International Journal of Scientific & Engineering Research, Volume 5, Issue 6, June-2014

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Sensor Review for Trace Detection of Explosives

Lama Mokalled, Mohammed Al-Husseini, Karim Y. Kabalan, Ali El-Hajj

AbstractThe detection of explosives covers a very important hazardous problem for people, due to the advancement of dangerous terroristic activities as well as of breakdowns in the production of these explosives. Border conflicts and terrorist attacks increased and hence detection of hidden bombs and explosives in lands, luggage, vehicles, aircrafts, and suspects became a must. The detection approach must take into consideration several factors including safety, sensitivity, accuracy, speed of recovery, and ease of implementation. This paper deals with a review of electronic/chemical sensors, optical sensors, and biosensors and their usa ge in tracing explosive devices and detecting landmines. Available techniques are covered which are characterized by a high degree of tech nological development. In addition, means of detection for vapor trace explosives is also presented in this review paper. Commercially available electronic/chemical trace explosive detection approaches are also presented with their characteristics.

Index TermsDetection, Explosives, Vapor, Trace, Detection, Electronic/Chemical, Optical, Sensors, Biosensors.

1 INTRODUCTION

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xplosives existing around the world pose serious prob- lems. Numerous solutions are explored and performed. The purpose behind such researches is to acquaint safe
and fast methods for mines detection to preserve precious human life, whether they were innocent civilians or part of the demining teams. Explosives are indiscriminate weapons, in other words, they do not distinguish between soldiers and civilians, or between adults and children. Hence the need for efficient, reliable, and fast methods for explosive detection has been an active area of research during the last few years to diminish the threat of terroristic activities [1]. Explosives have already injured or killed thousands of people all over the world hence inspiring researchers to dig for solutions. There exists a major concern about people security and environment protection which urged researchers for the development of sensors for explosive detection of compounds. Development of methods and instrumentations for explosive detection de- rived lots of attention. Several techniques have been explored and performed with the aim of implementing methods to pre- serve human lives [2].
Trace detectors are security tools that are able to detect ex- plosives of small magnitude by means of sensors. Detection is performed by sniffing vapors in a vapor explosive detector or by sampling traces of a particulate or even joining both meth- odologies based upon the need. Long time ago, and even nowadays, dogs are solely used to detect explosives through vapors [3], [ 4]. This technique is considered the most effective and efficient among other detection methods that are currently in use. Dogs undergo rigorous training in various operational fields with several types of explosives to be prepared for ex- plosive detection. However, a dog’s performance declines with age and overtime especially after extensive fieldwork [5].

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Lama Mokalled, Karim Y. Kabalan, and Ali El-Hajj are with the Depart- ment of Electrical and Computer Engineering, American University of Beiurt, Lebanon. E-mail: kabalan@aub.edu.lb

Mohammed Al-Husseini is currently with Beirut Research and Innovation Center, Lebanese Center for Studies & Research, Beirut, Lebanon. Email: husseini@ieee.org

Detection methodologies are divided into two major cate- gories: Trace detecton and bulk detection of explosives. Trace detection involves the chemical detection of explosives by gathering and analyzing small amounts of explosive vapor. Bulk detection requires the detection of a macroscopic mass of explosive material which is outside the scope of our review paper. The term trace detection refers to both vapor and par- ticulate forms. Vapor sampling requires no contact whilst par- ticulate sampling requires direct contact to remove explosive material particles from a contaminated surface [6], [7].
This paper presents a review of electronic/chemical sensors [8], optical sensors [9], biosensors [10], and their usages in tracing explosive devices and detecting landmines. Available techniques are covered which are characterized by a high de- gree of technological development. In addition, means of de- tection for vapor trace explosives is also thoroughly presented in this review paper. Commercially available electron- ic/chemical trace explosive detection approaches are also de- ployed with their characteristics. Explosives detected by each sensor type are also tabulated. The review ends with a com- parison between various detection types reaching conclusion and future work.

2 VAPOR DETETION METHODS

Solids and liquids emit vapor at certain temperature and pres- sure conditions. The amount of vapor emitted by each sub- stance characterizes its volatility. Sampling and analysis of saturated vapor is collected near the surface of explosive ma- terial without the need for contacting it. Sensitivity of vapor trace detection methods depends on several factors such as: vapor pressure, efficiency in collecting vapor, temperature, and wind strength. Vapors and traces are currently detected by means of electronic/chemical sensors, optical sensors, and biosensors. References [11], 12] present a survey and an over- view of commercially available explosive detection tech- niques. Fig. 1 represents a tree listing various types of vapor trace detection of explosives. Each type shall be covered in the coming three sections of the paper.

