Credit: © 2007 ACS

In electronic circuits, a diode restricts the flow of electric current to one direction. A nanofluidic diode performs a similar task, but controls the direction in which ions flow inside nanosized channels. Such devices can be used to control the delivery of chemicals in integrated 'lab-on-a-chip' systems.

Zuzanna Siwy and Ivan Vlassiouk1 at the University of California in Irvine have made a nanofluidic diode by patterning conical nanopores into a polymer membrane. The inner surface of the cone was chemically modified to make the region near one end positively charged and the rest of it negatively charged. When a solution of KCl enters the pore, the positive K ions and negative Cl ions accumulate in the regions with charges opposite to their own. The polarity of the voltage applied across the pore determines which way the ions move, resulting in either a high or low ionic current flowing though the nanopore. The ratio of the two currents measured with opposite bias — the rectification — can be in the order of several hundred.

Using a different approach, Rohit Karnik at the University of California, Berkeley and co-workers2, have used a new technique called diffusion-limited patterning, in which rectangular nanofluidic channels are half-filled with avidin, a highly cationic protein. The channels were pretreated with biotin, a small neutral molecule that bonds strongly with avidin. This leads to two distinct charge regions along the channel — positive where coated with avidin and neutral elsewhere — which controls the flow of ions.