On-Chip Electrokinetic Micropumping for Nanoparticle Impact Electrochemistry

Abstract

Single-entity electrochemistry is a powerful technique to study the interactions of nanoparticles at the liquid–solid interface. In this work, we exploit Faradaic (background) processes in electrolytes of moderate ionic strength to evoke electrokinetic transport and study its influence on nanoparticle impacts. We implemented an electrode array comprising a macroscopic electrode that surrounds a set of 62 spatially distributed microelectrodes. This configuration allowed us to alter the global electrokinetic transport characteristics by adjusting the potential at the macroscopic electrode, while we concomitantly recorded silver nanoparticle impacts at the microscopic detection electrodes. By focusing on temporal changes of the impact rates, we were able to reveal alterations in the macroscopic particle transport. Our findings indicate a potential-dependent micropumping effect. The highest impact rates were obtained for strongly negative macroelectrode potentials and alkaline solutions, albeit also positive potentials lead to an increase in particle impacts. We explain this finding by reversal of the pumping direction. Variations in the electrolyte composition were shown to play a critical role as the macroelectrode processes can lead to depletion of ions, which influences both the particle oxidation and the reactions that drive the transport. Our study highlights that controlled on-chip micropumping is possible, yet its optimization is not straightforward. Nevertheless, the utilization of electro- and diffusiokinetic transport phenomena might be an appealing strategy to enhance the performance in future impact-based sensing applications.

Utilizing the Oxygen Reduction Reaction in Particle Impact Electrochemistry: A Step toward Mediator-Free Digital Electrochemical Sensors

“…Charged particles can be seen to move along the surface of the glass shroud and enter the UME area (see also Figure 3 a). These trajectories 52 are in the opposite direction from the trajectories for an oxidation reaction (e.g., using a Fc-based mediator), in which particles typically approach the electrode from above ( Figure 3 b). 53 , 54 This is attributed to electroosmotic flows (EOF), in which the generation of charged species at the UME induces a tangential electric field that causes electroosmosis in the vicinity of the charged insulating material (glass).…”

“…Charged particles can be seen to move along the surface of the glass shroud and enter the UME area (see also Figure a). These trajectories are in the opposite direction from the trajectories for an oxidation reaction (e.g., using a Fc-based mediator), in which particles typically approach the electrode from above (Figure b). , This is attributed to electroosmotic flows (EOF), in which the generation of charged species at the UME induces a tangential electric field that causes electroosmosis in the vicinity of the charged insulating material (glass). , The fields induced by the oxidation reaction have the opposite polarity from the reduction case, leading to the reversal of both electrophoretic and EOF-induced forces on the particles. At high salt concentrations (Movie S1), positively charged particles move along the surface and adsorb onto the edge of the UME.…”

Recent Developments in Single-Entity Electrochemistry

“…Recent work has also demonstrated preparation of similar 3D ring MEAs using an inkjet printing approach . Additionally, on-chip MEAs embedded with a macroelectrode have been explored, with the 1000 times larger macroelectrode polarized to control particle motion towards the detecting microelectrodes, enabling studies of electrokinetic transport on impact electrochemistry . These works highlight the extensive applications of MEAs in stochastic collision electrochemistry.…”

Electronic Circuit Simulations as a Tool to Understand Distorted Signals in Single-Entity Electrochemistry

“…Thus, we suggest that the apparent mass transfer, with k about 45% higher than expected, may be attributable to migration of chloride ions, partial oxidation or asymmetric dissolution of silver nanoparticles. Enhancement through ion migration is likely, since nano impact experiments are often performed in nonfully supported electrolyte. , In addition to migration, electroosmotic convection might also play a role owing to the local electric field; however, it is expected to be minor, since the background current is relatively low compared to the impact current. ,, If the oxidation of the nanoparticle is not completed during the impact, the measured Q impact would correspond to only a fraction of the total charge required to fully oxidize the nanoparticle, leading to inaccurate calculation of the nanoparticle’s radius r 0,NP . With an underestimated initial condition, the simulation will produce an impact spike with a lower peak height.…”

“… 28 , 29 In addition to migration, electroosmotic convection might also play a role owing to the local electric field; however, it is expected to be minor, since the background current is relatively low compared to the impact current. 14 , 28 , 30 If the oxidation of the nanoparticle is not completed during the impact, the measured Q impact would correspond to only a fraction of the total charge required to fully oxidize the nanoparticle, leading to inaccurate calculation of the nanoparticle’s radius r 0,NP . With an underestimated initial condition, the simulation will produce an impact spike with a lower peak height.…”