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Fig. 1: Overview of vapor trace detection types [12]

2.1 Electronic/Chemical Trace Detection Method

2.1.1 Ion Mobility Spectrometry (IMS)

IMS is one of the most commonly used techniques for trace detection of explosives. It consists of an ion source region, an

ion gate, a drift region, and an ion detector as shown in Fig. 2.

Fig. 2: High Resolution Ion Mobility Spectrometer (HRIMS)
fitted with an electrospray ionization (ESI) source [13]
IMS detection is based on how fast ions move through the drift region reaching the detector. An applied electric field through the gas sample allows mobility of ions. Hence vapors are ionized at atmospheric pressure before reaching drift re- gion. Reaching drift region within a certain time interval is determined according to the mass/charge ratio of the ions and hence characterizes each component of the explosive material as shown in Fig. 3. An advantage of this method is its speed where measurements take only few seconds, but a disad- vantage is its low selectivity. A review of IMS for detection of explosives is presented in [13].

Fig. 3: Schematic of IMS operation [14]
Martin et al [15] presented a micro fabricated hotplate coated with a sorbent polymer as away to trap analytes of interest
prior to analysis with IMS; this technique enhanced sensitivity acquaintedly by at least one order of magnitude. Waltman et al [16] showed a distributed plasma atmospheric pressure ion- ization source that included the application of a high voltage alternating current through dielectric to produce plasma with- in which the sample was ionized. Tabrizchi and Ilbeigi [17] described a positive corona discharge technique using a cur- tain plate blocking the diffusion of NOx into the ionization region and accordingly allowing analysis in air as opposed to nitrogen. Fig. 4 shows some commercially available IMS detec- tion technologies.

Fig. 4: Pictures of commercial IMS technologies [11]

2.1.2 ChemiLuminescence (CL)

CL is the production and emission of light as the output of a chemical reaction. The produced light is proportional to the amount of NO present, which is related to the amount of the original nitrogen containing explosive material. CL can identi- fy the characteristic emission of radiation from a molecule, atom or effective fluorophore, in an excited state, produced in an exothermic chemical reaction. It can take place in gas, liq- uid, and solid state. The analyzed vapors of explosives (NO2 groups) are mixed with ozone, exciting NO2 molecules. Char- acteristic light emitted by these molecules is then detected [19]. A significant drawback of CL technique is its inability to detect explosives that are not nitro-based. Another disad- vantage of this method is lack of selectivity but this can be enhanced by coupling CL by means of different separation Ground Separation (GS) methods [20]. GS method uses fluo- rescent dyes by which a luminescent signal can be enhanced, depending on the distance separating the energy donor (emit- ting molecule) from the energy acceptor (dye molecule). Excit- ed state donor exchanges a high energy electron for one of lower energy, thus returning to the ground state. The ground state acceptor molecule loses the low energy electron and gains one of higher energy, thus entering an excited state. The rate of energy transfer depends on the concentration of accep- tor molecules. Fig. 5 represents mode of operation of a CL. In the market, products handheld and portable are shown in Fig.
6.

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Fig. 5: Image of CL operation [14]


Fig. 6: Commercially available CLs in the market: left- handheld, right-portable [11]

2.1.3 Electron Capture Detector (ECD)

ECD sensors can detect vapor or invisible particulates in the air that strongly capture thermal electrons. ECD is usually combined to a gas chromatograph for identification of explo- sives since an ECD sensor by itself can’t recognize individual explosive types when other interferents exist. The detector is characterized by its moderate sensitivity and small size in which it can fit in the palm of the hand [21, 22]. Theory of op- eration of an ECD is detailed in [23].
A prototype of ECD was built in the first place by James E. Lovelock during late 1950s [24]. Lovelock was trying to de- termine the damage that takes place to living cells when they are frozen. His instrument had low sensitivity so he tried to
implement a device to detect substances in the 10-15 grams range and hence ECD came to light. Fig. 7 shows Lovelock’s ECD during 1958. In 1962, Rachel Carson drew the public's attention to the detrimental effects of chlorinated hydrocar- bons and pesticides employed as economic poisons in agricul- ture on organisms [25]. A picture of a commercially available ECD is presented in Fig. 8.

