The filter consists of a polycarbonate membrane etched with tiny, evenly-sized pores less than 10 nanometers -- a few billionths of an inch -- in size. The pores are lined with a thin layer of gold and then with another layer of oily molecules called thiols. The thiols spontaneously arrange themselves into a membrane one molecule deep, with all the thiol molecules pointing the same way.
These thiols are chains of carbon atoms, with a sulfur atom at one end and an acidic region at the other end. The sulfur allows the thiol to stick to the gold layer, and the acidic end can then interact with whatever flows past. The final pores are less than nine nanometers wide.
UC Davis researchers Kyoung-Yong Chun and Pieter Stroeve found that by changing the pH on either side of the membrane, they could "open" or "close" the pores to different proteins even of similar size, using a method called electrostatic screening. Existing filters can only effectively separate proteins or biological molecules of different sizes.
"The switchable technology will be important for transport on the nano-scale, particularly for nano and micro-sensing, analysis on a chip and micro-fluidic devices," Stroeve said. Another application could be in controlled drug release, supplying drugs over a period of time when the body needs it, he said.
The work is published in the June 11 issue of the journal Langmuir.