February 15, 2007

Better Nanoscale membranes

This is the 1000th article posted on this site.

There are three new membranes with nanoscale dimensions that have been created. One is silicon membrane which could filter proteins and could be used for dialysis and one is a carbon nanotube membrane that can control water flow. Be able to control and purify molecules will be an important part of molecular nanotechnology systems. These could be bootstrapping technologies and they are interesting and have other potential as well. Another is a ceramic nanomesh which could filter HIV from blood.

MIT Technology Review and Physorg have articles about a new silicon membrane with nanoscale holes that can act roughly 10 times faster than current membranes used for blood dialysis, the artificial purification of blood.

A silicon wafer with 160 nanoporous silicon membranes. Each 15-nanometer-thick, 200-by-200-micrometers-square membrane is at the center of the 160 squares patterned into the wafer. Credit: University of Rochester

Current molecular-level filters use a polymer-based design that is a jumble of varying holes and tunnels. The sizes of holes in the polymer model vary greatly, and since its "holes" are really convoluted tunnels through the material, they require much more time for proteins to pass through, and they are prone to clogging.

The new membrane is 15 nanometers thick, so it filters faster without trapping the molecules that pass through it, which is important if researchers want to retain both the larger and smaller proteins. "Once a molecule gets to the membrane, it takes one step, and it's on the back side," McGrath says.

To make the membranes, the researchers employ tools that are used to create integrated circuit chips. This should make the filters easy to integrate into silicon-based microfluidic devices that are used for protein research, where they would be useful if scientists wanted to separate a particular protein of interest from a biological fluid sample. The researchers made the membranes by first depositing a stack of three thin layers--an amorphous silicon layer sandwiched between two silicon-dioxide layers--on a silicon wafer. Exposing the wafer to temperatures higher than 700 ºC crystallizes the amorphous silicon, and it forms pores. Then the researchers etch the wafer and silicon-dioxide layers to expose small squares of the nanoporous membrane that are 200 micrometers on each side. The temperature controls the pore diameter, allowing the researchers to fine-tune the membranes: at 715 ºC the membrane has an average pore size of 7 nanometers, while at 729 ºC the average is about 14 nanometers.

By fusing wet and dry nanotechnologies, researchers at Rensselaer Polytechnic Institute have found a way to control the flow of water through carbon nanotube membranes with an unprecedented level of precision. The research, which will be described in the March 14, 2007 issue of the journal Nano Letters, could inspire technologies designed to transform salt water into pure drinking water almost instantly, or to immediately separate a specific strand of DNA from the biological jumble.

Precise control of water transport through a nanotube membrane is demonstrated by a novel electro-chemical approach

HIV may one day be able to be filtered from human blood saving the lives of millions of people, thanks to a world-first nano-membrane innovation by Queensland University of Technology scientists. QUT scientists have developed specially designed ceramic membranes for nanofiltration, which are so advanced they have the potential to remove viruses from water, air and blood. Preliminary research had proved it successful in removing viruses from water.


Nils Larsson said...

One should be careful in their use of the term "nanotube membrane". The RPI work features nanotube "arrays", not true membranes with nanotubes as pores. The leap between this demonstration of electrochemically controlled water transport and desalination in a huge one and I fail to see the connection. Even with charged functional groups on the nanotube surface, the pores between these multiwall tubes are most certainly too large (10s nm) to permit efficient ion removal. Why put a bias on the nanotubes to electrochemically drive transport when you can simply apply pressure to drive water through a true nanotube membrane, as described by the groups at Livermore and Kentucky?