Multi-colorimetric sensor array for detection of explosives in gas and
liquid phase
N. Kosteshaa*, T.S. Alstrømb, C. Johnsenc, K.A. Nielsenc, J.O. Jeppesenc, J. Larsenb, A. Boisena and
M. H. Jakobsena
a
Department of Micro and Nanotechnology, Technical University of Denmark, Ørsteds Plads 345
East, DK-2800, Kgs. Lyngby, Denmark
b
Department of Informatics and Mathematical Modelling, Technical University of Denmark,
Richard Petersens Plads 321, 2800 Kgs. Lyngby, Denmark
c
Department of Physics and Chemistry, University of Southern Denmark, Campusvej 55, DK-5230,
Odense M, Denmark
ABSTRACT
In the framework of the research project “Xsense” at the Technical University of Denmark (DTU) we are developing a
simple colorimetric sensor array which can be useful in detection of explosives like DNT, TATP, HMX, RDX and
identification of reagents needed for making homemade explosives. The technology is based on an array of chemoselective compounds immobilized on a solid support. Upon exposure to the analyte in suspicion the colorimetric array
changes color. Each chosen compound reacts chemo-selectively with analytes of interest. A change in a color signature
indicates the presence of unknown explosives and volatile organic compounds (VOCs).
We are working towards the selection of compounds that undergo color changes in the presence of explosives and
VOCs, as well as the development of an immobilization method for the molecules. Digital imaging of the colorimetric
array before and after exposure to the analytes creates a color difference map which gives a unique fingerprint for each
explosive and VOCs. Such sensing technology can be used for screening relevant explosives in a complex background as
well as to distinguish mixtures of volatile organic compounds distributed in gas and liquid phases. This sensor array is
inexpensive, and can potentially be produced as single use disposable.
Keywords: colorimetric sensor array, DNT, RDX, HMX, TATP, chemo-selective compounds, principal component
analysis.
* Corresponding Author: Natalie Kostesha, [email protected]; Tel: + 45 45 25 81 40;
Fax: + 45 45 88 77 62; www.dtu.dk; http://www.xsense.dk/
1. INTRODUCTION
From the beginning of the 20th century to current day explosives have been the number-one tool for terrorists. To easily
detect a variety of military and industrial explosives, improvised explosives, energetic materials and materials needed to
make explosives new technologies must be developed. Major areas of applications for explosives sensors include: antiterrorism (screening luggage, mail packages, checking suspects and mass transit systems), demining and in the field of
Chemical, Biological, Radiological, Nuclear, and Explosives (CBRNE) Sensing XII,
Edited by Augustus W. Fountain III, Patrick J. Gardner, Proc. of SPIE Vol. 8018, 80181H
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environmental monitoring of hazardous compounds (TNT, DNT can easily enter the groundwater and is classified as
toxic to all life forms in concentrations above 2 ng/L). The sensing device should be portable, rapid, highly sensitive,
specific (minimize false positives) and have a low cost.
The tendency in efficient detection of explosives relies on the development of analytical methods which will be capable
to sense explosive molecules at low concentrations in gas or vapor phase. All compounds in both solid and liquid forms
give off some vapor denoted as a vapor pressure, which depends exponentially on the temperature. The detection limit
for sensors usually presents in terms of parts per billion (ppb) or parts per trillion (ppt) [1]. The respective vapor
pressure for most commonly used explosives is presented in [2, 3]. However, terrorists and distributors of explosives
always work towards the development of new composites and substances with low vapor pressures which make them
harder to detect. Well known, explosives in the liquid form have rather high vapor concentrations (ppm or above) and
will therefore have relatively high amounts of trace vapor for tracking. Some of the solid explosives require fairly high
sensitivities (ppb), especially when expecting to detect them from distances of several meters, where the vapor
concentration will drop several orders of magnitude, thus ppt sensitivity is needed.
Nowadays, vapors emanating by explosives are mainly detected by canines, electronic nose and sniffing probes [4-7].
