The development, detailed in an advance online publication today of the October 1 issue of the journal Nature Materials, will likely have wider commercial use in research and medical laboratories�in performing rapid biochemical assays, screening chemicals for potential new drugs and testing samples for toxic materials.

But because the technique permits rapid detection of the biological and chemical substances remotely, using a laser similar to a grocery scanner, it also could be employed as an advanced warning system for biological and chemical attacks.

"The idea is 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," says Michael J. Sailor, a professor of chemistry and biochemistry at UCSD who headed the research effort.

"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."

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 etch.

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," says Sailor. "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."

Other researchers involved in the development were Fr�d�rique Cunin and Thomas A. Schmedake, postdoctoral fellows in Sailor's laboratory; Jamie R. Link and Yang Yang Li, graduate students in Sailor's laboratory; Sangeeta N. Bhatia, an associate professor of bioengineering at UCSD and bioengineering graduate student Jennifer Koh. The project was supported by the Defense Advanced Projects Agency, or DARPA.

The encoding that takes place in these particles provides colors that are so sharp from the visible to the infrared that a laser can read thousands of distinct colors corresponding to separate chemicals.

In this way, the UCSD researchers say these coded particles can perform thousands of biochemical assays in a small beaker or a Petri dish, which should be useful in many medical and research applications, such as the discovery of new drugs, the diagnosis of disease and the controlled release of therapeutic drugs.

Because the smart-dust chips are fabricated from silicon, they can be easily made from existing computer chip technology. And the compatibility of porous silicon with living cells and the long-term stability and non-toxicity of this material makes them especially useful in biomedical applications.

"The big advantage of the method is that porous silicon is biocompatible and the use of these encoded silicon nanostructures in medical diagnosis may be significantly better than other methods that involve the use of potentially toxic materials, such as heavy metals," says Cunin, the first author of the paper.

"This is an example of marrying microtechnology, which is used to make microelectronic chips, with silicon chemistry and molecular and cell biology to create hybrid integrated chip platforms for medical applications," says Bhatia, who in addition to being a bioengineer has a medical degree.

For example, she says, if a patient has a cough, his or her blood sample could be sent to the laboratory for screening. DNA Probes for various types of infectious diseases could be coded with the crystals, and these probes could be mixed in with the patient's blood sample.

If the blood sample binds with one of the probes, its crystal code will exhibit a pattern that identifies the probe, and thus diagnose the disease that the patient carries.

"This technology offers two important advantages," Bhatia adds. "The encoding strategy is quite robust because it is hard-wired into the crystals and the crystals are built on a silicon platform, which we know is easily adapted to biological applications."

For the detection of chemical and biological warfare agents, the advantages of smart dust are numerous. Not only are the smart-dust crystals small in size, inconspicuous and capable of detecting thousands of possible agents at once, but they can detect potentially hazardous compounds remotely from a distance.

Unlike grocery store scanners, which typically must read bar codes only inches away, Sailor and his group have been able to get their laser to detect the color changes in the smart dust 20 meters away, the length of the hallway outside their research laboratory. With a more powerful laser, he adds, "we're planning to take this outside and see how far we can go. Our goal is one kilometer (or about .6 of a mile)."

The smart dust achievement is among a number of new silicon-based technologies developed in Sailor's laboratory in recent years that could be employed to thwart terrorists.

Working in collaboration with a team headed by William Trogler, a professor of chemistry and biochemistry at UCSD, Sailor and his group developed an inexpensive and 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.

The two also developed method of using tiny silicon wires in a solution to detect trace amounts of TNT and picric acid, a common explosive used by terrorists.

Earlier this year, Sailor's 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.

In addition, working in collaboration with Bhatia's group, Sailor and his team of 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.

Video of encoded porous silicon crystals in solution available here:

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