Advances in Optical Sensing of Explosive Vapours
Ross N. Gillanders, James M. E. Glackin, Iain A. Campbell, Ifor D. W. Samuel
and Graham A. Turnbull
Keywords: Organic Semiconductors, Conjugated Polymers, Instrumentation.
Abstract: Optical techniques for the detection of explosives are receiving increasing interest due to potentially fast-
responding, highly-sensitive systems. Conjugated polymers are suitable probe materials for this application
since their fluorescence is quenched by electronegative materials including explosives. This can be used to
make a sensor for explosive vapour, which can then give chemical information to help identify explosive
devices, and complements other approaches such as metal detectors and ground penetrating radar. Whilst the
principle has been known for some time, its practical implementation requires considerable development of
instrumentation and materials, including preconcentration materials. This paper reports our current efforts to
address these challenges, with particular emphasis on humanitarian demining and looking towards application
in Improvised Explosive Device (IED) detection.
1 INTRODUCTION
Of the many challenges in explosives detection,
detecting trace amounts of vapour from buried or
otherwise hidden explosives is one that could lead
to huge advances in the field. Conjugated polymers
are well-suited to explosive vapour detection since
they exhibit bright photoluminescence, are solution-
processed, and readily undergo photoluminescence
quenching by electron-deficient materials like
nitroaromatics when exposed to vapours (Rose et
al., 2005, Thomas et al., 2007).
The advantage of using photoluminescence for
sensing the chemical signature of explosives is that
this quenching effect can be monitored by a
photodiode and integrated into user-friendly,
inexpensive, portable instrumentation. Common
explosive/landmine detecting technologies have
disadvantages that can potentially be mitigated by
chemical sensing; for instance, metal detectors can
miss plastic mines, sniffer dogs can be
temperamental, and other methods such as Ion Mass
Spectrometry do not lend themselves to portability.
The use of photoluminescent conjugated polymer
films can be adapted for use in varying architectures
according to the specific requirement whether this
is vapour detection, aqueous environments, forensic
sampling, or moving towards methods for
specificity and selectivity.
This paper gives a brief overview of current
ongoing efforts on the optical detection of explosive
vapours being conducted in our laboratories. By
developing a suite of discrete methods that can
potentially be integrated, there is potential impact to
humanitarian demining and Improvised Explosive
Device (IED) detection.
2 INSTRUMENTATION
One of the main advantages of using
photoluminescent materials is the ability to monitor
emission intensity as it is quenched using
photodiode or CMOS detectors. This then can lead
to portable, modular, inexpensive and user-friendly
systems for detection of explosive vapours in the
field.
We have successfully developed
instrumentation to sense explosive vapours
(Gillanders, 2017), using photoluminescence
quenching from a conjugated polymer excited by an
LED. Photoluminescence was collected by a
Hamamatsu photodiode and data processed by an
off-the-shelf microprocessor (Arduino Uno). This
system successfully detected buried explosive
Gillanders, R., Glackin, J., Campbell, I., Samuel, I. and Turnbull, G.
Advances in Optical Sensing of Explosive Vapours.
DOI: 10.5220/0006729403230327
In Proceedings of the 6th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2018), pages 323-327
ISBN: 978-989-758-286-8
Copyright © 2018 by SCITEPRESS Science and Technology Publications, Lda. All rights reserved
323
vapours with a Limit of Detection (LoD) of 30 ppb
for 2,4-Dinitrotoluene (2,4-DNT), with promising
properties with respect to reproducibility, low-cost,
user-friendliness, and sensitivity.
We have also developed an optical system
based on a CMOS camera and Raspberry Pi
platform (Gillanders et al., 2016). The CMOS
camera-based system provides a route to array
sensing, since the camera is sensitive enough to
monitor photoluminescence quenching in real-time
from multiple polymer sensor films.
These open-source, inexpensive
microprocessors offer benefits to instrumentation
development, including rapid-prototyping, low-
cost, and portability. Close collaboration and
feedback from end-users helps direct the system
development for realistic field deployment
scenarios.