Sniffing dogs are trained to detect TNT, NG-based dynamites and plastic explosives all in concentrations as low as parts
per trillion (ppt), and even today this is the most cost effective applied detection technique. However, the sniffing dog
has some limitations; dogs are influenced negatively by environmental distractions – and further olfactory, audio or
visual disturbances can disrupt the work and provoke unpredicted behavior. Dogs are expensive to train and maintain,
and can only work for few hours. Dogs require a skilled handler whose training is also expensive and time consuming.
Finally, a dog’s response is not selective; animals can only indicate the presence or absence of explosives. General
advantages and disadvantages between instrumental explosive detection devices and trained detector dogs is presented
by Furton and Myers [5].
A number of detecting methods have been developed over the years, i.e amplifying fluorescent polymers, gas
chromatography, Raman spectrometry, mass spectrometry and ion mobility spectrometry. Trace detectors now exist that
can detect a wide range of chemical substances, however, these methods does not meet all the demands listed earlier.
Research groups are pursuing advanced solutions in detection of explosives. In the field of chemical sensors one of the
most promising technologies for explosive detection is based on use of amplifying fluorescent polymers (AFP) [8, 9].
Swager et al. has developed AFPs for detecting of a number of analytes (TNT, DNT, 2,3-dimethyl-2,3-dinitrobutane,
RDX, PETN) in solution and gas phase [1, 10]. The detecting principle is based on the non-bonding electrostatic
interactions between the electron-rich fluorescent polymer and electron-deficient TNT and DNT molecules. The
transduction mechanism is photo-induced charge transfer from the polymer donor to nitro-aromatic that binds via tight πcomplex to the conjugated polymer. AFP has shown the ability to detect low, in femtograms, of TNT in the gas phase
that corresponds to the ppq level (1×10-15 g/L) [11].
Ion mobility spectroscopy (IMS) has established itself as a world-wide, daily used technique in areas of chemistry,
medicine, manufacturing and detection of illicit compounds, like explosives and drugs. IMS for explosive monitoring
consists upon the favorable gas phase ionization chemistry for explosives at ambient pressure. Ions of an analyte
generated in the reactor are injected an electric field and undergoes separation according to their mobility through
traveling via a drift gas. There are positive and negative-ion analyses in IMS since the nature of analytes is different.
The negative-ion mobility spectra were obtained for NG, DNT, TNT, PETN in ambient air [12]. The minimum detection
limit was obtained by using IMS in the range of 30 ppb of DNT, 30-80 ppb of RDX, 10 ppb of TNT, 124 ppb of Tetryl,
180 ppb of PETN [13, 14]. In recent years IMS development has been mainly focused on the device miniaturization and
amplification of the method sensitivity.
Immunochemical approaches for detecting traces of explosives will probably find wide applications in the future.
Nowadays, identification systems are basically focused on the application of monoclonal or recombinant antibodies that
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are used for screening TNT or DNT molecules [15-17]. A TNT ELISA method has been reported as a highly sensitive,
selective and reproducible method for detecting of TNT in the range of 1 ppb. The cross-reactive reaction was not
observed in the presence of another explosives such as dinitro-2,4-dinitrophenol, 2,4-dinitrophenylhydrazine, mdinitrobenzene, 2,6-dinitrotoluene, 2,4-dinitroaniline, 2,6-dinitroaniline, and 2,4-dinitrotoluene or trinitro-2,4,6trinitroaniline, 2,4,6-trinitrophenol, and 1,3,5-trinitrobenzene explosives [15]. The immunochemical method is a very
applicable method due to high sensitivity and selectivity. This technique is widely used in the sea coast border controls
for detecting traces in soil and ground water, as well as for identification of illegal drugs.
One of the most traditional and common technologies that is involved in the detection and classification of volatile
organic compounds is an electronic nose [4, 18]. The electronic nose is a device that combines chemical-sensing and
pattern-recognition systems; biologically it could be the sensing organ of an animal like the nose of a bomb-sniffing dog.
The sensor can recognize specific molecules and is applicable in many areas such as food quality analysis, medical
diagnostics, explosives detection, toxins detection, and environmental monitoring. Further, electronic noses has shown
high capability for detecting explosives, such as ammonium nitrate, mineral explosives, and DNT in low concentrations
[19]. Nevertheless, the traditional electronic nose technique has limitations due to detection problems at low analyte
concentrations. Electronic noses has also proven to be faulty if the temperature is too high or too low, or if the humidity
is too high.
