Spectroscopic Investigation of the Selective Interaction of Mercuric and Cupric Ions with a Porphyrin Active Layer

The mechanisms of interaction between a tetrapyridyl-substituted porphyrin and Hg(II) and Cu(II) ions have been investigated by means of different spectroscopic techniques. Reflection spectroscopy investigations of the porphyrin Langmuir floating film and by polarization modulation infrared (IR) reflection adsorption spectroscopy at the air−water interface provided evidence of the active role played by the pyridyl substituents of the porphyrin in the interaction with the analyte. Such behavior seems to be very selective toward Cu2+ and Hg2+ ions, as demonstrated by the IR measurements in difference mode. UV−vis and IR characterizations suggest a deeply different interaction between the active molecules and the two analytes. In fact, the interaction of Hg2+ ions with the tetrapyrrolic derivative molecules involves both the pyridyl substituents and the central bite of the ring. On the other hand, in the case of Cu2+ ions, spectroscopic evidence suggests that the cupric ions interact with only the porphyrin peripheral substituents. A relevant fluorescence quenching of the Langmuir−Schaefer (LS) film is observed when even a 0.5 nM HgCl2 or 0.5 nM CuCl2 aqueous solution is fluxed on the LS film.