Department of Physics Detection of Explosives: Dogs vs. CMOS Capacitive Sensors SEMINAR 1a 1st year, 2nd cycle Urška Tomšič Mentor: Prof. Dr. Igor Muševič March 2013 Abstract This paper presents some basic characteristics of explosive materials that are significant for detection, continuing with an olfactory mechanism of a dog’s nose, which is still the most efficient and reliable in explosives detection. Therefore, a vast variety of detection methods based on principles of a dog's sense of smell have been developed. The most applicable, sensitive and effective methods are briefly described in this paper, followed by more thorough presentation of a new explosive detection technique based on CMOS capacitive sensors, that was developed by a group of scientists from Jožef Stefan Institute in collaboration with Electrical engineering department and Department of Chemistry.
Transcript
Department of Physics
Detection of Explosives: Dogs vs. CMOS Capacitive Sensors
SEMINAR 1a 1st year, 2nd cycle
Urška Tomšič
Mentor: Prof. Dr. Igor Muševič
March 2013
Abstract This paper presents some basic characteristics of explosive materials that are significant for detection, continuing with an olfactory mechanism of a dog’s nose, which is still the most efficient and reliable in explosives detection. Therefore, a vast variety of detection methods based on principles of a dog's sense of smell have been developed. The most applicable, sensitive and effective methods are briefly described in this paper, followed by more thorough presentation of a new explosive detection technique based on CMOS capacitive sensors, that was developed by a group of scientists from Jožef Stefan Institute in collaboration with Electrical engineering department and Department of Chemistry.
2 EXPLOSIVE MATERIALS: High explosives.......................................................................................... 3
2.1 Vapor pressure of explosives ............................................................................................................. 4
2.2 Nitroaromatic compounds: TNT an DNT .......................................................................................... 5
3 DOGS AND EXPLOSIVES ..................................................................................................................... 6
3.1 Sense of smell .................................................................................................................................... 6
3.2 Training and detection of high explosives ......................................................................................... 8
4 DETECTION OF EXPLOSIVE VAPOR TRACES ................................................................................. 9
1 INTRODUCTION During the last few years, the issue of explosives detection has been an active area of research. Concern about travelers carrying hidden explosive material on commercial airlines has become more and more intense. At the present, approximately 10 million items of luggage are scanned annually in major Airports. The demand for effective, reliable and rapid methods for screening luggage is increasing as the threat of terrorism increases, and is now becoming an important aspect of counter-terrorism activities [1]. Chemical sensors for rapid detection of explosives also have important potential applications such as global de-mining project, remediation of explosives manufacturing sites, forensic and criminal investigations like post-blast residue determinations, and other military applications. There is a growing concern about health risks associated with the release of explosives into the environment from military sites and former ammunition plants. Therefore, current security and environmental concerns have forced the development of new and innovative sensors for explosive compounds. Detection of vapor traces of explosives in the atmosphere is a potentially powerful method to reveal the presence of explosive devices and land mines. The principle of this detection method is based on the fact, that any explosive device will emit rather small but detectable number of gas-phase molecules constituting the explosive, due to its finite vapor pressure. Traditional detection of narcotics and explosive vapors relies on the olfactory system of dogs highly trained for such a purpose, and it is still the most effective and efficient method of detecting explosives in current use. Sniffing dogs do not just react to a particular chemical smell, but to a combination of many smells that make up an explosive or narcotic. Their extraordinary ability to smell the explosive charge present in landmines inspired development of numerous detection techniques that are available today [2]. Most of the high sensitivity methods involve taking air samples from the suspected area, followed by analysis using one of the several different means. Their common limitations are rather large sizes and weights, high power consumption, unreliable detection with false alarms, insufficient sensitivity or chemical selectivity, hyper-