In this paper we present a simple sensor [20], like an artificial nose, based on a colorimetric sensor array for detecting of
explosives, illicit drugs and volatile organic compounds. The multi-colorimetric sensor is able to detect the presence of
vapor emitting by a target and identify a class of specific analytes. Technique is based upon selective chemical binding
between a chemo-selective dye and an analyte resulting in change of color, changes in intensity and color or both.
We are working towards the development of a simple colorimetric sensor array which can be useful in detection and
identification of explosives like DNT, TNT, RDX, HMX and, TATP as well as VOCs in gas and liquid phases. This
technology relies on an array of a new class of chemo-selective compounds (CSCs) immobilized on a solid support. The
colorimetric sensor array contains 15 CSCs of 81 possible [21]. The main task of our work is selection of molecules that
undergo color changes in the presence of explosives, as well as the development of an immobilization method applicable
for such molecules. Using chemo-selective compounds we were able to apply the colorimetric sensor array for probing
explosives in the gas phase. Color change patterns manifest the presence of specific or given targets, in other words,
observed color change patterns indicate a particular vapor of the individual analyte or mixture of analytes.
2. MATERIALS AND METHODS
2.1 Reagents:
DNT was purchased from Sigma (St. Louise, MO, USA). RDX, HMX and TATP samples, a gift from Dr. Heiselberg
(Applied Research Branch Danish Defense Acquisition and Logistics Organization, Denmark), were used without further
purification. Explosives were tested separately using a working amount of 10 mg. For the colorimetric sensor array, 16
different compounds were selected from the new class of chemicals, and used as chemo-selective dyes. Stock solutions
of compounds were freshly prepared in DMSO (Sigma, St. Louis, USA) to obtain a final concentration of 1% (w/v) and
stored in lightproof flasks at room temperature.
2.2 Colorimetric sensor array:
A colorimetric sensor array was designed using chosen CSCs. CSCs were immobilized onto silica gel Kieselgel 60F254
plates (Merck KGaA, Germany). Position of compounds, are shown in Figure 1. The volume of applied chemo-selective
compounds was 1 µL per spot. Each individual spot was approximately 1.5 mm in diameter with the total size of the
sensor array of approximately 2.5 cm x 2.5 cm.
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Since explosives have a low vapor pressure at room temperature experiments were performed at the elevated temperature
of 100 °C. The experiments were of 2 minute duration. Pictures were scanned through an ordinary flatbed scanner
(Epson V750-M Pro Perfection scanner) immediately after immobilization of compounds and after exposure of analytes.
Pictures were obtained at 600 dots per inch in RGB color format. Digital imaging of the colorimetric array before and
after exposure was used for generating of the color difference map by pixel subtraction.
2.3 Colorimetric sensor array in liquid:
For detection of DNT in liquid phase CSC were immobilized on a solid support, a silica gel membrane using the same
technique. To perform the survey 50 mg of DNT was dissolved in 0.5 mL of 96.9 % ethanol (Sigma, St. Louise, MO,
USA), after DNT in ethanol was dissolved in 4.5 mL of Milli-Q water. Hydrochloric acid (Sigma, St. Louise, MO, USA)
was used in concentration of 1% in Milli-Q water. Experiments were performed at room temperature. The silica gel
membrane with immobilized CSCs was dipped into solutions of DNT and remained there for 5 sec. Pictures were
obtained as described above.
2.4 Data analysis:
For colorimetric sensors only the color change is of interest. To indentify the color change, the after exposure image is
aligned with the before exposure image using s a nearest neighbor interpolation scheme where no picture values are
actually changed. In order to extract the color code from each dye the position of each dye is then located. Each dye is
represented using the red, green, and blue color scheme. In this model every color is provided as red, green, and blue
color (RGB); RGB values are given in the 0-255 integer range. The minimum intensity of the color gives black (0;0;0)
and maximum white color (255;255;255). After the dye was located and converted to RGB values, we calculated the
median value of each. We used the median instead of the mean in order to be more robust to noise and outlier pixels
within each dye. The exact procedure that was used is described in a technical report [22].
