Novel experimental techniques allow for the manipulation and interrogation of biomolecules between metallic probes immersed in micro/nanofluidic channels. The behavior of ions in response to applied fields is a major issue in the use of these techniques in sensing applications. Here, we experimentally and theoretically elucidate the behavior of background currents in these systems. These large currents have a slowly decaying transient response, as well as noise that increases with ionic concentration. Using mechanically controllable break junctions (MCBJ), we study the ionic response in nanogaps with widths ranging from a few nanometers to millimeters. Moreover, we obtain an expression for the ionic current by solving time-dependent Nernst-Planck and Poisson equations. This expression shows that after turning on an applied voltage, ions rapidly respond to the strong fields near the electrode surface, screening the field in the process. Ions subsequently translocate in the weak electric field and slowly relax within the diffusion layer. Our theoretical results help to explain the short- and long-time behavior of the ionic response found in experiments, as well as the various length scales involved.

Novel approaches to DNA sequencing and detection require the measurement of electrical currents between metal probes immersed in ionic solution. Here, we experimentally demonstrate that these systems maintain large background currents with a transient response that decays very slowly in time and noise that increases with ionic concentration. Using a non-equilibrium stochastic model, we obtain an analytical expression for the ionic current that shows these results are due to a fast electrochemical reaction at the electrode surface followed by the slow formation of a diffusion layer. During the latter, ions translocate in the weak electric field generated after the initial rapid screening of the strong fields near the electrode surfaces. Our theoretical results are in very good agreement with experimental findings.