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sensitivity to mechanical influences and very high price. There is a need for miniature, portable, sensitive and chemically selective sensors, capable of detecting target molecules in real time. The advancement of MEMS technology has enabled the realization of miniaturized analytical instruments. Chemically surface modified MEMS sensors are currently the most promising and the most popular candidates for the ultrasensitive detection of low concentration of molecules in the atmosphere. An alternative to MEMS is to use the CMOS capacitive sensors with μm or sub-μm electrodes, which were developed recently for lab-on-chip and bio-sensing applications. 2 EXPLOSIVE MATERIALS: High explosives An explosive material is a reactive substance that contains a great amount of potential energy and can be initiated to undergo very rapid and self-propagating decomposition resulting in the formation of more stable material, great release of heat and light (large exothermic change) or the development of sudden pressure effect where great quantities of gases are released (a large positive entropy change). Thus, explosives are substances that contain a large amount of energy stored in chemical bonds. The energetic stability of the gaseous products and hence their generation comes from the formation of strongly bonded species like carbon monoxide, carbon dioxide, and (di)nitrogen, which contain strong double and triple bonds having bond strengths of nearly 1 MJ/mole. Consequently, most commercial explosives are organic compounds containing -𝑁𝑁𝑁𝑁2 , -𝑁𝑁𝑁𝑁𝑁𝑁2 and -𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁2 groups that, when detonated, release gases. Sensitivity of explosives refers to the ease with which an explosive can be ignited or detonated, i.e., the amount and intensity of shock, friction, or heat that is required [3]. Below are presented equation for the (i) explosive decomposition and (ii) the combustion in air of TNT explosive - 𝐶𝐶7𝑁𝑁5𝑁𝑁3𝑁𝑁6 (s):
In the combustion reaction, the oxidant comes from an external source (air). In the explosion, the oxidant is contained in the explosive material. Based on structure and performance explosives have been classified by explosive velocity, as low and high explosives, which are sub-divided by sensitivity, as primary and secondary explosives (Fig. 1). Low explosives (propellants, gunpowder, smokeless powder, pyrotechnics...) deflagrate or burn at relatively low rates cm/s, whereas high explosives detonate at velocities from 1 to 9 km/s. (Detonation is supersonic combustion powered by a self sustaining shockwave.) Primary explosives (lead azide, lead styphnate, etc.) are highly susceptible to initiation and are often referred as "initiating explosives" because they can be used to ignite secondary explosives.
Figure 1 – Basic classification of explosives. [17]
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Secondary explosives, which include nitroaromatics and nitramines are much more prevalent at military sites than primary explosives. They are often used as a main charge or bolstering explosives because they are formulated to detonate only under specific circumstances - until initiated by shock or heat. The energetic materials used by military as propellants and explosives are mostly organic compounds containing nitro (NO2) groups. The three major classes are nitroaromatics (e.g. TNT), nitramines (e.g. RDX) and nitrate esters (e.g. nitroglycerine, PETN). 2.1 Vapor pressure of explosives The saturated vapor pressure represents a maximum gas-phase pressure of explosive or maximum concentration of explosives that exists at equilibrium in the air above the surface of the explosive material at a given temperature. All solids and liquids emit a certain amount of vapor at all temperatures above absolute zero (−273 °𝐶𝐶), and at a given temperature the amount of vapor emitted is characteristic of the particular substance (Fig. 2). Explosives are substances with great molecular weight, often more than 150. For example at 25˚C, micrograms of DNT, nanograms of trinitrotoluene TNT and picograms of hexogene RDX are present in 1 gram of air (about 1𝑑𝑑𝑑𝑑3). For convenience, vapor pressures are often expressed as relative concentrations in saturated air. (Typical range of explosive vapor: 1 ppm/ppb/ppt/ppq - one part-per-million/billion/trillion/quadrillion or in relative concentration 10−6/10−9/10−12/10−15) Such concentrations are proportional to the true vapor pressure, and often provide a clearer picture of the amounts of vapor that are involved.
Relative values of the vapor pressures have important implications for the trace detection of explosives. The high-vapor-pressure explosives (DNT, NG) are relatively easy to detect from their vapor. The medium-vapor-pressure explosives (TNT) can also be detected from their vapor, but in many cases this will test the limits of sensitivity for gas-phase detection. The low-vapor-pressure explosives (RDX, PETN) produce such low amounts of vapor that detection from their vapor is possible in any but the most exceptional circumstances, so efforts to detect these compounds must focus on swipe collection of particulate material [5,6].
Figure 2 Left: Structural formula of TNT, RDX and PETN [3]. Right: The circles represent the position of different molecules in a 2D plot, where x axis represent molecular mass and y axis represent the number of molecules at vapor pressure of neat high explosives at 𝟐𝟐𝟐𝟐℃: NG (nitroglycerin), EGDN (ethylene glycol dinitrate) and DNT (2,4-dinitrotoluene), TNT (2,4,6-trinitrotoluene), RDX (cyclotrimethylenetrinitramine), PETN (pentaerythritol tetranitrate). For further references are on this diagram also presented: sensitivity, detection levels and comparison of various sensing systems [21].
