  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.
