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Time gated pSiNPs in mouse
Where are the nanoparticles? Time-gated imaging reveals porous Si nanoparticles in a "noisy" fluorescence background.
TIME-GATED IMAGING OF POROUS SILICON NANOPARTICLES
The imaging of fluorescent probes in vivo is often complicated by signals coming from the native fluorescence of tissues, known as tissue autofluorescence. Silicon nanoparticle probes display unusually long-lived excited states, which allows suppression of the interfering signals from tissue autofluorescence by time-gated fluorescence imaging. A mouse injected subcutaneously with luminescent porous Si nanoparticles on the right shoulder and a conventional molecular imaging dye (Cy3.5) on the left shoulder displays bright fluorescence from both the injection sites and from autofluorescent tissues in the abdomen/gut region (left image). That image was obtained using a conventional fluorescence imaging approach. When the same animal is imaged using a time-gated mode (acquisition of the fluorescence image 18 ns after the excitation pulse), the nanoparticles appear as a distinct spot (right image), and no signals from the fast-decaying Cy3.5 imaging dye or from tissue autofluorescence are observed. No background subtraction was performed for either of the images shown. The synthesis of porous Si nanoparticles allows one to tune the excited state lifetime, allowing the possibility of distinguishing between multiple nanoparticle formulations. This work was first reported in Gu, L.; Hall, D. J.; Qin, Z.; Anglin, E.; Joo, J.; Mooney, D. J.; Howell, S. B.; Sailor, M. J., "In vivo time-gated fluorescence imaging with biodegradable luminescent porous silicon nanoparticles." Nature Communications 2013, 4, 2326.

Images: Magnetic "smart dust" particles self-assembled on two drops of water (top image). The microscopic particles are photonic crystals of porous silicon that spontaneously assemble at the surface of the water drops. A magnetic field has been applied to bring the two drops together (lower image), resulting in a reaction between the chemicals contained in the two drops (silver ion in one, iodide ion in the other). The silver iodide precipitate that forms in the reaction is evident as a whitish cloud inside the drop. The reflectivity spectra of the particles (shown below) indicate the identity of each drop or of the combination.

MAGNETIC "SMART DUST" CHAPERONES
If you want to move a liquid around in the macroscopic world, you can use either a bucket or a pipe. In the chemical or biochemical laboratory, we use similar tools: beakers, syringes, pipets, and plastic tubing. One of the big challenges in nanotechnology is to move miniscule amounts of liquids around. There is an increasing need to do this, as the required time and cost of many medical and environmental analyses is directly proportional to sample volume. Fifteen years ago, the concept of the "lab on a chip" evolved as a marriage of the methods of analytical chemists and microbiologists with the tools developed in the semiconductor industry for microfabrication, in order to address the problem of efficiently manipulatiing small amounts of material.

In the world of microfluidics, the bucket is often preferable to the pipe; as the sample volume becomes smaller, the number of molecules that stick to the insides of a microchannel becomes a significant fraction of the total molecules in the sample. This problem spawned the so-called "lab-on-a-drop" concept. A sphere has the lowest ratio of surface area to volume, and if a droplet containing the sample of interest can be manipulated without it coming into contact with the walls of its container, the amount of material lost can be minimized. In this project, we use small porous Si particles to surround liquid droplets. The particles contain superparamagnetic iron oxide, and application of a magnetic field allows them to be manipulated. The method provides a general means for manipulating small volumes of liquids without a microfluidic container or use of a pump. Because the magnetic porous Si particles adhere to the surface of the drop, they do not require a specific payload composition such as a high ionic strength in order to effect liquid motion. The electrochemical synthesis of porous Si photonic crystals allows the incorporation of spectral “bar codes,” allowing the possibility of distinguishing between multiple distinct liquid drops in combinatorial libraries. Since either water or an organic liquid can be encapsulated by the amphiphilic particles, we think that the discovery will enable the manipulation of a wide range of inorganic molecules, organic molecules, mammalian cells, bacteria, etc. This work was first reported in Dorvee, J. R.; Derfus, A. M.; Bhatia, S. N.; Sailor, M. J., "Manipulation of liquid droplets using amphiphilic, magnetic 1-D photonic crystal chaperones." Nature Mater. 2004, 3, 896-899.

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Quicktime Movie of Smart Dust combining two water drops (2.6 Mb)

Self-assembled smart dust
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.
TARGETED "SMART DUST" NANOMACHINES
This work is aimed at developing robots the size of a grain of sand. The vision is to build devices that can move through their environment to specific targets, locate and detect chemical or biological compounds and report this information to the outside world. Examples of uses for such devices are: to monitor the purity of drinking water, to detect hazardous chemical or biological agents in the air, or to locate and destroy tumor cells in the body.
The main advance in this work is that we make microscopic photonic crystals that spontaneously assemble, orient, and sense their local environment. The technique involves a new method for making chemically modified nanostructures, involving two steps. In the first step, a photonic structure is produced by etching silicon using a special 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 target the interface between a drop of oil in water, but we hope to be able to apply the methodology to pollution particles, bacteria, or cells.
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 signaling 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, signaling 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.

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MSNBC Article

CBS News Article

Movie of Smart Dust assembling on a drop of oil (9Mb)

Very big movie of Smart Dust assembling on a drop of oil (92 Mb)

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More info on Smart Dust


Image:Polystyrene plastic molded from a porous Si photonic crystal template


Image:Polynorbornene/porous Si photonic crystal composite, made by in-situ polymerization. These films retain their photonic properties even after repeated flexing.