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Considering Clausius–Clapeyron equation that can be derived from the Gibbs equations, the vapor pressure of a substance can be expressed as:
(1) 𝑃𝑃𝑉𝑉 = 𝑃𝑃0 𝑒𝑒𝑒𝑒𝑒𝑒−∆𝑁𝑁 𝑅𝑅𝑅𝑅⁄ ,
where 𝑃𝑃𝑉𝑉 is the vapor pressure, 𝑃𝑃0 is a constant that is determined empirically for each substance, ∆𝑁𝑁 is the change in enthalpy during the phase transition, 𝑅𝑅 is the gas constant and 𝑅𝑅 is the temperature of a substance. From equation (1) can be gleaned that 𝑃𝑃𝑉𝑉 will increase exponentially with increasing temperature. For example, for solid TNT near room temperature, the vapor pressure approximately doubles with every increase of 5K [5,6]. Sensing of volatile components depends heavily on ambient temperature and humidity. Explosives have a tendency to adsorb strongly onto surfaces, such as wood, plastics, paper, and soil. Thus, moisture content plays a key role. Water competes with explosive molecules for binding sites on soil particles, causing more explosive to be released into the vapor space as soil moisture content rises. In case of landmine detection both dogs and chemical vapor sensors are more efficient at detection after rain [7]. Also, packaging plays an extremely important role in concealing explosives. Effective vapor pressure of explosive devices can be reduced by a factor of 1000 by sealing in plastics or by other coating surrounding. Vapor concentrations of volatile explosives near a bomb may be 2– 6 orders of magnitude less than their equilibrium vapor pressures due to their enclosure in a casing, adsorption to soil particles, and because mixtures of explosive materials have lower vapor pressures than their pure compounds [8]. Commonly used explosive devices include dynamite, landmines and plastic explosives. Dynamites usually contain EGDN and/or NG as an explosive ingredient. Plastic explosives, such as C-4, contain RDX and/or PETN as the explosive ingredient. In case of landmines the main component is usually TNT. Mixtures of high explosives are commonly used, and TNT - an inexpensive compound, is a component found in fifteen explosive compositions. 2.2 Nitroaromatic compounds: TNT an DNT Because of their widespread use and volatility, nitroaromatics comprise an important general class of explosives compounds for detection. One of the most commonly used high explosives in the last 100 years is 2,4,6-trinitrotoluene (TNT), which not only poses a security threat, but is also of great environmental concern because of soil and water contamination. For example, when a mine is buried in the soil, it will almost always gradually release explosives or chemical derivatives to the surrounding soil through either leakage from cracks and seams or vapor transport through the mine casing. An important characteristic of nitroaromatic compounds is their ability to rapidly penetrate the skin. TNT can readily enter groundwater supplies and has been classified as toxic as it presents harmful effects to all life forms [9]. It causes liver damage, aplastic anemia and toxic hepatitis that can lead to death. Detection of TNT is high priority in many fields and so the analyte of focus in most explosive detection research is TNT, or consequently DNT. 2,4-dinitrotoluene (DNT) is present in all military-grade TNT as a decomposition compound and ubiquitous major impurity, resulting from the manufacturing process. Effective detection of TNT can be performed by measurement of not only TNT itself, but also DNT, which has a higher vapor pressure and environmental stability. TNT has a very short half-life in soil (about a day at 22°C) because it is easily biodegraded and has very low vapor pressure; but DNT is much less easily biodegraded and has a higher vapor pressure. Though present is such small quantities, the significantly higher vapor pressure of DNT often makes it the target molecule for detection [10].
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3 DOGS AND EXPLOSIVES Dogs continue to be the gold standard against which other explosives detection methods are judged. Trained canines provide a reliable and time-proven method for detecting concealed explosives. There is in principle no explosive compound that dogs cannot be trained to sniff out. 3.1 Sense of smell The canine olfactory mechanism is an extremely sensitive chemical sensor. Elaborate experiments have been conducted [11] that place the dog's olfactory sense to be from hundred thousand to one million times more sensitive than a human's. A dog interprets the world predominantly by smell, whereas a human interprets it by sight. As the human brain is dominated by a large visual cortex, the dog brain is dominated by an olfactory cortex. While a dog's brain is only one-tenth the size of a human brain, the part that controls smell is, proportionally speaking, approximately 40 times larger than in humans. For one thing, dogs possess anywhere from 125 million up to 300 million olfactory receptors in their noses, compared to about 5 million in humans. These receptors occur in special sniffing cells deep in a dog's snout and are what allows a dog to "out-smell" humans [12]. Dog’s olfactory system includes soft tissue, bones, nerves, and parts of the brain. Nose consists of a pair of nostrils for inhaling air and odors. The soft tissue and bony structures make up the cavities, into which odor particles flow. These cavities contain a rich supply of olfactory receptor cells (specialized proteins), that extend throughout the entire layer of specialized olfactory epithelium on the upper part of nasal cavity (Fig. 3).
