IDEAL SENSORS FOR TERROR ATTACK DON'T EXIST YET
01 Apr 2003
Source: New York Times, April 1, 2003
Ideal Sensors for Terror Attack Don't Exist Yet
By KENNETH CHANG
If something poisonous wafts onto a battlefield in Iraq, American soldiers would want enough warning to put on their gas masks. If terrorists release nerve gas in a subway station, officials would want quick identification of the toxin.
If someone arrives at a hospital with suspicious symptoms, doctors would want a fast test to determine whether the sickness is anthrax or just the flu.
After the World Trade Center and anthrax attacks in 2001, the federal government has doubled financing for counterterrorism research, including improved detectors, in each of the last two years. The research has generated promising advances, but "the perfect system doesn't exist," said Dr. Duane L. Lindner, deputy director for chem/bio programs at the Sandia National Laboratories site in Livermore, Calif.
Accurate systems are slow, bulky and expensive. The simple quick tests are prone to "false positives," finding danger where none exists.
Nonetheless, sensors have been installed in some crowded public spaces like Washington Metro stations and San Francisco International Airport. In New York and elsewhere, stations used by the Environmental Protection Agency to measure air quality are being modified to sniff for dangerous germs, as well.
Because no current technique fits the ideal — fast, accurate, simple and cheap — "people are forced to make various compromises," Dr. Lindner said.
For example, shining an ultraviolet laser in the air can disclose an approaching cloud of biological particles, because certain light frequencies cause biological molecules to glow faintly. That technique may provide a few minutes of warning to soldiers of a possible attack — time to put on masks. But it is ineffectual as a warning system in a city, where the particles would much more likely be pollen rather than anthrax.
In pursuit of better detectors, scientists are trying to exploit particles of gold, diamond film, lasers and even plants. Many researchers have set up companies, hoping to turn their endeavors into commercial products in a few years.
In the current war, Iraq has not yet fired biological or chemical weapons, so it is not known how well the American battlefield detectors will provide early warning, but they at least do not appear to be raising rampant false alarms. In the first gulf war in 1991, the chemical detectors repeatedly sent false alarms, mistaking diesel fumes and insecticides for chemical weapons.
Most current sensors for chemical agents use one of two techniques. One, surface wave acoustic detection, uses a thin membrane, usually made of quartz, vibrating at high frequencies. The membrane surface is coated to attract certain chemicals. If present, those chemicals stick to the membrane, slowing its vibrations.
The second technique, ion mobility spectroscopy, adds and subtracts electrons from the chemical molecules, making them electrically charged, and then pushes the charged molecules with an electric field. The speed that the molecules are pushed, bouncing through a gas, gives a measure of their size.
Neither technique is infallible. Surface wave detectors can be fooled by other molecules that also happen to stick to the membrane. Ion mobility detectors cannot easily differentiate between a nerve gas molecule and any other molecule of the same size.
Another technique, mass spectroscopy, which breaks apart a molecule, accelerates the charged fragments and bends their paths in a magnetic field, provides much surer identification. But it has not been widely used to detect chemical weapons, because most mass spectrometers are huge, weighing half a ton to several tons.
Dr. R. Graham Cooks, a professor of chemistry at the University of Notre Dame, is among the researchers who are trying to make something more portable. Using a variation of mass spectroscopy that does not require a magnet, he has built a mass spectrometer that weighs 36 pounds. "We're on our way to 25 pounds this summer," Dr. Cooks said, adding that the eventual goal is a couple of pounds.
Other reliable techniques for identifying chemicals, like gas chromatography, have usually required a skilled technician working in a laboratory.
Advances in microfluidics, or channeling liquids through a maze of pipes about as wide as a human hair, will allow the same analysis to be conducted by a hand-held device, like one made by Sandia that operates with the push of a button.
A more novel technique is Dr. Michael J. Sailor's "smart dust," shards of silicon that have been perforated with holes one ten-millionth of an inch wide, or about as wide as 20 hydrogen atoms side by side. The holes give the dust-size shards an iridescent sheen like that of soap bubble film. "They look like glitter," said Dr. Sailor, a professor of chemistry and biochemistry at the University of California at San Diego. The dust could be sprinkled outside or glued to a wall.
