Aluminum(III) Porphyrins as Ionophores for Fluoride Selective Polymeric Membrane Electrodes

Abstract

Aluminum(III) porphyrins are examined as potential fluoride selective ionophores in polymeric membrane type ionselective electrodes. Membranes formulated with Al(III) tetraphenyl (TPP) or octaethyl (OEP) porphyrins are shown to exhibit enhanced potentiometric selectivity for fluoride over more lipophilic anions, including perchlorate and thiocyanate. However, such membrane electrodes display undesirable super-Nernstian behavior, with concomitant slow response and recovery times. By employing a sterically hindered Al(III) picket fence porphyrin (PFP) complex as the membrane active species, fully reversible and Nernstian response toward fluoride is achieved. This finding suggests that the super-Nernstian behavior observed with the nonpicket fence metalloporphyrins is due to the formation of aggregate porphyrin species (likely dimers) within the membrane phase. The steric hindrance of the PFP ligand structure eliminates such chemistry, thus leading to theoretical response slopes toward fluoride. Addition of lipophilic anionic sites into the organic membranes enhances response and selectivity, indicating that the Al(III) porphyrin ionophores function as charged carrier type ionophores. Optimized membranes formulated with Al(III)-PFP in an o-nitrophenyloctyl ether plasticized PVC film exhibit fast response to fluoride down to 40 mM, with very high selectivity over SCN, Br À and NO 3 À (k pot < 10 À3 for all anions tested). With further refinements in the membrane chemistry, it is anticipated that Al(III) porphyrin-based membrane electrodes can exhibit potentiometric fluoride response and selectivity that approaches that of the classical solid-state LaF 3 crystal-based fluoride sensor.

Polymeric membrane electrodes with improved fluoride selectivity and lifetime based on Zr(IV)- and Al(III)-tetraphenylporphyrin derivatives

“…The structure of these species prevent unwanted metalloporphyrin dimer-monomer equilibria within the membrane phase of the electrode, with such chemistry leading to non-ideal response properties. Similar studies with Al(III) picket fence porphyrin (Al[PFP]) to prevent dimer-monomer chemistry have also been reported [14]. However, it was found that Al[PFP] easily forms insoluble crystals within the membrane phase and also leaches from the membrane to the water sample; hence, in spite of excellent selectivity towards fluoride, electrodes based on Al(III)-[PFP] doped membranes are not analytically useful because of their continuous loss in slope, response range and selectivity over just a few days of operation.…”

“…Similarly, thus far, relatively few useful fluoride-selective polymeric membrane electrodes have been reported [8][9][10][11][12][13][14][15]. Most suffer from practical problems such as high level of interference by lipophilic anions [11,12,15], super-nernstian response slopes, sluggish and not fully reversible response properties [8,9,10,14] and short use lifetimes of the electrodes [14].…”

“…Most suffer from practical problems such as high level of interference by lipophilic anions [11,12,15], super-nernstian response slopes, sluggish and not fully reversible response properties [8,9,10,14] and short use lifetimes of the electrodes [14]. Recent research has suggested that membranes formulated with Al(III) octaethylporhyrin (Al [OEP]) and Al(III) tetraphenylporphyrin (Al[TPP]) [14] exhibit very high potentiometric selectivity towards fluoride anions, and this has led also to the design of highly selective optical fluoride sensors with sub-micromolar fluoride ion detection limit [16,17]. However, when these same ionophores are employed in the potentiometric detection mode [14], the common problems cited above (sluggish response, non-nernstian response, etc.)…”

“…Recent research has suggested that membranes formulated with Al(III) octaethylporhyrin (Al [OEP]) and Al(III) tetraphenylporphyrin (Al[TPP]) [14] exhibit very high potentiometric selectivity towards fluoride anions, and this has led also to the design of highly selective optical fluoride sensors with sub-micromolar fluoride ion detection limit [16,17]. However, when these same ionophores are employed in the potentiometric detection mode [14], the common problems cited above (sluggish response, non-nernstian response, etc.) are often observed.…”

Potentiometric response and mechanism of anionic recognition of heterocalixarene-based ion selective electrodes

“…ISEs are also of obvious interest because they can help translate the chemistry of new substrate binding systems into tools that can be used to recognize selectively various targeted species in the presence of potentially interfering analytes. In the specific case of anion recognition, this approach has been explored extensively by Umezawa, Meyerhoff, Simon, Reinhoudt, Schmidtchen and others using a range of receptors including bis-guanidinium [1][2], porphyrins [3], protonated polyamines [4][5][6][7], and protonated sapphyrins [8][9][10], as well as a variety of Lewis acidic systems such as calixarenes [11][12][13][14][15], uranylsalenophenes [16][17], metalloporphyrins [18][19][20][21][22], metallocenes [23], other organometallic derivatives [24][25][26], and fluorinated compounds [27].…”