When dogs inhale and the air flow enters the nose, it splits into two different flow paths, one for olfaction and one for respiration (Fig. 4a). A fold of tissue just inside its nostril helps to separate these two different functions. When a dog breathes normally, only about 12% of the inspired air detours into a recessed area in the back of the nose that is dedicated to olfaction, while the rest of the incoming air sweeps past that nook and disappears down through the pharynx to the lungs. The other function, sniffing, is a big part of smelling and it is accomplished through a series of rapid, short inhalations and exhalations, which are use to maximize detection of odors. When a dog takes a deep sniff, the shape of the nostril openings change. Structure just inside the nostrils called the alar fold opens, allowing air to flow through the upper area into the olfactory epithelium, where most of the olfactory cells are located. A bony pocket traps odor molecules that are unrecognizable in a single sniff and permits them to accumulate and interact with olfactory receptors. The smell is retained in the maze of scent receptors and is not expelled. Because when a dog exhales, the alar fold closes off the upper part and pushes air down and out backwards
Figure 3 – Anatomy of a dog’s nose [13].
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through the slits on the side of the nose. The manner in which the exhaled air swirls out helps usher new odors into the dog's nose (Fig. 5). The odorant-laden air within nasal cavity passes through a labyrinth of exquisite complexity - nature’s solution for packing a large surface area in a small volume, crucial for delivering odors to millions of olfactory receptors (Fig. 4b). The air flow over the mucous-coated receptors is remarkably smooth due to sniffing, which presents consistent signal to the receptors [14]. Odor molecules in the olfactory epithelium of the nasal cavity are dissolved in the mucous layer that covers scent receptors and diffuse to the cilia of receptor neurons. When scent molecules hit these cilia, the cell is stimulated, sending electrical signals along tiny nerves, which form olfactory nerves, to the highly developed olfactory bulb and then to the appropriate parts of the cortex that interpret them as smells (Fig. 3). The biochemistry of smell is just beginning to be understood and remains under study. Some of the more important factors appear to be the overall size and shape of the molecule. Additionally the stereo chemistry of the molecule and certain chemical (polarity and the nature of functional groups) and physical (solubility and volatility) properties must be important. Receptors bond to one or several molecules depending on their shape. Many will respond to the same odorant. Complex scents are made up of many odorant molecules. These different molecules cause a different chemical deposition patterns to occur, which is unique to the total odor. [12,14,15].
The wetness of a dog’s nose is essential for determining the direction of the air current containing the smell. This is why dogs typically tilt their snouts upward when trying to detect a smell. If dogs have a dry nose they may lick it to aid them in scent. Dog’s nostrils are highly mobile and dogs can wiggle them independently. When sniffing, each nostril pulls in a separate odor sample. This, along with the fact that the so-called aerodynamic reach of each nostril is smaller than the distance between them (Fig. 5), helps dogs to determine which nostril is pulling in the scent and consequently allowing them to know what direction a smell is coming from - which direction to go when tracking [16]. Dogs can smell hundreds of different scents at a time and this helps them pick out one particular smell.
Figure 4 – a) When a dog breathes in, the air separates into distinct paths, one (red) flowing into the olfactory area and the other (blue) passing through the pharynx (black) to the lungs. b) - In the rear of a dog’s nose lies the olfactory
region (green), with its scroll-like tissues bristling with smell receptors. Respiratory regions appear in pink. [14]
Figure 5 - When a dog breathes in (upper left), it can tell which nostril an odor arrived in because of each nostril's “aerodynamic reach” is small (blue). When a dog breathes out (upper left), the expired air blows out the side slits in such a way as to augment the sampling of new odors.
The two pictures below show real dog’s nose in two different positions. [14]
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There is a great initiative to build a mechanical nose that closely simulates the entire canine olfactory system. Scientists are trying to reverse-engineer the canine nose in part to aid in the design of artificial "noses" that can sniff out odors as well as dogs can. Compared to current technology-based “sniffer” systems, dogs have the advantages of superior mobility and the ability to rapidly follow a scent directly to its source. Because of these advantages, canines are an excellent choice for explosives detection applications that involve a significant search component. 3.2 Training and detection of high explosives
Canines have been used for landmine detection for decades.(Fig. 6, 7) They are an excellent choice for explosive detections that include the search of vehicles, warehouses, luggage and cargo, aircraft, buildings, parking lots, property perimeters, etc. We can also train them for jobs such as tracking and rescue. Dogs are proven to work really well in many scenarios and under many environmental conditions, and are capable of detecting explosive vapors at concentrations
far lower than those measurable by the best chemical sensors. The lower limit at which they can detect explosives is uncertain and it varies with the breed of a dog. Dogs are specifically and rigorously trained to detect the chemical signature of explosives. Like other methods that rely on vapor detection. These dogs consider their job a game. By offering dogs a reward of play, favorite toy or food each time they successfully sniff the target scent, they can be trained to signal when they smell explosive. They alert their handlers to the presence of these things by pawing, barking, or in the case of something dangerous, sitting quietly. In the mine detection context, dogs walk ahead of their handlers, noses to the ground, and sit at the first scent of a mine. A manual deminer then follows and investigates the area with a probe (Fig. 7). Dogs used in the military are typically trained to detect nine different explosives. Depending on training and experience of the dog, skill of the handler, and a number of environmental factors, accuracy rates may range from 60% to 95%. [11, 17] Canine detection, however, suffers from drawbacks such as the high cost associated with expensive training and maintaining, their narrow attention span and inability to work round the clock, behavioral and mood variations. There is also a need for regular retraining. Dogs require proper care and are easily fatigued. When a dog is overheated and actively panting, its sense of smell is reduced by as much as 40 percent as it uses the air to cool itself rather than for smelling [11, 17].
