Selective membranes using metalloporphyrins

L. Gorski, E. Malinowska, P. Parzuchowski, W. Zhang, and M.E. Meyerhoff, Recognition of anions using metalloporphyrin-based ion-selective membranes state-of-the-art. Electroanalysis 15, 1229-1235 (2003). 

The solvent polymeric membranes used for EMF measurements typic ly consisted of 1 wt.% metalloporphyrin, 33 wt. % poly-(vinyl chloride) (PVC) and 66 wt % plasticizer (dibutylsebacate). If the porphyrin complex central metal oxidation state was (-I- 2) or (-t- 4), addition of lipophilic cationic sites or anionic sites respectively was required to achieve optimal anion response and selectivity. Tridodecyl-methylammonium cation (TDMAC) or teti s[bis (3,5-trifluoromethyl) phenyl] anion (at a concentration 10-20% mol relative to the metalloporphyrin) are used for this purpose. The total weight of the components varies according to the desired memln ane thickness (typically 150-250 pm) and diameter (7-10 mm). 

Several authors have also reported the potential use of electropolyme-rized metalloporphyrin films as new electrode materials for anion detection and pH measurements. For example, Daunert et al reported that anion-selective membrane electrodes can be prepared by electropolymerizing aniline-substituted cobalt tetraphenylporphyrin complexes onto a glassy carbon surface. It was shown that the resulting electrodes are highly selective towards the detection of thiocyanate and nitrite anions. The mechanism by which these metalloporphyrin electrode sensors operate is probably related to the well-known axial complexation capability of the central metal cation of the porphyrin towards different kind of anions and bases. The same authors also reported a detection limit of 5 x 10 M for these ion-selective electrodes which have lifetimes of more than 2 months... 

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. " - ... 

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