To create a traditional difference map from the values of red, green or blue the absolute value of the color change is used
as the RGB color scheme does not allow negative values. To visualize the color change a further enhancement is
sometimes made. Usually this involves scaling and shifting of the pixel values [20]. However, elimination of negative
values and the need for scaling in order to make the differences visible imply that color the difference map does not
produce unique representations and further that sign-information is lost. As an alternative we suggest to use bar plots
where each bar represents the cumulative density function (cdf) of color change values.
2.5 Statistical analysis of data.
Data obtained with the colorimetric sensor array has been evaluated by using the principal component analysis (PCA)
method [23]. PCA is a simple, non-parametric method which is relevant to extract data among different analytes into the
minimum number of dimensions. In order to apply the method the difference maps must be represented as a matrix. Each
difference map is represented using 48 color numbers hence each difference map corresponds to a vector is a 48
dimensional space (each map contains 16 dyes and each dye yields 3 values) [20]. We then construct a data matrix where
each column corresponds to a difference map. PCA was applied to this matrix.
3. RESULTS AND DISCUSSIONS
3.1 The sensor working principle.
The colorimetric sensor array described in this paper has been already used to detect and identify different chemical
compounds belongs to the different classes, like amines, alcohols, acids, ketones, aldehydes [20].
Suslick et al. demonstrated the capability of a colorimetric sensor array to sense different analytes, even a mixture of
analytes: formaldehyde emanating by formaldehyde-based resins with detection limit of 20 ppm [24], amines with
detection limit of 600 ppm [7]. There are several examples, like assessment of ethanol concentrations and organic
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molecules in the beer [25], sugar in soft drink samples [26], even explosive, like TATP [27]. This group successfully
uses three of the most known classes of chemo-selective compounds: Lewis acid or base dyes (i.e. metal ion containing
dyes), Brønsted acidic or basic dyes (i.e. pH indicators), and dyes with large permanent dipoles (i.e. zwitterionic
solvatochromic dyes). The specific interaction between molecules and atoms, such us dipolar and multipolar interactions,
acid-based interaction, van der Waals interaction and physical adsorption cost the changes in colors [28]. This
identification method can provide a color pattern which is unique for each specific analyte. The colorimetric sensor array
technique presented by Suslick et al. showed the great potential for real-time monitoring of analytes such as amines,
carboxylic acids, and thiols with extended sensitivity below 6-60 ppm [29].
Using similar detecting principle and a new class of chemo-selective compounds we were able to apply the colorimetric
sensor array not only for screening volatile organic compounds, like acetone, acetic acid, ethanol, formic acid,
hydrochloric acid, methanol, propanol, and toluene, but also for probing DNT, RDX, HMX, and TATP in the gas phase
and DNT in the liquid phase.
The colorimetric sensor array contains 16 chemo-selective compounds immobilized on a solid support, silica gel
membrane, as shown in Figure 1. After the exposure of DNT the colorimetric array has changed color (Fig. 1). The
changes in a color signature indicate the presence of explosives or VOCs. Almost each chosen CSC reacted chemoselectively with the analyte of interest. Data extraction technique is presented in the Material and Method section.
Figure 1. A difference map of the colorimetric sensor array obtained before and after the exposure of DNT in the gas phase.
The image was generated after the mathematical analysis of the color changes. A difference map presents the difference in
absolute value of RGB colors obtained from the absolute value of red, green or blue color after the exposure of DNT minus
the absolute value of red, green or blue color before the exposure of DNT.
3.2 Detection of explosives.
In order to develop sensitive and fast techniques that are able to obtain the important information about hidden
explosives, we first of all need to analyze basic physical and chemical properties of such material, especially vapor
pressure. Since explosives have the low vapor pressure at room temperature [2, 3], to increase the analyte vapor DNT,
RDX, HMX, and TATP was heated up to 100 °C. The colorimetric sensor with the ordinary immobilized dyes was
heated up to 100 °C during 2 minutes together with analytes. Control experiments were performed without presence of
explosives at the room temperature and elevated temperature 100 °C. Results obtained from the analysis presented in the
Figure 2.