Figure 7 – Example of a land mine search. Pictures present acts of detecting landmines with help of a dog,
investigation with a probe and a careful removal of a found landmine. [17]
Figure 6 – Different types of mines. [17]
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4 DETECTION OF EXPLOSIVE VAPOR TRACES Extensive efforts have been devoted to the development of innovative and effective sensors. In gas molecule detection systems, certain gas trace components can easily go undetected. This is due to ultralow yet dangerous concentrations combined with limitations of the detection methods. Ideally, vapor phase detection methodologies for explosive compounds should, (a) have minimal space and power requirements, making them adaptable to portable devices; (b) be extremely sensitive for the compound being detected; (c) be selective for the compound of interest, minimizing the likelihood of false alarms; and (d) be able to perform real-time high-throughput analysis. Analytical procedures in use today for the trace detection of explosives typically involve the collection of vapor samples and their analysis by using sensitive methods. Some such methods include ion mobility spectrometry, mass spectrometry, surface acoustic wave detectors, nuclear quadrupole resonance, chemiluminescence, electron capture detection, surface enhanced Raman spectroscopy and other electronic noses and biosensors [4,18]. Most known are roughly described below. And then, more attention is paid to the latest and more suitable innovation based on MEMS technology.
• ION MOBILITY SPECTROMETER (IMS) Ion mobility spectrometry has become the most commonly used technology for the detection of trace levels of nitro-organic explosives in airports. The principle of operation of an IMS is shown in Fig.8.
The spectrometer consists of two main sections: the ionization region and the drift region. In a typical IMS, ambient air is drawn into an inlet port at the rate of a few hundred cm3/min. The air first enters the ionization region, where electrons interact with the incoming molecules to form positive or negative ions. In the case of explosives, it is negative ions that are formed. The source of the ionizing electrons is a small, sealed piece of metal that has been coated with a radioactive material, usually nickel-63. Once ions are formed, they are periodically admitted into the drift region through an electronically shuttered gate. The ions are drawn through the gate by a static electric field, which pulls them towards a metal collection plate at the far end of the drift region. This “drift” of the ions from one end of the drift region to the other occurs at atmospheric pressure, with many collisions between the ions and the various molecules present. The time that it takes the ions to travel the length of the drift region is called the drift time, and is a complex function of the charge, mass, and size of the ion. By determining the mass/charge ratio, it is possible to identify components within the sample through comparison with known standards. Typical drift times are on the order of a few milliseconds. The current collected at the metal plate is measured as a function of time, and an IMS spectrum is a plot of ion current versus time, with different peaks representing different specific ions. IMS has sensitivity in the picogram to
Figure 8 - Schematic presentation of ion mobility spectrometer (IMS) operation
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nanogram range, but it is also expensive, operator dependent, prone to false positives, and spectrometers must be frequently calibrated.
• MASS SPECTROMETER (MS) Mass spectrometry has long been one of the most powerful techniques available for laboratory chemical analysis, although it is rarely used in routine field applications. Samples are drawn from the air into a mass spectrometer where molecules are first ionized and then passed through a magnetic filter, which allows ions to be identified based on their charge-to-mass (m/e) ratio. In some systems, the MS is connected to a front-end gas chromatograph. Mass spectrometers have excellent specificity for identifying different ions, and some systems have sub-picogram sensitivity, but mass spectrometers tend to be expensive.
