Single-emulsion toluene oil droplets (femtoliter) containing a hydrophobic redox probe that are dispersed in water stochastically collide with an ultramicroelectrode (UME). The fast-scan cyclic voltammetry (FSCV) or Fourier-transformed sinusoidal voltammetry (FTSV) is applied: the UME was scanned with a fast, repetitive triangular, or sinusoidal potential, and its current in time/frequency domains were monitored. The electron transfer at the UME/oil interface is coupled with ion transfer at the oil/water interface. Thus, the obtained transient voltammograms of a myriad of ions were used to estimate thermodynamics of ion transfer at the toluene/water interface. Additionally, the single-droplet voltammogram combined with finite element simulations reveal the droplet's size and shape distributions. Four collision mechanisms with new physical insights were also uncovered via comprehensive analysis of phase angle in the frequency domain, time domain FSCVs, and finite element simulations.

An ion transfer-resolved configuration has been introduced to study the fusion impact of a tiny (femtoliter) water-in-oil droplet probed at the micro liquid/liquid (L/L) interface. A periodic linear potential signal with a high frequency is applied in this configuration, and typical (facilitated) ion transfer voltammograms and capacitive voltammograms of single fusion events have been obtained. The responses with voltammetric characteristics allow us to identify ion transfer potentials and elucidate the mechanism in single fusion events. The aqueous droplets encapsulating femtomoles of different quaternary ammonium ions are successfully tracked by fusing with the L/L interface and are identified by their ion transfer potentials. In addition, a capacitive fusion impact is recorded in the obtained current response when no biphasic ion translocation event occurs during a single fusion process, revealing a new mechanism of charge exchange between the electrode surface and polarized interface in the alternating electric field. In addition, a single fusion impact revealed by the facilitating ion transfer is reported, and the obvious discrepancy of fusion event voltammograms with and without ionophore participation is established. By providing new physiochemical insights, this study enables extra trace analysis at the femtomole scale and provides a possible method to electrochemically characterize micro-/nanoscale heterogeneous media.

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