Image:Poly(lactide) molded from a porous Si photonic crystal template. This polymer contains the drug caffeine. Decay of the optical spectrum (the green color visible in the image) from this fixture provides a surrogate measure of the drug delivery rate.

NANOIMPRINTED POLYMERS FROM POROUS Si PHOTONIC CRYSTAL TEMPLATES. This method provides a general means of constructing an elaborate optical nanostructure out of almost any plastic. The process used to create these flexible, polymer-based materials is similar to the injection-molding process that manufacturers use to make plastic parts, and derives from template-based synthesis techniques pioneered by Charles R. Martin at the University of Florida. We start by treating a silicon wafer with an electrochemical etch to produce a porous silicon chip containing a precise array of tiny, nanometer-sized holes. This gives the chip the optical properties of a photonic crystal—a crystal with a periodic structure that can precisely control the transmission of light much as a semiconductor controls the transmission of electrons.
We then cast a molten or dissolved plastic into the pores of the finished porous silicon photonic chip. The silicon chip mold is dissolved away, leaving behind a flexible “replica” of the porous silicon chip. Detection of volatile organic compounds (VOCs) was demonstrated with these materials. The strength of this approach lies in the ability to make a photonic crystal sensor from a robust, chemically stable material that can be compatible with the environment or with living systems.
IMPLANTABLE, BIOCOMPATIBLE PHOTONIC CRYSTALS. If a biocompatible polymer is used, the resulting casting could be used to make a variety of self-reporting implantable medical devices. For example, the optical spectrum can provide a measure of the amount of drug contained in a biocompatible implant. This spectrum can be read non-invasively through human tissue with a beam of light. This could allow a physician to directly see how much of a drug has been delivered to a patient, or how much strain is being placed on a newly implanted joint.
The flexible composite materials were reported in M. S. Yoon et al., Chem. Commun. 2003, 680 - 681 and the nanocasting of melt- and solution-processible polymers containing drugs was reported in Y. Y. Li et al., Science 2003, 299, 2045.

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FAQs

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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QUICKTIME MOVIE OF POROUS SILICON PHOTONIC CRYSTAL SENSOR (CHIP-BASED SYSTEM (3.4 Mb)download]

Discover Magazine article

New York Times article

Science News article

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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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QUICKTIME MOVIE DUST PARTICLES IN WATER:
--(1.7 Mb) [download]

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Image: An exploding chip. EXPLOSIVE NANOCRYSTALLINE SILICON The objective of this effort is to develop a safe, non-toxic explosive for use in portable analytical devices, on-chip propulsion systems for microelectromechanical (MEMS) systems, and for the ultimate in secure microcircuitry. The silicon on a wafer is made into a nanocrystalline powder using a special electrochemical process. It then provides the fuel for a high-tech version of gunpowder. The work was sponsored by the Defense Advanced Research Projects Agency. The DARPA point of contact for this project is Dr. Ed Carapezza. Published in Mikulec, F. V.; Kirtland, J. D.; Sailor, M. J. Adv. Mater. 2002, 14, 38-41. Yes it could also make your stolen cell phone blow up (New Scientist).

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QUICKTIME MOVIE OF EXPLODING CHIP:
--Small (11 Mb) [download]

QUICKTIME MOVIE OF FLYING CHIP (1 Mb) [download]

Image: Liver cells in nanostructured wells on a silicon chip.
LIVER ON A CHIP In collaboration with Professor Sangeeta Bhatia in the Bioengineering division, we are trying to build micro- bioreactors from living cells and inorganic materials. Such composites may be useful for artificial organs and for the rapid production and purification of drugs and vaccines. In the artificial liver project we characterized the attachment, viability, and function of liver cells from a rat, a notoriously difficult cell type to grow in culture. Published in Chin, V.; Collins, B. E.; Sailor, M. J.; Bhatia, S. N. Adv. Mater. 2001, 13, 1877-1880.

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Image: Cover of Cell Magazine. NERVE CELLS WIRED TO A SILICON CHIP In a collaboration with neurobiologists Yukiko Goda and Michael Colicos, we developed a method of stimulating nerve cells in a manner that mimicks their stimulation in the brain. This involves using the photoconductive properties of silicon in a way that allows us to deliver a short pulse of electricity to a specific area of a neuron on a silicon chip by simply shining light on that area. Light excitation in that area of the silicon creates a temporary electrical connection between the neuron and the chip. Published in Colicos, M. A.; Collins, B. E.; Sailor, M. J.; Goda, Y. Cell 2001, 107, 605-616.

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Image: TNT-Contaminated Thumbprints darknes when imaged with luminscent polysilole contrast agent.
NANOWIRES USED TO DETECT EXPLOSIVES In a collaboration with Professor William Trogler, wires of silicon that are only a single atom thick are used to detect explosives such as TNT and picric acid (the explosive often used in letter bombs). Published in Sohn, H.; Calhoun, R. M.; Sailor, M. J.; Trogler, W. C. Angew. Chem. Int. Ed. 2001, 40, 2104-5.

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Image: Prototype of a portable chemical sensor device. MICROSENSORS INCORPORATING NANOSTRUCTURED SILICON USED TO DETECT NERVE WARFARE AGENTS Portable, low cost and low-power sensors for chemical and biological warfare agents are currently of great interest. In a collaboration with chemist William Trogler and electrical engineer Yeshayu Fainman, we developed thin film silicon sensors that are being used to detect a range of pollutants, biological and chemical agents, including Sarin. Published in Sohn, H.; Létant, S.; Sailor, M. J.; Trogler, W. C. J. Am. Chem. Soc. 2000, 122, 5399-5400.

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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