3 MATERIALS & METHODS
3.1 Conjugated Polymer Properties
A number of conjugated polymers are
commercially-available, including Super Yellow
from Merck, and F8BT and Polyfluorene (PFO)
from American Dye Source (Wang et al., 2012,
Bolse et al., 2017). While all three have
demonstrated good sensitivity to nitroaromatics,
each polymer has a distinctive response to various
explosive vapours. For instance, Super Yellow,
illustrated in Figure 1, exhibits strong sensitivity to
2,4-DNT, a common by-product of TNT and
constituent found in many landmines. F8BT, on the
other hand, shows a higher quenching response to
TNT. This difference in response from each
polymer can enable a multi-sensor approach to
“fingerprint” a specific explosive vapour, for
instance by using the CMOS camera-based system
described above to monitor multiple sensing
polymers simultaneously. Thus, by developing a
library of vapour-polymer responses, identification
of the nitroaromatic vapour could be achieved
helping to reduce false positives in the field, which
might arise from distractants such as pesticides.
Films based on these materials have some
drawbacks including lumophore degradation caused
mainly by photo-oxidation when excited under
ambient conditions. However, this may be mitigated
in part by monitoring gradual photoluminescent
degradation of a pristine film, in comparison with
the faster exponential decay of an analyte quenching
event (Gillanders, 2017).
Figure 1: Structure of Merck Super Yellow.
3.2 Rest Sampling
Remote Explosive Scent Tracing (REST) is a
common method for technical surveys of an area to
determine the presence of explosive materials, and
is used after clearance activities for Quality
Assurance (QA) (Lugo et al., 2017). Typical
practice involves sample collection of air and dust
across a suspected site into a polyethylene net filter
prior to sending the sample to a dedicated centre to
be analysed with sniffer dogs. This method is time-
consuming and may be subject to inconsistencies
from the canine behaviour. By adapting the optical
system for use on-site, the timeline of sampling
could potentially be decreased from weeks to hours.
We adapted a method used previously for
Surface Acoustic Wave devices (Houser et al.,
2001, Voiculescu et al., 2006) and separation
science (Egorov et al., 2006, Grate et al., 2007)
where a polymeric material sorbs vapours, which
can then be thermally desorbed by heating to around
60°C. This technique, known as preconcentration,
enables a higher mass of explosive particles to be
collected and delivered to the sensor material.
Figure 2: Sorption of explosive materials to a
fluoropolymer-spotted paper substrate via air-sampling
and subsequent thermal desorption for delivery to sensor.
We used a commercially-available polymer
known as Aflas as the preconcentrating polymer. As
illustrated in Figure 2, the polymer was spotted on a
filter paper substrate then placed in the nozzle of an
air pump and air was sampled for 10 minutes at 60
AOMatSens 2018 - Special Session in Advanced Optical Materials, Sensors and Devices
324
L/min. The filter was transferred to a sealed heating
chamber, heated to 60°C, then a valve was opened
exposing the atmosphere of the preconcentrating
chamber to the Super Yellow sensing film, and the
photoluminescence intensity recorded.
Figure 3: Benchtop tests with an unexposed Aflas-spotted
filter paper (Control, black line), uncoated filter paper
(Paper, green line), and an Aflas-spotted filter paper (red
line) exposed to DNT vapour.
The quenching response of Super Yellow to
preconcentrated 2,4-DNT vapour is shown in Figure
3. The Control line, which was a clean Aflas-spotted
filter paper, gives a quenching response of around
25%, which can be attributed to thermal
degradation. The unspotted filter paper gives a
slightly higher quenching response of 30%, which
suggests the filter paper can sorb explosives to some
degree. The Aflas-spotted substrate gives a
quenching response of over 50%, indicating that the
material is well-suited as a preconcentrating
material. The advantage to this is that the material is
inexpensive, solution-processed, and can be
prepared in large batches by a variety of coating
methods, including spotting, dip-coating, and blade-
coating.
3.3 Swabbing
Swabbing is widely-used in security applications or
in post-blast forensics (Borusiewicz et al., 2013,
Ceco et al., 2014) to sample trace particles of
explosives. Swabbing is commonly seen in airports,
where hand-luggage is swabbed prior to analysis by
Ion Mass Spectrometry.
