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

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