• SURFACE ACOUSTIC WAVE SENSORS (SAW) Surface acoustic wave sensors detect a chemical by measuring the disturbance it causes in sound waves across a tiny quartz crystal. Chemical vapor sensors use the application of a thin film polymer which selectively absorbs the gas or gases of interest. An acoustic wave confined to the surface of a piezoelectric substrate material is generated and allowed to propagate. If a vapor is present on the same surface, then the wave and substances in the vapor will interact to alter the properties of the wave (e.g. amplitude, phase, harmonic content, etc.). The measurement of changes in the surface wave characteristics is a sensitive indicator of the properties of the vapor. The most sensitive of saw sensors exhibits detection limit for DNT in low ppt ranges. Though highly sensitive, these detection systems have common limitations such as rather large sizes and weights, high power consumption, unreliable detection with false alarms, insufficient sensitivity or chemical selectivity, hyper-sensitivity to mechanical influences and very high price. Some methods require sophisticated instrumentation that is not easily applied to on-site field testing. Furthermore, since the concentrations of target compounds are often in the lower sub-ppt range, which is below the detection limits of most instruments, a pre-concentration step may be required. 4.1 MEMS microcantilever sensors
MEMS technology has made it possible to miniaturize detecting devices. Micro-electro-mechanical systems (MEMS) have been emerging as platform for the development of sensors for optical or electrical detection with extreme high sensitivity and are currently the most promising and popular candidates for the ultrasensitive detection of low concentration of molecules in the atmosphere. The detection method is based on monitoring the changes in surface stress, bulk stress and mass of micro-cantilever (MC), which are caused by the adsorption of target molecules on the chemically modified surface of the cantilever [19].
a) Surface stress (Fig. 9a) I case of surface stress, molecules adsorbed on one side of the cantilever cause the cantilever to deflect. With modification of the relationships originally derived by Stoney’s equation the adsorbate-induced deformations of thin plates can be accurately predicted (Fig. 9d):
(2)
∆𝑧𝑧 =12𝑙𝑙2
𝑅𝑅 =3𝑙𝑙2(1− 𝜈𝜈)
𝐸𝐸𝑡𝑡2 ∆𝜎𝜎 , where 𝑅𝑅 is the radius of MC curvature, ∆𝑧𝑧 is the tip displacement of MC with length 𝑙𝑙 and thickness 𝑡𝑡, 𝐸𝐸 Young’s module for the substrate, 𝜈𝜈 is Poisson ratio and ∆𝜎𝜎 is the differential surface stress on the MC [20,21].
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b) Mass changes (Fig. 9b)
Here is used the fact that the resonant frequency of the cantilever depends on the mass of the cantilever and the resonant frequency drops as the mass increases. It is possible to make indirect mass change estimations in the atto- to zepto-gram range by following the resonant frequency change of the cantilever.
c) Bulk stress (Fig. 9c) The cantilever can be used as a tool to evaluate stress changes in the bulk of the cantilever material, for example, caused by humidity or temperature changes resulting from a chemical reaction. Cantilevers consisting of two materials with different thermal expansion coefficients will bend as a function of temperature due to the bimorph effect. Stress changes are explored in the realization of extremely sensitive thermometers. This effect can be used to measure temperature changes down to 10−5K.
The change in cantilever deflection or change in vibrational amplitude is most commonly measured by so-called optical leverage technique which is well known from AFM, where basically a laser is reflected off the backside of the cantilever and onto a position sensitive quadrant photodiode. Deflection can also be measured by the change of capacitance or resistance of the cantilever. The detection sensitivity of chemically functionalized MEMS is in the range 1: 109 and more, which makes MEMS devices extremely appealing for the realization of the electronic nose. But, optical cantilever position measurement is difficult, and the apparatus is bulky when precision optics with long optical path has to be used for the precision required. Furthermore, the measurements of cantilever bending are very sensitive to environment influences like temperature, vibrations, mechanical shock and acceleration. Therefore, such measurements are considered non-practical [19,20,21]. 4.2 MEMS COMB CAPACITIVE SENSORS In this section is presented a miniature detection system that is based on surface-functionalized array of COMB capacitive sensors using MEMS. The instrument is sensitive and selective, consumes a minimum amount of energy, is very small and cheap to produce in large quantities, and is insensitive to mechanical influences. It is possible to detect less than 3.5ppt of TNT, and ten times better for RDX, in the atmosphere at 25℃ in one second using very small volume (few mm3) and only approx. 20mA current from a 5V supply voltage [22].
