In this study, we report the new application of single nanodroplet electrochemistry to in situ monitor the interfacial transfer kinetics of electroactive species across liquid/liquid interface. Interfacial kinetic information is crucial in drug delivery and membrane transport. However, interfacial information has been mainly studied thermodynamically, such as partition coefficient, which could not manifest a speed of transfer. Herein, we measure the phase-transfer kinetic constant via the steady-state electrochemistry of an extracted redox species in a single nanodroplet. The redox species were transferred from the continuous oil phase to the water nanodroplet by partition equilibrium. The transferred redox species are selectively electrolyzed within the droplet when the droplet contacts with an ultramicroelectrode, while the electrochemical reaction of the redox species outside the droplet (i.e., organic solvent) is effectively suppressed by adjusting the electrolyte composition. The redox species in the water droplets can quickly attain a steady state during electrolysis owing to an extensive mass transfer by radial diffusion, and the steady-state current can be analyzed to obtain kinetic information with help from the finite-element method. Finally, a quick calculation method is suggested to estimate the kinetic constant of phase transfer without simulation.

We report enhanced electrochemical detection of single water-in-oil emulsion droplets using the nano-impact method. To detect the emulsion droplets, the water molecules in the droplets were directly oxidized (i.e., water splitting) without additional electroactive species when the droplets collided with the ultramicroelectrode. The water molecules in the emulsion droplet cannot be directly electrolyzed in an organic solvent because the emulsifier does not require a hydrophobic electrolyte. To enhance the signal intensity, the electrochemistry of sub-microscale single droplets was investigated considering the charge neutrality and limiting reagent. Therefore, effective electrolysis of the droplets was achieved. Approximately 10% of water molecules in the droplet (55.6 M H2O) were oxidized based on calculations from the electrochemical peak analysis and DLS measurements.

Herein, we report the electrochemical monitoring of attoliters of water droplets in an organic medium by the electrolysis of an extracted redox species from the continuous phase upon collisional events on an ultramicroelectrode. To obtain information about a redox-free water droplet in an organic solvent, redox species with certain concentrations need to be contained inside it. The redox species inside the droplet were delivered by a partitioning equilibrium between the organic phase and the water droplets. The mass transfer of the redox species from the surrounding organic phase to the droplet is very fast because of the radial diffusion, which resultantly establishes the equilibrium. Upon the collisional contact between the droplet and the electrode, the extracted redox species in the water droplets were selectively electrolyzed, even though the redox species in the organic continuous phase remained unreacted because of the different solvent environments. The electrolysis of the redox species in the droplets, where the concentration is determined by the equilibrium constant of the redox species in water/oil, can be used to estimate the size of single water droplets in an organic solution.

No abstract