Amperometric Sensors Based on Mercaptopyridine−Montmorillonite Intercalation Compounds

The objective of this work is the study of the intercalation of 2- and 4- mercapto-substituted pyridines (2Mpy and 4Mpy, respectively) in Na + −montmorillonite, providing intercalation compounds with interesting functional properties derived from the presence of the mercapto group. CHN chemical analysis, X-ray diffraction, Fourier transform infrared spectroscopy, and 13 C NMR spectroscopy have been employed for the characterization of the intercalation compounds. The molecular arrangement of such species in the interlayer space depends on the intercalation medium (neutral or acidic) and on the position of the mercapto group in the starting mercaptopyridine. In methanol medium, only 4Mpy can be intercalated, the pyridine ring interacting with the interlayer cations through water bridges. 2Mpy and 4Mpy can be intercalated by cation exchange from the corresponding protonated (pyridinium) derivatives, formed by addition of 1 M HCl to the intercalation medium. Tautomeric equilibrium between thiol and thione species of such mercaptopyridines must be taken into account to explain the arrangement of molecular aggregates and their particular orientation in the interlayer space. We have tried to correlate the structural arrangements of the intercalated species with the complexing ability of mercapto groups toward heavy metal ions. In this way, these materials have been tested as the active phase of carbon paste electrodes (CPEs) for the amperometric determination of Cd(II), Pb(II), Cu(II), and Hg(II) ions in aqueous solutions. CPEs based on pure 2- or 4-mercaptopyridines alone do not give an electroanalytical response toward heavy metal ions, whereas CPEs of the nanostructured materials show good electroanalytical behavior due to the particular arrangement of the guest species in the interlayer space of the clay.