Surface hydrophobicity governs collision dynamics of emulsion droplets on bare and SAMs-modified electrodes

Single-entity electrochemistry (SEE) provides abundant information at the single-particle level. The applications of self-assembled monolayers (SAMs) modified electrodes further improve the sensitivity and selectivity of electrodes in SEE. However, soft particles collision at the SAMs-modified electrodes have rarely been studied, the surface chemistry effect of which also remains unclear. Here, dynamic interactions between o/w-type nitrobenzene emulsion droplets and bare Au, bare C and SAMs-modified Au UMEs, as well as electron transfer (ET) kinetics were investigated by SEE. Three important conclusions have be obtained: (1) Soft particle-mediated ET: emulsion droplets colliding on the SAMs-Au UME could trigger and mediate ET by tunneling through the insulating SAMs. (2) Surface hydrophobicity effect: sharp collision current spikes changed to be single- or multi-peaks with the prolonged residence time in the later stage of measurement on the hydrophobic C UME and SAMs-Au UME, while kept consistent on the hydrophilic Au UME. (3) Hydrophobicity modulation mechanism: organic layers form and stay on the hydrophobic surface of the electrode due to the accumulation of the colliding emulsions, which hinder the ET via tunneling. This study highlights the hydrophobicity effect on the dynamic emulsion-electrode interaction and ET kinetics, which not only enriches the knowledge of SEE but also enlightens the design of modified electrodes and electrochemical sensors.