When chemicals are trapped in the holes, the wavelength of reflected light shifts. Different pieces of dust could be tagged to detect different chemicals.
"Very much like the bar code at a supermarket," Dr. Sailor said. "They have chemical smarts and tell you what they're seeing by changing color."
At a nature preserve near San Diego in November, Dr. Sailor and his colleagues showed that the smart dust could detect harmless ethanol vapors, shining a laser on them from 80 feet away. Dr. Sailor said it would take several years for the technology to evolve into effective weapons sensors.
The smart dust could also be used to detect biological weapons by putting antibodies in the pores that would hook onto viruses or germs.
In most cases, detectors for biological weapons are entirely different from those for chemical weapons. A nerve gas molecule is but a few ten-millionths of an inch wide. A virus like smallpox is about 100 times as wide, and an anthrax spore is several times larger yet.
Detecting biological weapons usually involves two steps, latching onto the germ and reporting that it has been latched onto. Typically, a crucial component in a biological sensor consists of an antibody, to hook onto a protein in the germ, that is attached to a fluorescent molecule that lights up.
To automate the testing is not easy. The samples collected by the E.P.A. stations have to be analyzed in laboratories, potentially delaying the discovery of a biological attack for up to 24 hours. Because most diseases take several days to incubate, a one-day delay would still leave time for treatment, but quicker notice would help.
A system developed at the Lawrence Livermore National Laboratory draws in air, traps particles in a liquid and flows them past beads coated with antibodies. "At this point, we have taken it to a government laboratory where we can test it with real pathogens, and that was successful," said Dr. Richard G. Langlois, a senior biomedical scientist who heads the project.
Livermore is working on licensing the technology to an aerospace company. Dr. Langlois estimated it could be a commercial product in a year, costing $50,000 to $100,000. "It's definitely not something everybody would have in their house," he said. "It's something for airports or convention centers."
Improved sensors might use pieces of DNA that would hook onto the germ's genetic material instead of antibodies. DNA is less prone to false positives, but the sensors would have to break apart the germs to release the DNA. The Pacific Northwest National Laboratory makes DNA-coated beads that could be used in such a sensor.
Dr. Panos Datskos, a scientist at the Oak Ridge National Laboratory in Tennessee, takes an entirely different approach. He uses micromachine technology, building a main detection element that is a strip one-250th of an inch long, one five-hundredth of an inch wide and one fifty-thousandth of an inch thick that bends when heated. The device is irradiated with infrared light.
Chemical bonds in molecules that stick to the strip heat up at certain frequencies, providing a fingerprint of the molecule in a few hundredths of a second. "The spectrum is very characteristic of something absorbed on the surface," Dr. Datkos said.
A more unusual idea is using plants and their natural sensors. Researchers at Penn State are trying to figure out the functions of more than 600 receptors in arabidopsis, an often-studied flowering plant in the mustard family. They hope that some combination of the receptors react to specific chemical and biological agents and that they can genetically engineer the plants to glow green if they detect a harmful substance.
Unlike mechanical and electronic devices, such plant sentinels would require no maintenance beyond usual gardening. Also, "They are inconspicuous," said a researcher, Dr. Jack Schultz, a professor of entomology. "You can put them anywhere you want. Nobody will notice them."
Perhaps even more important than the type of sensors is determining their locations. Before the new international terminal at the San Francisco airport opened, Sandia scientists blew smoke through it to see where germs and toxins would drift. "We've extensively characterized the facility," Dr. Lindner of Sandia said. The airport is testing a variety of sensors.
When Sandia started work with the Washington Metro four years ago on a chemical warning system, it began examining air movement through tunnels and stations. In a chemical attack, should trains stop or rush to a station? Should ventilation be closed to prevent blowing toxin or turned up to dissipate the toxin to harmless concentrations?
"There is no simple answer," Dr. Lindner said. The answers, he added, depend on the layout of the stations and other conditions.