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Nanotechnology based sensors for rapid, sensitive and reliable detection of explosives

In recent years, spread of terrorism has assumed an alarming proportion across the globe, because of the simplicity and variety of schemes by which these explosive-based weapons can be deployed causing enormous damage to public safety and environmental pollution. This has emphasized the need for rapid, sensitive and reliable detection of concealed explosives, to protect the human life and property.
The explosives typically used by terrorists and insurgents only give off incredibly small amounts of detectable gas. Existing systems which are capable of detecting compounds in the gas phase at such low levels are large, unwieldy and very expensive. They also have limited sensitivity and selectivity – there is no way to be certain which explosive substance has been detected.
In a warzone, the ability to detect toxic chemicals and explosives is imperative. Warfare agents include chemical and explosive compounds such as nerve agents like sarin, soman, and tabun, which are clear, colorless liquids without strong odors, which can cause loss of consciousness, seizures, and eventual death.
Sensing of trace explosives such as 2,4,6-trinitrotoluene (TNT) is a complex and challenging task due to the lack of inexpensive sensors with high selectivity and sensitivity, the lack of easily detectable signals, and wide selection of explosive compositions.
Trained dogs are still one of the most widely employed detection systems as they provide unmatched specificity.  Recently UK Aviation security is  deploying Dogs that are capable of detecting minute traces of explosives concealed in air freight  in cargo sheds at British airports as the government introduces its latest measure to reinforce the UK’s aviation security. Free running explosive detection dogs (FREDDs) working in close partnership with their human handlers are being used to check freight for a range of explosive materials. The dogs are being used alongside existing screening methods, offering a rapid and effective way of screening large volumes of air cargo. One of the emerging areas of research is artificial olefaction also known as electronic noses. These electronic noses can assist security agencies to  detect explosives in venues like stadiums, schools, airports, hospitals or any other crowded place, which offer an attractive target to  terrorists.
The rising concerns about explosives and their trafficking have encouraged many researchers around the world to explore new and effective ways to detect these materials in environments and places where they could represent a serious threat to people. Developments for miniaturisation, portability, field-ruggedisation and improvements in stand-off distances, selectivity and sensitivity have been necessary to develop and improve techniques.
As nanomaterials become more readily available for commercial devices, great interest has been shown in using them to develop trace-level detection systems for explosives which overcome all of these issues. Because of the unique nature and tuneable properties of nanomaterials like carbon nanotubes, nanowires, and other nanostructures, handheld or portable systems which are sensitive down to the molecular level could well be possible. Carbon nanotubes, nanowires, and other nanostructured materials have very high surface areas and a unique set of optical, mechanical and electrical properties, which make them ideal for exploitation in high-sensitivity detection of molecules.

Explosive Sensors

Explosives are chemical-based compounds, which, on being initiated to undergo self-propagating decay, result in a sudden release of heat and pressure. Based on the burn rates, they are classified as low or high intensity explosives. Explosives include but are not limited to metal acetylides, organic peroxides, nitrated aromatic and aliphatic compounds, and fuel oxidizer mixtures.


Nitroaromatic compounds including 2,4-dinitrotoluene (DNT) and 2,4,6-trinitrotoluene (TNT), which are associated with landmines and bombs used in military activities and terrorist attacks,are present as contaminants in
groundwater and soils at munitions processing sites and military ranges.


Explosives sensors are broadly categorised into electrochemical, mass, optical and biosensors depending on the type of measurement. Recent advances in this area of research including vapor and trace detection techniques (chemiluminescence, mass spectrometry, ion mobility spectrometry, electrochemical methods and micromechanical sensors, such as microcantilevers) and bulk detection techniques (neutron techniques, nuclear quadrupole resonance, x-ray diffraction imaging, millimeter-wave imaging, terahertz imaging and laser techniques).


Nitroaromatic compounds comprise an important class of explosive compounds for detection due to their widespread use. Due to the low volatility of explosives, absorptive materials such as polymers have been employed as explosive vapour sensors, with a variety of transduction schemes.


Spectrophotometric methods that are used for detecting explosives include photoluminescence, fluorescence, laser-induced breakdown spectroscopy and Tera-Hertz spectroscopy.


Ion mobility spectrometer (IMS) has been widely deployed for on-site detection of explosives in Airports worldwide. The common nitro-based explosives are usually detected by negative IMS while the emerging peroxide-based explosives are better detected by positive IMS.


Much of its popularity is derived from its successful application in the detection of conventional explosives including 2,4,6-trinitrotoluene (TNT), cyclo-1,3,5-trimethylene-2,4,6-trinitramine (RDX), pentaerythritol tetranitrate (PETN), ammonium nitrate fuel oil (ANFO) and so on, which are usually detected by negative ion molecules. In recent years, the peroxide-based explosives, such as triacetone triperoxide (TATP) and hexamethylene triperoxide diamine (HMTD), have been implicated in the terrorist activities due to their easy preparation. These explosives were better detected by positive ion mode IMS. Therefore, to quickly and fully identify different explosive species, a fast polarity-switchable IMS is highly desirable. The necessity of physical sample handling in IMS hampers automated screening as well as screening of large objects.


Nanotechnology based sensors

Nanotechnology-enabled sensors offer significant advantages over conventional sensors. This may be in terms of better sensitivity and selectivity, lower production costs, reduced power consumption as well as improved stability. The unique properties of nanoscale materials, such as increased surface area, confinement effects, etc., make them ideal for sensing. Nanomaterials can be integrated into existing sensing technologies or can be used to develop new devices. Various types of sensors, such as physical sensors, electro-sensors, chemical sensors and biosensors, have greatly benefitted from nanotechnology.


