Chloride-Selective Electrodes Based on Two-Wall Aryl-Extended Calix[4]Pyrroles: Combining Hydrogen Bonds and Anion-π Interactions to Achieve Optimum Performance

The performance of chloride-selective electrodes based on two-wall aryl-extended calix[4] pyrroles and multiwall carbon nanotubes is presented. The calix[4] pyrrole receptors bear two phenyl groups at opposite meso-positions. When the meso-phenyl groups are decorated with strong electron-withdrawing substituents, attractive anion-pi interactions may exist between the receptor's aromatic walls and the sandwiched anion. These anion-pi interactions are shown to significantly affect the selectivity of the electrodes. Calix[4] pyrrole, bearing a p-nitro withdrawing group on each of the meso-phenyl rings, afforded sensors that display anti-Hofmeister behavior against the lipophilic salicylate and nitrate anions. Based on the experimental data, a series of principles that help in predicting the suitability of synthetic receptors for use as anion-specific ionophores is discussed. Finally, the sensors deliver excellent results in the direct detection of chloride in bodily fluids.