Nanoscale Carbonate Ion-Selective Amperometric/Voltammetric Probes Based on Ion-Ionophore Recognition at the Organic/Water Interface: Hidden Pieces of the Puzzle in the Nanoscale Phase.

Here, we report on the successful demonstration and application of carbonate (CO) ion-selective amperometric/voltammetric nanoprobes based on facilitated ion transfer (IT) at the nanoscale interface between two immiscible electrolyte solutions. This electrochemical study reveals critical factors to govern CO-selective nanoprobes using broadly available Simon-type ionophores forming a covalent bond with CO, i.e., slow dissolution of lipophilic ionophores in the organic phase, activation of hydrated ionophores, peculiar solubility of a hydrated ion-ionophore complex near the interface, and cleanness at the nanoscale interface. These factors are experimentally confirmed by nanopipet voltammetry, where a facilitated CO IT is studied with a nanopipet filled with an organic phase containing the trifluoroacetophenone derivative COionophore (COionophore VII) by voltammetrically and amperometrically sensing CO in water. Theoretical assessments of reproducible voltammetric data confirm that the dynamics of CO ionophore VII-facilitated ITs (FITs) follows the one-step electrochemical (E) mechanism controlled by both water-finger formation/dissociation and ion-ionophore complexation/dissociation during interfacial ITs. The yielded rate constant, = 0.048 cm/s, is very similar to the reported values of other FIT reactions using ionophores forming non-covalent bonds with ions, implying that a weak binding between CO ion-ionophore enables us to observe FITs by fast nanopipet voltammetry regardless of the nature of bondings between the ion and ionophore. The analytical utility of CO-selective amperometric nanoprobes is further demonstrated by measuring the CO concentration produced by metal-reducing bacteria MR-1 as a result of organic fuel oxidation in bacterial growth media in the presence of various interferents such as HPO, Cl, and SO.