Theoretical Study of Non-equilibrium Ionic Response Near Electrode Surface

Abstract

Recently, single-molecule detection using nanofluidic devices have attracted much attention, such as DNA sequencing via biological or solid-state nanopores. However, there are some difficulties to achieve it, since single molecules are tremendously affected by surrounding environments, such as thermal fluctuations and external perturbations. In such a nano-scaled confined space, electrophoretic or electroosmotic flows become effective. Moreover, due to very weak electrical signals of a single molecule, high-resolution current measurement is also crucial. On the other hand, noise in the background current should be suppressed properly to clearly recognize signals. In order for the optimal design, the ionic flow has to be understood in more detail. Herein, focusing on ionic currents in aqueous solution, we develop a theoretical model and analyze responses of ions near a biased electrode surface. Diffusion and migration of ions in non-uniform electric fields can be expressed by coupling the Nernst-Planck equation and the Poisson equation. Solving both equations self-consistently, density distributions can be determined. In this study, transient responses of some kinds of aqueous solution are investigated. As a result, rapid increase and subsequent slow decay of ionic current can be replicated well. It is found that the response time depends on rapid screening of a strong field near the electrode surface and slow change of a diffusion layer far from the surface. This is a reason why it takes long time period until the solution gets to a steady current state.