Graphene-based planar nanofluidic rectifiers

Structurally symmetric two-dimensional multilayered graphene oxide films, which facilitate ion transport through "nanochannels" comprising the interstitial spaces between each stacked sheet within the film, are for the first time shown to exhibit peculiar ion current rectification and nonlinear current-voltage characteristics below a critical electrolyte concentration when the interstitial spacing becomes comparable to the Debye screening length such that the film becomes permselective. We attribute the unexpected rectification behavior to the fore-aft asymmetry that arises in the diffusion boundary layer on both sides of the millimeter long film upon reversal between the high resistance positive bias state and the low resistance negative bias state, the asymmetry being primarily a consequence of the trapping and release of counterions within the film, compounded by the nonuniform electric field that occurs in the tortuous nanochannels within the film. In addition to elucidating the influence of the electrolyte concentration and applied bias voltage, we demonstrate the possibility of tuning the ion selectivity and hence the rectification behavior through the solution pH. These first graphene-based nanofluidic rectifiers, which are easily synthesized, therefore offer a flexible, robust, low cost, and facile large-scale alternative to conventional nanochannels that require elaborate and sophisticated nanofabrication.