Saturday, January 30, 2010

Alexander Graham Bell Meets Nanotechnology: Photoacoustic Spectroscopy and Quantum Dots Used for Remote Detection of Explosives and Chemical Weapons


Commercial explosives and dangerous chemicals often contain taggants that allow the materials to be easily identified.  However terrorists don’t use such chemical markers.  Nano-Proprietary, Inc has developed a remote photoacoustic/quantum dot method to detect terror weapons even if a barrier is present.

One of the specific photoacoustic methods capable of detecting outgassed by-products or vapors of explosive materials is photoacoustic spectroscopy (PAS). Initially discovered by Alexander Graham Bell in 1880, this method was further developed theoretically by Rosencwaig and Gersho in 1973. Now Nano-Proprietary, Inc. (Austin, TX)  has developed a PAS system that uses quantum dots for the remote detection of explosives and dangerous chemicals.  The system is capable of detecting the vapors from explosive materials from a safe distance, according to Dr. Zvi Yaniv, President and CEO of Applied Nanotech, Inc in U.S. Patent Application 20100022009Nano-Proprietary, Inc. is a subsidiary of Applied Nanotech Holdings Inc.

The idea behind the photoacoustic effect is based on the detection of sound waves produced by a gas surrounding an object which absorbs the light from a light source modulated by amplitude. The object can be in a solid state, liquid, or gas phase.

Yaniv devised strategies for chemically sensing explosives, chemical weapons, and other harmful agents by exploiting the high vapor pressures that many of them possess and the emission of nitrogen- and phosphorus-containing free radicals from the explosives, chemical weapons, and other harmful agents. This is the case for phosphorus-containing chemical nerve agents like sarin, soman, tabun, and VX and for nitro-containing explosives like trinitrotoluene (TNT) and nitroglycerine. Chemical sensing, such as utilizing spectral characteristics, could be used to detect such harmful materials in public places like airports, subways, shopping malls, etc. This would allow for the pre-emptive identification of harmful materials, before they have inflicted any damage.

FIG. 1 illustrates, in general terms,  Applied Nanotech’s  process of using nanoparticles for chemical sensing; detection and monitoring of an environment. Photoacoustic techniques have a number of advantages such as a very strong signal-to-noise ratio, and can be tuned to very specifically detect the explosive materials with sensitivities in the range of about 1 part per million (ppm) to approximately 1 part per billion (ppb) in certain situations such as for plastic explosives.
FIG. 2 depicts a photoacoustic detection system useful for remote sensing of chemical and hazardous materials. The photoacoustic system comprises a light source 201 that passes through a chopper 202 and into a photoacoustic cell 203 comprising the analyte gas. Pressure waves within the cell are detected as sound waves by microphones 204. The signal produced by the microphones can then be amplified and transmitted to a remote location, typically via a wireless means.
Nanoparticle-Based Detection

The process uses nanometer-size particles (also known as nanoparticles or nanocrystals, in the sensing and identification of chemicals and harmful agents by exposing such chemicals and harmful agents to the nanoparticles. Such exposure could be in the gas phase, the liquid phase, or the solid phase, and could include mixed phase exposures. Such sensing exploits unique properties of the nanoparticles, specifically unique photoluminescence properties of the nanoparticles known as quantum dots.

Nanoparticle, are particles comprising finite bandgap materials, and having particle diameters which are generally less than about 100 nm. Finite bandgap materials, in contrast to zero bandgap and infinite bandgap materials, can be categorized as semimetals, semiconductors, insulators, and combinations thereof. Examples of finite bandgap materials include, but are not limited to, silicon (Si), gallium arsenide (GaAs), cadmium sulfide (CdS), cadmium selenide (CdSe), titanium dioxide (TiO.sub.2), diamond, cerium oxide (CeO2), silicon oxide (SiO2), aluminum oxide (Al2O3), and the like and combinations thereof.

Photoluminescence (PL) comprises all forms of luminescence including fluorescence, phosphorescence, and combinations thereof. The excitation radiation, which induces photoluminescence, is typically in the ultraviolet (UV) region of the electromagnetic (EM) spectrum, but can generally be in any or all regions of the electromagnetic spectrum capable of inducing photoluminescence in the nanoparticles. Photoluminescence is typically in the visible (optical) region of the electromagnetic spectrum, but can generally be in any or all regions of the electromagnetic spectrum.

Photoluminescence of the nanoparticles, in the process,  is induced when the nanoparticles are irradiated with light, particularly with wavelengths found in the UV region of the electromagnetic spectrum. The emitted radiation (the photoluminescence) is generally in the visible (optical) region of the electromagnetic spectrum. When a chemical species adsorbs onto the surfaces of the nanoparticles, the photoluminescence properties of the nanoparticles are altered. Chemical sensing is accomplished by detecting and  analyzing the altered photoluminescence properties. Sensing includes detecting, analyzing, monitoring, and the like and combinations thereof.

Harmful materials could be detected by spraying a suspect item (e.g., luggage or mail) with an aerosol of nanoparticles (e.g., silicon nanoparticles) having one or more pre-defined altered photoluminescence properties, illuminating the suspect item with a UV laser in the process of spraying it with the aerosol of nanoparticles, measuring the photoluminescence shift or change, i.e., measuring the altered photoluminescence properties, and observing whether or not there is a pre-defined shift or change in the photoluminescence spectra corresponding to a known--and already evaluated--chemical agent (sarin, for example), i.e., comparing the altered photoluminescence properties to the one or more pre-defined altered photoluminescence properties.

Such a process could be carried out remotely from a distance. In the case of sarin, the high vapor pressure of this nerve agent might render the environment in the immediate vicinity of the article to be relatively high in sarin content-even if it were enclosed in some type of crude container that permitted the escape of merely trace amounts. The sarin vapor would then cause a predetermined shift or change in the photoluminescence spectra of the nanoparticles on account of the altered chemical environment. A variation on this embodiment would be to use the nanoparticle aerosol in the vicinity of a military weapons depot, whereby leaks in containers containing explosives and chemical weapons could be detected and identified.

The use of such photoacoustic processes in such  remote sensing applications can have numerous variations. For example, in some embodiments, a photoacoustic detector is installed in location of interest and then the signal is wirelessly transmitted to a remote location when a certain gas/species is detected. In some or other embodiments, if wireless transmission is prohibited due to certain limitations in the field or in the area of interest, a tunable UV laser can interrogate the photoacoustic cavity from a large distance by tuning the laser to specific well-defined frequencies with which the photoacoustic effect can be triggered and a specific gas identified. Devices using the photoacoustic effect in such a way can be easily miniaturized, and one can envision portability and even distribution of the device to individual soldiers in the field. In some or other such embodiments, in a way that is somewhat analogous to the way microwaves are used,

 Applicants believe that this photoacoustic phenomena can be used without a dedicated cavity by shining an infra-red (IR) laser on an area of interest and then trying to listen to the specific acoustic effect with the help of special remotely-located microphones. Further, by shining a laser beam for a dedicated gas on an area of interest and comparing it to the beam reflected from an area in the vicinity that does not contain the specific gas in question and then comparing the two reflected beams, the presence of the harmful gas can be detected and the signal-to-noise ratio can be increased.

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