3,3-Disubstituted 1-acylthioureas as ionophores for Pb(II)-ion selective electrodes: physical and chemical characterization of the sensing membranes

Solid-state Pb(II) ion selective electrodes were developed, which are based on 3,3-disubstituted 1-acylthioureas as ionophores and use PVC membranes. 1-Benzoyl, 1-(2-furoyl), 3,3-diethyl and 3,3-diphenyl were the substituent groups on the thioureide core. The analytical parameters for the constructed electrodes were studied. The electrodes based on 3,3-diethyl substituted thiourea derivatives showed better analytical response to Pb(II) ions with suitable sensitivity and an average lifetime of more than 30 days. Cu(II), Hg(II) and Ag(I) ions were interfering species during Pb(II) detection. Scanning electron microscopy and atomic force microscopy micrographs of the activated sensing membranes revealed morphological changes, which have been associated with the performance of the sensors. The studies have suggested that the detection of few large size particles or aggregates at the membrane surface are responsible for inadequate functioning of the prepared electrodes. This pattern was mainly observed for the sensing membranes based on 3,3-diphenyl groups as substituents and is related to unfavorable steric and electronic factors. Furthermore, X-ray photoelectron spectroscopy profiles S2p and Pb4f supported chemically the analytical response displayed for the four membrane systems. The X-ray photoelectron spectroscopy results combined with scanning electron microscopy and atomic force microscopy studies shed light on the working mechanism of the studied electrodes.