A Tube of Tubes

With just benzene, ferrocene, and a simple laboratory apparatus, scientists have made robust filters composed entirely of multiwalled carbon nanotubes. Shaped like hollow cylinders, the filters are remarkably utilitarian. Easy to clean and reusable, they can remove bacteria and viruses from water, eliminate heavy hydrocarbons from petroleum, and even separate a mixture of benzene and naphthalene [Nat. Mater., published online Aug. 1, http://dx.doi.org/10.1038/nmat1192].

Onkar N. Srivastava, a physics professor at Banaras Hindu University, in India, and Pulickel M. Ajayan, a materials science professor at Rensselaer Polytechnic Institute, Troy, N.Y., collaborated to create the nanotube-based structures.

 It’s easy to see why Srivastava and Ajayan would think of carbon nanotubes as ideal building blocks for membranes and filters. By virtue of their tiny diameters, an array of nanotubes standing side by side could easily separate small particles from solution.

 The overall architecture of the filter also is important, the researchers say. It has a tubular shape in which the nanotubes are radially aligned, like the fibers in a roll of carpet. The researchers seal one end of the hollow cylinder and pass a solution through its interior. Any unwanted material remains on the filter’s inner walls or within its pores. Srivastava and Ajayan say that this tubular arrangement makes the filter easy to clean: They simply remove the seal and wash off any debris.

 “In general, this is a nearly ideal geometry of aligned carbon nanotubes that can have numerous applications in chemical separations,” comments Bruce J. Hinds, an assistant professor of chemical and materials engineering at the University of Kentucky who has made nanoporous membranes out of aligned multiwalled carbon nanotubes [Science, 303, 62 (2004)]. “In particular, the overall tubular geometry is ideal for cross-flow filtration that reduces fouling and cleaning cycles, which is of significant practical importance.”

 Srivastava and Ajayan’s team build the structures using a standard chemical vapor deposition (CVD) process. They have made filters in a range of sizes, the largest measuring several centimeters in length and diameter, with walls that are between 300 and 500 m thick. Ajayan says that, in terms of size, the group is limited by what can be set up in the lab. “I don’t foresee any limits in the length,” he adds, but reckons that because of the CVD process, “the thickness of the wall might be limited to some extent.”

 The filters look like heavy rubber tubing, but Ajayan says they feel like a brittle ceramic. They’re robust enough to handle but will break if crushed. “It’s all made of carbon, so you can heat it,” he explains. “That’s an advantage over polymer or cellulose filters.” The filters function up to 400 °C, whereas conventional polymer membrane filters can’t go much hotter than 50 °C. The nanotube filters can also withstand ultrasonication and autoclaving cleaning routines.

 So far, the team has used the filters for both organic and aqueous separations. They’ve separated heavy hydrocarbons out of petroleum—an important step in gasoline production. The group also used the filters to make water contaminated with Escherichia coli bacteria safe enough to drink. Perhaps the most remarkable separation was the removal of poliovirus, which is less than 30 nm wide, from water.

 The researchers say they aren’t exactly certain how the actual filtering takes place, although they speculate that the primary conduit is through the spaces in between the nanotubes. There’s probably some adsorption along the nanotube walls, they argue, and some filtrate may flow through the nanotubes themselves, although catalyst particles often clog that route.

 The group’s report highlights potential high-volume applications, such as petroleum and water filtrations, but Ajayan admits that because of costs, the filters probably won’t be used in those areas. Instead, he says, they’re far more likely to be adopted for specialized separations where highly controlled pore size is prized.