In this talk, we introduce the first successful application of facilitated ion transfers (ITs) based on ion-ionophore recognition to nanoscale amperometric/voltammetric probes at the nanopipet supported organic/water interface. We address three pivotal factors to realize the facilitated IT across the nanopipet-supported interface, i.e., nanoscale cleanness at the interface, peculiar solubility of a lipophilic ionophore and its ion-complex in the organic phase, and the quantitative analysis of IT kinetics. As a model, CO32−-selective system is studied, where voltammetry is performed with a CO32−ionophore VII filled nanopipet to sense CO32− in water. Both experimentally controlled cleanness at the nanoscale organic/water interface, and theoretically predicted solubility decrease of CO32− ionophore and its complex in the organic phase near the nanoscale interface reveal the hidden obstacles to achieving nanoscale ion-selective amperometric/ voltammetric probes and practical utilization of them. Also, the kinetic analysis by the finite-element method corroborates our mechanistic understanding of a facilitated IT reaction involving a water-finger formation as a rate-determining step, thus yielding the apparent rate constant as k0 = 0.048 cm/s, 5 ~ 100 fold faster than reported values determined at microscale interface. We further validate our fundamental understanding by practically measuring the actual level of CO32− produced by metal-reducing bacteria, Shewanella oneidensis MR-1 as a result of organic fuel oxidation. Importantly, our finding will broadly impact on developing various types of amperometric/voltammetric nanoprobes utilizing ion-ionophore recognition for clinical, physiological, industrial, and environmental analysis, and fabricating amperometric probes for scanning electrochemical microscopy at nanoscale spatial resolution.

Unraveling hidden barriers to the development of nanoscale ion-selective amperometric-voltammetric probes

ACS Spring 2022

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