Ion-channel-mimetic sensing of hydrophilic anions based on monolayers of ahydrogen bond-forming receptor

Ion-channel-mimetic sensing based on monolayers of a hydrogen bond-forming bis(thiourea) receptor was used to detect inorganic anions, Receptor monola yers were formed at the air-water interface and subsequently contacted with a highly oriented pyrolytic graphite electrode. Horizontal touch cyclic vo ltammetry was performed with subphase solutions containing various electroi nactive analyte anions and [Fe(CN)(6)](4-) as electroactive marker. Binding of analyte anions to the receptor monolayer was found to inhibit [Fe(CN)(6 )](4-) oxidation. The influences of the analyte anions on the cyclic voltam mograms were largest for HPO42- and decreased in the order of HPO42- F- app roximate to SO42- > CH3COO- > Cl-, whereas ion-selective electrodes (ISEs) containing the same receptor respond with a selectivity order of Cl- > SO42 - > CH3COO- > H2PO4-/HPO42-. Because the bis(thiourea) receptor does not bi nd to all potentially hydrogen-bonding sites of most of these anions, it is apparent that several of the larger anions, and in particular phosphate an d sulfate, are still substantially hydrated while being bound to the interf acial receptor layer. This distinct feature of interfacial molecular recogn ition seems to explain why selectivities of these ion-channel-mimetic senso rs differ so strongly from the selectivities for the complete anion transfe r from aqueous to organic phases, as represented by the ISE selectivity. Th e results in the present work suggest that ion-channel-mimetic sensors are particularly promising for the analysis of very hydrophilic, relatively lar ge analytes, for which hosts that encapsulate the analyte and do not allow the analyte in this complex to be hydrated are difficult to synthesize.