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 3 2- ) 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 3 2- -selective nanoprobes using broadly available Simon-type ionophores forming a covalent bond with CO 3 2- , 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 3 2- IT is studied with a nanopipet filled with an organic phase containing the trifluoroacetophenone derivative CO 3 2- ionophore (CO 3 2- ionophore VII) by voltammetrically and amperometrically sensing CO 3 2- in water. Theoretical assessments of reproducible voltammetric data confirm that the dynamics of CO 3 2- 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, k 0 = 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 3 2- 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 3 2- -selective amperometric nanoprobes is further demonstrated by measuring the CO 3 2- concentration produced by metal-reducing bacteria Shewanella oneidensis MR-1 as a result of organic fuel oxidation in bacterial growth media in the presence of various interferents such as H 2 PO 4 - , Cl - , and SO 4 2- .