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

Abstract

Here, we report on the successful demonstration and application of carbonate (CO3 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 CO3 2–-selective nanoprobes using broadly available Simon-type ionophores forming a covalent bond with CO3 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 CO3 2– IT is studied with a nanopipet filled with an organic phase containing the trifluoroacetophenone derivative CO3 2–ionophore (CO3 2–ionophore VII) by voltammetrically and amperometrically sensing CO3 2– in water. Theoretical assessments of reproducible voltammetric data confirm that the dynamics of CO3 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 CO3 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 CO3 2–-selective amperometric nanoprobes is further demonstrated by measuring the CO3 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 H2PO4 –, Cl–, and SO4 2–.

Nanoelectrochemistry at liquid/liquid interfaces for analytical, biological, and material applications

“…27 The selectivity and sensitivity of these ITIES-based nanosensors were high enough for practical applications to detect the target ions in complicated biological media around living neuronal cells 28,29 and bacteria. 26,27 Moreover, the nanopipet-supported ITIES was applied for fundamental studies to measure fast IT kinetics 10,11 and electrodeposit metal nanoparticles 30 at the nanoscale interface. Experimentally, a nanopipet is usually filled with a waterimmiscible organic solution of highly hydrophobic electrolytes and immersed in an aqueous electrolyte solution to form a nanometer-sized interface.…”

“…10 A recent study also demonstrated that the nanopipet-supported ITIES was protected from adventitious contaminants from the metal electrode in the organic phase and from ambient air to obtain reproducible current responses. 27…”

“…10A). 27 The recognition of CO 3 2À was mediated by highly selective molecular-tweezers-type CO 3 2À ionophore VII via covalent bond formation. 130 The fundamental understanding to realize this nanoscale CO 3 2À probe was newly addressed through both theoretical and experimental studies.…”

“…For instance, the highly selective ionophores developed for potentiometry 142 were successfully applied in amperometry/voltammetry at the microscale ITIES 125,131,143 but at the nanoscale counterpart only recently. 27 This nanotechnological breakthrough will allow for the utilization of not only commercially available ionophores but also the ionophores that have been developed in the field of supramolecular chemistry. In comparison with equilibrium potentiometry, the highly selective detection of multiple ions is achieved by dynamic voltammetry based on both thermodynamics and kinetics of ionophore-facilitated/accelerated IT reactions.…”

“…In comparison with equilibrium potentiometry, the highly selective detection of multiple ions is achieved by dynamic voltammetry based on both thermodynamics and kinetics of ionophore-facilitated/accelerated IT reactions. [93][94][95] The fast nanoscale IT reaction facilitated by a highly selective ionophore 27 is promising for the development of the ion sensor based on an array of the nanoscale ITIES. The incorporation of ionophores into NEs is also feasible 144 and attractive for sensing applications of single-entity electrochemistry.…”