Fig. 7: ECD image invented in 1958 [24]

Fig.8: Commercially available ECD [18]

2.1.4 Thermo-Redox

Thermo-redox technology is an electrochemical method based on the thermal decomposition of explosive molecules and the subsequent reduction of NO2 groups. A sample is drawn into the system and is passed through a tube, which traps explosive materials. The sample is heated to release NO2 molecules, and these molecules are detected using appropriate technology. The method is used to detect the presence of NO2 molecules in explosive materials. A critical pitfall in this method is that sensors can neither detect non nitrogen explo- sives; nor distinguish explosives in other substances contain- ing NO2 groups [26]. A handheld device of this methodology is shown in Fig. 9.

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Fig. 9: Handheld thermo-redox explosive detection system [18]

2.1.5 SURFACE ACOUSTIC WAVE/GAS CHROMATOGRAPHY

(SAW/GC)

SAW/GC is technological method that allows excel for hun- dreds of individual SAW sensors. SAW sensors are oscillator circuits whose resonance frequencies are controlled by SAW delay line or resonator devices in the feedback path [27]. SAW devices are functionalized for chemical vapor sorption by de- positing broadly selective polymer thin films in the acoustic wave propagation region. Exposure to vapor produces shift in oscillator frequency, and is considered to be the chemical sig- nal. The measurement of changes in the surface wave charac- teristics such as amplitude, phase, frequency, etc is a sensitive indicator of the properties of the vapor. Fig. 10 illustrates the performance of the SAW approach.

Fig. 10: Schematic of a simple SAW device [14]
[28] and [29] employ the principle of zNose approach where a gas mixture is separated using a chromatographic column where the outlet from each column is detected by a SAW sen- sor. A SAW/GC is developed for detecting explosives using a resonator crystal functioning at 500MHz.The crystal is subject to the gas exit of a capillary column. When condensable ana- lyte vapors influence the active area of the SAW crystal, a fre- quency shift exists that is proportional to the mass of the ana- lyte, the temperature of the crystal, and the chemical nature of the crystal surface [30, 31].The main advantage of using SAW/GC is its ability to detect chemicals in addition to explo- sives. Fig. 11 shows a commercially available SAW system.

Fig. 11: Portable commercial SAW system [11]

2.1.6 MASS SPECTROMETRY (MS)

MS divides and analyses the chemical composition of a speci- men by ionizing molecules and passing them through a filter. Ions are thus identified according to the molecule mass to charge ratio. Desorption Electro Spray Ionization (DESI) al- lows detection of the substance of interest in its ambient envi- ronment by bombarding it with a mist of electrically charged droplets hence creating ions which are drawn into the MS with a vacuum [32]. In an another technique to DESI, named Direct Analysis in Real Time (DART), a voltage is applied to a carrier gas and the resulting excited state species release mole- cules from the sample which are introduced into the MS [32,
33]. Mode of operation of an MS is viewed in Fig. 12 while Fig.
13 shows portable MS portable detection systems.

Fig. 12: MS working principle [14]

Fig. 13: Portable commercial MS explosive detection systems
[11]

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2.1.7 MICRO ELECTRO MECHANICAL SYSTEMS

(MEMS)

MEMS technique uses an integration of mechanical elements, sensors, actuators, and electronics on a common silicon sub- strate by micro fabrication. Fig. 14 displays the contents of a MEMS sensor. MEMS sensors called microcantilevers, which are hair like silicon based devices that can detect and measure relative humidity, temperature, pressure, flow, viscosity, sound, ultraviolet and infrared radiation, chemicals, and bio- molecules.

Fig. 14: Components of a MEMS sensor [14]
An operational mode for MEMS that includes change in the vibration frequency of a heated polymer coated silicon based cantilever is discussed in [34, 35]. These changes are a result of nano explosions of the detected explosive vapors. Detecting the temperature of the cantilever allows pointing various ex- plosive types according to their temperature changes. MEMS based microcantilever sensors are rugged, reusable, and ex- tremely sensitive, yet they can be low-cost, and consume little power. Another advantage in using the sensors is that they work in air, vacuum, or under liquid environments. A pro- posal for a handheld MEMS system is viewed in Fig. 15.

