Have you ever noticed the sheen on a beetle wing? If you have, you were probably struck by its unusual iridescence. Despite what your eye sees, however, that color is not a “natural” one. Beetles “don’t have a pigment,” says Michael Sailor, a professor in the department of chemistry and biochemistry at the University of California, San Diego. Instead, he explains, the color is produced by two other properties: optical interference-the same phenomenon behind the colors in rainbows and soap bubbles-and elaborate structures in the wing surface.By artificially mimicking this phenomenon, Sailor intends to make more than rainbows. With funding from the Defense Advanced Research Projects Agency (DARPA), he’s working to turn nanoparticles imbued with iridescent colors into “fingerprints” that can be added to explosives and other chemicals-making it possible to trace a bomb or an illegal drug back to a single manufacturer. He’s also working to make the particles reflect signature colors when they encounter specific pathogens in air or water-to create a cheap, disposable sensor for detecting chemical and biological weapons.
To make the particles, which Sailor calls “smart dust,” he first creates a filter for light in the surface of a silicon wafer about the size of a quarter. He places the wafer in a conductive solution, and then electrochemically corrodes it with an alternating current. Sailor says, “as [the corrosion] drills down into the silicon, it bottlenecks and opens up again, then bottlenecks and opens up again.” The result is a delicately etched network of parallel pores about two nanometers in diameter. Using ultrasound vibrations, Sailor then crumbles the wafer into particles about the width of a hair.
When dispersed in air or water, ordinary dust particles scatter light in every direction. But when illuminated with a laser, Sailor explains, the smart dust appears quite different. “You’ll get this one sharp, very precise wavelength of light for a given angle coming in and bouncing off that surface,” he says. “The colors that result are incredibly vibrant, strong [and] highly reflective.” By varying the current, the length of the process and the composition of the solution, Sailor can create filters that produce millions of specific colors. Each color is determined by the refractive index of those complex layers in the silicon. Sailor says the refractive index is like a barcode a laser can read to determine the composition of the dust.
Sailor’s work has caught the interest of DARPA because of its battlefield and counter-terror applications. The particles could be applied as a “tag” to certain bomb-making materials, so that when a bomb blows up, investigators can scan a crime scene for the specific smart-dust particles. “Most of the stuff that is used in terrorism activities is diverted from legitimate purposes,” says Sailor. If different manufacturers incorporated uniquely coded smart dust, the type of dust found at the bomb scene would indicate where the bomb materials were purchased and provide a clue to the identity of the terrorists who made the bomb.