Surface-enhanced Raman spectroscopy (SERS) is a highly sensitive and selective spectroscopic technique used to identify analyte molecules via their vibrational fingerprints. This technique takes advantage of the strong field enhancement of plasmonic nanostructures (aka. SERS substrates) that can enhance Raman signals of molecules in their proximity by 4 to 10 orders of magnitude. SERS substrates based on noble metal nanoparticles such as gold (Au NPs) and silver (Ag NPs) offer numerous advantages for explosive detection such as high chemical stability, cost-effectiveness, rapid response, and little or no sample preparation.


A large number of nanosensor platforms such as micro- and nanofabricated structures, quantum dots, nanowires, nanotubes, nanobelts, etc. are being used for sensing explosives.


Nano sensor technology when leveraged to form a massive network has potential for detecting trace explosives.


Electronic Nose

Many research groups in academia and industry are focusing on the performance improvement of electronic nose (E-nose) systems mainly involving three optimizations, which are sensitive material selection and sensor array optimization, enhanced feature extraction methods and pattern recognition method selection. Researchers are also trying to make them affordable and widely available.
In recent works many alternatives have been tried, such as the design of microsensors on nanostructured metaloxides, the use of organic field effect transistors (OFET), sensor arrays and sampling systems, conducting polymer gas sensors, quartz microbalance sensors, surface acoustic wave [SAW] gas sensors etc. Biological neurons from rats have also been used as sensors and integrated with electronic circuitory.

Black silicon can help detect explosives

Scientists from Far Eastern Federal University (FEFU), Far Eastern Branch of the Russian Academy of Sciences, Swinburne University of Technology, and Melbourne Center for Nanofabrication developed an ultrasensitive detector based on black silicon. The device is able to detect trace amounts of nitroaromatic compounds and can be applied to identify the majority of explosives or highly toxic pollutants for medical and forensic evaluations. The related article was published in ACS Sensors.


The novel sensor is based on the so-called “black silicon” that is fabricated by high-performing reactive etching of commercially available silicon substrates. Such etched silicon has a nanostructured spiky surface exhibiting unique optical properties. After etching, the surface is covered with a monolayer of carbazole molecules. This process is called chemical functionalization since the attached molecules impart the substrate a certain important function, namely, the ability to bind and concentrate nitroaromatic compounds on the surface. The carbazole monolayer renders the device sensitive to such widely spread nitroaromatic substances as nitrobenzene, o-nitrotoluene, 2.4-dinitrotoluene, etc. However, the sensor does not react to the presence of other molecules, such as benzene, toluene, tetrachloromethane, methanol, ethanol, and so on.


“Nitroaromatic compounds can be found in the waste waters of paint plants or military facilities and are extremely dangerous for the environment. Moreover, they are parts of many explosives as well. Their detection in trace concentrtion represents an important and complex practical task. Our sensor platform identifies the presence of nitroaromatic compounds by means of registering the changes in the luminescence spectrum of the functional layer of carbazole that selectively reacts to nitroaromatic molecules,” said Alexander Kuchmizhak, a research associate at the VR and AR Center of the Science and Technology, FEFU.


According to the scientist, nanostructured black silicon used as the basis of the device gives it high sensitivity and an unprecedented dynamic measurement range. In the lab the sensor is able to provide information about the presence of toxic molecules in liquids or gases within several minutes.


“Combination of unique morphological and optical properties of black silicon being combined with easy-to-implement methods of surface chemistry used to functionalize silicon surface with carbazole molecules allowed to achieve unprecedented sensitivity. Our sensor is able to detect nitroaromatic compounds at concentrations down to ppt (part per trillion or 10-10 %). Extremely broad dynamic measurement range is caused by the unique spiky morphology of black silicon that provides uneven local concentration of carbazole molecules creating surface sites with different sensitivity,” explained Alexander Mironenko, the designer of the sensor, and a senior research associate at the Institute of Chemistry, FEB RAS.


Scientists stated the manufacture of the new sensor platform is expected to be quite cheap compared to the existing analogs. Moreover, the same sensor can be used multiple times. It can become a part of gas sensor systems that secure public and ecological safety.


Electronic chip sensor

A revolutionary new electronic chip sensor that can detect and identify explosives in real time and with great accuracy has been devised by Prof. Fernando Patolsky of Tel Aviv University’s School of Chemistry and Center for Nanoscience and Nanotechnology. The tiny chip consists of hundreds of nano-sized transistors, which are extremely sensitive to explosives in the air at concentrations as low as a few molecules per 1,000 trillion, which is four to five orders of magnitude more sensitive than any existing technological method, and two to three orders of magnitude more sensitive than a dog’s nose.


The device which is still at prototype stage, has been tested successfully from stand-off distance of few meters against explosives like TNT, RDX, and HMX, used in commercial blasting and military applications, as well as peroxide-based explosives like TATP and HMTD. The latter are commonly used in homemade bombs and are very difficult to detect using existing technology.


The device would enhance the security of airports and other public areas against terrorist attacks by enabling security forces to identify different explosives in real time with great accuracy, differentiate explosives with other nohazardous material, and decrease their reliance on sophisticated equipment, trained personnel, and detection dogs. Tracense has invested over $10M in research and development of the device since 2007, which is undergoing multiple and extensive field tests at present and is expected to be marketed next year.


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