NEWS OF THE WEEK MATERIALS
SCIENCE
LAYERED MATERIAL HOLDS MORE DATA Onionlike polymer particles ideal for secure encryption and identification ENCAPSULATED In a three-dye system, core, shell, and matrix are labeled with separate dyes.
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basis of a new material that is said to be ideal for secure data encryption and dense optical information storage [Adv. Mater, 16,516 (2004)1. The material consists of a lattice of onionlike spheres in which the particle core and its layers each contain a different dye. The material can hold four or more pieces of information in one spot—not just two as in binary optical data storage. And it opens a door to high-density three-dimensional optical data storage. "The approach is really sim-
ple," says lead researcher Eugenia Kumacheva, associate professor of chemistry at the University of Toronto, who worked with postdocs Ilya Gourevich and Hung H. Pham and microscopist James E. N. Jonkman. They start with colored colloids—polymeric nanospheres labeled with a dye —for example, an ultraviolet dye. Then they envelop the nanosphere, what Kumacheva calls the core, with a shell of another polymer labeled with a dye that has a spectrum entirely distinct from the first—say, a visible dye. Any
ARCHAEOLOGY
Quartz Hydration Fills Gap In Dating Artifacts
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y measuring the thickness s diffuses into the fresh surface to of the so-called hydration S form a hydration layer that inlayer of quartz-containing | creases in thickness with age. artifacts and other materials, rex Recognizing this phenomenon, searchers can determine their § archaeological scientist age with a high degree of accura1 Jonathon E. Ericson of the Unicy, according to a new study U £ versity of California, Irvine, and ArchaeoL ScL, 31,883 (2004); | coworkers at Johann Wolfgang http://www.sciencedirect. | Goethe University, Frankfurt, com/science/journal/0305U03]. 2 bombarded quartz samples with The new method—quartz hy2 15N ions, which react with hydrodration dating—can date artifacts | gen in water or other molecular from 100 to 100,000 years old, § species to produce 12C, an alpha filling a chronological gap for arparticle, and gamma rays. tifacts and other materials that The gamma-ray yield is are between 50,000 and 100,000 measured at various energy levyears old, and for which current els to create a concentration MINERAL dating techniques are mostly curve that represents the depth MEASUREMENTS ineffective. of the hydration layer. The team This quartz pendant then developed a general equaMaterials containing quartz are made by an ancient tion for the rate of diffusion of Mexican civilization is common and have been used for water into the quartz surface. dated at A.D. 1 using millennia to make tools, houseComparing the diffusion rate the quartz hydration wares, and decorative objects. with the layer depth allows reWhen quartz crystals are frac- technique. searchers to determine a samtured or carved, the researchers ple's age.—VICTORIA GILMAN who developed the technique explain, water 10
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number of dye-polymer shells can be added. The last shell then becomes the matrix that holds the layered particles. What this means, Kumacheva says, is that when the embedded nanospheres are lit with UV light, the UV dye shows up. Visible light brings out a different hue. With two dyes, "we have four different ways to write and then read on a single spot," Kumacheva says: no dye, dye one, dye two, and both dyes together. Three dyes offer eight (23) variations, and so on. The system, says Younan Xia, associate professor of chemistry at the University ofWashington, "can be easily extended into multishelled colloids to include as many types of dyes as one would like to have." For example, Kumacheva has added a near-infrared dye to a third polymer. Other materials scientists have tried a variety of methods to place one or more dyes in the same spot. Kumacheva's advance is in her successful creation of coreshell particles embedded in a matrix. She judiciously chose for the inner core a polymer, poly(methyl methacrylate), that would remain solid while a shell ofpoly[(methyl methacrylate)-^-(butyl methacrylate)} formed around it. "So when we start heating the arrays of particles, the shell softens, flows, and forms a matrix while the cores remain intact." The particles and matrix can be configured for high-density 3D data storage. They can also be considered "intriguing building blocks for fabricating photonic crystals," Xia says. And, Kumacheva notes, the technology is well suited for creating identification cards nearly impossible to fake. Canada and the U.S., she says, have an interest in passports and immigration cards that cannot be counterfeited. She imagines a card that shows a picture under visible light, a signature under UV illumination, and a fingerprint under near-IR.—LO UIS A DALTON
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