Fig. 15: A future handheld MEMS explosive detector [36]

2.1.8 ELECTRONIC NOSE (E-NOSE)

An electronic nose sensor, as its name indicates, is a device used to detect odors and flavors. The aim of this electronic sensor or e-sensor is to imitate human nose sensing capability. With the addition of nano-enhanced sensors and improve- ments in pattern recognition systems, such as neural network
technologies, e-sensors have undergone important shifts from a technical and commercial perspective. An electronic nose has the capability of detection and identification of miniature amounts of explosive chemicals. A detailed description of elec- tronic noses and their application to explosive detection is re- viewed in [37]. Fig. 16 shows an e-nose sensor.

Fig. 16: An e-nose sensor [14]

2.1.8.1 COMPONENTS OF AN ELECTRONICS NOSE

An electronic nose is basically composed of a chemical sensing system and a pattern recognition system. Each vapor intro- duced to the system produces a signature or fingerprint. Pre- senting many different chemicals to the sensor forms a data- base of fingerprints, which the pattern recognition system uses to identify each chemical. Sensor arrays offer several ad- vantages over single sensors mentioning; better selectivity, multicomponent analysis, and analyte recognition. Some elec- tronic noses use fluorescent polymers, fiber optic cables [38], arrays of different polymeric thin film sensors [39], gold nanoclusters deposited on interdigital micro electrode arrays [40], surface acoustic wave [41], [42], quartz crystal microbal- ance gas sensors, and micro electromechanical systems [43]. In future, these systems will replace some of the larger and more expensive detection devices. E-Nose can detect an electronic change of about 1 part per million [44].Some of the e-nose techniques used will be presented in the coming sections of the review. [45] gives insight investigation of e-nose tech- niques used in explosive detection that can be of great value to readers. The components of an e-nose are shown in Fig. 17.

Fig. 17: Components of an e-nose [14]

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2.1.8.2 AMLIFYING CHROMOPHORE QUENCHING

Some electronic noses use fluorescent polymers as chemical detectors for explosives that react to volatile chemicals such as nitrogen based components. Commonly used fluorescence detection techniques usually measure a variation in fluores- cence intensity or a wavelength shift that occurs when a single molecule of the analyte interacts with an isolated chromo- phore. In that case, only the chromophore that interacts direct- ly with the analyte molecule is suppressed, whilst the remain- ing chromophormes continue to fluoresce. Fig. 18 is a sche- matic of the basic sensor design used in explosive detection by

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electronic nose.

Fig. 18: Schematic of basic sensor design [45]
Swager et al [46], [47], [48], developed a polymer which reacts with nitrocompounds and thin films to reveal high fluores- cence of TNT and DNT vapors emanating from landmines. The backbone of the polymer acts as a molecular wire enabling propagation of a quantum of electronic energy through the polymer chain. The sensory elements of the detection system are made up of thin films coated into a substrate.

2.1.8.3 FIBER OPTICS AND BEADS

For the sake of imitating a biological nose, a detector with mil- lions of sensors is required. Walt et al [49[, [50], [51], [52], pre- sented an explosive sniffer that uses a sensor array of fiber optic cables. The sniffer is used to sample the air and watch for a color change of the sensor. Each sensor produces a different reaction for the same chemical. Hence, each odor creates a dif- ferent pattern that can be stored on a computer database.
Fig. 19 shows three test tubes A, B, and C each containing a different type of polymer sensor suspended in a solution. A combination of them is formed by mixing the three constitu- ents and a new compound is formed. A drop of the mixture is placed onto the distal tip of an etched imaging fiber. Conse- quently, beads rest in random localizations through the well array. They are identified by their characteristic response to a test vapor pulse. The beads are self-encoded and the signal of each bead is used to identify it and map its position in the ar- ray
Fig. 19: Self encoded bead array concept [45]

2.1.8.4 POLYMERIC THIN FILMS

Lewis et al [53[, [54], [55], developed a new type of electronic nose based upon an array of different polymeric thin film. It is based upon multisensing principle, where individual sensors are not specific to any one compound. Each detector of the sensor array consists of a conductive carbon black mixed with a non-conducting polymer.
The detector materials are deposited as thin films on an alumina substrate across each of two electrical leads to create conducting chemiresistors. Detector is exposed to an analyte vapor which allows polymer to act as a sponge and absorb the analyte. When the analyte is removed, the polymer sponge returns to its original configuration, the film shrinks, and the conductive pathways are reestablished. The baseline resistance (R baseline) of the device is measured while a representative background vapor flows over the array. The response from the chemiresistor during exposure to an analyte is measured as a relative resistance change (R/R baseline). Fig. 20 reveals the response of a collection of incrementally different sensors used to generate a complex pattern or finger print, characteristic of a given analyte.

