By Salvatore Salamone
November 15, 2002 | An experimental high-throughput screening method uses what its inventors call "smart dust" to quickly identify chemically or biologically relevant elements in a solution.
The smart dust technique was developed by scientists at the University of California at San Diego, primarily for the speedy detection of bioterrorism attacks. But the researchers, who recently published information about the technique in Nature Materials, believe smart dust can also be used to perform rapid biochemical assays and aid in drug discovery research.
“The technique is based on building some intelligence into a small particle so it can identify specific chemical and biological agents,” says Michael Sailor, a professor of chemistry and biochemistry at UCSD and a coauthor of the Nature Materials paper.
Sailor and his UCSD colleagues have developed a method to construct silicon beads the size of dust particles, that, when illuminated, reflect different light patterns similar to a grocery store bar code. The beads are known as photonic crystals, which are micron-sized particles that reflect light of a very specific wavelength. Each wavelength can be thought of as comparable to a single bar of a bar code, according to UCSD researchers.
The bar code on a silicon dust particle is a specific wavelength of light, or color, reflected from the surfaces of a bead after thin films layered on the silicon bead chemically react to a specific chemical or biological agent. If a specific wavelength of reflected light is present, that means the bead has bonded to an agent associated with that wavelength -- identifying that particular agent as present in a solution.
“We can illuminate a beaker with a beam of light; the reflected light can be read using a scanner similar to a bar code reader in a grocery store,” says Sailor. Knowing the bar code light pattern, a researcher can then identify the chemical or biological agents in that solution.
To create the beads, researchers use silicon wafers similar to the ones used to make computer chips. Researchers then layer nanometer-thick porous films on the wafers containing the beads. This layered -- or etched -- structure gives the bead its unique reflective optical properties.
“So the [bar] code is an integrated part of the bead,” says Sailor. Because of the etching “this [bar] code does not dissolve away or leach out.” That last point is a reference to some previous techniques that tried to identify agents in a solution using reflected light. For instance, one common attempt in the industry is to use fluorescent dyes on small beads; the dyes sometimes leach out of the beads, however, making it hard to detect the presence of the bead when light is shined onto the solution.
UCSD’s smart dust approach gets around this leaching problem because of the permanency of the etchings on the beads, according to Sailor.