  Nanopores that exhibit ionic current rectification (ICR) behave like diodes,
such that they transport ions more efficiently in one direction than the other.
Conical nanopores have been shown to rectify ionic current, but only those with
at least 500 nm in length exhibit significant ICR. Here, through the finite
element method, we show how ICR of conical nanopores with length below 200 nm
can be tuned by controlling individual charged surfaces i.e. inner pore surface
(surface_inner), and exterior pore surfaces on the tip and base side
(surface_tip and surface_base). The charged surface_inner and surface_tip can
induce obvious ICR individually, while the effects of the charged surface_base
on ICR can be ignored. The fully charged surface_inner alone could render the
nanopore counterion-selective and induces significant ion concentration
polarization in the tip region, which causes reverse ICR compared to nanopores
with all surface charged. In addition, the direction and degree of
rectification can be further tuned by the depth of the charged surface_inner.
When considering the exterior membrane surface only, the charged surface_tip
causes intra-pore ionic enrichment and depletion under opposite biases which
results in significant ICR. Its effective region is within ~40 nm beyond the
tip orifice. We also found that individual charged parts of the pore system
contributed to ICR in an additive way due to the additive effect on the ion
concentration regulation along the pore axis. With various combinations of
fully/partially charged surface_inner and surface_tip, diverse ICR ratios from
~2 to ~170 can be achieved. Our findings shed light on the mechanism of ionic
current rectification in ultra-short conical nanopores, and provide a useful
guide to the design and modification of ultra-short conical nanopores in ionic
circuits and nanofluidic sensors.