The preconcentration materials described in
Section 3.2 may also be deployed in a swabbing
procedure where it could be used to pick up
explosive residues on luggage, packages, door
handles and other objects. This has particular
potential for IED detection where fast positive
detection can be crucial in the timeline of a terrorist
plot. Initial results indicate this is a promising tool
for fast detection of explosive residues on common
objects. Work is ongoing to improve
instrumentation to allow in-field detection with
minimum false positives.
3.4 Explosives in Aqueous
Environments
Detection of explosives in aqueous environments is
a challenge spanning landmines in humid or
waterlogged environments, port security,
wastewater monitoring for detection of IED
factories, post-detonation forensics, and water
quality monitoring in contaminated sites, including
munitions factories. However, conjugated polymer
films typically are not robust enough for aqueous
deployment, especially over long periods of time.
Ormosils have been applied to
photoluminescent polymer films as barrier coatings
to exclude water from the sensor layer (Gillanders
et al., 2018). However, the structure of the material
allows the analyte to diffuse through the pores,
resulting in a reversible, robust explosives sensor
for aqueous environment. Figure 4 shows the typical
photoluminescence response of a Super Yellow film
coated in an ormosil layer to concentrations of 2,4-
DNT from 7.2 M to 7.2 mM, with a Limit of
Detection of 8.24 M (0.22mg/L).
Figure 4: Photoluminescenct quenching response of an
ormosil-coated Super Yellow layer to concentrations of
2,4-DNT from 7.2 M to 7.2 mM.
Preliminary work has been performed on
extraction of explosive materials from soil for
detection by optical interrogation. This allows for
soil samples to be analysed for the presence of
explosives, and potentially identified using
specificity or selectivity tests. Figure 5 shows
extraction of 2,4-DNT from soil. The concentrations
spiked into the soil samples were 600 mg/L, with
106 mg/L, 228 mg/L and 260 mg/L recovered. For
Advances in Optical Sensing of Explosive Vapours
325
comparison, the typical level of TNT found in
wastewater of a TNT factory is 156 mg/L (Barreto-
Rodrigues et al., 2009). With the sensitivity shown
by the ormosil-coated aqueous explosives sensor,
these typical levels of nitroaromatic compounds can
be detected in the field.
Figure 5: Calibration curve of DNT in Acetonitrile. Three
identical vials of 3g soil spiked with 600 mg/L DNT in
water were added (absolute mass approx. 2 mg DNT per
vial), left overnight, sonicated with MeCN then the
solvent extracted and run through a HPLC.
4 CONCLUSIONS & OUTLOOK
The use of light-emitting polymers for nitroaromatic
sensing has been described in several different
configurations, including vapour sampling, REST
sampling, and aqueous detection, to help address the
many challenges in explosives detection. While all
of the strands in this research are in progress, efforts
have been made particularly towards portable
optical sensing systems which have been developed
and characterised, and further development of sol-
gel barrier films for aqueous sensing, and imprinting
for specificity are being investigated. Counter-IED
applications of these technologies and methods are
under increasing focus since global interest in
explosives detection is tending towards IED
detection in areas including anti-terrorism activities,
airport security, and crowd screening. Parallel
development of instrumentation with materials and
methods for increased specificity and selectivity can
lead to reliable in-field detection, which can
potentially have a positive impact on humanitarian
demining and Counter-IED activities across the
world.
ACKNOWLEDGEMENTS
This project has received funding from the
European Union’s Seventh Framework Programme
for research, technological development and
demonstration under agreement no 284747, and the
EPSRC under EP/K503940/1, EP/K503162/1, and
EP/N509759/1. IDWS acknowledges a Royal
Society Wolfson Research Merit Award.
REFERENCES
Barreto-Rodrigues, M., Silva, F. T. & Paiva, T. C. B.
2009. Characterization of wastewater from the
Brazilian TNT industry. Journal of Hazardous
Materials, 164, 385-388.
Bolse, N., Eckstein, R., Schend, M., Habermehl, A.,
Eschenbaum, C., Hernandez-Sosa, G. & Lemmer, U.
