Origin of non nernstian anion response slopes of metalloporphyrin-based liquid/polymer membrane electrodes

The origin of the non-Nernstian potentiometric anion response behavior exhi bited by several metalloporphyrin-based liquid/polymeric membrane electrode s is examined. UV-visible spectrophotometry of organic-phase solutions and thin plasticized PVC films containing In(III) and Ga(III) octaethylporphyri ns suggests that, in the absence of preferred axial coordination anions, th e metalloporphyrins form hydroxide ion bridged dimers within the organic ph ases, as indicated by a significant blue shift of the Soret band in the vis ible spectrum. As increasing levels of the preferred anions are added, the degree of dimerization decreases and the intensity of the Soret band corres ponding to the monomer species increases. Observation of Nernstian response s with membranes doped with picket fence-type In(III) and Ga(III) porphyrin s not capable of forming hydroxide bridged structures las determined by W-v isible spectroscopy) confirms that dimerization is likely responsible for t he super-Nernstian slopes of membrane electrodes formulated with the non-pi cket fence species. A phase boundary model based on simultaneous binding eq uilibria of hydroxide ions with two metalloporphyrins to form the dimeric s pecies, while the target anions bind with metalloporphyrins to form neutral 1:1 complexes, is shown to fully predict the observed non-Nernstian behavi or. The prospect of utilizing this anion-dependent dimer-monomer metallopor phyrin equilibrium to fabricate anion-selective optical sensors using thin films of metalloporphyrin-doped polymers is also discussed.