Optical and Electrochemical Anion Sensors based on Metalloporphyrins and Metallotexaphyrins.

Abstract

The development of new anion sensors using metalloporphyrins was investigated. This involved both tuning the nature of the metal cation by modifying ligand structures, as well as exploiting conformational changes such as dimer-monomer equilibria of given metalloporphyrins. These factors are critical when metalloporphyrins are incorporated in plasticized films as anion ionophores. Scandium porphyrins proved to be useful as fluoride-selective ionophores in optical and electrochemical fluoride sensors. These sensors exhibited excellent selectivity for fluoride over other anions. A novel bis(µ-fluoride) dimer structure was identified by X-ray crystallography and optical selectivity was further improved based on the Soret band shift induced by the dimer-monomer equilibrium in response to fluoride. Since gallium porphyrins were found to complex fluoride in both monomeric and dimeric forms within plasticized PVC films, the dimer-monomer equilibrium was actively utilized to improve sensitivity and selectivity for detecting fluoride. Weakening dimerization by doping the film with appropriate additives led a lowered detection limit for optical fluoride sensing. However, enhancing dimerization by covalently connecting two porphyrins improved selectivity for fluoride over other anions. This study demonstrated alternative approaches to improve sensor properties when a dimer-monomer equilibrium is present. The dimer-monomer response mechanism was examined using a sub-micron thick optical film in the planar waveguide configuration. This study was based on the dimer-monomer equilibria of indium porphyrins as a model system. The fast response time (< 2 sec) for detection of aqueous chloride ion and gaseous amines showed this equilibrium is fast enough to be potentially applied to prepare flow-through optical detectors for analytical instruments such as GC and LC. Finally, the effect of ligand structure around the metal cation as an anion binding site was investigated by comparing indium texaphyrin to indium porphyrins. A lipophilized indium texaphyrin was prepared and characterized as an anion ionophore when doped in plasticized PVC films. The selective potentiometric response toward thiocyanate and fluoride suggests that indium texaphyrins behave more like gallium porphyrins than indium porphyrins. This result provides valuable insights on tuning the anion selectivity by switching ligands.