Biomolecular screening with encoded porous-silicon photonic crystals

Strategies to encode or label small particles or beads for use in high-throughput screening and bioassay applications' focus on either spatially differentiated, on-chip arrays2-4 or random distributions of encoded beads5,6. Attempts to encode large numbers of polymeric, metallic or glass beads in random arrays or in fluid suspension have used a variety of entities to provide coded elements (bits) - fluorescent molecules, molecules with specific vibrational signatures7,8, quantum dots9, or discrete metallic layers10. Here we report a method for optically encoding micrometre-sized nanostructured particles of porous silicon. We generate multilayered porous films in crystalline silicon using a periodic electrochemical etch. This results in photonic crystals with well-resolved and narrow optical reflectivity features, whose wavelengths are determined by the etching parameters11. Millions of possible codes can be prepared this way. Micrometre-sized particles are then produced by ultrasonic fracture12, mechanical grinding or by lithographic means. A simple antibody-based bioassay using fluorescently tagged proteins demonstrates the encoding strategy in biologically relevant media.