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Image: "Smart dust" particles self-assembled on drops of oil (dichloromethane) in water. The microscopic particles are nanostructured flakes of porous silicon that spontaneously assemble, orient, sense, and report on their local environment. Each particle contains two highly reflective colored mirrors, red on one side and green on the other. The red side is facing the viewer in this photo. Each mirror is also chemically modified so that it can stick to a pre-determined type of surface. The particles in this photo were designed to target and align themselves at an oil/water interface with the green side facing the oil phase and the red side facing the viewer. When they orient in such a way, the nanostructure in the particles senses the liquids and adjusts the color of each mirror in a predictable fashion, such that it indicates to the observer that it has found its intended target..

In order to spontaneously assemble and orient the micron-sized porous Si "smart dust," we couple chemical modification with the electrochemical machining process used to prepare the nanostructures. The process involves two steps, see the scheme below. In the first step, a porous photonic structure is produced by etching silicon with an electrochemical machining process. This step imparts a highly reflective and specific color-code to the material, that acts like an address, or identifying bar-code for the particles. The second step involves chemically modifying the porous silicon photonic structure so that it will find and stick to the desired target. In the present case, we use chemistry that will target the interface between a drop of oil in water, but we hope to be able to apply the methodology to pollution particles, pathogenic bacteria, and cancer cells. The two steps (etch and modify) are repeated with a different color and a different chemistry, yielding two-sided films. The films are broken up into particles about the size of a human hair. With the chemistry shown below, the particles seek out and attach themselves to an oil drop, presenting their red surface to the outside world and their green surface towards the inside of the drop.

Once they find the interface for which they were programmed, the individual mirrored particles begin to line up, or "tile" themselves on the surface of the target. As an individual, each particle is too small for one to observe the color code. However, when they tile at the interface, the optical properties of the ensemble combine to give a mirror whose characteristic color is easily observed. This collective behavior provides a means of amplifying the molecular recognition event that occurs at the surface of each individual particle.
As a means of signalling their presence at the interface, the particles change color. As the nanostructure comes in contact with the oil drop, some of the liquid from the target is absorbed into it. The liquid only wicks into the regions of the nanostructure that have been modified with the appropriate chemistry. The presence of the liquid in the nanostructure causes a predictable change in the color code, signalling to the outside observer that the correct target has been located. This work was first reported in J. R. Link, and M. J. Sailor, Proc. Nat Acad. Sci. 2003 100, 10607-10610.

Cartoon of smart dust

Image:Artist's conception of how encoded porous Si "Smart Dust" particles might be used to detect chemicals in the field.

NANOSTRUCTURED "SMART DUST" CHEMICAL SENSORS. The objective of this effort is to construct sensors for chemical or biological molecules that use no power, are the size of dust particles, and can be probed at a remote distance using visible or infrared laser scanner technology. The particles are made of porous silicon, and they are constructed with of an elaborate layered structure that gives the materials photonic crystal properties. To use them as chemical sensors, the porous nanostructure is chemically modified so that its code changes in a predictable fashion when it is exposed to chosen molecules. Detection of volatile organic compounds (VOCs) was demonstrated in T. A. Schmedake, F. Cunin, J. R. Link, and M. J. Sailor, Adv. Mater. 2002, 14, 1270-1272.

HOW IT WORKS: Nanoscale holes in the porous matrix act as concentrators, condensing vapors of the analyte. Catalytic reactions take place in small micellar nanoreactors, converting the analyte into a chemical that can be recognized specifically by the matrix (see J. Am. Chem. Soc. 2000, (22), 5399-5400). The particle has a layered structure that provides an optical code, much like a barcode, that can be read by a remote laser beam. The reactions in the nanoreactors change the code in a predictable fashion, providing a signal that scales with dose. In the example shown in the image, the catalytic reaction is chosen to detect G-type nerve warfare agents, such as Sarin (the agent used in the Tokyo subway attack perpetrated by the Aum Shinrikyo cult).


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Image: Microscopic image of porous Si "Smart Dust" particles encoded with two different colors. These particles, each roughly the size of a human hair, can be used in applications designed to rapidly screen for new drugs or genetic markers for disease.
SILICON "SMART DUST" FOR HIGH THROUGHPUT SCREENING. The objective of this effort is to construct particles that contain distinct codes. The particles are made of porous silicon, and they are constructed with of an elaborate layered structure. This gives the materials photonic crystal properties; only very specific wavelengths of light are reflected from them. These specific wavelengths are like a code that can be read with a laser or an optical spectrometer, similar to how a bar code is scanned at a grocery store checkout counter. Use of these encoded materials in biological screening applications appeared in F. Cunin,T. A. Schmedake, J. R. Link, Y. Y. Li, J. Koh, S. N. Bhatia and M. J. Sailor, Nature Materials 2002, 1, 39-41. They may also be useful as non-toxic tags, or tracers, for a variety of forensic or environmental applications.


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Designed by Andrea Tao.
Main address: Department of Chemistry, University of California, San Diego, 9500 Gilman Dr., La Jolla, CA 92093-0358 (858) 534-0227
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Last modified Monday, February 17, 2003