"SMART-DUST: Silicon nanoparticles eyed for chemical detection"
Smart dust results from nanostructuring micron-sized porous-silicon particles so that they have a selective response to light when sensing dangerous chemicals or biological agents. Developed here at the University of California, the process creates a special reflective layer � called a rugate filter � on the surface of the particles. The layer only reflects light in a narrow spectral band. The reflection bands vary, creating a medium that has a 20-bit code, enabling millions of tests to be performed simultaneously. Any given chemical will change the reflectivity of some particles in a cloud, creating a unique signature that can be detected from a distance.

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Story in EE Times: http://eet.com/at/news/OEG20020909S0082