text: It requires a channel to be narrow and long for it to have a good selectivity. In other words, a channel with a high aspect ratio has a better selectivity. To further increase its selectivity, it is required to have a highly charged wall.
  The performance of ionic selectivity also largely related to the applied bias. With a low bias, a high selectivity is observed. With the increase of the bias voltage, there is an apparent decrease in the selectivity. For a nanochannel with a low aspect ratio, high selectivity is possible when the bias voltage is low.