We report a new application of the single-entity electrochemistry (SEE) to in situ measure a partition coefficient at intact nanoemulsions (NEs). The partition coefficient at intact NEs is the most crucial physicochemical property to determine the uptake of delivery molecules inside NEs. It, however, has not been unequivocally elucidated by currently existing techniques based on ex situ measurements. Herein, we apply the single-entity electrochemistry (SEE) to directly and quantitatively measure the partition coefficient at NEs in situ. In this work, we use NEs featured with amphiphilic triblock copolymer (Pluronic F-127) as a model system to extract/preconcentrate 2-aminobiphenyl (2-ABP) dissolved in the water and demonstrate a new application of SEE to in situ quantitatively estimate the amounts of 2-ABP distributed into each intact NE. Our SEE measurements reveal that the partitioning is governed by extraction of 2-ABP inside NEs rather than its adsorption on the NE surface, and this extraction is remarkably efficient with up to ∼8 orders of magnitude of the preconcentration factor, thus leading to the unprecedentedly large partition coefficient of 1.9 (±1.4) × 1010. This result implies that not only the thermodynamic distribution but also the intermolecular interaction of extracted compounds inside NEs could play a significant role in the apparent partition coefficient (P = 1.9 (±1.4) × 1010). The experimentally determined partition coefficient was validated by molecular dynamics (MD) simulations with showing a stabilizing role of intermolecular interaction in the partitioned system. We further verified our methodology with other compounds exhibiting aromatic properties, e.g., ferrocenemethanol. Significantly, our new approach can be readily applicable to investigate practical NEs commercially marketed for drug, food, and cosmetics.

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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–.