Entrance effect on ion transport in nanochannels

We investigate the effect of the surface charge at channel entrances upon ion conductance, which has been overlooked in the study of nanofluidics. Nonlinear ion transport behaviors were observed in 20-nm thick nanochannels having opposite surface charge polarity on the entrance side-walls with respect to that in the nanochannel. The heterogeneous distribution of surface charge at the channel entrance functions as a parasitic diode, which can cause ion current saturation under high voltage biases. Such effect becomes crucial at low bath concentration at which the electric double layers originated from the bath sidewalls pinch off the channel entrance. The experimental results are clarified by theoretical calculations based on 2D Poisson-Nernst-Planck equations. With such strong effect on ionic conductance of nanochannels, the change of surface charge polarity at the entrance sidewalls may find applications in chemical and biological sensing.