Fig.20: Patterns produced by an array of broadly responsive vapor detectors [56]

2.1.8.5 GOLD NANOCLUSTERS

A new nanometer scale, low power and solid state device is being utilized for the detection of explosives. This chemical vapor sensor is composed of nanometer sized gold particles encapsulated by monomolecular layers of alkanethiol surfac- tant deposited as thin films on interdigitated microelectrodes as shown in Fig. 21.

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Fig. 21: A picture showing a gold nanocluster [60]
Jiang et al [57], described a simple colorimetric visualization of TNT. It is based on the interaction between TNT and primary amines named cysteamines. The cysteamine acts as a stabilizer to the gold nanoparticles (Au NP).Depositing TNT into the aqueous solution caused aggregation of the amine covered Au NPs which resulted in a color change from red to violet. Da- sary et al [58] described a Surface Enhanced Raman Scattering (SERS) probe coated with gold nanoparticle cysteine conju- gates; this system was able to form aggregating Meisenheimer complexes within water in the presence of TNT. A similar ap- proach was taken by Yang et al in [59] using functionalized silver nanoparticles coated on silver molybdate nanowires.

2.9 OPTICAL TRACE DETECTION METHOD

2.9.1 ULTRA VIOLET-VISIBLE_NEAR_INFRARED (UV- VIS-NIR) SPECROSCOPY

UV-VIS-NIR, also called Differential Reflectometry (DR), measures the differential reflection from materials at multiple wavelengths. In [61], Ultra Violet (UV) light sweeps two adja- cent zones with reflectivities R1 and R2on a piece of luggage placed on a moving conveyer belt as shown in Fig. 22. Re- flected light is collected using spectrograph and CCD (Charge Couple Device) camera. A computer processes the resulting data and produces in turn a differential reflection spectrum. This technique is fast, can potentially scan large quantities of parcels or luggage for surface explosives threads with a fairly sensitive level.
Hatab et al [62], developed a project using Raman Spec- troscopy (SERS) as the interaction process of inelastic scatter- ing of photons by molecules of a substance. The Raman spec- trum consists of bands shifted with respect to the line of excit- ing radiation. This shift and shape of spectral bands are the fingerprints of detected material.
Fig. 22: Schematic representation of a DR system [61]

2.9.2 PHOTO ACOUSTIC SPECTROSCOPY (PAS)

PAS is a form of spectroscopy that uses sound to identify sample components. It is a part of a class of photothermal techniques, in which an incoming light beam is absorbed and affects the thermal state of the sample. However, if the incom- ing light is modulated, the sample warms and cools in a cycle. If the cycle is so fast, that the sample does not have time to expand and contract in response to the modulated light, a change in pressure develops. This pressure wave can lead to the production of a sound wave [63].
Fig. 23 shows a general setup for the photoacoustic spec- troscopy of a gas sample. When a species absorbs some of the incoming light, it increases translation energy of the gas parti- cles and allows their heating [64]. Varying the wavelength of the incoming light will change the amount of light absorbed. The amount of pressure changes also and accordingly the amount of sound produced. One advantage of PAS is that it can be performed on all phases of matter. A disadvantage can be noted as a result of narrow laser light bandwidth, where the analyte molecule must absorb some light from the source for the sake of detection.

Fig. 23: Experimental setup that shows PAS performance on a gas [64]

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2.9.3 CAVITY RING DOWN SPECTROSCOPY

This technique is based upon measurement of absorption rate rather than magnitude of absorption of a light pulse. A short pulse of light is inserted into a resonant cavity which is sur- rounded by highly reflective mirrors. When sufficient radia- tion is build up within the cavity, laser is turned off and expo- nential decay for light intensity is measured as a function of time. The decay time for an empty cavity is then compared with one containing the sample. The molecular absorbance and hence concentration can be derived from the rate of decay [65 -66].
Ramos and Dagdigian [67], described a study into the use of ultra-violet CRDS as a means to improve on the low sensi- tivities related to infra-red CDRS. The issue, however, is that this technique is unable to decisively identify specific samples due to its poor selectivity in the 240-260 nm spectral range [67].
A typical experimental setup for CRDS is shown in Fig. 24. The ring down cavity is formed by two planoconcave mirrors placed at a distance slightly less than twice their radius of cur- vature. The mirrors are coated for an optimum reflectivity in the desired wavelength range. The mirrors often act at the same time as windows for the closed cell, which contains the specimen to be studied. Photo Multiplier Tube (PMT) is used to ensure that all transverse modes are detected [68]. The out- put signal is then amplified and recorded using a fast and high resolution digitizer through General Purpose Interface Bus (GPIB).