2017. A digitally printed optoelectronic nose for the
selective trace detection of nitroaromatic explosive
vapours using fluorescence quenching. Flexible and
Printed Electronics, 2, 024001.
Borusiewicz, R., Zadora, G. & Zieba-Palus, J. 2013.
Chemical analysis of post explosion samples obtained
as a result of model field experiments. Talanta, 116,
630-636.
Ceco, E., Onnerud, H., Menning, D., Gilljam, J. L., Baath,
P. & Ostmark, H. 2014. Stand-off imaging Raman
spectroscopy for forensic analysis of post-blast scenes
- Trace detection of ammonium nitrate and 2,4,6-
trinitrotoluene. SPIE Defense, Security + Sensing
Symposium / 15th Annual Meeting on Chemical,
Biological, Radiological, Nuclear, and Explosives
(CBRNE) Sensing, May 05-09 2014 Baltimore, MD.
BELLINGHAM: Spie-Int Soc Optical Engineering.
Egorov, O. B., O'Hara, M. J. & Grate, J. W. 2006.
Equilibration-based preconcentrating minicolumn
sensors for trace level monitoring of radionuclides
and metal ions in water without consumable reagents.
Analytical Chemistry, 78, 5480-5490.
Gillanders, R. N., Campbell, I. A., Chen, F., Morawska,
P., Samuel, I. D. W. & Turnbull, G. A. A Portable,
Low-cost System for Optical Explosive Detection
based on a CMOS Camera. Proceedings of the 4th
International Conference on Photonics, Optics and
Laser Technology - Volume 1: PHOTOPTICS, 2016.
130-134.
Gillanders, R. N., Campbell, I. A., Glackin, J. M. E.,
Samuel, I. D. W. & Turnbull, G. A. 2018. Ormosil-
coated conjugated polymers for the detection of
explosives in aqueous environments. Talanta, 179,
426-429.
Gillanders, R. N., Samuel, I.D.W., Turnbull, G.A. 2017.
A Low-Cost, Portable Optical Explosive-vapour
Sensor. Sensors and Actuators B-Chemical, 245, 334-
340.
AOMatSens 2018 - Special Session in Advanced Optical Materials, Sensors and Devices
326
Grate, J. W., Ozanich, R., Hartman, J. S., O'Hara, M. J.,
Egorov, O. B. & IEEE 2007. Preconcentrating
minicolumn sensors for trace environmental
monitoring. 2007 Ieee Sensors, Vols 1-3.
Houser, E. J., Mlsna, T. E., Nguyen, V. K., Chung, R.,
Mowery, R. L. & McGill, R. A. 2001. Rational
materials design of sorbent coatings for explosives:
applications with chemical sensors. Talanta, 54, 469-
485.
Lugo, J. H., Zoppi, M. & Molfino, R. 2017. Design and
Kinematic Modeling of a Screw-Propelled Mobile
Robot to Perform Remote Explosive Scent Tracing
Filter Sampling in Forest during Humanitarian
Demining. Advances in Cooperative Robotics, 699-
715.
Rose, A., Zhu, Z. G., Madigan, C. F., Swager, T. M. &
Bulovic, V. 2005. Sensitivity gains in chemosensing
by lasing action in organic polymers. Nature, 434,
876-879.
Thomas, S. W., III, Joly, G. D. & Swager, T. M. 2007.
Chemical sensors based on amplifying fluorescent
conjugated polymers. Chemical Reviews, 107, 1339-
1386.
Voiculescu, I., McGill, R. A., Zaghloul, M. E., Mott, D.,
Stepnowski, J., Stepnowski, S., Summers, H.,
Nguyen, V., Ross, S., Walsh, K. & Martin, M. 2006.
Micropreconcentrator for enhanced trace detection of
explosives and chemical agents. Ieee Sensors Journal,
6, 1094-1104.
Wang, Y., Yang, Y., Turnbull, G. A. & Samuel, I. D. W.
2012. Explosive Sensing Using Polymer Lasers.
Molecular Crystals and Liquid Crystals, 554, 103-
110.
Advances in Optical Sensing of Explosive Vapours
327