Potentiometric Porphyrinic Sensors for Detection of Ions

Most potentiometric anion-selective electrodes are based on anion exchangers such as quaternary ammonium salts. The selectivity pattern of these sensors correlates with anion lipophilicity. Highly hydrated anions such as fluoride, bicarbonate and chloride are difficult to monitor due to significant interference from more lipophilic anions like perchlorate, salicylate and nitrate which may be present in the analyzed sample. The anions can be classified according to their lipophilicity resulting in the classical Hofmeister series (ClOi SCN salicylate I NOJ Br  

In order to improve the practical application of potentiometric sensors, much effort must be devoted to developing membrane materials whose selectivity deviates fi-om the Hofmeister series. Chloride- and nitrite-selective electrodes based on lipophylic vitamin B12 derivatives have been reported by Simon.xhese electrodes were the first anion-selective electrodes of which selectivity sequence did not obey the Hofmeister series. Since then, a number of anion-selective liquid-membrane electrodes have been developed using a variety of hosts metallocenes, diphosphonium dication salts, diquartemary ammonium dication salts, bisthiourea derivatives, metalloporphyrins, lipophylic macrocyclic polyamines, cytosine-dependent triamine and metallophtalocyamines. Among this large variety of compounds studied, the metalloporphyrins seem to be the most promising for the preparation of effective ion-selective potentiometric sensors. -  

Metalloporphyrins, one carbon larger but structurally analogous to the corrin ring of vitamin B12, exhibited unique anion ionophore properties when incorporated into a polymer membrane. Mn(III), Co(III), Ru(II), and Sn(IV) porphyrin-based ion-selective sensors exhibit high selectivity for thiocyanate, nitrite/thiocyanate, thiocyanate and siicylate, respectively. doo-io7,i 10-114 nature of the 

The potentiometric response of membranes doped with metalloporphyrins involves the direct interaction of the anion as an axial ligand. So, changes in the porphyrin structure, both in terms of initial ligation as well as substitution on the porphyrin ring, can influence the ability of given anions to bind as axial ligands and thereby alter the potentiometric anion selectivity. When a relatively simple 

Expanded-porphyrin electrodes can discriminate anions base on their size. Sapphyrin with a core size of roughly 5.5 A diamet is well suited for the binding of the central moiety about the size of fluoride with strong hydrogenbonding interactions. The response order by the sapphyrin-based electrode is F Br C1 indicating that the sapphyrin-based electrode is more selective for fluoride than chloride and bromide. The potentiometric discriminations of cisitrans geometrical (maleate over frimarate) and positional (phthalate over isophtalate and terephtalate) 

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