Micron-size, chemically asymmetric photonic crystals that were developed at the University of California, San Diego, in La Jolla target organic liquid/water interfaces and orient themselves to display discrete reflectivity spectra. The particles of porous silicon, called "smart dust," may have applications in chemical and biological sensing, drug discovery, telecommunications and information display.University researchers create the smart dust by electrochemically etching rugate filters with different periodicities into the opposite sides of a single-crystal silicon film so that one side displays green peak reflectivity and the other red. They chemically modify the two sides so that one is hydrophilic and the other hydrophobic, and fracture the film into micron-size pieces by ultrasonication. When in solution, the chemically treated particles cluster at an organic liquid/water interface and orient themselves according to their chemical nature, shifting their reflectivity spectra. The researchers presented the work in the Sept. 16 issue of PNAS.