Nanofiltration

[Image Credit: Song et al. 1996. Science 274, 1859.]

One clear application of engineering on the scale of individual atoms and molecules is simply filtration -- or materials selection. Currently, a number of researchers across the world have already developed the hardware for filtering certain minute particles out of a slurry of other, perhaps unwanted, particles. Various methods and tools have been created in the past three years by which pores the size of only a nanometer or two can be effectively and efficiently produced. In short, the ability to filter effectively on the scale of nanometers already exists; however, the potentially thousands of applications of this technology are only beginning to be discovered.

In 2006, a team of researchers at the University of California, San Diego found that using a form of nanofilter could be a fast and easy way to sequence DNA.

According to the researchers, although the ability to sequence the human gnome exists -- in fact, the entire human gnome has already been mapped -- it is far too slow to make it technically feasible to synthesize taylor-made treatments to particular human diseases, or to particular humans.

When it comes to nanofabricated pores, the advantage is not simply that they are so small; it is that they can be constructed in such a way as to take advantage of the electromagnetic and even quantum properties of the filtering materials. DNA, according to the researchers, can have its natural electromagnetic vibrations minimized by forcing it into close proximity with certain nanoporous materials. This means that DNA can be pushed through the pours with less noise and greater efficiency.

Still, this is just one of many applications of nano-constructed filtration mechanisms.

Doug Gin, professor in Chemistry and Biochemistry, as well as Chemical and Biological Engineering, has recently developed a new process for constructing what he calls designer polymer liquid crystals.

These particular liquid crystals can be shaped into tools for filtering virtually anything out of water, he said – including salt molecules.

“We design special soap molecules so that they form a table size that is basically three-quarters of a nanometer deep,” Gin said.

Soaps, which are a type of fatty-acid, can form many types of crystalline structures.

The soap molecules Gin uses, which come in the shape of a vast series of hexagons, fit together like puzzle pieces upon a flat surface substrate. At the center of each minute molecule is an even smaller hole or pour, causing each individual molecule to resemble a nano-scale donut.

"This is the only technology we know of that can get uniform pour sizes of less than a nanometer,” he said.

Gin has found that his single-molecule-think nanostuctures manage to filter-out 95-99.9 percent of all dissolved salt ions, neutral molecules and other large molecules, as well as most molecular ions between the sizes of 0.64 and 1.2 nanometers in just one pass.

Ultimately, with cheap fabrication techniques and the exceptionally high efficiencies Gin is getting, filtering salt water, brackish water, or polluted water could be a significantly more surmountable endeavor, for regions of the world lacking in clean fresh water reserves.