Marriage Peril
Dust in Time: Detecting Bioterror with Tiny Particles

‘Smart dust’ technology offers possibity of sensing biochemical agents from one kilometer away.

by Douglas Page, 2003

A post office is closed when workers see white powder spilling from mail.

A suspect package purporting to contain anthrax is removed from the dean’s office and 12 people are taken to the hospital as a precautionary measure.

A newspaper office is evacuated after staff opens a letter containing a white powder.

Within a month of the September 11, 2001 terrorist attacks there were more than 2,000 anthrax alerts. While most anthrax letter threats proved to be hoaxes, enough letters actually contaminated with anthrax organisms were delivered to put police and other first responders on red alert in every instance.

When law enforcement is dispatched to the scene of an suspected anthrax attack, or other questionable bioterror incident, officers have no way of knowing immediately whether it’s a ruse or the real thing.

In a perfect world, first responders would have at their disposal a test to quickly determine whether the substance is anthrax or body talc.

When the Dust Settles

Just such a technology may be at hand.

Researchers at the University of California, San Diego have developed dust-sized chips of silicon capable of rapidly and remotely detecting a variety of biological and chemical agents, including substances that a terrorist might dissolve in drinking water, spray into the atmosphere, or fold into a letter.

Although researchers believe the new development will have wider commercial application in research and medical laboratories performing rapid biochemical assays, screening chemicals for potential new drugs, or testing samples for toxic materials, the most urgent appeal of the UCSD technology may be to law enforcement or homeland security personnel as an advanced warning system for biological and chemical attack.

"The idea that you can have something that's as small as a piece of dust with some intelligence built into it so that it could be inconspicuously stuck to paint on a wall or to the side of a truck or dispersed into cloud of gas to detect toxic chemicals or biological materials has obvious law enforcement potential," said Michael J. Sailor, a professor of chemistry and biochemistry at the University of California at San Diego who headed the research effort.

Dust Buster

The objective of this effort is to construct sensors for chemical or biological molecules that use no power, are the size of dust particles, and can be probed at a distance using visible or infrared laser scanning technology, Sailor said.

The particles are made of porous silicon, and are constructed using a delicate layered structure that gives the materials photonic crystal properties, meaning they possess a quality called a periodic index of refraction - an interesting property that allows one to manipulate the flow of light.

In order to use them as chemical sensors, the porous nanostructure is chemically modified so that its code changes in a predictable fashion when it is exposed to chosen molecules, Sailor said. Detection of volatile organic compounds (VOCs) was demonstrated in earlier work. (T. A. Schmedake, et. al. Adv. Mater. 2002, 14, 1270-1272.)

"When the dust recognizes what kinds of chemicals or biological agents are present, that information can be read like a series of bar codes by a laser that's similar to a grocery store scanner to tell us if the cloud that's coming toward us is filled with anthrax bacteria or if the tank of drinking water into which we've sprinkled the smart dust is toxic," Sailor said.

The "bar code" on the silicon dust particles is basically a specific wavelength of light, or color, reflected from their surfaces after thin films layered on the silicon chip chemically react to a specific chemical or biological agent.

The scientists start with silicon wafers similar to those used in the manufacture of computer chips, then "encode" them by generating layers of nanometer-thick porous films on the wafers using a special electrochemical etching process. This layered structure on the dust-sized particles, which are created by breaking apart the wafer using ultrasound, imparts unusual optical properties to the particles.

Referred to as photonic crystals, these micron-sized

particles are able to reflect light of very precise colors, each one of which can be thought of as a single bar of a grocery store bar code.

"When you're looking for chemical or biological warfare agents, you're going to want to search for thousands of different chemicals," Sailor said. "Since the particles can be encoded for millions of possible reactions, it's possible to test for the presence of thousands of chemicals at the same time."

Long Distance Remedy

The advantages of smart dust technology are numerous. The crystals are not only small in size, inconspicuous, and capable of detecting thousands of possible agents at once, but they can also detect potentially hazardous compounds from a considerable distance. Most sensors designed to detect toxicity must be in close proximity to the suspect substance.

"Unlike grocery store scanners, which typically must read bar codes only inches away, we’ve been able to get our laser to detect the color changes in the smart dust from 20 meters away," Sailor said. Twenty meters happens to be the length of the hallway outside Sailor’s research laboratory.

Once they perfect a more powerful laser scanner, the researchers plan to take the technology outside to see how far away from the smart dust they can get and still obtain a reliable reading.

"Our goal is one kilometer, or about .6 of a mile," he said.

The advantage to law enforcement, hazmat crews, or homeland protection teams of being able to detect biohazards from a half mile away would be extraordinary.

A miniature laser capable of transmitting data 13 miles across San Francisco Bay has already been tested by University of California, Berkeley, scientists in other research.

Once perfected, Sailor’s smart dust could be sprayed almost anywhere, into and around government buildings, hospitals, vehicles, aircraft, or mixed into samples of drinking water, and then scanned for thousands of hazardous chemicals simultaneously.

Smart dust coated with specific compounds could also be useful as an inexpensive mobile molecular detector — screening a DNA sample for particular genes, for instance, or identifying pathogens in a patient's blood sample.

The detectors are already proficient at distinguishing several chemicals. Now the researchers have started work on recognizing biological agents.

Th smart dust idea originated in a Defense Advanced Research Projects Agency (DARPA) project run by Ed Carapezza on micro unattended ground sensors, an effort in which Sailor participated.

"In that program we developed the capability for detection of Sarin, TNT, and picric acid," Sailor said.

The current smart dust project is likewise supported by DARPA. Since the Pentagon is paying for the research, smart dust will probably find its first uses in military tactics. Imagine scattering clouds of these minute sensors around the desert in Iraq, for example, looking for the biological weapons Saddam Hussein supposedly has hidden somewhere.

Sailor has been busy in recent years developing a number of technologies that could also be used to thwart terrorism.

Working in collaboration with a team headed by another UCSD professor of chemistry and biochemistry, William Trogler, Sailor and his group developed an inexpensive, portable nerve gas detector that uses a CD laser to detect the changes of a catalyst on the surface of a tiny silicon chip that reacts to sarin and other nerve agents.

Sailor and Trogler also worked together to develop a method of using tiny silicon wires in a solution to detect trace amounts of TNT and picric acid, a common explosive used by terrorists.

Then, early in 2002, the Sailor group devised a method of using the explosive properties of silicon in a way that would allow computer chips with valuable security information to self destruct or allow for the explosive propulsion of tiny information-collecting chips.

Also, working in collaboration with Sangeeta N. Bhatia, an associate professor of bioengineering at UCSD, Sailor and his team of busy scientists developed porous silicon chips capable of maintaining fully functioning liver cells, an important advance in the effort to keep liver cells alive outside of the human body.

The smart dust technology is being modified for high throughput pharmaceutical screening applications in collaboration with Illumina, Inc. The biosensor aspects are being developed in collaboration with Trex Industries.

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