Fig. 24: Scheme for the experimental setup of CRDS [68]

2.9.4 LIGHT DETECTION AND RANGING (LIDAR)

LIDAR is the acronym of Light Detection and Ranging and started with the innovation of laser. It is a remote sensing technology that measures distance by illuminating a target with a laser and analyzing the reflected light. A narrow laser beam is used to map physical features with very high resolu- tion. The structure of LIDAR system consists of transmitting beam, receiving optics and signal processing.
One of the adequate applications of the inline type LIDAR
is a Raman LIDAR [69]. It is developed for the detection of hydrogen leak gas. The hydrogen concentration is estimated by calculating the ratio between the nitrogen echo and hydro- gen echo intensities. The receiving characteristics coincided well with the results estimated on the optical designs and in
the observation range. The compact Raman LIDAR estimates low hydrogen concentration in the order less than 1%. Fig. 25 shows Raman LIDAR setup.

Fig. 25: Compact Raman LIDAR for hydrogen gas detection
[69]

2.9.4 DIFFERENTIAL ABSOORPTION LIDR (DIAL)

DIAL is a technique for the remote sensing of atmospheric gases. DIAL lasers transmit into the atmosphere pulses of ra- diation at two wavelengths: one of which is absorbed by the gas to be measured and the other is not. The difference be- tween the return signals from atmospheric backscattering of the absorbed and non-absorbed wavelengths is used as a di- rect measure of the concentration of the absorbing species.
A DIAL system that is capable of measuring natural gas pipeline leaks as an aircraft flies over the surveyed pipeline location is introduced in [70]. The DIAL is deployed in an aircraft so that long segments of pipeline can be rapidly sur- veyed. Fig. 26 shows an example of pipeline leak detection. DIAL sensor has greatly advanced the capability for remote detection of trace concentrations of gases.

Fig. 26: Leakage detected in a pipe line using a DIAL [70]

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2.10 BIOSENSORS

2.10.1 BIOSENSOR DEFINITION

The most commonly used definition of a biosensor is: a self- contained analytical device that incorporates a biologically active material in intimate contact with an appropriate trans- duction element for the purpose of detecting (reversibly and selectively) the concentration or activity of chemical species in any type of sample [71]. The first biosensor, an enzyme-based glucose sensor, was developed by Clark and Lyons [72].Since then, hundreds of biosensors have been developed in many research laboratories around the world [73], [74], [75], [76]. Several research papers and books about biosensors are pub- lished reviewing the principles of operation and fabrication in addition to potential applications in food and agricultural in- dustries in particular [77]. Fig. 27 shows a biosensor in opera- tion mode on an analyte.

2.10.4 REQUIREMENTS FOR BIOSENSORS

To be commercially successful, a biosensor has to meet the general requirements of commercial sensors [78]. These are:

Accuracy and repeatability

Sensitivity and resolution

Testing and calibration

Reliability and self checking

Physical robustness and service requirements

Safety and speed of response

Dynamic range and user acceptability, and

Insensitivity to temperature, electrical or environmen- tal variations

2.10.5 HISTORY OF BIOSENSORS

Table 1, shown below, includes the history of biosensors from year 1956 reaching current year [79].
Table 1: History of biosensors

Analy te

Respon se

Analys is

Sig nal

Detecti on

Sample handling/ preparation

Fig. 27: Biosensor acting on an analyte [14]

2.10.2. COMPONENTS OF BIOSENSOR

A biosensor consists of two components: a bioreceptor and a transducer. The bioreceptor is a biomolecule that recognizes the target analyte whereas the transducer converts the recog- nition event into a measurable signal. The uniqueness of a bio- sensor is that the two components are integrated into one sin- gle sensor. The simplicity and the speed of measurement are the main advantages of a biosensor.

