Grimes and his team at Penn State's Materials Research Institute have come up with an etching method that turns a flat titanium surface into a densely packed forest of tiny metal towers, or nanotubes. The spaces between these tubules, it turns out, make a perfect trap for hydrogen.

"Historically," Grimes explains, "we have viewed sensor technology from the point of view of surface area." The greater the surface area of a reactive material, the more interaction with gaseous molecules. With titania nanotubes, however, "The sensitivity comes from the nanoarchitecture," Grimes says. Hydrogen molecules caught in the cracks between the tubules are forced to react instead of bouncing away. Thus, doubling the surface area by etching all those tubules does more than double the sensitivity: It increases it 200-fold.

To make the tubules, Grimes explains,a flat sheet of titanium foil is wired to an electrical current and dipped in an acid bath. Oxygen molecules in the water immediately react with the metal, forming a protective coating across the surface. Then the acid etches millions of tiny pits through the new surface layer. With the aid of the electrical current, the exposed metal at the bottom of each depression dissolves faster than the oxide layer above. Over time, the pits grow into deep pores, and the metal around the outside walls of the pores dissolves too, leaving millions of chimney-shaped tubules.

Once mounted in a sensor, the nano-tubes interact with infinitesimal traces of hydrogen gas. When the gas molecules, which are composed of two hydrogen atoms, hit the surface of the sensor, they divide, and the uncoupled atoms slip down into the spaces between the tubes. There, the surface absorbs electrons from each atom, causing the electrical resistance of the sensor to decrease. Since electrical resistance is easy to measure with current technology - and H₂ levels well under one part per million cause a measurable response in the nanotubes - highly sensitive detectors could be developed for only $200, Grimes says, a tremendous savings over the $50,000 required for a traditional gas chromatograph with comparable function.

Small, affordable, and supersensitive hydrogen sensors could be put to use in a wide variety of fields, including medicine, he adds. Many bacterial infections of the digestive system produce hydrogen gas. In adult problems like lactose intolerance and celiac disease, standard detectors are good enough for diagnosis, but require a long procedure, while a bandage-like nanotube sensor could easily take measurements during daily life. In addition, during the early stages of necrotizing enterocolitis, a leading cause of infant mortality, levels of H₂ are currently undetectable, and the disease usually progresses until more severe symptoms are noticed. Grimes is currently working with James Kendig and Charles Palmer of Penn State College of Medicine to determine whether titania nanotube sensors can pick up hydrogen at this low level. If the sensors pass this test, they could become standard equipment in hospital wards worldwide, saving thousands of new lives every year.

-Joseph Gyekis

Craig Grimes, Ph.D., is associate professor of materials science and engineering in the Department of Electrical Engineering and is head of a research team at the Materials Research Institute, 217 Materials Research Laboratory, University Park, PA 16802; 814-865-9142; cgrimes@engr.psu.edu. James W. Kendig, M.D., is professor of pediatrics at Penn State College of Medicine and attending neonatologist at the Penn State Children's Hospital, Milton S. Hershey Medical Center, 500 University Drive, H085 Main Building, Hershey, PA 17033-0850; 717-531-8412; jkendig@hmc.psu.edu. Charles Palmer, M.B. Ch. B., is a staff physician in Neonatology Professional Services at the Hershey S. Medical Center, 500 University Drive, H085 Main Building, Hershey, PA 17033-0850; 717-531-8412 cxp5@psu.edu.