Single-Entity Electrochemistry of Nanoemulsion: The Nanostructural Effect on Its Electrochemical Behavior.

New electrochemical approaches have been applied to investigate nanoemulsions (NEs) for their nanostructures and the relevant electrochemical activity by single-entity electrochemistry (SEE). Herein, we make highly monodisperse NEs with ∼40 nm diameter, composed of biocompatible surfactants, castor oil as plasticizers, and ion exchangers. Dynamic light scattering (DLS) measurements with periodically varying surfactant to oil ratios provide us with a structural implication about uneven distributions of incorporating components inside NEs. To support this structural insight, we apply SEE and selectively monitor electron-transfer reactions occurring at individual NEs containing ferrocene upon each collision onto a Pt ultramicroelectrode. The quantitative analysis of the nanoelectrochemical results along with DLS and transmission electron microscopy (TEM) measurements reveal nanostructured compartments of incorporating components inside NEs and their effect on the electrochemical behavior. Indeed, a tunneling barrier inside NEs could be formed depending on the NE composition, thus determining an electrochemical behavior of NEs, which cannot be differentiated by a general morphological study such as DLS and TEM but by our SEE measurements. Furthermore, by employing the nanopipet voltammetry with an interface between two immiscible electrolyte solutions (ITIES) to mimic the NE interface, we could explicitly investigate that the electron-transfer reaction occurring inside NEs is facilitated by the ion-transfer reaction. Overall, these comprehensive electrochemical approaches enable us to elucidate the relation between structures and the electrochemical functionality of NEs and provide quantitative criteria for the proper compositions of NEs regarding their activity in the electrochemical applications. Also, this finding should be a prerequisite for suitable biomedical/electrochemical applications of NEs.