Metalloporphyrins emerged recently as particularly interesting ionophore species¹. The selectivity pattern of this group of ionophores is significantly different from the sequence based on ions free energy of hydration (so-called Hofmeister selectivity pattern²). It was shown that the unique potentiometric selectivities observed for membranes doped with various metalloporphyrins result from selective interaction of analyte anion with the metal center of the porphyrin structure. Metalloporphyrin – based ISEs could serve as detectors in flow injection analysis (FIA) systems. Example of such system is setup developed for glucose determination with metalloporphyrin-based polymeric-membrane ISE selective to fluoride as a detector³.

Fig. 1. Structure of dichloro[5,10,15,20-tetra(tertbutylphenyl)porphyrinato]zirconium(IV).

               The scope of this work was to study the influence of porphyrin structure and polymer membrane composition on the acetate response of ISEs doped with Zr(IV) – porphyrins. Among few tested ionophores, the best response parameters (selectivity, sensitivity, LDL) towards acetate were obtained for electrodes with membranes containing Zr(IV)-tTPP (see Fig. 1). Electrodes with membranes based on the chosen compound were then used as detectors in FIA systems for acetylcholine determination and acetylcholinesterase inhibition studies.

Fig. 2. FIA measurement setup for acetylcholine determination used in this work.

                The method based on the enzymatic hydrolysis reaction, where choline and acetate are produced, was applied for acetylcholine determination. Enzyme (acetylcholinesterase) was covalently immobilized on amberlite XAD7 and placed in PVC tubing forming flow-through reactor.  Newly designed system (see Fig. 2), is the only one, described in literature to date⁴, that uses potentiometric detection of acetate for acetylcholine determination.  Proposed FIA system allowed for determination of acetylcholine in the range of 10⁻⁴ to 10⁻¹ M, with the throughput of 60 samples per hour. The lifetime of enzymatic reactor was approximately 9 days. The electrode response parameters stayed unchanged during this period.

            The same system was used for determination of chlorpyrifos – the organophosphorus  insecticide compound which can function as inhibitor of acetylcholinesterase, resulting in decrease of acetate-dependent signals. The preliminary studies showed that such system allows for determination of inhibitor compound at the level of 10⁻⁵ M.

1. Ł. Gorski, E. Malinowska, P. Parzuchowski, W. Zhang, M.E. Meyerhoff, Electroanalysis 15 (2003) 1229.

2. F. Hofmeister, Arch. Exp. Pathol. Pharmakol. 24 (1888) 247

3. Ł. Gorski, D. Klimaszewska, M. Pietrzak, E. Malinowska, Anal. Bioanal. Chem. 389 (2007) 533.

4. Ł. Gorski, M. Mroczkiewicz, M. Pietrzak, E. Malinowska, Analytica Chimica Acta 644 (2009) 30–35

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