxamidecontainingside arms in the 1,3-positions on the lower rim isemployed for the selective sensing of Hg2+ at low concentrationlevels in water/MeCN (1:1, v/v) solutions containing Pb2+and Cd2+. All three metal ions quench the fluorescence of theligand in pure MeCN. However, in water/MeCN mixed solvent,the recognition of such cations occurs differently as onlyHg2+ complexation quenches the fluorescence of the calixarene.Experiments carried out in the presence of an acid and abulky non-complexing cation shows that the quenching of thecalixarene fluorescence upon Hg2+ addition is likely due toproton displacement from the proton-ionizable side arms ofthe ligand. The system may be employed as a simple tool forthe selective and efficient mercury sensing in mixed water/organic solvent.