Standoff Detection of Chemicals Using Porous Silicon “Smart Dust” Particles

“Smart dust”—small particles of optically encoded microporous Si—can detect organic vapors by measurement of the intensity of reflected light using a laser probe and a detector stationed 20 m away (see Figure and inside front cover). The porous particles absorb condensable vapors, producing a change in refractive index and consequently a quantitative change in optical reflectivity at the wavelength of the laser.