2.10.3. CONSIDERATIONS IN BIOSENSOR DEVELOPMENT

Once a target analyte is identified, the major tasks in develop- ing a biosensor involve [78]:

Selecting a bioreceptor molecule

Selecting a transducer

Designing a biosensor considering measurement range, linearity, and minimization of interference , and

Packaging of biosensor

3. COMMERTIALLY AVAILABLE SETECTION

METHODOLOGIES

Table 2, shown below, summarizes the specs for electronic/ chemical trace detection methods [11]. The cost is as reported in [11] which is subject to changes with years and innovation of new technologies

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Table 2: Commercially available electronic/chemical trace de- tection equipment

tem and hence affecting fluorescence intensity. The Fido sys- tem is matched to a communication box and mounted on the surface of a robot to keep the operator at far away distance.

Portability Ease of Use Throughput

Rate

Sample

Collection

Cost

Results are displayed on the robot control unit for the sake of analyses and detection.

IMS handheld,

operated by a

two to three

vapor and

handheld:

3 COMMON EXPOLSIVE TYPES

Explosives are chemical or nuclear materials which can un- dergo pretty fast self-propagating decomposition which result in the production of heat or development of abrupt pressure effect. Explosives require a stimulus to liberate energy and hence produce explosion. The various stimuli to which every explosive material responds separately allow classification of explosives into several types. Fig. 28 shows a tree representing explosive types. Explosives are mainly classied as low and high explosives. Low explosives burn at relatively low rates of the chemical reaction in the range of cms, whereas high explo- sives detonate at velocities of kms [84], [85].

Fig. 28: Explosive Types [84]
Other in the market products can be also listed mentioning Vreeland Direct Reading Spectroscope, where the specimen is illuminated by continuous spectra of light and the recordings are saved on transparent films that are embedded inside the instrument. Comparison of the obtained film takes place with a master film and allows identification of the explosive sub- stance [80]. On the other hand, EX-Detect XD-2 is a color indi- cator for explosives it uses colorimetric screening for detection of explosives. A swipe is rubbed over the surface to be tested and then it is placed into a clamp at the center of the detector and power is turned on. Two solutions are present; dispensing solutions in a certain manner and then heating allows color change as explained thoroughly in the reference [81] and hence detection.
Seeker XDU is an easy to use, light weight colorimetric device. It is made up of seven button keypad that allows navi- gation on the LCD screen of the XDU. Moving to the opera- tion, a swipe card is to be selected in the first place and the sample should be rubbed against it. After that, bar code scan- ning takes place prior to inserting the card into the device. Finally, result analyses can be witnessed on graph [82]. Fido On Board [83] is based upon amplifying chromophore quench- ing where the sensing element or fluorescent material is placed inside the detector. Air sample is passed into the sys-
Low explosives or propellants contain oxygen for combustion which in turn produce a gas that forms an explosion. Explo- sives belonging to this category vary widely in the rate at which they deliver energy mentioning black powder, smoke- less powder, and flash powder [86].
High explosives are subdivided into two groups: primary explosives and secondary explosives. Primary explosives are highly susceptible to initiation and are often referred as initiat- ing explosives because they can be used to ignite secondary explosives. They explode when heated or exposed to shock. Materials differ in the amount of heat and brisance they pro- duce during explosion. Examples of primary explosives in- clude lead azide, lead styphnate, mercury fulminate, Diazodi- nitrophenol (DDNP), tetrazenelead salts of picric acid and trinitroresorcinol, m-nitrophenyldiazonium perchlo- rate, tetracene, nitrogen sulfide, copper acetylide, fulminating gold, nitrosoguanidine, mixtures of potassium chlorate with red phosphorus or with various other substances, the tar- tarates and oxalates of mercury and silver [87].
Secondary explosives are insensitive to heat, friction, and shock. They are often called base explosives or bolstering ex- plosives and are formulated to detonate only under specic circumstances. Secondary explosives can be categorized into melt-pour explosives which are based on nitroaromatics, suchastrinitrotoluene (TNT), dinitrotoluene (DNT) and plastic bonded explosives which are based on a binder and crystalline

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explosive, such as hexahydro-1,3,5 trinitroazine (RDX). One can also state tetryl, picric acid, nitrocellulose, nitroglycerin, liquid oxygen mixed with wood pulp, fuming nitric acid mixed with nitrobenzene, compressed acetylene and cyano- gen, nitroguanidine, ammonium nitrate and perchlorate as secondary explosives [88].