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Figure 2. Difference maps of the colorimetric sensor array obtained in the presence of DNT, RDX, HMX, and TATP
at elevated temperature 100 °C. Images were generated after the mathematical calculations of the color changes; and
presented as the difference map obtained from the absolute values of RGB values of each dye spot before and after
the exposure of targets.
According to the evaluated data, CSCs not only were able to detect specific analytes, also they were able to change the
color pattern specifically at the high temperature. This phenomenon can be used for developing a thermal sensor that can
be used together with the chemical sensor.
A difference map was able to compose a unique fingerprint for each explosive. To evaluate the color changes of chemoselective compounds in the presence of explosives, a similar data analysis was performed as described in [20]. The
changes in a color signature of chemo-selective compounds indicate the presence of explosives in gas phase. By using
array of chemo-selective compounds was possible to identify DNT (twelve CSCs gave the signal), TATP (five CSCs
gave the signal), some compounds selectively changed color in the presence of RDX (seven CSCs shown the signal) and
HMX (twelve CSCs gave the signal) (Fig. 2 and 3). We were able to observe that the signal could be achieved much
faster than in 2 minutes within 30-60 sec with a detection limit of 50 ppb and lower.
The simple colorimetric sensor array can be useful in detection and/or identification of volatile organic compounds in
air. Chemo-selective molecules have capability for recognizing specific analytes; this recognition is a function of
intermolecular interactions, basically weak, non-covalent interactions or donor-acceptor interactions. However, only
weak, non-covalent interactions can occur between CSCs and neutral target molecules like DNT, TNT, HMX, RDX or
other explosives. In order to increase sensing properties of CSCs, molecules were merged together with redox-active
groups; the diversity of CSCs compound is presented in [21]. CSCs were successfully applied in the detection of 1,3,5trinitrobenzene, tetrafluoro- p-benzoquinone, tetrachloro-p-benzoquinone, p-benzoquinone, and 1,3,5-trinitrophenole.
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3.3 Data analysis.
However, the traditional approach to drawing color difference map presented in Fig 1 and 2 (with eliminated negative
values) requires scaling in order to make all the differences visible. The signal results can instead be presented as bar
plots (Fig. 3). The Figure 3 shows 48 bars for DNT, TATP, HMX, RDX and control in extracted color code. The bars are
grouped by chemo-selective compound and CSCs are scanned horizontally starting from the left most CSC – i.e. the first
3 bars represent the RBG values of the first dye (the top left) and so forth. This type of plot offers some advantages over
the traditional difference map analysis as no scaling is needed and negative values can be plotted as well.
Each bar is drawn as the experimental cumulative density function (cdf) - a well known function from statistics. In the
case of repeated experiments the results can be encoded into a bar plot as well. The cdf has an initial value of 0 (in the
area below the bars the value of the cdf is 0) and once n event(s) has occurred it will change its value to n/N, where N is
to total amount of events. The value of the cdf can be marked by a color. Further details can be found in the technical
report [22]. A color signature obtained after the mathematical survey indicates that by using the colorimetric sensor array
it is possible to determinate differences not only among various group of chemicals and also distinguish the individual
analyte within the same chemical group.
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DNT
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Figure 3. Analysis and comparison of responses from chemo-selective compounds after exposure of DNT, TATP, RDX and HMX
using the cdf difference plots. Controls are presented as responses obtained at room temperature and at 100 °C. Each bar represents
the cumulative density function (CDF) of the color difference for a particular CSC and color channel R,G,B.
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3.4 Detection in liquid.
The colorimetric sensor array can be also applied for screening different varieties of explosives in the real-time format in
liquid phase. Main characteristics for the colorimetric sensor are based on such parameters as detection limit, the
response time and sensor selectivity. Those characteristics are very important and essential in order to prevent terroristic
attacks, distribution of explosives and in the environmental control. The colorimetric survey can be performed in liquid
phase, for example in water or in organic solvents; analysis in liquid phase can enhance detection properties of the
sensor. Liquids are a good environment for screening of analytes in various concentrations. The signal of relevant
reactions which depending on color changes can be obtained faster in liquid phase than in gas phase. Since our chemoselective compounds have strong donor-acceptor properties it is important to evaluate the interaction between chemoselective compounds, used solvents and membrane stability in liquid phase.
