Metalloporphyrins as anion carriers in membrane electrodes.

Fundamental and applied studies regarding the use of metalloporphyrins as membrane active carrier molecules in the design of new solvent polymeric membrane anion-selective electrodes are described. Several of the metalloporphyrin-based electrodes examined exhibit anion selectivities markedly different than those obtained with classical dissociated ion-exchangers. The potentiometric selectivity patterns observed are dependent on the specific porphyrin used, as well as on the nature and oxidation state of the central metal. In certain cases, the presence of competing lipophilic ligand species in the membrane phase drastically diminishes the potentiometric anion response of the membrane systems. These results, combined with spectrophotometric and additional electrochemical data, suggest that reversible ligation of the detected anion with the metal center of the porphyrin at the membrane/sample interface is directly responsible for the potentiometric anion responses observed. Detailed investigations of Mn(III)-based porphyrin systems indicate that the selectivity of membranes doped with these species is heavily dependent on the peripheral structure of the porphyrin. Response towards large lipophilic inorganic anions (e.g. ClO$\sb4\sp-$, IO$\sb4\sp-$ can be nearly eliminated by employing porphyrins that can sterically inhibit the coordination with the Mn(III) center. On the other hand, the response and selectivity of such Mn(III) porphyrins toward small linear anions such as SCN$\sp-$ is dramatically enhanced. The effect of varying the central metal on the potentiometric anion selectivity is also demonstrated. Exchanging Sn(IV) for Mn(III) in a simple tetraphenylporphyrin (TPP) structure significantly alters the anion selectivity observed. The Sn (TPP) Cl$\sb2$ carrier exhibits rather high selectivity for salicylate ion, over most other anions tested. Experiments suggest that this porphyrin molecule functions via a charged carrier mechanism by reversibly exchanging its original chloride ligands with salicylate, and/or hydroxide. The latter results in a membrane with significant pH response, mandating the use of buffered samples for detection of salicylate. The Sn (TPP) Cl$\sb2$-doped polymeric membrane electrode can be used to measure salicylate accurately in diluted biological samples. Electrode salicylate measurements in serum and urine samples correlate well with the Trinder colorimetric method for samples spiked with salicylate in the therapeutically important range.Ph.D.Analytical chemistryPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/128314/2/8920512.pd