The hitherto known methods for detecting explosive materials are mainly directed towards the detection of nitrogen containing compounds. These methods usually require concentrating vapors of explosive nitro-compounds followed by their decomposition to produce gases of nitric oxide (NO) and/or nitric dioxide (NO2). These gases can subsequently be detected using a variety of techniques including gas, capillary electrophoresis and high performance liquid chromatography, mass spectrometry, and ion mobility analyzer. U.S. Pat. Nos. 5,092,218; 5,109,691; 6,571,649; and 6,840,120 disclose exemplary uses of said techniques for explosive detection.
Other commonly used techniques include x-ray scattering, neutron analysis, nuclear quadrupole resonance, FTIR and Raman spectrometry, and immunoassays (Wang, Analy. Chimi. Acta, 2004, 507: 3). U.S. Pat. No. 5,801,297 discloses methods and devices for the detection of odorous substances including explosives comprising a plurality of gas sensors selected from semiconductor gas sensors, conductive polymer gas sensors, and acoustic surface wave gas sensors. U.S. Pat. No. 6,872,786 discloses a molecularly imprinted polymeric explosives sensor, which possesses selective binding affinity for explosives. U.S. Pat. No. 5,585,575 discloses an explosive detection screening system which comprises a concentration and analyzing system for the purification of the collected vapor and/or particulate emissions and their subsequent detailed chemical analysis. U.S. Pat. No. 7,224,345 discloses a system for electrochemical detection based on carbon or carbon/gold working electrode having a modified surface to detect trace amounts of nitro-aromatic compounds.
The most frequently used sensing devices for detecting explosive materials are based on the lock-and-key approach, wherein each sensor detects one explosive material. In this manner, the sensors are designed to detect very specific target molecules resulting in restricted applicability.
Electronic nose devices perform odor detection through the use of an array of cross-reactive sensors in conjunction with pattern recognition methods. In contrast to the “lock-and-key” model, each sensor in the electronic nose device is widely responsive to a variety of odorants. In this architecture, each analyte produces a distinct signature from the array of broadly cross-reactive sensors. This configuration allows to considerably widen the variety of compounds to which a given matrix is sensitive, to increase the degree of component identification and, in specific cases, to perform an analysis of individual components in complex multi-component mixtures. Pattern recognition algorithms can then be applied to the entire set of signals, obtained simultaneously from all the sensors in the array, in order to glean information on the identity, properties and concentration of the vapor exposed to the sensor array.
Nanoparticles possess several features which render them advantageous as sensing devices designated for explosive detection. A particularly important feature is their enhanced surface to bulk ratio. Furthermore, molecules that are attached to the surface of the nanoparticles play a key role in determining the physical and chemical properties of these particles. Such molecules are often referred to as organic coating that can be tailored to serve either one of several functionalities. Most importantly, the organic coating can modify the electronic properties of “bare” nanoparticles. It is further possible to obtain electron transport between the nanoparticles and the organic coating thus introducing cooperative effects. Nanoparticles capped with an organic coating (NPCOC) are therefore of high technological importance, particularly since both nanoparticles and their organic coating may be selected from a wide variety of compounds.
Devices based on nanoparticles and methods of use thereof for detecting, inter alfa, explosive compounds are disclosed in e.g. U.S. Pat. Nos. 7,171,312; 7,144,553; 7,034,677; 6,839,636; 6,773,926; 6,759,010; in U.S. Patent Application Nos. 2007/0264719; 2007/0231790; 2007/0165217; 2007/0132043; 2007/0059211; 2006/0160134; 2006/0040318; 2005/0263394; 2005/0150778; 2004/0204915 and 2001/0041366 and in Wohltjen et al., Anal. Chem., 1998, 70(14): 2856; and Evans et al., J. Mater. Chem., 2000, 10(1): 183.
U.S. Pat. No. 7,052,854 discloses systems and methods for detecting a target analyte/biomarker including an explosive, using nanostructure-based assemblies comprising a nanoparticle, a means for detecting a target analyte/biomarker, and a surrogate marker. The sensor technology is based on the detection of the surrogate marker which indicates the presence of the target analyte/biomarker in a sample.
Sensors based on changes in the physical and/or electrical properties of films composed of spherical NPCOC(SNPCOC) have been illustrated in the literature. Theoretical as well as experimental measurements indicate that the sensitivity of SNPCOC based sensors toward analytes is limited to a concentration range of 100-1000 parts per billion (ppb). This limitation has been attributed to two main reasons. First, while voids between adjacent (spherical) nanoparticles can host analyte molecules during the exposure process, they do not contribute to the obtained sensing signal. Second, the contact interface between adjacent spherical nanoparticles, onto which analyte molecules adsorb and induce sensing signals (e.g., by inducing swelling/aggregation of the film), is limited to a very small area in comparison to the total surface area of the SNPCOCs.
For the reasons mentioned hereinabove, obtaining high sensing performance requires increased sensitivity which is often met by pre-concentrating the explosive vapors prior to their measurement thus leading to lengthier measurements. Alternatively, in order to achieve high sensitivity, an increase in film thickness can be employed. However, such an increase results in intensified diffusion limitations, thus reducing the response time. Hence, there is an unmet need for fast responsive sensors having improved sensitivity as well as selectivity allowing real-time measurement of minute quantities of volatile or non-volatile compounds derived from explosive materials.