Electrochemically generating electric field gradients in the absence of buffer for membrane-free separations

Abstract

The work described herein focuses on modulating the electric field within microelectrochemical devices for electrokinetic separations. Specifically, two new electrochemical approaches were investigated for forming electric field gradients which are useful for manipulating ion motion. The first method involved the integration and electrochemical reduction of the intercalation material Prussian blue within a microfluidic device. The results showed that the reduction of Prussian blue and concomitant ion intercalation from solution selectively formed an ion depletion zone and corresponding electric field gradient. This electric field gradient proved useful for the separation and enrichment of a charged fluorophore in solution, representing the first step towards successful integration and use of intercalation materials for efficient and selective separations. The second electrochemical approach utilized water electrolysis at a bipolar electrode in the absence of buffer to locally vary solution conductivity and the amount of ionic current that passed through a microfluidic device. Experiments and finite element simulations were performed to confirm the presence of sharp electric field gradients in solution. Additionally, the electric field gradients near the bipolar electrode were shown to be useful for filtering and continuously separating anionic microplastic particles from solution. Subsequently, the electric field gradients formed near bipolar electrodes were used to enrich cations at specific locations within microelectrochemical devices. The cation enrichment proved to be a dynamic process due to the interrelationship between current passing through the bipolar electrode and solution conductivity. Finally, cation enrichment was performed in highly conductive, buffer-free solutions, demonstrating the broad utility of this electrochemical method for manipulating charged species. The results presented here introduce new methods for forming and utilizing electric field gradients within microelectrochemical devices. Importantly, these methods expand the scope of electrochemical separations leveraging electric field gradients, which is significant when considering future separation applications in solutions of interest like seawater or blood