Control of the ion diffusion coefficient in a nanochannel

In this study, we first propose a simple yet novel method to measure the diffusion coefficient of ions through a nanochannel. Back-side track etching is used for the fabrication of a nanochannel on an n-type silicon substrate. A metal-oxide semiconductor field-effect transistor (MOSFET) like device named the metal-semiconductor-solution field-effect transistor (MSSFET) is implemented to control the ion diffusion current. When a negative gate voltage is applied, positive ions that travel along the nanochannel are confined to the central zone of the nanochannel allowing the radial Brownian movements to be reduced. The effect is equivalent to an increase of the diffusion coefficient. However, a positive gate voltage can produce an opposite Zeta potential on the nanochannel surface. The cations in the nanochannel are dragged to the channel surface. This condition can be regarded as a decrease of the diffusion coefficient. Experimental results illustrate that the transfer characteristics of the MSSFET are similar to those of a p-channel depletion-type MOSFET. The ion diffusion coefficient in a nanochannel can be controlled when the the initial ion concentration difference across a nanochannel is larger than a certain threshold.