Direct Electrolysis and Detection of Single Nanosized Water Emulsion Droplets in Organic Solvent Using Stochastic Collisions

Electrochemical Analysis of Attoliter Water Droplets in Organic Solutions through Partitioning Equilibrium

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.

Effects of Water Removal from Palm Oil Reactant by Electrolysis on the Fuel Properties of Biodiesel

Biodiesel, which is composed of mono-alkyl esters of long carbon-chained fatty acids, is used as an alternative fuel to petro-diesel. The water content of the reactant mixture of feedstock oil influences the extent of transesterification and thus the fuel characteristics. Lower water content in feedstock oil is generally suggested for successful transesterification. This experimental study removed water from the reactant mixture of feedstock palm oil and methanol during transesterification using various systems composed of either electrodes or molecular sieves with rotary vibration. The effect of input electrical energy, number of electrodes, vibration modes, and operating time on the amount of water removed from the reactant mixture and the fuel properties of the final biodiesel product were analyzed and compared with those achieved using molecular sieves. The results show that the biodiesel—after water was removed during transesterification—appeared to have increased kinematic viscosity, cetane index, distillation temperature, and acid value, while the heating value, flash point, ignition point, and water content decreased with an increase in the input electrical energy of the electrodes responsible for electrolyzing water away. Electrolysis by the double-pair electrodes was more effective at reducing acid value and water content than that performed by the single-pair electrodes under the same input electrical energy. The biodiesel was found to have the lowest water content (0.0304 wt.%) and the highest water-removal rate (0.011 wt.%) when water was removed during transesterification by the double-pair electrodes with an input electrical energy of 9 J/(g palm oil). The water-removal rate of the rotary-vibrating molecular sieves was 11.24 times that of the single-pair electrodes. The biodiesel was found to have increased kinematic viscosity with higher input electrical energy, reaching 5.15 mm2/s when the double-pair electrodes with an input electrical energy of 11 J/(g palm oil) were used. Longer carbon-chained fatty acids, ranging from C20 to C24, amounted to 0.74 wt.% of the biodiesel produced using the double-pair electrodes, which was greater than that seen for the single-pair electrodes. However, the molecular sieve method consumed more energy than the double-pair electrodes did to remove the same amount of water from the palm oil reactant mixture via transesterification.

In Situ Probing Liquid/Liquid Interfacial Kinetics through Single Nanodroplet Electrochemistry

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.

In Situ Monitoring of Collision and Recollision Events of Single Attoliter Droplets via Single-Entity Electrochemistry

We describe a simple method for real-time observation of collision and recollision behavior of a single aqueous attoliter droplet in an organic solvent through single-entity electrochemistry. The dynamics and morphology of the droplet after the collision event at the Au ultramicroelectrode (Au-UME) were monitored by consecutive cyclic voltammetry and amperometric current-time measurements. By sequentially applying oxidative potential and reductive potential at the Au-UME in the presence of attoliter droplets containing reversible redox species (e.g., ferrocyanide), we successfully detected the oxidative collision spike and its reductive recollision spike successively owing to the reversible redox reactions inside the droplet. Because the redox species was dissolved in a reduced form, the reductive collision spikes observed are the direct evidence that the water droplets colliding at the electrode surface are detached after the oxidation reaction. The collided droplet properties, such as size change and contact area, are also investigated and discussed.

Determination of Serotonin Concentration in Single Human Platelets through Single-Entity Electrochemistry

This research introduces a method to directly detect serotonin in a single platelet through single-entity electrochemistry. Platelets isolated from human blood were analyzed by cyclic voltammetry and current-time measurements. When a single platelet collides with an ultramicroelectrode, serotonin inside the platelet is oxidized at the electrode surface, and an anodic current peak is consequently observed during measurement. The concentration of serotonin can be determined by integrating this peak current. In addition, this method can be used to determine the platelet concentration. Analysis of the collision frequency of platelets can provide information about the platelet concentration in the blood. As a result, platelet levels and serotonin concentrations in single platelets can be measured quickly and easily.