Figure 9 - Schematic representation of various modes of operation of MCs: a) Surface stress due to absorption of molecules causing static deflection; b) Dynamic resonance frequency shift mode due to change in effective mass; c) Heat sensing mode due to differential thermal expansion. Picture d) is a Schematic representation of MC deflecting under surface stress, and e) presents the ‘‘optical lever’’ readout commonly used to measure deflections of microfabricated cantilevers. [20,21]
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4.2.1 Principles of operation The central part of the detection measurement system is a surface-modified, differential capacitive sensor with electrical equivalent circuit (Fig. 10). It is composed of two COMB capacitors, each covered with a thin layer of silicon dioxide: 𝐶𝐶𝑒𝑒 is chemically modified in order to enhance the surface adsorption of target molecules, while 𝐶𝐶𝑒𝑒 is not. They are very close to each other so on average, equal numbers of target molecules are present in the space between the plates of each capacitor. There are also other molecules in the air around the sensors, but only target molecules adsorb preferentially for a short time to the surface of the modified capacitor Cp. Because of adsorption, the relative dielectric constant of the left capacitor changes, resulting in a slight change in the capacitance, until the surface-adsorbed molecules desorbs. Electronic circuit converts the change in capacitance to a change in voltage, which is then appropriately amplified, converted into a digital signal and limited to the bandwidth of 1Hz. The response is in principle proportional to the capacitance change due to adsorption and amplification factor (gain). Using appropriate sensing signals and extremely low noise electronics, one can measure the difference of the two capacitors, even in case of small number of adsorbed target molecules [22].
• The Langmuir Adsorption model:
Whenever a gas is in contact with a solid there will be an equilibrium established between the molecules in the gasphase and the corresponding adsorbed molecules which are bound to the surface of the solid. The Langmuir adsorption model is the most common model used due to its simplicity and its ability to fit a variety of adsorption data. The Langmuir equation (also known as the Langmuir adsorption isotherm) describes the dependence of the surface coverage or adsorption of molecules on a solid surface to gas pressure or concentration of a medium above the solid surface at a given temperature. It is a semi-empirical isotherm with a kinetic basis and was derived based on statistical thermodynamics. Langmuir’s isotherm describing the adsorption of adsorbate A(g) onto the surface of the adsorbant B(s) requires the following assumptions: (i) All of the adsorption sites are equivalent and each site can only accommodate one molecule, (ii) the surface is energetically homogeneous and adsorbed molecules do not interact, (iii) there are no phase transitions and (iv) at the maximum adsorption only a monolayer is formed. The chemical reaction for monolayer adsorption can be represented as follows:
(3) 𝐴𝐴 + 𝐵𝐵 ⇌ 𝐴𝐴𝐵𝐵 ,
Figure 10 - A pair of COMB capacitors, representing the difference in the capacitance due to preferential adsorption of target molecules on functionalized electrodes of left capacitor. [22]
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where AB is adsorbed gas molecule. The direct and inverse rate constants are 𝑘𝑘 and 𝑘𝑘−1. If we define surface coverage 𝜃𝜃 as the fraction of the adsorption sites occupied and considering that the number of filled surface sites (AB) is proportional to 𝜃𝜃, the number of unfilled sites (A) is proportional to 1 − 𝜃𝜃 and the number of particles is proportional to the gas pressure or concentration (p), the equilibrium constant 𝐾𝐾𝑎𝑎𝑑𝑑𝑠𝑠 for reaction (3) can be rewritten as:
(4)
𝐾𝐾𝑎𝑎𝑑𝑑𝑠𝑠 =𝑘𝑘𝑘𝑘−1
=[𝐴𝐴𝐵𝐵]
[𝐴𝐴][𝐵𝐵] =𝜃𝜃
(1 − 𝜃𝜃) 𝑒𝑒
Rearranging, we obtain the final form of the Langmuir adsorption isotherm:
(5)
𝜃𝜃 =𝑒𝑒 𝐾𝐾𝑎𝑎𝑑𝑑𝑠𝑠
1 + 𝑒𝑒 𝐾𝐾𝑎𝑎𝑑𝑑𝑠𝑠
The constant 𝐾𝐾𝑎𝑎𝑑𝑑𝑠𝑠 is the Langmuir adsorption constant and increases with an increase in the binding energy of adsorption and with a decrease in temperature. [23] 4.2.2 Sensor design and fabrication One capacitor consists of 51 poly silicon COMB fingers with a length of approximately 350μm. Fingers are 1μm apart and 25μm high; on the bottom, they are attached to the thick layer of silicon dioxide and covered with approximately 10nm of silicon dioxide (Fig. 11). Fingers are interconnected using Al metal lines forming the capacitor with approximately 0.5pF capacitance. In the process following MEMS fabrication, both capacitors are first chemically modified with TNT receptor molecules. The modification layer on one capacitor is later removed using selective laser erosion process, to obtain two chemically different capacitors. After processing, the capacitors do not have exactly the same capacitance. A 3𝜎𝜎 statistical variation of matching accuracy can reach up to 5%; therefore, the initial difference might be as big as 25fF. This difference is reduced during the automatic calibration procedure at the beginning of the measurement cycle. (Option to calibrate the capacitors is built into the ASIC). It is possible to reduce the initial difference with the accuracy better than 0.5 fF. The surface used for molecular sensing is a SiO2 surface covered with a monolayer of trialcoxyalkylamino and trialcoxyarylamino silanes. The modification of the surface was carried out by dipping the sensors into a diluted solution of the corresponding silane in organic solvent for an appropriate period of time (approx. 5 hrs.) at room temperature. Depending on the silane, solvents with different polarity were used to assure its solubility [24]. Upon completion, the modified MEMS sensors were removed from the solution and rinsed several times with dimethylformamide and methanol to remove any organic residue. Finally, the sensors were thoroughly dried under the argon stream. Finally, modified surfaces were investigated by XPS-spectroscopy, where measurements confirmed the presence of an amino group on the SiO2 sensor surface, thus, conforming successful modification with organosilanes [22].
