Metalloporphyrin-based polymer membrane electrode with high selectivity for 2-hydroxybenzhydroxamate

Solvent/polymeric membrane electrodes formulated with either Sn(IV)-tetraphenylporphyrin (SnTTP) or Sn(IV)-tetrakis(p-fluorophenyl)porphyrin (SnTFPP) within plasticized PVC membranes are shown to exhibit high potentiometric anion selectivity toward 2-hydroxybenzhydroxamate (HBHA), a drug used to prevent formation of kidney stones. The resulting sensors exhibit good sensitivity toward HBHA (detection limit = 30 μM), with unusually high selectivity over more lipophilic anions (such as salicylate, perchlorate, thiocyanate, iodide). Addition of lipophilic anionic sites in the form of tetrakis[bis(3,5-trifluoromethyl)phenyl]borate within the membranes is shown to further enhance the observed potentiometric selectivity, confirming that the Sn(IV)-porphyrins serve as a charged carrier type ionophores for HBHA. The basicity of various anionic oxo-ligands, as defined by their pKa values, is correlated with the electrodesʹ potentiometric selectivity toward these species. Oxo-ligands with higher pKa values, such as HBHA, bind more strongly to the central Sn(IV) of the porphyrin and hence are the preferred anions for axial site coordination with metalloporphyrin. Such binding is confirmed via UV-visible spectral shifts in the absorption spectra of the metalloporphyrins in the presence of different anions, and the retention times of various anions on an high-performance liquid chromatographic column packed with a Sn(IV)-tetraphenyl-porphyrin-silica stationary phase. The porphyrin-based sensor is shown to be useful for the assay of HBHA in pharmaceutical formulations with an average recovery of 101% and mean standard deviations of 1.8%.