4 EXPLOSIVES DETECTED VY EACH SENSOR TYPE

Comparative assessment of sensor performance for detection of explosives is represented in Table 3. Useful information can also be found in review paper [89] and in references [90], [91], [ 92].
Table 3: Sensor types and the corresponding detected explo- sives
Table 4: Sensitivity of trace detection of explosives
An important characteristic of each explosive type is sensitivi- ty. Table 4 below shows Electronic/ Chemical, Optical, and Biosensor sensitivities depending upon field of application and explosive material detected [87].

5 COMPARISON OF VARIOUS TRACE DETECTION

TYPES

Each technology of the above mentioned trace explosive detec- tion techniques has its own advantages and disadvantages. The electronic/chemical sensors are sensitive and rapid in addition to being inexpensive, but they suffer from low selec- tivity meaning that different electronic/chemical sensors might be required for different materials. For instance, IMS systems have attractive features due to their moderate cost, ease of use, and portability. IMS instruments in contrast con- tain radioactive material as ionizing source and have low se- lectivity [93]. CL method contains no radioactive source whilst inability to detect explosives that are not nitro-based. ECD detector has a fast response time, moderate sensitivity, small size, low cost, and light weight. Two problems arise with ECD which are the need for radioactive ionization source and the presence of the carrier gas which can put limits on field appli- cations [94]. Thermo-redox contains no radioactive source yet detects NO2 groups only [93]. No radioactive source exists in SAW/GC either but its major disadvantage is being nonspecif- ic, hence the presence of other chemicals will make detection of explosives more complex [93]. MS detectors do not utilize radioactive source and are specific with the drawback of long sample analysis time [93]. MEMS have the pros of high repro- ducibility with cheap manufacture cost and cons of small test masses and inertial forces as the case of accelerometers [95].The biggest benefit of E-Nose relies in calibration simplici- ty on the other hand; they are severely affected by environ- mental conditions mentioning temperature and humidity [96].

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Optical trace detection methods generally have high sen- sitivity and fast response but they could falsely detect the composition of the material in question. UV-VIS-NIR or DR Spectroscopy is characterized by fast sample analysis whilst confusing optical constants [97]. PAS can be applied to all phases of matter but limitation exists in narrow laser band- width [98]. CRDS measurements are dependent upon rate of signal decay and are insensitive to laser power fluctuations. Its main disadvantage is being unspecific to a certain point in the plasma [99]. LIDAR reduces costs for field measurements over wide areas and helps locate useful field data positions though it requires specialized skills and software for operation [100]. DIAL has the ability of attaining direct measurements of cer- tain gases irrespective of day, time, or season. A potential drawback is the amount of energy needed to illuminate the desired target [101].
Biosensors, on the other hand, have a complicated design in terms of their integration of a biological material on top of an appropriate transduction element. However, the design of the biological material (e.g. antibody) itself is easier with the possibility of replacing it on the same transduction element to detect a different material composition.

6 CONCLUSION AND FUTURE WORK

The presented work focuses on a very important problem of hazard for people, due to the danger of terroristic activities as well as of breakdowns in the production of explosives or dan- gerous materials. It is evident that almost all presented tech- niques in this review have undergone significant changes to improve one or more aspects of their working practices in- cluding, for example: sensitivity, specificity, cost, and ease of use. Further advancement will be necessary to provide a sys- tem that incorporates all aspects of an ideal explosive identifi- cation technique due to the inherent issues that are associated with low volatility of explosive vapors, concealment, interfer- ences and the actual damage caused by a false response.
A system for the remote detection and localization of landmines will be designed in the near future. This system will be based on the use of wireless biosensors. Several (usual- ly a large number of) biosensors shall randomly be deployed in a field with suspected existence of landmines. The biosen- sors will be equipped with wireless functionality, including an antenna. The biosensors will form a sensor network, and using some routing algorithm, they will relay their location infor- mation to a Data Processing Unit (DPU), e.g. a computer, lo- cated outside the field, which will determine the location of each biosensor using localization algorithms. husThe sensors will later register and route their readings of the explosive particles concentrations to the DPU. After that, remote explo- sion for the materials will take place to remove the threat of mines without human interaction.

ACKNOWLEDGMENT

The authors gratefully acknowledge the support of the Leb- anse National Council for Scientific Research and the AUB Alumni Association Dubai & Northern Emirate.

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