Figure 11 - SEM micrograph of a COMB MEMS sensor. [22]
14
4.2.3 Measurements and results The development and testing of detectors requires a reliable gas generator capable of producing known, controllable and very low concentrations of explosive vapors. For laboratory measurements a TNT vapor generator was built. The mixture of N2 gas with vapor pressure TNT molecules is delivered via Teflon tube and is pumped to the sensor using miniature piezo-electric pumps. For laboratory experiments, the gas input is switched between dry N2 gas and N2 contaminated with target molecules in equal proportions. At room temperature, the vapor pressure of TNT is estimated to 6 ∙ 10−4 Pa, which means that the density of target molecules relative to the N2 molecules is in a range of 𝑋𝑋targ = 10−9. In Fig. 12 in shown measured response of a COMB capacitive sensor to the gas switched between pure 𝑁𝑁2 and 𝑁𝑁2 contaminated with TNT at vapor pressure in equal proportions. Estimated number of TNT molecules in the mixture was 0.5 ppb. From the difference between two readings ∆𝑁𝑁, standard deviation of the measurements 𝜎𝜎 (the noise of electronics is presented as the standard deviation of consecutive readings of one or other measurements, 𝑁𝑁2 or 𝑁𝑁2+TNT) and bandwidth 𝐵𝐵𝐵𝐵 one can estimate the normalized sensitivity of the detector in 1Hz and thus the detection level using equation: (6)
𝑆𝑆TNT ≅ �0.5 𝑋𝑋targ 𝜎𝜎∆𝑁𝑁TNT√𝐵𝐵𝐵𝐵
� = 3.5 ∙ 10−12/√𝑁𝑁𝑧𝑧
The lower level of the response on the 𝑦𝑦 axis of Fig. 12 is proportional to the intrinsic difference between capacitors 𝐶𝐶𝑒𝑒 and 𝐶𝐶𝑛𝑛 , while the change due to adsorbed molecules is proportional to ∆𝑁𝑁. Long measurement times are a consequence of slow gas flow through piezo-electric pumps and small diameter of the tubes used. Good results open up many exciting application possibilities for the detection of vapor traces of different molecules in the air. Such instrument could be massively deployed for detection of hazardous molecules in the air. The sensitivity of this sensor is still at least 3 orders of magnitude worse than the sensitivity of a dog's nose (Fig. 2), but there is predicted future work directed towards improving the sensitivity and the selectivity by changing the sensor geometry and size and by reducing further the analog channel noise.
Sensor sensibility is influenced by the following parameters:
• Sensor: The geometry of the sensor, excitation voltage, size of the molecules which are adsorbed, the size of the surface modified coverage, relative change in dielectric constant caused by adsorbed molecules and the ratio of useful capacitance to parasitic capacitance.
• Electronics: the signal-to-noise ratio, the size of excitation signal on capacitive sensor, the gain of entire measuring path and the ratio of useful capacitance to parasitic capacitance of electronics.
Figure 12 – Measured response of a COMB capacitive sensor to gas switched between pure N2 and N2 contaminated with TNT concentration of 0.5 ppb (left); The sensor-measurement system implementation on a
printed circuit board (right). [22]
15
5 CONCLUSION There has been a great increase in the development of trace and ultra-trace explosive detection in the last decade, mainly because of the globalization of terrorist acts, and the reclamation of contaminated land previously used for military purposes. In search of a better artificial nose, scientists are going back to the dogs. They are still far better vapor detectors than any currently available technology. Their olfactory mechanism is distinguished by exceptional level of sensitivity and selectivity and can perceive many different scents concurrently, a property that has proven difficult to replicate artificially. It is high time to make progress towards the development of portable, automated, low cost sensors which can mimic the olfaction of the dogs without having their drawbacks and will overcome the limitations of various other methods of explosive detection. A step forward could definitely be CMOS capacitive sensor described above. Detection methods for traces of explosives continue to be hampered by the low volatility of the analytes, and thus the analytical problem remains challenging.
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