As presented herein, the resulting colorimetric sensor was used for detecting DNT and hydrochloric acid; chemoselective compounds changed color in the presence of analytes (Fig. 4). The strong signal was obtained in the presence
of acids, the color-changes profile shows a significant different between inorganic acid and DNT. By using the chemoselective compounds array was possible to identify the presence of DNT (six CSCs shown the signal) and hydrochloric
acid (seven CSCs shown the signal).
Figure 4. The representation of the colorimetric sensor array obtained in gas and liquid phases before and after the exposure of
DNT and hydrochloric acid. The experiments were of 5 sec in duration. Pictures were scanned through an ordinary flatbed
scanner (Epson V750-M Pro Perfection scanner) immediately after immobilization of chemo-selective compounds – gas phase,
dipping the membrane into Milli-Q water without analytes and with DNT and HCl – liquid phase. Pictures were obtained at 600
dots
per inch in RGB color format.
3.5 Statistical analysis
Data obtained with the colorimetric sensor array has been statistically evaluated by using the principal component
analysis (PCA) method. Figure 5 demonstrates the sensor selectivity, the sensor is capable to detect and identify different
chemical compounds belongs to different classes. According to the statistical analysis the overlap in the response for
different analytes is insignificant (Fig.5). Figure 5 shows the PCA plot for the 1st and 2nd principal components (PC) for
drugs, explosives and environmental measurements. We see that there separation between the three classes can be
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obtained using a few linear cuts except for one measurement with explosives.. However a non-linear or cluster
classification algorithm should be able group the measurements with a low amount of classification errors.
Figure 5. The Principal Component Analysis of the colorimetric sensor array obtained from the response results of illicit drugs
and explosives.
4. CONCLUSION
In this paper we demonstrated different methods such as the color difference map, the CDF difference plot, the PCA
analysis, and those methods are a good alternative for data evaluation obtained from the colorimetric sensor. Since the
effective real-time surveys of explosives and VOCs in low concentrations under the ambient conditions are relatively
scarce. This colorimetric sensor array can be applied for fast, precise screening of the most commonly used explosive in
the real-time format in gas and liquid phases. In the future this array will be probed for detecting of different analytes in
various concentrations and the kinetics of relevant reactions will be calculated.
In practical use the array can be modified for sensing of chemicals poisonous, toxins, illegal compounds, drugs, and
narcotics. However, the progress in this area could clearly be reached with the development of tools, which will combine
inside different sensor system based on the application of classical methods and modern sophisticated techniques with
capability for fast and precise detection of explosives and VOCs in real-time regime as well as quantify the type and
place of hidden compounds.
Colorimetry is a simple method, based on using of CSCs immobilized on the solid support. CSCs can react to exposure
to different molecules. The color difference between the before and after exposure will give a fingerprint of an analyte
and can be used to identify the unknown substance. Another advantage of this technique is that the colorimetric sensor
array is inexpensive approach, and can potentially be produced as single use disposable.
The colorimetric sensor array has been already shown promising results in detection and identification of different
chemical compounds belongs to different classes, like amines, cyanides, alcohols, arenes, ketones, aldehydes, and acids.
In this paper we expanded the application area of CSCs and demonstrated possibilities in detection of DNT, TNT, TATP,
RDX and HMX explosives.
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In practical use the colorimetric sensor array can be deployed in strategic locations such as airports, border inspection
stations and public places to provide real-time and online monitoring of explosives. In the future, the colorimetric sensor
can be developed for screening of food freshness, poisoning compounds and heavy metals in water, poison plastic
materials in industry or houses, in medicine and in environmental monitoring.
5. ACKNOWLEDGEMENTS
We acknowledge the support from the Danish Agency for Science and Technology’s, Program Commission on
Nanoscience Biotechnology and IT (NABIIT). Case number: 2106-07-0031 - Miniaturized sensors for explosives
detection in air. We thank Dr. Heiselberg (Applied Research Branch Danish Defense Acquisition and Logistics
Organization, Denmark) for stimulating and fruitful discussions.
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