Abstract
We report here the electrochemistry of emulsion droplets by observing single emulsion droplet collisions with selective electrochemical reduction on an ultramicroelectrode (UME). With appropriately applied potentials at an UME, we can observe the electrochemical effects of single collision signals from the complete electrolysis of single emulsion droplets, or selective electrolysis of redox species in single emulsion droplets. This was observed with nitrobenzene (NB), 7,7,8,8-tetracyanoquinodimethane (TCNQ), and ionic liquid. The NB, TCNQ, and ionic liquid act as emulsion material, redox specie, and emulsifier (and electrolyte), respectively. NB emulsions and NB (TCNQ) emulsions were made by ultrasonic processing. During the amperometric current-time (i-t) curve measurement with NB/water emulsion at -0.65 V, reduction of NB emulsion droplets was measured. In the case of less negative potentials, e.g., at -0.45 V with a NB (TCNQ) emulsion, selective reduction of TCNQ in NB droplet was measured. Spike-like responses from electrolysis of NB or TCNQ in each experiment were observed. From these single-particle collision results of NB and NB (TCNQ) emulsions, the collision frequency, size distribution, i-t decay behavior of emulsion droplets, and possible mechanisms are discussed.
Highlights
There has been considerable activity in the field of singleparticle collisions for over a decade, since it allows one to obtain important information on single-particle properties, e.g., catalytic activity, size, and lifetime, that is not available in ensemble results.[1−9] So far, most single-particle collision experiments have been focused on “hard” particles such as metal nanoparticles (Pt, Au, Cu, Ag),[1−5] oxides (IrO2, TiO2),[6,7] and dielectrics.[8,9] Recently, we extended the single-particle collision research to “soft” particles such as emulsions
We used a high concentration of hydrophilic redox molecules (200 mM potassium ferrocyanide) in the aqueous continuous phase containing the same emulsion droplets, where the UME is biased at the potential for the oxidation of ferrocyanide
Cyclic voltammetry (CV) on a 10 μm C-UME was measured to demonstrate the electrochemical reduction of 20 mM TCNQ in bulk NB solution with 200 mM IL-PP for comparison with the same reaction within an emulsion droplet (Figure 2)
Summary
There has been considerable activity in the field of singleparticle collisions for over a decade, since it allows one to obtain important information on single-particle properties, e.g., catalytic activity, size, and lifetime, that is not available in ensemble results.[1−9] So far, most single-particle collision experiments have been focused on “hard” particles such as metal nanoparticles (Pt, Au, Cu, Ag),[1−5] oxides (IrO2, TiO2),[6,7] and dielectrics (polystyrene, SiO2).[8,9] Recently, we extended the single-particle collision research to “soft” (or “liquid”) particles such as emulsions. Once the emulsion droplets collide, redox molecules inside the oil emulsion droplet start to be electrolyzed, and a spike type of current increase is observed during the current−time (i−t) measurements. These EDR and EDB methods gave a collision signal from a single emulsion droplet, which enables one to gain information about the stochastic events such as droplet size distribution, collision frequency, and droplet contents These suggested concepts can provide a better understanding for the collision of other types of single “soft” particles with an UME in previously published papers based on single liposome and vesicle detection methods.11−16Also, we discuss the ion transfer across the droplet interface between organic and water phase as reported by Scholz for his 2 μL droplet.[17]. Some redox-active organic molecules (e.g., NB) are hydrophobic enough to form an emulsion droplet and simultaneously behave as a redox mediator for an electrolysis reaction The average diameter of a NB droplet in the emulsion was approximately calculated from the total NB volume (0.1 mL) divided by the average emulsion droplet volume (5.96 aL, assumed to be spherical) gave a diameter 225 nm
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