Researchers at the University of California, San Diego, in La Jolla have demonstrated the use of optically encoded silicon crystals for bioassays. Such "smart dust" may have applications in drug discovery, medical diagnosis and screening for biological agents on the battlefield.As reported in the September issue of Nature Materials, the process begins with the anodic etching of pseudosinusoidal waveforms into a P-type silicon wafer to create a rugate filter with a sharp and distinctive reflection peak in the visible or near-IR, depending on the waveform. The researchers separate the filter layer from the substrate with another current pulse and mechanically or ultrasonically break it into micron-size particles, which may then be treated with antibodies.Using fluorescence and spectral reflectance microscopy to monitor changes in the reflectivity spectra, they can detect any binding of the analyte to the particles. Thus far, they have obtained results with the sample 20 m from the detection apparatus. For the remote detection of chemical and biological agents, they hope to increase the sampling range to 1 km.