Impact of Surface Chemistry on Emulsion–Electrode Interactions and Electron-Transfer Kinetics in the Single-Entity Electrochemistry

Abstract

Single-entity electrochemistry (SEE) provides powerful means to track a single particle, single cell, and even single molecule from the nano to microscale. The electrode serves as not only the detector of collision but also the surface supplier in SEE, and the fundamental understanding of the electrode surface chemistry on the dynamic particle−electrode interactions and electrochemical responses of a single particle still remains unexplored, particularly for soft particles. Herein, dynamic interactions of microemulsions and the interaction-controlled electron-transfer (ET) kinetics are studied employing SEE and fluorescence spectroscopy. The o/w-type nitrobenzene emulsions were prepared with the surfactant-type room temperature ionic liquids (RTILs). Biased the electrode potential for the reduction of 7,7,8,8-tetracyanoquinodimethane within emulsions, it is surprising to see the distinct collision current signals on the carbon fiber ultramicroelectrode (C UME) and Au ultramicroelectrode (Au UME) in the late stage of chronoamperometric measurements. Theoretical understanding was made to determine the ET kinetics behind the disparate current signals. It is believed that the electrode surface chemistry, i.e., the surface energy, has a great influence on the dynamic emulsion− electrode interactions and ET kinetics. On the hydrophilic surface of Au UME, emulsions tend to decompose/detach from the electrode surface immediately after colliding. In contrast, on the lipophilic surface of C UME with lower surface energy, a layer of oil phase accumulated by the coalescence of emulsions and the migration of the precedent colliding emulsions, which would serve as a barrier to block ET via tunneling as manifested by the gradual slowdown of ET rate and the reduced collision frequency in the late stage of measurement. The impacts of the emulsion size and amphiphilicity of RTILs on the C UME-emulsion interactions and ET kinetics were also investigated.

Recent Developments in Single-Entity Electrochemistry

“…This approach allows for atomic restructuring and dynamic changes to chemical reactivity to be monitored as the applied electrochemical potential is cycled. In a different approach, Du et al formed Nile Red-doped emulsions of nitrobenzene in water, allowing SE collisions to be imaged using fluorescence microscopy . Electrochemical data of the collision of the emulsions with either gold or carbon ultramicroelectrodes showed a difference in the number of collisions and electron-transfer kinetics based on the electrode material.…”

“…In a different approach, Du et al formed Nile Red-doped emulsions of nitrobenzene in water, allowing SE collisions to be imaged using fluorescence microscopy. 231 Electrochemical data of the collision of the emulsions with either gold or carbon ultramicroelectrodes showed a difference in the number of collisions and electron-transfer kinetics based on the electrode material. Fluorescence microscopy data revealed that the slower electron-transfer kinetics and reduced number of collisions on the carbon ultramicroelectrode surface were due to the adsorption and coalescence of the emulsions, which formed a thin organic layer that blocked the surface of the electrode, slowing electron-transfer rates in subsequent collision events.…”